Learning Outcomes
* Describe the development of muscle tissue
* Describe the structure, location in the body, and function of skeletal, cardiac, and smooth muscle tissue
* Compare and contrast the general microscopic characteristics of skeletal, cardiac, and smooth muscle
Lecture Video: Muscle Tissue
Subtopic: Development of muscle tissue
* Most muscle tissue develops from a common cell called a myoblast (a muscle-forming stem cell)
* Myoblast come the embryonic mesoderm.
* The myoblast develop into three types of muscle tissue: smooth, cardiac, or skeletal muscle tissue.
* Uninucleate (one nucleus) cardiac and smooth muscle cells originate from myoblasts (do not fuse, hence one nucleus per cell)
* Multinucleated (many nuclei) muscle fiber forms from hundreds of myoblasts (fuse during development becoming a mature muscle fiber).
Subtopic: Smooth, Cardiac, and skeletal Muscles
Muscle Type
Smooth Muscle Tissue
Cardiac Muscle Tissue
Skeletal Muscle Tissue
Structure
* Fusiform
* Short
* Nonstriated
Contain only one centrally located nucleus
* Short, bifurcated, and striated
* One or two centrally located nuclei
* Intercalated discs between cells
* Long, cylindrical, parallel, and unbranched
* Multinucleated with nuclei along periphery
Function
* Involuntary muscle movements and motions
* Moves materials through organs
* Involuntary contraction and relaxation pump blood into the heart
* Moves along skeleton
* Responsible for body movements
* Locomotion
* Heat production
Location
* Walls of hollow internal organs
* Vessels
* Airways
* Stomach
* Bladder
* Uterus
* Heart wall (myocardium)
* Attaches to bone, and sometimes to skin
* Facial muscles
* Found in voluntary sphincters- lips, urethra, and anus.
Subtopic: Microscopic
1. Skeletal Muscle: myofilaments (thread-like muscle proteins) give the muscle a striated appearance.
2. Cardiac Muscle: has intercalated disks which helps hold the cardiac cells together when pumping blood (powerful cardiac contractions) Video: Cardiac Muscle
3. Smooth Muscle: uninucleated, nonstriated; smooth appearance, tapered ends.
Subtopic: Functions of Muscles
1. Producing body movements
2. Stabilizing body positions
3. Storing and moving substances within the body
4. Generating heat
Learning Outcomes
* Describe the gross structure of skeletal muscle
* Name the connective tissue layers that surround each skeletal muscle fiber, fascicle, entire muscle, and a group of muscles and indicate the specific type of connective tissue that composes each of these layers
* Describe the organization of skeletal muscle, from the cell (skeletal muscle fiber) to whole muscle
* Describe the components within a skeletal muscle fiber (e.g., sarcolemma, transverse [T] tubules, sarcoplasmic reticulum, myofibrils, thick [myosin] myofilaments, thin [actin] myofilaments, troponin, tropomyosin)
* Define sarcomere
* Identify major bands and zones in a sarcomere (A-band, I-band, H-zone, M-line, zone of overlap, etc.)
* Describe the functions of skeletal muscle proteins
Lecture Video: Skeletal Muscle Tissue
Subtopic: Muscle Structure
* Fascia is the most superficial connective tissue of skeletal muscles and anchors muscles and other organs of the body.
* Fascicles: bundles of muscle fibers; covered by the perimysium; Inside skeletal muscles;
* Muscle fiber covered by endomysium; composed of many fibrils; gives striated appearance. Skeletal muscle fiber is surrounded by a plasma membrane called the sarcolemma, which houses sarcoplasm, the cytoplasm of muscle cells.
Subtopic: The Three Connective Tissue Layers
* Each skeletal muscle has three layers of connective tissues that bind the muscle cells together.
* Epimysium: outermost covering of muscle; allows the muscle to contract and move powerfully; separates adjacent muscles to move independently.
* Perimysium: the connective tissue that separates fascicles.
* Endomysium: Muscle fibers are covered by endomysium; contains the extracellular fluid and nutrients supplied by the blood to support the muscle fiber.
In skeletal muscles, the three connective tissue layers fuse at the ends of the muscle organ turning into a tendon or an aponeurosis.
* Tendons are rope-shaped connective tissues that tie (anchor) muscles to bone.
* An aponeurosis is a sheet-like (flat) connective tissue that attaches muscle to bone or another muscle. (ex: broad sheet of connective tissue anchoring the lattisimus dorsi)
Subtopic: Anatomy of Skeletal Muscle
Cell Parts:
1. Plasma membrane = (of muscle fibers) is called the sarcolemma.
* T tubules are extensions of the sarcolemma that go deep into the muscle fiber and allows action potentials to reach further into the interior of the cell.
2. Cytoplasm = Sarcoplasm
3. Smooth endoplasmic reticulum= is called the sarcoplasmic reticulum. Ca+ is released from SR.
4. Myofibrils are specialized contractile organelles found within muscle fibers. (made up of 3 types of proteins)
* Proteins: contractile, regulatory, and structural.
* Contractile proteins: generate force during muscle contraction (myosin and actin).
* Regulatory proteins: determine when a muscle contracts and relaxes (troponin and tropomyosin.)
* Structural proteins: keep the thick and thin filaments in alignment and link the myofibrils to the sarcolemma. (Titin, Nebulin, Alpha-actin, Myomesin, Dystropohn)
Subtopic: Sarcomere
* Sarcomere: region from one Z-line to the next Z-line (functional unit of a skeletal muscle fiber)
* Sarcomeres are linked together (end to end) creating myofibrils.
* Sarcomere proteins: highly organized arrangement of proteins:
* Sarcomere proteins: actin (thin) filaments and the myosin (thick) filaments.
* Sarcomeres are divided into bands and zones depending on the area and contain thick filaments, thin filaments, or both.
* A Band: darker middle part includes the entire length of the thick filaments. Creates darker striations.
* I Band: lighter; contains only thin filaments.
* Z discs pass through the center of each I band; Z discs are the end of the sarcomere.
* H zone: narrow area in the center of each A band that contains thick filaments.
* M line: region in the center of the H zone that contains proteins that hold thick filaments together at the center of the sarcomere.
Subtopic: Proteins
Three different types of proteins: contractile, regulatory, and structural.
* Contractile proteins: generate force during muscle contraction (myosin and actin.)
* Regulatory proteins: determine when a muscle contracts and relaxes (troponin and tropomyosin).
* Structural proteins keep the thick and thin filaments in alignment and link the myofibrils to the sarcolemma.
Muscle Fiber contraction & Relaxation
Learning Outcomes
* Explain the resting membrane potential and the distribution of charges in and out of the cell
* Describe how muscle action potentials arise at the neuromuscular junction
* Describe how action potentials result in the release of calcium contraction
* Describe the contraction cycling events in muscle relaxation
Video: Neuromuscular Junction
Video: Muscle Contraction- Sliding Filament Mechanism
Subtopic: Membrane potential
* Resting membrane Potential: ++ and — on either side of a membrane
* Proteins embedded in the sarcolemma include channels, gates, and pumps; these proteins regulate the concentration of ions (namely Ca2+, Na+, K+, and Cl–) in and out of the cell.
* Ion channel proteins selectively allow particular ions to pass into or out of the cell by diffusion.
* There are specific channels for each ion (Na+ channels, K+ channels, etc).
* Gates are similar to channels with gates that open and close. Just like channels, ion gates only allow diffusion of specific ions (Na+ gates, K+ gates) but some will allow diffusion of more than one type of ion (Na+/K+ gates).
* Some gates open when they must bind a ligand (ligand-activated gate) or they might open in response to a change in electric charge (voltage-activated gate).
* The process of diffusion describes the movement of materials from high to low concentration.
* Example: if more Na+ is outside the cell than inside, Na+ will diffuse through a channel or a gate (if it’s open) into the cell. Pumps do the opposite. These membrane proteins concentrate ions in or out of the cell. The most common pump is the Na+/K+ pump which kicks out 3 Na+ while bringing in 2 K+ with each pumping cycle.
* Because Na+/K+ pumps kick out three + charges while only pumping in two, the electrical charge difference (voltage) between the inside and outside of the membrane is usually around -60 to -90 mV.
* This is referred to as a cell’s resting membrane potential and results from Na+/K+ pumps and Na+ and K+ channels reaching an equilibrium.
* The resting membrane potential changes to an excited state when gates open and allow a massive influx or outflux of ions.
* Neurons and muscle cells use this change in membrane potentials to generate and relay electrical signals. In the case of muscle fibers, this results in muscle contraction.
Subtopic: Muscle Contraction-Sliding Filament Mechanism
Before contraction begins:
* The sarcoplasmic reticulum releases Ca2+ into the sarcoplasm.
* The Ca2+ binds to troponin, changes its shape, and moves the tropomyosin strands out of the way, exposing the myosin-binding sites. The contraction cycle can now begin.
Four steps of the contraction cycle are presented below.
1. ATP hydrolysis. Myosin heads include an ATP-binding site. This location functions as an ATP-ase (an enzyme that helps break apart ATP), hydrolyzing ATP into ADP (adenosine diphosphate) and a phosphate group. The energy released from this reaction is then stored within the myosin head to be used in later phases of the contraction cycle. The myosin head is now “cocked”.
2. Cross-Bridge- attachment of myosin to actin. The myosin head can now attach to the myosin-binding site on actin.
3. Power stroke. Once the cross-bridge has been formed, the myosin head utilizes the energy derived from the hydrolysis of ATP and pulls the actin filament towards the center of the sarcomere.
4. Detachment of myosin from actin. Once the power stroke is complete, the myosin head remains attached to the binding site. Once another molecule of ATP binds to the myosin head it can detach from actin. As long as ATP is available, it readily attaches to myosin, the contraction cycle can recur, and muscle contraction can continue. In the absence of ATP, the myosin head will not detach from the binding site.
Subtopic: Relaxation of a Muscle Fiber
* Ca++ ions are pumped back into the SR > causes the tropomyosin to reshield the binding sites on the actin strands.
* A muscle may also stop contracting when it runs out of ATP and becomes fatigued.
4 Steps:
1) Events at the Neuromuscular junction
2) Muscle fiber excitation
3) Excitation-Contraction Coupling
4) Contraction Cycle
Subtopic: Events at the Neuromuscular Junction
1. Action potentials in neurons cause the release of ACh molecules into the synaptic cleft.
2. ACh diffuses across the NMJ and binds gated-Na+/K+ channels
3. gated-Na+/K+ channels open, causing the diffusion of Na+ into the cell and K+ out.
4. The diffusion of Na+ into the cell depolarizes the membrane of the muscle fiber in and around the motor endplate.
Muscle Metabolism and Energy
Learning Outcomes
* Describe the reactions by which muscle fibers produce ATP
* Distinguish between anaerobic glycolysis and aerobic respiration
* Describe the effects of exercise on different types of skeletal muscle fibers
Subtopic: ATP
* ATP supplies the energy for muscle contraction. (muscle contraction will not occur without sufficient ATP).
* When ATP is broken down - it must be regenerated & replaced quickly to allow for sustained contraction.
* Three mechanisms by which ATP can be regenerated:
* creatine phosphate metabolism:
* Creatin phosphate is a molecule that can store energy in its phosphate bonds
* Resting muscle = excess ATP transfers its energy to creatine, producing ADP and creatine phosphate.
* acts as an energy reserve that can be used to quickly create more ATP
* Muscle contracts and needs energy = creatine phosphate transfers its phosphate back to ADP to form ATP and creatine.
* anaerobic glycolysis
* Anaerobic glycolysis is a non-oxygen-dependent process that breaks down glucose (sugar) to produce ATP.
* aerobic respiration
* Lactic acid
* exercise has stopped and oxygen is being delivered.
Subtopic: Muscle Metabolism [OpenStax] Figure 10.13
1. Some ATP is stored in a resting muscle.
2. As contraction starts, it is used up in seconds.
3. More ATP is generated from creatine phosphate for about 15 seconds.
4. Each glucose molecule produces two ATP and two molecules of pyruvic acid (used in aerobic respiration or converted to lactic acid).
5. If oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue.
6. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle.
7. Aerobic respiration is the breakdown of glucose in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP.
8. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria. [OpenStax]
Control of Muscle Tension
Learning Outcomes
* Explain the phases of a twitch contraction
* Describe how the frequency of stimulation affects muscle tension, and how muscle tone is produced
* Distinguish between isotonic and isometric contractions
* Describe the structure and function of a motor unit
* Describe motor unit recruitment
Subtopic: Twitch contraction
Video: Twitch Contraction
Twitch Contraction: a brief contraction of all muscle fibers in a motor unit in response to an action potential.
Latent Period: delay following an action potential.
Contraction Period: Ca++ binds to troponin; peak contraction
Relaxation Period: Ca++ is transported back into the sarcoplasmic reticulum.
Refractory Period: the period of lost excitability.
Subtopic: Isotonic & Isometric Contractions
Video: Types of Muscle Contractions
* Isometric: muscle length does not change. (Ex: Biceps Curl- holding the weights at 90 degrees elbow flexion)
* Isotonic: muscle length changes.
* Concentric: The muscle length shortens. (Ex: Biceps Curl- Raising the dumbbells up with increased elbow flexion)
* Eccentric: The muscle length lengthens. (Ex: Biceps Curl- Lowering the dumbbells with increased elbow extension)
Subtopic: Motor Unit
Video: Motor Unit
* A motor unit consists of one motor neuron and all the muscle fibers it innervates.
* Every skeletal muscle fiber is supplied by a motor neuron
* Motor Unit Recruitment: takes place when the number of active motor units increases.
* weakest motor units are recruited first, followed by stronger motor units.
* motor units contract alternately in order to sustain a contraction (for longer periods of time).
Subtopic: Muscle Fibers
Lecture Video: Muscle Fiber Types
Slow oxidative (SO) fibers:
* endurance exercises; requires little force (numerous repetitions over a longer period)
* SO fibers use aerobic metabolism to generate mass amounts of ATP
* Examples: Cross Country, marathon runners.
* Endurance exercises can trigger the formation of more extensive capillary networks around the fiber (called angiogenesis). This increases the oxygen supply and waste removal to/from cells.
Fast Glycolytic (FG)
* high amounts of glycogen; can be broken down by glycolysis to quickly generate glucose and ATP.
* FG fibers are used to produce rapid, forceful contractions to make quick, powerful movements.
* FG fibers fatigue rapidly (short periods) due to the lack of O2 and burning through their glycogen and ATP stores quickly
* Example: sprinters
Fast oxidative (FO) fibers
* Also known as intermediate fibers (have characteristics between fast fibers and slow fibers)
* FO fibers produce ATP faster than SO fibers.
* They are oxidative because they produce ATP aerobically. (high amounts of mitochondria, and do not fatigue quickly)
* Not a significant amount of myoglobin (give them the a lighter color- pinkish)
* Examples: walking
* More fatigue-resistant than fast glycolytic fibers.
Characteristic
Fast Glycolytic
Fast Oxidative
Slow Oxidative
Other names
Type IIx, Fast Twitch
Type IIa, Fast Twitch
Type I, Slow Twitch
Number of mitochondria
Low
High/moderate
High
Resistance to fatigue
Low
High/moderate
High
Predominant energy system
Anaerobic
Combination
Aerobic
ATPase activity
Highest/fastest
High
Low/slowest
Speed of shortening (Vmax)
Highest
High
Low
Efficiency
Low
Moderate
High
Strength (Specific tension)
High
High
Moderate
Myoglobin
Low
Moderate
High
Glycogen
High
Moderate
Low
Learning Outcomes
* Explain how muscle fibers regenerate
* Describe the factors that contribute to muscle fatigue
* Explain the effects of aging on skeletal muscle.
Lecture Video: 10.7
Video: Muscle Regeneration
Subtopic: Muscle size and regeneration
* Muscle cells can change in size
* New cells are not formed when muscles grow.
* Hypertrophy: new thick and thin filaments are produced and added to existing muscle fibers in a process.
* This additional amount of fibers adds mass and bulk to a muscle fiber, so the cell diameter of the fiber increases.
* Atrophy: structural proteins are lost and muscle mass decreases.
Subtopic: Exercise & Muscle Fatigue
Slow oxidative (SO) fibers: does not fatigue easily.
Fast Glycolytic (FG): FG fibers fatigue rapidly (short periods) due to the lack of O2 and burning through their glycogen and ATP stores quickly
Fast oxidative (FO) fibers: More fatigue-resistant than fast glycolytic fibers.
Characteristic
Fast Glycolytic
Fast Oxidative
Slow Oxidative
Resistance to fatigue
Low
High/moderate
High
Subtopic: Aging
* Atrophy due to disuse can typically be reversed with exercise.
* Sarcopenia: muscle atrophy with age (irreversible). As we age, muscle fiber die and are replaced with connective tissue (and adipose tissue).
* We have decreased contractions as we age.
* Connective tissue and adipose tissue do not generate as much force as the muscle fibers
* Decrease in strength also leads to decrease in posture and mobility.
* Loss of strength
* Reduction in FG fibers (that produce short, powerful contractions).
* Muscles in older people have more SO fibers (longer contractions)
* Reduction in the size of motor units (fewer muscle fibers being stimulated).