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# The Bottom Line: Integument, Fascia, and Anatomical Spaces
Integumentary system: The integumentary system (the skin) consists of the epidermis, dermis, and specialized structures (hair follicles, sebaceous glands, and sweat glands). The skin: plays important roles in protection, containment, heat regulation, and sensation;
synthesizes and stores vitamin D; and features tension lines, relating to the predominant direction of collagen fibers in the skin, that have implications for surgery and wound healing. Subcutaneous tissue, located beneath the dermis, contains most of the bodys fat stores.
Fascias and bursae: Deep fascia is an organized connective tissue layer that completely envelops the body beneath the subcutaneous tissue underlying the skin. Extensions and modifications of the deep fascia: divide muscles into groups (intermuscular septa),
invest individual muscles and neurovascular bundles (investing fascia), lie between musculoskeletal walls and the serous membranes lining body cavities (subserous fascia), and hold tendons in place during joint movements (retinacula). Bursae are closed sacs formed of serous membrane that occur in locations subject to friction; they enable one structure to move freely over another.
# SKELETAL SYSTEM
The skeletal system may be divided into two functional parts (Fig. 1.11): The axial skeleton consists of the bones of the head (cranium or skull), neck (hyoid bone and cervical vertebrae), and trunk (ribs, sternum, vertebrae, and sacrum). The appendicular skeleton consists of the bones of the limbs, including those forming the pectoral (shoulder) and pelvic girdles. FIGURE 1.11. Skeletal system.
# Cartilage and Bones
The skeleton is composed of cartilages and bones. Cartilage is a resilient, semirigid form of connective tissue that forms parts of the skeleton where more flexibility is requiredfor example, where the costal cartilages attach the ribs to the sternum. Also, the articulating surfaces (bearing surfaces) of bones participating in a synovial joint are capped with articular cartilage that provides smooth, low-friction, gliding surfaces for free movement (see Fig. 1.16A). Blood vessels do not enter cartilage (i.e., it is avascular); consequently, its cells obtain oxygen and nutrients by diffusion. The proportion of bone and cartilage in the skeleton changes as the body grows; the younger a person is, the more cartilage he or she has. The bones of a newborn are soft and flexible because they are mostly composed of cartilage.
Bone , a living tissue, is a highly specialized, hard form of connective tissue that makes up most of the skeleton. Bones of the adult skeleton provide: Support for the body and its vital cavities; it is the chief supporting tissue of the body Protection for vital structures (e.g., the heart) The mechanical basis for movement (leverage) Storage for salts (e.g., calcium) A continuous supply of new blood cells (produced by the marrow in the medullary cavity of many bones) A fibrous connective tissue covering surrounds each skeletal element like a sleeve, except where articular cartilage occurs; that surrounding bones is periosteum (see Fig. 1.15), whereas
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that around cartilage is perichondrium . The periosteum and perichondrium nourish the external aspects of the skeletal tissue. They are capable of laying down more cartilage or bone (particularly during fracture healing) and provide the interface for attachment of tendons and ligaments. The two types of bone are compact bone and spongy (trabecular) bone . They are distinguished by the relative amount of solid matter and by the number and size of the spaces they contain (Fig. 1.12). All bones have a superficial thin layer of compact bone around a central mass of spongy bone, except where the latter is replaced by a medullary (marrow) cavity. Within the medullary cavity of adult bones, and between the spicules (trabeculae) of spongy bone, yellow (fatty) or red (blood cell and platelet forming) bone marrowor a combination of both is found.
> FIGURE 1.12. Transverse sections of femur. The shaft of a living bone is a tube of compact bone that surrounds a medullary cavity.
The architecture and proportion of compact and spongy bone vary according to function. Compact bone provides strength for weight bearing. In long bones designed for rigidity and attachment of muscles and ligaments, the amount of compact bone is greatest near the middle of the shaft where the bones are liable to buckle. In addition, long bones have elevations (e.g., ridges, crests, and tubercles) that serve as buttresses (supports) where large muscles attach. Living bones have some elasticity (flexibility) and great rigidity (hardness). Classification of Bones
Bones are classified according to their shape. Long bones are tubular (e.g., the humerus in the arm). Short bones are cuboidal and are found only in the tarsus (ankle) and carpus (wrist). Flat bones usually serve protective functions (e.g., the flat bones of the cranium protect the brain). Irregular bones have various shapes other than long, short, or flat (e.g., bones of the face). Sesamoid bones (e.g., the patella or knee cap) develop in certain tendons and are found where tendons cross the ends of long bones in the limbs; they protect the tendons from excessive wear and often change the angle of the tendons as they pass to their attachments.
# Bone Markings and Formations
Bone markings appear wherever tendons, ligaments, and fascias are attached or where arteries lie adjacent to or enter bones. Other formations occur in relation to the passage of a tendon (often to direct the tendon or improve its leverage) or to control the type of movement occurring at a joint. Some of the various markings and features of bones are (Fig. 1.13): Body: the principal mass of a bone; with long bones, the shaft of the bone; with vertebrae, the anterior, weight-bearing portions between interventricular discs Capitulum: small, round, articular head (e.g., capitulum of the humerus) Condyle: rounded, knuckle-like articular area, often occurring in pairs (e.g., the lateral and medial femoral condyles) Crest: ridge of bone (e.g., the iliac crest) Epicondyle: eminence superior or adjacent to a condyle (e.g., lateral epicondyle of the humerus) Facet: smooth flat area, usually covered with cartilage, where a bone articulates with another bone (e.g., superior costal facet on the body of a vertebra for articulation with a rib) Foramen: passage through a bone (e.g., obturator foramen) Fossa: hollow or depressed area (e.g., infraspinous fossa of the scapula) Groove: elongated depression or furrow (e.g., radial groove of the humerus) Head (L. caput): large, round articular end (e.g., head of the humerus) Line: linear elevation, sometimes called a ridge (e.g., soleal line of the tibia). Malleolus: rounded process (e.g., lateral malleolus of the fibula) Neck: relatively narrow portion adjacent to the head Notch: indentation at the edge of a bone (e.g., greater sciatic notch) Process: an extension or projection serving a particular purpose, having a characteristic shape, or extending in a particular direction (e.g., articular process, spinous process, or transverse process of a vertebra) Protuberance: a bulge or projection of bone (e.g., external occipital protuberance) Shaft: the diaphysis, or body, of a long bone
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Spine: thorn-like process (e.g., the spine of the scapula) Trochanter: large blunt elevation (e.g., greater trochanter of the femur) Trochlea: spool-like articular process or process that acts as a pulley (e.g., trochlea of the humerus) Tubercle: small raised eminence (e.g., greater tubercle of the humerus) Tuberosity: large rounded elevation (e.g., ischial tuberosity)
> FIGURE 1.13. Bone markings and formations. Markings appear on bones wherever tendons, ligaments, and fascia attach. Other formations relate to joints, the passage of tendons, and the provision of increased leverage.
# Bone Development
Most bones take many years to grow and mature. The humerus (arm bone), for example, begins to ossify at the end of the embryonic period (8 weeks); however, ossification is not complete until age 20. All bones derive from mesenchyme (embryonic connective tissue) by two different processes: intramembranous ossification (directly from mesenchyme) and endochondral ossification (from cartilage derived from mesenchyme). The histology (microscopic structure) of a bone is the same by either process (Pawlina, 2020). The two processes of bone development proceed as follows: In intramembranous ossification (membranous bone formation), mesenchymal models of bones form during the embryonic period, and direct ossification of the mesenchyme begins in the fetal period. In endochondral ossification (cartilaginous bone formation), cartilage models of the bones form from mesenchyme during the fetal period, and bone subsequently replaces most of the cartilage. A brief description of endochondral ossification helps explain how long bones grow (Fig. 1.14). The mesenchymal cells condense and differentiate into chondroblasts, dividing cells in growing cartilage tissue, thereby forming a cartilaginous bone model. In the midregion of the model, the cartilage calcifies (becomes impregnated with calcium salts), and periosteal capillaries (capillaries from the fibrous sheath surrounding the model) grow into the calcified cartilage of the bone model and supply its interior. These blood vessels, together with associated osteogenic (bone-forming) cells, form a periosteal bud (Fig. 1.14A). The capillaries initiate the
primary ossification center , so named because the bone tissue it forms replaces most of the cartilage in the main body of the bone model. The shaft of a bone ossified from the primary ossification center is the diaphysis , which grows as the bone develops.
> FIGURE 1.14. Development and growth of a long bone. A. Ossification centers. The formation of primary and secondary ossification centers is shown. B. Growth of long bones. Growth in length occurs on both sides of the cartilaginous epiphysial plates (arrowheads). The bone formed from the primary center in the diaphysis does not fuse with that formed from the secondary centers in the epiphyses until the bone reaches its adult size. When growth ceases, the depleted epiphysial plate is replaced by a synostosis (bone-to-bone fusion), observed as an epiphysial line in radiographs
ALGRAWANY and sectioned bone.
Most secondary ossification centers appear in other parts of the developing bone after birth; the parts of a bone ossified from these centers are epiphyses . The chondrocytes in the middle of the epiphysis hypertrophy, and the bone matrix (extracellular substance) between them calcifies. Epiphysial arteries grow into the developing cavities with associated osteogenic cells. The flared part of the diaphysis nearest the epiphysis is the metaphysis . For growth to continue, the bone formed from the primary center in the diaphysis does not fuse with that formed from the secondary centers in the epiphyses until the bone reaches its adult size. Thus, during growth of a long bone, cartilaginous epiphysial plates intervene between the diaphysis and epiphyses (Fig. 1.14B). These growth plates are eventually replaced by bone at each of its two sides, diaphysial and epiphysial. When this occurs, bone growth ceases and the diaphysis fuses with the epiphyses. The seam formed during this fusion process (synostosis) is particularly dense and is recognizable in sectioned bone or radiographs as an epiphysial line (Fig. 1.15). The epiphysial fusion of bones occurs progressively from puberty to maturity. Ossification of short bones is similar to that of the primary ossification center of long bones, and only one short bone, the calcaneus (heel bone), develops a secondary ossification center.
> FIGURE 1.15. Vasculature and innervation of a long bone.
# Vasculature and Innervation of Bones
Bones are richly supplied with blood vessels. Most apparent are the nutrient arteries (one or more per bone) that arise as independent branches of adjacent arteries outside the periosteum and pass obliquely through the compact bone of the shaft of a long bone via nutrient foramina . The nutrient artery divides in the medullary cavity into longitudinal branches that proceed toward each end, supplying the bone marrow, spongy bone, and deeper portions of the compact bone (Fig. 1.15). However, many small branches from the periosteal arteries of the periosteum are responsible for nourishment of most of the compact bone. Consequently, a bone from which the periosteum has been removed dies. Blood reaches the osteocytes (bone cells) in the compact bone by means of haversian systems or osteons (microscopic canal systems) that house small blood vessels. The ends of the bones are supplied by metaphyseal and epiphysial arteries that arise mainly from the arteries that supply the joints. In the limbs, these arteries are typically part of a peri-articular arterial plexus, which surrounds the joint, ensuring blood flow distal to the joint regardless of the position assumed by the joint. Veins accompany arteries through the nutrient foramina. Many large veins also leave through foramina near the articular ends of the bones. Bones containing red bone marrow have numerous large veins. Lymphatic vessels are also abundant in the periosteum. Nerves accompany blood vessels supplying bones. The periosteum is richly supplied with sensory nerves periosteal nerves that carry pain fibers. The periosteum is especially sensitive to tearing or tension, which explains the acute pain from bone fractures. Bone itself is relatively sparsely supplied with sensory endings. Within bones, vasomotor nerves cause constriction or dilation of blood vessels, regulating blood flow through the bone marrow.
# C L I N I C A L B O X
# BONES Accessory (Supernumerary) Bones
Accessory (supernumerary) bones develop when additional ossification centers appear and form extra bones. Many bones develop from several centers of ossification, and the separate parts normally fuse. Sometimes one of these centers fails to fuse with the main bone, giving the appearance of an extra bone. Careful study shows that the apparent extra bone is a missing part of the main bone. Circumscribed areas of bone are often seen along the sutures of the cranium where the flat bones abut, particularly those related to the parietal bone (see Chapter 8, Head). These small, irregular, worm-like bones are sutural bones (wormian bones). It is important to know that accessory bones are common in the foot to avoid mistaking them for bone fragments in radiographs and other medical images.
# Heterotopic Bones
Bones sometimes form in soft tissues where they are not normally present (e.g., in scars). Horse riders often develop heterotopic bones in their thighs (riders bones),
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probably because of chronic muscle strain resulting in small hemorrhagic (bloody) areas that undergo calcification and eventual ossification.
# Trauma to Bone and Bone Changes
Bones are living organs that cause pain when injured, bleed when fractured, remodel in relationship to stresses placed on them, and change with age. Like other organs, bones have blood vessels, lymphatic vessels, and nerves, and they may become diseased. Unused bones, such as in a paralyzed limb, atrophy (decrease in size). Bone may be absorbed, which occurs in the mandible when teeth are extracted. Bones hypertrophy (enlarge) when they support increased weight for a long period. Trauma to a bone may break it. For the fracture to heal properly, the broken ends must be brought together, approximating their normal position. This is called reduction of a fracture. During bone healing, the surrounding fibroblasts (connective tissue cells) proliferate and secrete collagen, which forms a collar of callus to hold the bones together (Fig. B1.4). Bone remodeling occurs in the fracture area, and the callus calcifies. Eventually, the callus is resorbed and replaced by bone. After several months, little evidence of the fracture remains, especially in young people. Fractures are more common in children than in adults because of the combination of their slender, growing bones and carefree activities. Fortunately, many of these breaks are greenstick fractures (incomplete breaks caused by bending of the bones). Fractures in growing bones heal faster than those in adult bones.
> FIGURE B1.4. Bone fracture healing.
# Osteoporosis
During the aging process, the organic and inorganic components of bone both decrease, often resulting in osteoporosis, a reduction in the quantity of bone, or atrophy of skeletal tissue (Fig. B1.5). Hence, the bones become brittle, lose their elasticity, and fracture easily. Bone scanning is an imaging method used to assess normal and diminished bone mass (see Medical Imaging Techniques at the end of this chapter).
> FIGURE B1.5. Osteoporosis.
# Sternal Puncture
Examination of bone marrow provides valuable information for evaluating hematological (blood) diseases. Because it lies just beneath the skin (i.e., is subcutaneous) and is easily accessible, the sternum (breast bone) is a commonly used site for harvesting bone marrow. During a sternal puncture, a wide-bore (large-diameter) needle is inserted through the thin cortical bone into the spongy bone. A sample of red bone marrow is aspirated with a syringe for laboratory examination. Bone marrow transplantation is sometimes performed in the treatment of leukemia. If vascular collapse has occurred in a patient in shock, fluids may be rapidly infused by needle into the bone marrow of the tibia (preferred) or the sternum.
# Bone Growth and Assessment of Bone Age
Knowledge of the sites where ossification centers occur, the times of their appearance, the rates at which they grow, and the times of fusion of the sites (times when synostosis occurs) is important in clinical medicine, forensic science, and anthropology. A general index of growth during infancy, childhood, and adolescence is indicated by bone age, as determined from radiographs, usually of the hands (Fig. B1.6). The age of a young person can be determined by studying the ossification centers in the bones. The main criteria are (1) the appearance of calcified material in ossification centers, such as the diaphysis and/or epiphyses of long bones, and (2) the narrowing and disappearance of the radiolucent (dark) line representing the epiphysial plate (absence of this line indicates that epiphysial fusion has occurred; fusion occurs at specific times for each epiphysis). The fusion of epiphyses with the diaphysis occurs 1 to 2 years earlier in females than in males. Determining bone age can be helpful in predicting adult height in early- or late-maturing adolescents. Assessment of bone age also helps establish the approximate age of human skeletal remains in medicolegal cases.
> ALGRAWANY FIGURE B1.6. Anterior radiographs. Right hand of a 2.5-year-old ( A) and an 11-year-old ( B).
# Effects of Disease, Diet, and Trauma on Bone Growth
Some diseases produce early epiphysial fusion (ossification time) compared with what is normal for the persons chronological age; other diseases result in delayed fusion. The growing skeleton is sensitive to relatively slight and transient illnesses and to periods of malnutrition. Proliferation of cartilage at the metaphyses slows down during starvation and illness, but the degeneration of cartilage cells in the columns continues, producing a dense line of provisional calcification. These lines later become bone with thickened trabeculae, or lines of arrested growth. Fractures involving the epiphyses may cause stunting of bone growth.
# Displacement and Separation of Epiphyses
Without knowledge of bone growth and the appearance of bones in radiographic and other diagnostic images at various ages, a displaced epiphysial plate could be mistaken for a fracture, and separation of an epiphysis could be interpreted as a displaced piece of a fractured bone. Knowing the patients age and the location of epiphyses can prevent these anatomical errors. The edges of the diaphysis and epiphysis are smoothly curved in the region of the epiphysial plate. Bone fractures always leave a sharp, often uneven edge of bone. An injury that causes a fracture in an adult usually causes the displacement of an epiphysis in a child.
# Avascular Necrosis
Loss of arterial supply to an epiphysis or other parts of a bone results in the death of bone tissueavascular necrosis (G. nekrosis, deadness). After every fracture, small areas of adjacent bone undergo necrosis. In some fractures, avascular necrosis of a large fragment of bone may occur. A number of clinical disorders of epiphyses in children result from avascular necrosis of unknown etiology (cause). These disorders are referred to as osteochondroses.
# Joints
Joints (articulations) are unions or junctions between two or more bones or rigid parts of the skeleton. Joints exhibit a variety of forms and functions. Some joints have no movement, such as the epiphysial plates between the epiphysis and diaphysis of a growing long bone; others allow only slight movement, such as teeth within their sockets; and some are freely movable, such as the glenohumeral (shoulder) joint.
CLASSIFICATION OF JOINTS
Three classes of joints are described, based on the manner or type of material by which the articulating bones are united (Fig. 1.16): 1. The articulating bones of synovial joints are united by a joint (articular) capsule (composed of an outer fibrous layer lined by a serous synovial membrane ) spanning and enclosing a joint or articular cavity. The joint cavity of a synovial joint, like the knee, is a potential space that contains a small amount of lubricating synovial fluid , secreted by the synovial membrane. Inside the capsule, articular cartilage covers the articulating surfaces of the bones; all other internal surfaces are covered by synovial membrane. The bones in Figure 1.16A, normally closely apposed, have been pulled apart for demonstration, and the joint capsule has been inflated. Consequently, the normally potential joint cavity is exaggerated. The periosteum investing the participating bones external to the joint blends with the fibrous layer of the joint capsule. 2. The articulating bones of fibrous joints are united by fibrous tissue. The amount of movement occurring at a fibrous joint depends in most cases on the length of the fibers uniting the articulating bones. The sutures of the cranium are examples of fibrous joints (Fig. 1.16B). These bones are held close together, either interlocking along a wavy line or overlapping. A syndesmosis type of fibrous joint unites the bones with a sheet of fibrous tissue, either a ligament or a fibrous membrane. Consequently, this type of joint is partially movable. The interosseous membrane in the forearm is a sheet of fibrous tissue that joins the radius and ulna in a syndesmosis. A dento-alveolar syndesmosis (gomphosis or socket) is a fibrous joint in which a peg-like process fits into a socket, forming an articulation between the root of the tooth and the alveolar process of the jaw. Mobility of this joint (a loose tooth) indicates a pathological state affecting the supporting tissues of the tooth. However, microscopic movements here give us information (via the sense of proprioception) about how hard we are biting or clenching our teeth and whether we have a particle stuck between our teeth. 3. The articulating structures of cartilaginous joints are united by hyaline cartilage or fibrocartilage. In primary cartilaginous joints , or synchondroses, the bones are united by hyaline cartilage, which permits slight bending during early life. Primary cartilaginous joints
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are usually temporary unions, such as those present during the development of a long bone (Figs. 1.14 and 1.16C), where the bony epiphysis and the shaft are joined by an epiphysial plate. Primary cartilaginous joints permit growth in the length of a bone. When full growth is achieved, the epiphysial plate converts to bone and the epiphyses fuse with the diaphysis.
Secondary cartilaginous joints , or symphyses, are strong, slightly movable joints united by fibrocartilage. The fibrocartilaginous intervertebral discs (Fig. 1.16C) between the vertebrae consist of binding connective tissue that joins the vertebrae together. Cumulatively, these joints provide strength and shock absorption as well as considerable flexibility to the vertebral column (spine).
> FIGURE 1.16. Classes of joints with examples. A. Synovial (freely moveable) joints. A schematic model and the more complex shoulder joint are shown. B. Fibrous joints. Three types of this class are shown. C. Cartilaginous joints. Primary and secondary cartilaginous joints are shown.
Synovial joints, the most common type of joint, provide free movement between the bones they join; they are joints of locomotion, typical of nearly all limb joints. Synovial joints are usually reinforced by accessory ligaments that are either separate (extrinsic) or are a thickening of a portion of the joint capsule (intrinsic). Some synovial joints have other distinguishing features, such as a fibrocartilaginous articular disc or meniscus, which are present when the articulating surfaces of the bones are incongruous (Fig. 1.16A). The six major types of synovial joints are classified according to the shape of the articulating surfaces and/or the type of movement they permit (Fig. 1.17): 1. Plane joints permit gliding or sliding movements in the plane of the articular surfaces. The opposed surfaces of the bones are flat or almost flat, with movement limited by their tight joint capsules. Plane joints are numerous and are nearly always small. An example is the acromioclavicular joint between the acromion of the scapula and the clavicle. 2. Hinge joints permit flexion and extension only, movements that occur in one plane (sagittal) around a single axis that runs transversely; thus, hinge joints are uniaxial joints. The joint capsule of these joints is thin and lax anteriorly and posteriorly where movement occurs; however, the bones are joined by strong, laterally placed collateral ligaments. The elbow joint is a hinge joint. 3. Saddle joints permit abduction and adduction as well as flexion and extension, movements occurring around two axes at right angles to each other; thus, saddle joints are biaxial joints that allow movement in two planes, sagittal and frontal. The performance of these movements in a circular sequence (circumduction) is also possible. The opposing articular surfaces are shaped like a saddle (i.e., they are reciprocally concave and convex). The carpometacarpal joint at the base of the 1st digit (thumb) is a saddle joint. 4. Condyloid joints permit flexion and extension as well as abduction and adduction; thus, condyloid joints are also biaxial. However, movement in one plane (sagittal) is usually greater (freer) than in the other. Circumduction, more restricted than that of saddle joints, is also possible. The metacarpophalangeal joints (knuckle joints) are condyloid joints. 5. Ball and socket joints allow movement in multiple axes and planes: flexion and extension, abduction and adduction, medial and lateral rotation, and circumduction; thus, ball and socket joints are multiaxial joints. In these highly mobile joints, the spheroidal surface of one bone moves within the socket of another. The hip joint is a ball and socket joint in which the spherical head of the femur rotates within the socket formed by the acetabulum of the hip bone. 6. Pivot joints permit rotation around a central axis; thus, they are uniaxial. In these joints, a rounded process of bone rotates within a sleeve or ring. The median atlanto-axial joint is a pivot joint in which the atlas (C1 vertebra) rotates around a finger-like process, the dens of the axis (C2 vertebra), during rotation of the head.
> ALGRAWANY FIGURE 1.17. Six types of synovial joints. Synovial joints are classified according to the shape of their articulating surfaces and/or the type of movement they permit.
JOINT VASCULATURE AND INNERVATION
Joints receive blood from articular arteries that arise from the vessels around the joint. The arteries often anastomose (communicate) to form networks (peri-articular arterial anastomoses) to ensure a blood supply to and across the joint in the various positions assumed by the joint.
Articular veins are communicating veins that accompany arteries (L. venae comitantes) and, like the arteries, are located in the joint capsule, mostly in the synovial membrane. Joints have a rich nerve supply provided by articular nerves with sensory nerve endings in the joint capsule. In the distal parts of the limbs (hands and feet), the articular nerves are branches of the cutaneous nerves supplying the overlying skin. However, most articular nerves are branches of nerves that supply the muscles that cross and therefore move the joint. The Hilton law 1
(more a rule of thumb) indicates that the nerves supplying a joint also supply the muscles moving the joint and, added later, the skin covering their distal attachments (Ellis & Mahadevan, 2019). Articular nerves transmit sensory impulses from the joint that contribute to the sense of
proprioception , which provides an awareness of movement and position of the parts of the body. The synovial membrane is relatively insensitive. Pain fibers are numerous in the fibrous layer of the joint capsule and the accessory ligaments, causing considerable pain when the joint is injured. The sensory nerve endings respond to the twisting and stretching that occurs during sports activities.
# C L I N I C A L B O X
# JOINTS Joints of Newborn Cranium
The bones of the calvaria (skullcap) of a newborn infants cranium do not make full contact with each other (Fig. B1.7). At these sites, the sutures form wide areas of fibrous tissue called fontanelles. The anterior fontanelle is the most prominent; laypeople call it the soft spot. The fontanelles in a newborn are often felt as ridges because of the overlapping of the cranial bones by molding of the calvaria as it passes through the birth canal. Normally, the anterior fontanelle is flat. A bulging fontanelle may indicate increased intracranial pressure; however, the fontanelle normally bulges during crying. Pulsations of the fontanelle reflect the pulse of cerebral arteries. A depressed fontanelle may be observed when the neonate is dehydrated (Swartz, 2021).
> FIGURE B1.7. Neonatal (newborn) cranium.
# Degenerative Joint Disease
Synovial joints are well designed to withstand wear, but heavy use over several years can cause degenerative changes. Some destruction is inevitable during such activities as jogging, which wears away the articular cartilages and sometimes erodes the underlying articulating surfaces of the bones. The normal aging of articular cartilage begins early in adult life and progresses slowly thereafter, occurring on the ends of
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the articulating bones, particularly those of the hip, knee, vertebral column, and hands (Salter, 1998). These irreversible degenerative changes in joints result in the articular cartilage becoming a less effective shock absorber and lubricated surface. As a result, the articulation becomes increasingly vulnerable to the repeated friction that occurs during joint movements. In some people, these changes do not produce significant symptoms; in others, they cause considerable pain. Degenerative joint disease or osteoarthritis is often accompanied by stiffness, discomfort, and pain. Osteoarthritis is common in older people and usually affects joints that support the weight of their bodies (e.g., the hips and knees). Most substances in the bloodstream, normal or pathological, easily enter the joint cavity. Similarly, traumatic infection of a joint may be followed by arthritis, inflammation of a joint, and septicemia, blood poisoning.
# Arthroscopy
The cavity of a synovial joint can be examined by inserting a cannula and an arthroscope (a small telescope) into it. This surgical procedurearthroscopy enables orthopedic surgeons to examine joints for abnormalities, such as torn menisci (partial articular discs of the knee joint). Some surgical procedures can also be performed during arthroscopy (e.g., by inserting instruments through small puncture incisions). Because the opening in the joint capsule for inserting the arthroscope is small, healing is more rapid after this procedure than after traditional joint surgery.
# The Bottom Line: Skeletal System
Cartilage and bones: The skeletal system can be divided into the axial (bones of the head, neck, and trunk) and appendicular skeletons (bones of the limbs). The skeleton itself is composed of several types of tissue: cartilage, a semirigid connective tissue; bone, a hard form of connective tissue that provides support, protection, movement, storage (of certain electrolytes), and synthesis of blood cells; and periosteum, which surrounds bones, and perichondrium, which surrounds cartilage, provide nourishment for these tissues and are the sites of new cartilage and bone formation. Two types of bone, spongy and compact, are distinguished by the amount of solid matter and the size and number of spaces they contain. Bones can be classified as long, short, flat, irregular, or sesamoid. Standard terms for specific bone markings and features are used when describing the structure of individual bones. Most bones take many years to grow. Bones grow through the processes of intramembranous ossification, in which mesenchymal bone models are formed during the embryonic and prenatal periods, and endochondral ossification, in which cartilage models are formed during the fetal period, with bone subsequently replacing most of the cartilage after birth.
Joints: A joint is a union between two or more bones or rigid parts of the skeleton. Three general types of joints are recognized: fibrous, cartilaginous, and synovial. Freely moveable synovial joints are the most common type; can be classified into plane, hinge, saddle, condyloid, ball and socket, and pivot; receive their blood supply from articular arteries that often form networks; are drained by articular veins originating in the synovial membrane; and are richly innervated by articular nerves that transmit the sensation of proprioception, an awareness of movement and position of parts of the body.
# MUSCLE TISSUE AND MUSCULAR SYSTEM
The muscular system consists of all the muscles of the body. Voluntary skeletal muscles constitute the great majority of the named muscles. All skeletal muscles are composed of one specific type of muscle tissue. However, other types of muscle tissue constitute a few named muscles (e.g., the ciliary and detrusor muscles and the arrector muscles of hairs) and form important components of the organs of other systems, including the cardiovascular, alimentary, genitourinary, integumentary, and visual systems.
# Types of Muscle (Muscle Tissue)
Muscle cells, often called muscle fibers because they are long and narrow when relaxed, are specialized contractile cells. They are organized into tissues that move body parts or temporarily alter the shape (reduce the circumference of all or part) of internal organs. Associated connective tissue conveys nerve fibers and capillaries to the muscle cells as it binds them into bundles or fascicles. Three types of muscle are described based on distinct characteristics relating to whether it is normally willfully controlled (voluntary vs. involuntary) whether it appears striped or unstriped when viewed under a microscope (striated vs. smooth or unstriated) whether it is located in the body wall (soma) and limbs or makes up the hollow organs (viscera, e.g., the heart) of the body cavities or blood vessels (somatic vs. visceral) There are three muscle types (Table 1.1): 1. Skeletal striated muscle is voluntary somatic muscle that makes up the gross skeletal muscles that compose the muscular system, moving or stabilizing bones and other structures (e.g., the eyeballs). 2. Cardiac striated muscle is involuntary visceral muscle that forms most of the walls of the heart and adjacent parts of the great vessels, such as the aorta, and pumps blood. 3. Smooth muscle (unstriated muscle) is involuntary visceral muscle that forms part of the walls of most vessels and hollow organs (viscera), moving substances through them by coordinated
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sequential contractions (pulsations or peristaltic contractions).
TABLE 1.1. TYPES OF MUSCLE (MUSCLE TISSUE)
Muscle Type Location Appearance of Cells Type of Activity Stimulation
Composes gross, named muscles (e.g., biceps of the arm) attached to skeleton and fascia of limbs, body wall, and head/neck Large, very long, unbranched, cylindrical fibers with transverse striations (stripes) arranged in parallel bundles; multiple, peripherally located nuclei Intermittent (phasic) contraction above a baseline tonus; acts primarily to produce movement (isotonic contraction) through shortening (concentric contraction) or controlled lengthening (eccentric contraction), or to maintain position against gravity or other resisting force without movement (isometric contraction) Voluntary (or reflexive) by somatic nervous system
Muscle of the heart (myocardium) and adjacent portions of great vessels (aorta, vena cava) Branching and anastomosing shorter fibers with transverse striations (stripes) running parallel and connected end to end by complex junctions (intercalated discs); single, central nucleus Strong, quick, continuous rhythmic contraction; acts to pump blood from the heart Involuntary; intrinsically (myogenically) stimulated and propagated; rate and strength of contraction modified by the autonomic nervous system
Walls of hollow viscera and blood vessels, iris, and ciliary body of the eye; attached to hair follicles of the skin (arrector muscle of hair) Single or agglomerated; small, spindle-shaped fibers without striations; single central nucleus Weak, slow, rhythmic, or sustained tonic contraction; acts mainly to propel substances (peristalsis, vascular pulsation) and to restrict flow (vasoconstriction and sphincteric activity) Involuntary by autonomic or enteric nervous systems Skeletal Muscles
FORM, FEATURES, AND NAMING OF MUSCLES
All skeletal muscles, commonly referred to simply as muscles, have fleshy, reddish, contractile portions (one or more heads or bellies) composed of skeletal striated muscle. Some muscles are fleshy throughout, but most also have white noncontractile portions (tendons), composed mainly of organized collagen bundles, that provide a means of attachment (Fig. 1.18).
> FIGURE 1.18. Architecture and shape of skeletal muscles. The architecture and shape of a skeletal muscle depend on the arrangement of its fibers.
When referring to the length of a muscle, both the belly and the tendons are included. In other words, a muscles length is the distance between its attachments. Most skeletal muscles are attached directly or indirectly to bones, cartilages, ligaments, or fascias or to some combination of these structures. Some muscles are attached to organs (e.g., the eyeball), skin (such as facial muscles), and mucous membranes (intrinsic tongue muscles). Muscles are organs of locomotion (movement), but they also provide static support, give form to the body, and provide heat. Figure 1.19 identifies the skeletal muscles that lie most superficially. The deep muscles are identified when each region is studied.
> ALGRAWANY FIGURE 1.19. Superficial skeletal muscles. Most of the muscles shown move the skeleton for locomotion, but some musclesespecially those of the headmove other structures (e.g., the eyeballs, scalp, eyelids, skin of face, and tongue). The sheath of the left rectus abdominis, formed by aponeuroses of the flat abdominal muscles, has been removed to reveal the muscle. Retinacula are deep fascial thickenings that tether tendons to underlying bones as they cross joints.
The architecture and shape of muscles vary (Fig. 1.18). The tendons of some muscles form flat sheets, or aponeuroses , that anchor the muscle to the skeleton (usually a ridge or a series of spinous processes) and/or to deep fascia (such as the latissimus dorsi muscle of the back) or to the aponeurosis of another muscle (such as the oblique muscles of the anterolateral abdominal wall). Most muscles are named on the basis of their function or the bones to which they are attached. The abductor digiti minimi muscle, for example, abducts the little finger. The sternocleidomastoid muscle (G. kleidos, bolt or bar, referring to the clavicle) attaches inferiorly to the sternum and clavicle and superiorly to the mastoid process of the temporal bone of the cranium. Other muscles are named on the basis of their position (medial, lateral, anterior, posterior) or length (brevis, short; longus, long). Muscles may be described or classified according to their shape, for which a muscle may also be named: Flat muscles have parallel fibers often with an aponeurosisfor example, the external oblique (broad flat muscle). The sartorius is a narrow flat muscle with parallel fibers. Pennate muscles are feather-like (L. pennatus, feather) in the arrangement of their fascicles and may be unipennate, bipennate, or multipennatefor example, extensor digitorum longus (unipennate), rectus femoris (bipennate), and deltoid (multipennate). Fusiform muscles are spindle shaped with a round, thick belly (or bellies) and tapered ends for example, biceps brachii. Convergent muscles arise from a broad area and converge to form a single tendonfor example, pectoralis major. Quadrate muscles have four equal sides (L. quadratus, square)for example, the rectus abdominis, between its tendinous intersections. Circular or sphincteral muscles surround a body opening or orifice, constricting it when contractedfor example, orbicularis oculi (closes the eyelids). Multiheaded or multibellied muscles have more than one head of attachment or more than one contractile belly, respectively. Biceps muscles have two heads of attachment (e.g., biceps brachii), triceps muscles have three heads (e.g., triceps brachii), and the digastric and gastrocnemius muscles have two bellies. (Those of the former are arranged in tandem; those of the latter lie parallel.)
CONTRACTION OF MUSCLES
Skeletal muscles function by contracting; they pull and never push. However, certain phenomena such as popping of the ears to equalize air pressure and the musculovenous pump (see Fig. 1.26)take advantage of the expansion of muscle bellies during contraction. When a muscle contracts and shortens, one of its attachments usually remains fixed while the other (more mobile) attachment is pulled toward it, often resulting in movement. Attachments of muscles are commonly described as the origin and insertion; the origin is usually the proximal end of the muscle, which remains fixed during muscular contraction, and the insertion is usually the distal end of the muscle, which is movable. However, this is not always the case. Some muscles can act in both directions under different circumstances. For example, when doing push-ups, the distal end of the upper limb (the hand) is fixed (on the floor), and the proximal end of the limb and the trunk (of the body) are being moved. Therefore, this book usually uses the terms proximal and distal or medial and lateral when describing most muscle attachments. Note that if the attachments of a muscle are known, the action of the muscle can usually be deduced (rather than memorized). When studying muscle attachments, act out the action; you are more likely to learn things you have experienced.
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Reflexive Contraction. Although skeletal muscles are also referred to as voluntary muscles, certain aspects of their activity are automatic ( reflexive ) and therefore not voluntarily controlled. Examples are the respiratory movements of the diaphragm, controlled most of the time by reflexes stimulated by the levels of oxygen and carbon dioxide in the blood (although we can willfully control it within limits), and the myotatic reflex, which results in movement after a muscle stretch produced by tapping a tendon with a reflex hammer.
Tonic Contraction. Even when relaxed, the muscles of a conscious individual are almost always slightly contracted. This slight contraction, called tonic contraction or muscle tone
(tonus), does not produce movement or active resistance (as phasic contraction does) but gives the muscle a certain firmness, assisting the stability of joints and the maintenance of posture, while keeping the muscle ready to respond to appropriate stimuli. Muscle tone is usually absent only when unconscious (as during deep sleep or under general anesthesia) or after a nerve lesion resulting in paralysis.
Phasic Contraction. There are two main types of phasic (active) muscle contractions : (1)
isotonic contractions , in which the muscle changes length in relationship to the production of movement, and (2) isometric contractions , in which muscle length remains the sameno movement occurs, but the force (muscle tension) is increased above tonic levels to resist gravity or other antagonistic force (Fig. 1.20). The latter type of contraction is important in maintaining upright posture and when muscles act as fixators or shunt muscles as described below.
> FIGURE 1.20. Types of skeletal muscle contraction. A. Isometric contraction. The position of the joint is sustained without producing movement. B and C. Isotonic contractions. Muscles change in length, resulting in movement. Concentric contraction ( B) shortens muscle length, while eccentric contraction ( C) actively increases muscle length.
There are two types of isotonic contractions. The type we most commonly think of is
concentric contraction , in which movement occurs as a result of the muscle shorteningfor example, when lifting a cup, pushing a door, or striking a blow. The ability to apply exceptional force by means of concentric contraction often is what distinguishes an athlete from an amateur. The other type of isotonic contraction is eccentric contraction , in which a contracting muscle lengthensthat is, it undergoes a controlled and gradual lengthening while continually exerting a (diminishing) force, like playing out a rope. Although people are generally not as aware of them, eccentric contractions are as important as concentric contractions for coordinated, functional movements such as walking, running, and setting objects (or ones self) down. Often, when the main muscle of a particular movement (the prime mover) is undergoing a concentric contraction, its antagonist is undergoing a coordinated eccentric contraction. In walking, we contract concentrically to pull our center of gravity forward, and then as it passes ahead of the limb, we contract eccentrically to prevent a lurching during the transfer of weight to the other limb. Eccentric contractions require less metabolic energy at the same load but, with a maximal contraction, are capable of generating much higher tension levels than concentric contractionsas much as 50% higher (Marieb & Hoehn, 2019). Whereas the structural unit of a muscle is a skeletal striated muscle fiber, the functional unit of a muscle is a motor unit , consisting of a motor neuron and the muscle fibers it controls (Fig. 1.21). When a motor neuron in the spinal cord is stimulated, it initiates an impulse that causes all the muscle fibers supplied by that motor unit to contract simultaneously. The number of muscle fibers in a motor unit varies from one to several hundred. The number of fibers varies according to the size and function of the muscle. Large motor units, in which one neuron supplies several hundred muscle fibers, are in the large trunk and thigh muscles. In smaller eye and hand muscles, where precision movements are required, the motor units include only a few muscle fibers. Movement (phasic contraction) results from the activation of an increasing number of motor units, above the level required to maintain muscle tone.
> ALGRAWANY FIGURE 1.21. Structure of skeletal muscle and motor units. A. Motor unit. A motor unit consists of a single motor neuron and the muscle fibers innervated by it. B. Skeletal muscle structure. Epimysium is the same as investing fascia. Actin (thin) and myosin (thick) filaments are contractile elements in the muscle fibers.
FUNCTIONS OF MUSCLES
Muscles serve specific functions in moving and positioning the body: A prime mover (agonist) is the main muscle responsible for producing a specific movement of the body. It contracts concentrically to produce the desired movement, doing most of the work (expending most of the energy) required. In most movements, there is a single prime mover, but some movements involve two prime movers working in equal measure. A fixator steadies the proximal parts of a limb through isometric contraction while movements are occurring in distal parts. A synergist complements the action of a prime mover. It may directly assist a prime mover, providing a weaker or less mechanically advantaged component of the same movement, or it may assist indirectly, by serving as a fixator of an intervening joint when a prime mover passes over more than one joint, for example. It is not unusual to have several synergists assisting a prime mover in a particular movement. An antagonist is a muscle that opposes the action of another muscle. A primary antagonist directly opposes the prime mover, but synergists may also be opposed by secondary antagonists. As the active movers concentrically contract to produce a movement, antagonists eccentrically contract, relaxing progressively in coordination to produce a smooth movement. The same muscle may act as a prime mover, antagonist, synergist, or fixator under different conditions. Note also that the actual prime mover in a given position may be gravity. In such cases, a paradoxical situation may exist in which the prime mover usually described as being responsible for the movement is inactive (passive), while the controlled relaxation (eccentric contraction) of the antigravity antagonist(s) is the active (energy requiring) component in the movement. An example is lowering (adducting) the upper limbs from the abducted position (stretched out laterally at 90 to the trunk) when standing erect (see Fig. 1.20C). The prime mover (adductor) is gravity; the muscles described as the prime movers for this movement (pectoralis major and latissimus dorsi) are inactive or passive; and the muscle being actively innervated (contracting eccentrically) is the deltoid (an abductor, typically described as the antagonist for this movement). When a muscles pull is exerted along a line that parallels the axis of the bones to which it is attached, it is at a disadvantage for producing movement. Instead it acts to maintain contact between the articular surfaces of the joint it crosses (i.e., it resists dislocating forces); this type of muscle is a shunt muscle . For example, when the arms are at ones sides, the deltoid functions as a shunt muscle. The more oblique a muscles line of pull is oriented to the bone it moves (i.e., the less parallel the line of pull is to the long axis of the bone, for example, the biceps brachii when the elbow is flexed), the more capable it is of rapid and effective movement; this type of muscle is a spurt muscle . The deltoid becomes increasingly effective as a spurt muscle after other muscles have initiated abduction of the arm.
NERVES AND ARTERIES TO MUSCLES
Variation in the nerve supply of muscles is rare; it is a nearly constant relationship. In the limb, muscles of similar actions are generally contained within a common fascial compartment and share innervation by the same nerves (see Fig. 1.9); therefore, you should learn the innervation of limb muscles in terms of the functional groups, making it necessary to memorize only the exceptions. Nerves supplying skeletal muscles ( motor nerves ) usually enter the fleshy portion of the muscle (vs. the tendon), almost always from the deep aspect (so the nerve is protected by the muscle it supplies). The few exceptions are pointed out later in the text. When a nerve pierces a muscle, by passing through its fleshy portion or between its two heads of attachment, it usually supplies that muscle. Exceptions are the sensory branches that innervate the skin of the back after penetrating the superficial muscles of the back. The blood supply of muscles is not as constant as the nerve supply and is usually multiple. Arteries generally supply the structures they contact. Thus, you should learn the course of the arteries and deduce that a muscle is supplied by all the arteries in its vicinity.
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# SKELETAL MUSCLES Muscle Testing
Muscle testing helps examiners diagnose nerve injuries. There are two common testing methods: The person performs movements that resist those of the examiner. For example, the person keeps the forearm flexed while the examiner attempts to extend it. This technique enables the examiner to gauge the power of the persons movements. The examiner performs movements that resist those of the person. When testing flexion of the forearm, the examiner asks the person to flex his or her forearm while the examiner resists the efforts. Usually, muscles are tested in bilateral pairs for comparison. Electromyography (EMG), the electrical recording of muscles, is another method for testing muscle action. The examiner places surface electrodes over a muscle, asks the person to perform certain movements, and then amplifies and records the differences in electrical action potentials of the muscles. A normal resting muscle shows only a baseline activity (muscle tone), which disappears only during deep sleep, during paralysis, and when under anesthesia. Contracting muscles demonstrate variable peaks of phasic activity. EMG makes it possible to analyze the activity of an individual muscle during different movements. EMG may also be part of the treatment program for restoring the action of muscles.
# Muscle Dysfunction and Paralysis
Wasting (atrophy) of muscle may result from a primary disorder of the muscle or from a lesion of the nerve that supplies it. Muscular atrophy may also be caused by immobilization of a limb, such as with a cast. From the clinical perspective, it is important not only to think in terms of the action normally produced by a given muscle but also to consider what loss of function would occur if the muscle failed to function (paralysis). How would the dysfunction of a given muscle or muscle group be manifest (i.e., what are the visible signs)?
# Absence of Muscle Tone
Resting muscle tone, although a gentle force, can have important effects: The tonus of muscles in the lips helps keep the teeth aligned, for instance. When this gentle but constant pressure is absent (due to paralysis or a short lip that leaves the teeth exposed), teeth migrate, becoming everted (buck teeth). The absence of muscle tone in an unconscious patient (e.g., under a general anesthetic) combined with the absence of normal protective reflexes may allow joints to be dislocated as the patient is being lifted or positioned. When a muscle is denervated (loses its nerve supply), it becomes paralyzed (flaccid, lacking both its tonus and its ability to contract phasically on demand or reflexively). In the absence of a muscles normal tonus, that of opposing (antagonist) muscle(s) may cause a limb to assume an abnormal resting position. In addition, the denervated muscle will become fibrotic and lose its elasticity, also contributing to the abnormal position at rest.
# Muscle Soreness and Pulled Muscles
Eccentric contractions that are either excessive or associated with a novel task are often the cause of delayed-onset muscle soreness. Thus, walking down many flights of stairs would actually result in more soreness, owing to the eccentric contractions, than walking up the same flights of stairs. The muscle stretching that occurs during the lengthening type of eccentric contraction appears to be more likely to produce microtears in the muscles and/or periosteal irritation than that associated with concentric contraction (shortening of the muscle belly). Skeletal muscles are limited in their ability to lengthen. Usually, muscles cannot elongate beyond one third of their resting length without sustaining damage. This is reflected in their attachments to the skeleton, which usually do not permit excessive lengthening. An exception is the hamstring muscles of the posterior thigh. When the knee is extended, the hamstrings typically reach their maximum length before the hip is fully flexed (i.e., flexion at the hip is limited by the hamstrings ability to elongate). Undoubtedly, this, as well as forces related to their eccentric contraction, explains why hamstring muscles are pulled (sustain tears) more commonly than other muscles (Fig. B1.8).
> ALGRAWANY FIGURE B1.8. Tear of hamstring tendon.
# Growth and Regeneration of Skeletal Muscle
Skeletal striated muscle fibers cannot divide, but they can be replaced individually by new muscle fibers derived from satellite cells of skeletal muscle (see skeletal muscle figure, Table 1.1). Satellite cells represent a potential source of myoblasts, precursors of muscle cells, which are capable of fusing with each other to form new skeletal muscle fibers if required (Pawlina, 2020). The number of new fibers that can be produced is insufficient to compensate for major muscle degeneration or trauma. Instead of regenerating effectively, the new skeletal muscle is composed of a disorganized mixture of muscle fibers and fibrous scar tissue. Skeletal muscles are able to grow larger in response to frequent strenuous exercise, such as body building. This growth results from hypertrophy of existing fibers, not from the addition of new muscle fibers. Hypertrophy lengthens and increases the myofibrils within the muscle fibers (see Fig. 1.21), thereby increasing the amount of work the muscle can perform.
# Cardiac Striated Muscle
Cardiac striated muscle forms the muscular wall of the heart, the myocardium. Some cardiac muscle is also present in the walls of the aorta, pulmonary vein, and superior vena cava. Cardiac striated muscle contractions are not under voluntary control. Heart rate is regulated intrinsically by a pacemaker, an impulse-conducting system composed of specialized cardiac muscle fibers; they, in turn, are influenced by the autonomic nervous system (ANS) (discussed later in this chapter). Cardiac striated muscle has a distinctly striped appearance under microscopy (Table 1.1). Both types of striated muscleskeletal and cardiacare further characterized by the immediacy, rapidity, and strength of their contractions. Note: Even though the trait applies to both skeletal and cardiac striated muscle, in common usage, the terms striated and striped are used to designate voluntary skeletal striated muscle. As demonstrated in Table 1.1, cardiac striated muscle is distinct from skeletal striated muscle in its location, appearance, type of activity, and means of stimulation. To support its continuous level of high activity, the blood supply to cardiac striated muscle is twice as rich as that to skeletal striated muscle.
# Smooth Muscle
Smooth muscle , named for the absence of striations in the appearance of the muscle fibers under microscopy, forms a large part of the middle coat or layer (tunica media) of the walls of blood vessels (above the capillary level) (see Fig. 1.23; Table 1.1). Consequently, it occurs in all vascularized tissue. It also makes up the muscular parts of the walls of the alimentary tract and ducts. Smooth muscle is found in skin, forming the arrector muscles of hairs associated with hair follicles (see Fig. 1.6), and in the eyeball, where it controls lens thickness and pupil size. Like cardiac striated muscle, smooth muscle is involuntary muscle; however, it is directly innervated by the ANS. Its contraction can also be initiated by hormonal stimulation or by local stimuli, such as stretching. Smooth muscle responds more slowly than striated muscle and with a delayed and more leisurely contraction. It can undergo partial contraction for long periods and has a much greater ability than striated muscle to elongate without suffering paralyzing injury. Both of these factors are important in regulating the size of sphincters and the caliber of the lumina (interior spaces) of tubular structures (e.g., blood vessels or intestines). In the walls of the alimentary tract, uterine tubes, and ureters, smooth muscle cells are responsible for peristalsis, rhythmic contractions that propel the contents along these tubular structures.
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# CARDIAC AND SMOOTH MUSCLE Hypertrophy of Myocardium and Myocardial Infarction
In compensatory hypertrophy, the myocardium responds to increased demands by increasing the size of its fibers. When cardiac striated muscle fibers are damaged by loss of their blood supply during a heart attack, the tissue becomes necrotic (dies) and the fibrous scar tissue that develops forms a myocardial infarct (MI), an area of myocardial necrosis (pathological death of cardiac tissue). Muscle cells that degenerate are
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not replaced because cardiac muscle cells do not divide. Cardiac progenitor (stem) cells have been identified in the heart, but their potential to significantly generate cardiac muscle fibers in the manner of the satellite cells of skeletal muscle has not been established.
# Hypertrophy and Hyperplasia of Smooth Muscle
Smooth muscle cells undergo compensatory hypertrophy in response to increased demands. Smooth muscle cells in the uterine wall during pregnancy increase not only in size but also in number (hyperplasia) because these cells retain the capacity for cell division. In addition, new smooth muscle cells can develop from incompletely differentiated cells (pericytes) that are located along small blood vessels (Pawlina, 2020).
# The Bottom Line: Muscle Tissue and Muscular System
Skeletal muscles: Muscles are categorized as skeletal striated, cardiac striated, or smooth.
# Skeletal muscles are further classified according to their shape as flat, pennate, fusiform, quadrate, circular or sphincteral, and multiheaded or multibellied. Skeletal muscle functions by contracting, enabling automatic (reflexive) movements, maintaining muscle tone (tonic contraction), and providing for phasic (active) contraction with (isotonic) or without (isometric) change in muscle length. Isotonic movements are either concentric (producing movement by shortening) or eccentric (allowing movement by controlled relaxation). Prime movers are the muscles primarily responsible for particular movements. Fixators fix a part of a limb while another part of the limb is moving.
Synergists augment the action of prime movers. Antagonists oppose the actions of another muscle.
Cardiac and smooth muscle: Cardiac muscle is a striated muscle type found in the walls of the heart, or myocardium, as well as in some major blood vessels. Contraction of cardiac muscle is not under voluntary control but is instead activated by specialized cardiac muscle fibers forming the pacemaker, the activity of which is regulated by the autonomic nervous system (ANS). Smooth muscle does not have striations. It occurs in most vascular tissues and in the walls of the alimentary tract and other organs. Smooth muscle is directly innervated by the ANS and thus is not under voluntary control.
# CARDIOVASCULAR SYSTEM
The circulatory system transports fluids throughout the body; it consists of the cardiovascular and lymphatic systems. The heart and blood vessels make up the blood transportation network, the cardiovascular system. Through this system, the heart pumps blood through the bodys vast system of blood vessels. The blood carries nutrients, oxygen, and waste products to and from the cells.
# Vascular Circuits
The heart consists of two muscular pumps that, although adjacently located, act in series, dividing the circulation into two components: the pulmonary and systemic circulations or circuits (Fig. 1.22A, B). The right ventricle of the heart propels low-oxygen blood returning from the systemic circulation into the lungs via the pulmonary arteries. Carbon dioxide is exchanged for oxygen in the capillaries of the lungs, and then the oxygen-rich blood is returned via the pulmonary veins of the lungs to the hearts left atrium. This circuit, from the right ventricle through the lungs to the left atrium, is the pulmonary circulation . The left ventricle propels the oxygen-rich blood returned to the heart from the pulmonary circulation through systemic arteries (the aorta and its branches), exchanging oxygen and nutrients for carbon dioxide in the remainder of the bodys capillaries. Low-oxygen blood returns to the hearts right atrium via
systemic veins (tributaries of the superior and inferior vena cavae). This circuit, from left ventricle to right atrium, is the systemic circulation .
> ALGRAWANY FIGURE 1.22. Circulation. A. Schematic of the anatomical arrangement of the two muscular pumps (right and left heart) serving the pulmonary and systemic circulations. B. Schematic of the bodys circulation, with the right and left heart depicted as two pumps in series. The pulmonary and systemic circulations are actually serial components of one continuous loop. C. A more detailed schematic demonstrating that the systemic circulation actually consists of many parallel circuits serving the various organs and regions of the body.
The systemic circulation actually consists of many parallel circuits serving the various regions and/or organ systems of the body (Fig. 1.22C).
# Blood Vessels
There are three types of blood vessels: arteries, veins, and capillaries (Fig. 1.23). Blood under high pressure leaves the heart and is distributed to the body by a branching system of thick-walled arteries. The final distributing vessels, arterioles, deliver oxygen-rich blood to capillaries. Capillaries form a capillary bed, where the interchange of oxygen, nutrients, waste products, and other substances with the extracellular fluid occurs. Blood from the capillary bed passes into thin-walled venules, which resemble wide capillaries. Venules drain into small veins that open into larger veins. The largest veins, the superior and inferior venae cavae, return low-oxygen blood to the heart. FIGURE 1.23. Blood vessel structure. A. The walls of most blood vessels have three concentric layers of tissue, called tunics (L. tunicae, coats). With less muscle, veins are thinner walled than their companion arteries and have wide lumens (L. luminae) that usually appear flattened in tissue sections. B. Muscular artery and vein (low power). C. Arteriole and venule (high power).
Most vessels of the circulatory system have three coats, or tunics: Tunica intima , an inner lining consisting of a single layer of extremely flattened epithelial cells, the endothelium , supported by delicate connective tissue. Capillaries consist only of this tunic, with blood capillaries also having a supporting basement membrane. Tunica media , a middle layer consisting primarily of smooth muscle Tunica adventitia , an outer connective tissue layer or sheath The tunica media is the most variable coat. Arteries, veins, and lymphatic ducts are distinguished by the thickness of this layer relative to the size of the lumen, its organization, and, in the case of arteries, the presence of variable amounts of elastic fibers.
ARTERIES
Arteries are blood vessels that carry blood under relatively high pressure (compared to the corresponding veins) from the heart and distribute it to the body (Fig. 1.24A). The blood passes through arteries of decreasing caliber. The different types of arteries are distinguished from each other on the basis of overall size, relative amounts of elastic tissue or muscle in the tunica media (Fig. 1.23), the thickness of the wall relative to the lumen, and function. Artery size and type are a continuumthat is, there is a gradual change in morphological characteristics from one type to another. There are three types of arteries: Large elastic arteries (conducting arteries) have many elastic layers (sheets of elastic fibers)
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in their walls. These large arteries initially receive the cardiac output. Their elasticity enables them to expand when they receive the cardiac output from the ventricles, minimizing the pressure change, and return to normal size between ventricular contractions, as they continue to push the blood into the medium arteries downstream. This maintains the blood pressure in the arterial system between cardiac contractions (at a time when ventricular pressure falls to zero). Overall, this minimizes the ebb in blood pressure as the heart contracts and relaxes. Examples of large elastic arteries are the aorta, the arteries that originate from the arch of the aorta (brachiocephalic trunk, subclavian and carotid arteries), and the pulmonary trunk and arteries (Fig. 1.24A). Medium muscular arteries (distributing arteries) have walls that consist chiefly of circularly disposed smooth muscle fibers. Their ability to decrease their diameter (vasoconstrict) regulates the flow of blood to different parts of the body as required by circumstance (e.g., activity, thermoregulation). Pulsatile contractions of their muscular walls (regardless of lumen caliber) temporarily and rhythmically constrict their lumina in progressive sequence, propelling and distributing blood to various parts of the body. Most of the named arteries, including those observed in the body wall and limbs during dissection such as the brachial or femoral arteries, are medium muscular arteries. Small arteries and arterioles have relatively narrow lumina and thick muscular walls. The degree of filling of the capillary beds and level of arterial pressure within the vascular system are regulated mainly by the degree of tonus (firmness) in the smooth muscle of the arteriolar walls. If the tonus is above normal, hypertension (high blood pressure) results. Small arteries are usually not named or specifically identified during dissection, and arterioles can be observed only under magnification. FIGURE 1.24. Systemic portion of cardiovascular system. A. Systemic arteries. Systemic arteries carry oxygen-rich blood from the heart to the systemic capillary beds. B. Systemic veins. Systemic veins return oxygen-depleted blood from the systemic capillary beds to the heart. Although commonly depicted and considered as single vessels, as shown here, the deep veins of the limbs usually occur as pairs of accompanying veins. Together, systemic arteries, veins, and capillary beds constitute the systemic circulation.
Anastomoses (communications) between multiple branches of an artery provide numerous potential detours for blood flow in case the usual pathway is obstructed by compression due to the position of a joint, pathology, or surgical ligation. If a main channel is occluded, the smaller alternate channels can usually increase in size over a period of time, providing a collateral circulation or alternate pathway that ensures the blood supply to structures distal to the blockage. However, collateral pathways require time to open adequately; they are usually insufficient to compensate for sudden occlusion or ligation. There are areas, however, where collateral circulation does not exist or is inadequate to replace the main channel. Arteries that do not anastomose with adjacent arteries are true
(anatomical) terminal arteries (end arteries). Occlusion of an end artery interrupts the blood supply to the structure or segment of an organ it supplies. True terminal arteries supply the retina, for example, where occlusion will result in blindness. While not true terminal arteries, functional terminal arteries (arteries with ineffectual anastomoses) supply segments of the brain, liver, kidneys, spleen, and intestines; they may also exist in the heart.
ALGRAWANY VEINS
Veins generally return low-oxygen blood from the capillary beds to the heart, which gives the veins a dark blue appearance (Fig. 1.24B). The large pulmonary veins are atypical in that they carry oxygen-rich blood from the lungs to the heart. Because of the lower blood pressure in the venous system, the walls (specifically, the tunica media) of veins are thinner than those of their companion arteries (Fig. 1.23). Normally, veins do not pulsate and do not squirt or spurt blood when severed. There are three sizes of veins: Venules are the smallest veins. Venules drain capillary beds and join similar vessels to form small veins. Magnification is required to observe venules. Small veins are the tributaries of larger veins that unite to form venous plexuses (networks of veins), such as the dorsal venous arch of the foot (Fig. 1.24B). Small veins are unnamed. Medium veins drain venous plexuses and accompany medium arteries. In the limbs, and in some other locations where the flow of blood is opposed by the pull of gravity, the medium veins have valves. Venous valves are cusps (passive flaps) of endothelium with cup-like
valvular sinuses that fill from above. When they are full, the valve cusps occlude the lumen of the vein, thereby preventing reflux of blood distally, making flow unidirectional (toward the heart but not in the reverse direction; see Fig. 1.26). The valvular mechanism also breaks columns of blood in the veins into shorter segments, reducing back pressure. Both effects make it easier for the musculovenous pump to overcome the force of gravity to return blood to the heart. Examples of medium veins include the named superficial veins (cephalic and basilic veins of the upper limbs and great and small saphenous veins of the lower limbs) and the accompanying veins that are named according to the artery they accompany (Fig. 1.24B). Large veins are characterized by wide bundles of longitudinal smooth muscle and a well-developed tunica adventitia. An example is the superior vena cava. Veins are more abundant than arteries. Although their walls are thinner, their diameters are usually larger than those of the corresponding artery. The thin walls allow veins to have a large capacity for expansion and do so when blood return to the heart is impeded by compression or internal pressures (e.g., after taking a large breath and holding it; this is called the Valsalva maneuver). Since the arteries and veins make up a circuit, it might be expected that half the blood volume would be in the arteries and half in the veins. Because of the veins larger diameter and ability to expand, typically, only 20% of the blood occupies arteries, whereas 80% is in the veins. Although often depicted as single vessels in illustrations for simplicity, veins tend to be double or multiple. Those that accompany deep arteries accompanying veins (L. venae comitantes)surround them in an irregular branching network (Fig. 1.25). This arrangement serves as a countercurrent heat exchanger, the warm arterial blood warming the cooler venous blood as it returns to the heart from a cold limb. The accompanying veins occupy a relatively unyielding fascial vascular sheath with the artery they accompany. As a result, they are stretched and flattened as the artery expands during contraction of the heart, which aids in driving venous blood toward the heartan arteriovenous pump. FIGURE 1.25. Accompanying veins. Although most veins of the trunk occur as large single vessels, veins in the limbs occur as two or more smaller vessels that accompany an artery in a common vascular sheath.
Systemic veins are more variable than arteries, and venous anastomosesnatural communications, direct or indirect, between two veinsoccur more often between them. The outward expansion of the bellies of contracting skeletal muscles in the limbs, limited by the surrounding deep fascia, compresses the deep veins within and around the skeletal muscle inside the deep fascia, milking the blood superiorly toward the heart; another (musculovenous) type of venous pump (Fig. 1.26). The valves of the veins break up the columns of blood, thus relieving the more dependent parts of excessive pressure, allowing venous blood to flow only toward the heart. The venous congestion that hot and tired feet experience at the end of a busy day is relieved by resting the feet on a footstool that is higher than the trunk (of the body). This position of the feet also helps the veins return blood to the heart.
> FIGURE 1.26. Musculovenous pump. Muscular contractions in the limbs function with the venous valves to move blood toward the heart. The outward expansion of the bellies of contracting muscles is limited by deep fascia and becomes a compressive force, propelling the blood against gravity.
Superficial veins of the limbs are external to the deep fascia and so are not affected by
ALGRAWANY