Overview
This lecture explores the types, properties, structure, organization, and functions of muscle tissue, focusing on skeletal muscle. It covers muscle contraction, fiber types, biomechanics, muscle naming, and the characteristics of cardiac and smooth muscle, as well as muscle development.
Properties and Types of Muscle Tissue
- There are three types of muscle tissue:
- Skeletal muscle: voluntary control.
- Cardiac muscle: involuntary.
- Smooth muscle: involuntary.
- All muscle tissue shares these properties:
- Excitability: ability to respond to stimuli.
- Conductivity: ability to transmit electrical impulses along the cell membrane.
- Contractility: ability to generate tension and shorten.
- Elasticity: ability to return to resting length after being stretched or contracted.
- Extensibility: ability to stretch beyond resting length.
Skeletal Muscle Structure and Function
- Each skeletal muscle is considered an organ and contains all four tissue types.
- Muscles are usually attached to bones and are striated.
- Functions of skeletal muscle:
- Body movement: muscles pull on bones to move the body.
- Maintain posture: continuous contraction keeps the body upright.
- Protection and support: muscle layers protect and support internal organs.
- Regulate elimination: sphincters control passage of materials through tracts.
- Produce heat: muscle contraction generates heat to maintain body temperature.
- Structural organization:
- Muscle: surrounded by epimysium.
- Fascicle: bundle of muscle fibers, surrounded by perimysium.
- Muscle fiber (cell): elongated, multinucleated, surrounded by endomysium.
- Myofibril: cylindrical organelle within muscle fiber.
- Myofilaments: thick (myosin) and thin (actin, tropomyosin, troponin) filaments.
Microscopic Anatomy of Muscle Fibers
- Sarcolemma: plasma membrane of muscle fiber.
- Sarcoplasm: cytoplasm of muscle fiber, rich in mitochondria for energy.
- Unique structures:
- Transverse tubules (T-tubules): deep invaginations of sarcolemma that transmit impulses.
- Sarcoplasmic reticulum: stores calcium ions; includes terminal cisternae.
- Triad: T-tubule flanked by two terminal cisternae.
- Muscle fibers form by fusion of embryonic myoblasts, resulting in multinucleated cells.
- Satellite cells: myoblasts that remain for muscle repair and regeneration.
Sarcomere and Contraction Mechanism
- Sarcomere: the basic contractile unit, spanning from one Z disc to the next.
- Composed of overlapping thick and thin filaments.
- Sarcomere regions:
- I band: contains only thin filaments; bisected by Z disc; shortens during contraction.
- A band: contains thick filaments; central region; remains constant in width.
- H zone: center of A band; thick filaments only; disappears during contraction.
- M line: center of H zone; aligns thick filaments.
- Key structural proteins:
- Connectin (titin): provides elasticity and alignment.
- Nebulin: helps with sarcomere assembly.
- Dystrophin: links myofibrils to the sarcolemma; defects cause muscular dystrophy.
- Sliding filament theory: thin filaments slide past thick filaments, shortening the sarcomere and causing muscle contraction. Filament lengths do not change, only their positions.
Neuromuscular Junction and Contraction Events
- Neuromuscular junction: site where a motor neuron communicates with a muscle fiber.
- Synaptic knob: end of neuron axon.
- Synaptic vesicles: contain acetylcholine (ACh).
- Synaptic cleft: gap between neuron and muscle.
- Motor end plate: region of sarcolemma with ACh receptors.
- Acetylcholinesterase (AChE): enzyme that breaks down ACh.
- Sequence of contraction:
- Nerve impulse triggers release of ACh into synaptic cleft.
- ACh binds to receptors, initiating a muscle impulse in the sarcolemma.
- Impulse travels along T-tubules, causing calcium release from terminal cisternae.
- Calcium binds to troponin, moving tropomyosin and exposing actin sites.
- Myosin heads bind to actin, forming crossbridges; ATP is used for the power stroke.
- Repeated cycles of attach-pivot-detach-return slide filaments and shorten the sarcomere.
- When stimulation stops, calcium is pumped back into the sarcoplasmic reticulum, and the muscle relaxes.
Types of Muscle Fibers
- Skeletal muscles contain three main fiber types:
- Slow oxidative (SO, Type I): small, aerobic, high fatigue resistance, dark red, many mitochondria, used for endurance and posture (e.g., trunk, lower limbs).
- Fast oxidative (FO, Type IIa): intermediate size, fast contraction, aerobic, moderate fatigue resistance, lighter red, used for moderate movement (e.g., walking, biking).
- Fast glycolytic (FG, Type IIb): large, anaerobic, contract quickly for short bursts, fatigue rapidly, pale color, few mitochondria, used for intense, brief movements (e.g., sprinting, upper limbs).
- Each motor unit contains only one fiber type.
- Distribution varies by muscle function: postural muscles have more slow fibers; muscles for quick, brief actions have more fast fibers.
Muscle Fiber Organization and Architecture
- Muscle fibers are grouped into fascicles, which are arranged in four main patterns:
- Circular: fascicles arranged around an opening (e.g., sphincters).
- Parallel: fascicles run parallel to muscle's long axis; high endurance, less force.
- Convergent: triangular shape with a common attachment; direction of pull can change.
- Pennate: fascicles at an angle to the tendon; stronger pull than parallel muscles.
- Unipennate: fibers on one side of tendon.
- Bipennate: fibers on both sides of tendon.
- Multipennate: tendon branches within muscle.
Muscle Adaptation and Biomechanics
- Muscle hypertrophy: increase in muscle fiber size (not number), more myofibrils, mitochondria, and glycogen; results from repetitive, intense stimulation.
- Muscle atrophy: decrease in muscle fiber size, tone, and power due to reduced stimulation.
- Muscles act as levers:
- Lever: rigid bar that rotates around a fulcrum.
- Effort arm: between fulcrum and effort.
- Resistance arm: between fulcrum and resistance.
- Mechanical advantage depends on the ratio of arm lengths.
- Three classes of levers exist in the body.
- Muscle roles:
- Agonist (prime mover): produces movement.
- Antagonist: opposes the agonist.
- Synergist: assists the agonist, includes stabilizing fixators.
Naming of Muscles
- Muscles are named based on several criteria:
- Action: e.g., adductor magnus (adducts thigh).
- Body region: e.g., tibialis anterior (front of tibia).
- Attachments: e.g., sternocleidomastoid (attaches to sternum, clavicle, mastoid).
- Fiber orientation: e.g., rectus abdominis (straight fibers).
- Shape and size: e.g., trapezius (trapezoid shape), adductor magnus (large).
- Number of heads/tendons: e.g., biceps brachii (two heads), triceps brachii (three heads).
Cardiac and Smooth Muscle Traits
- Cardiac muscle:
- Striated, one or two nuclei per cell, many mitochondria.
- Cells branch and connect via intercalated discs (gap junctions and desmosomes).
- Autorhythmic: can generate impulses without nerve input.
- Involuntary control by the autonomic nervous system.
- Smooth muscle:
- Found in walls of viscera and blood vessels.
- Short, fusiform cells with a single central nucleus; no striations.
- Thin filaments attach to dense bodies.
- Sparse sarcoplasmic reticulum, no T-tubules.
- Uses calmodulin (not troponin) for calcium binding.
- Contractions are slow, efficient, and fatigue-resistant.
Muscle System Development
- Skeletal muscle tissue develops from mesodermal somites during the fourth week of embryonic development.
- Somites differentiate into:
- Sclerotome: forms skeleton.
- Dermatome: forms connective tissue of skin.
- Myotome: forms skeletal muscles.
- Myoblasts in the myotome fuse to form multinucleated muscle fibers.
- Migrating myotomes in limbs form dorsal (extensor) and ventral (flexor) muscle masses.
- Satellite cells (unfused myoblasts) remain to aid in muscle repair.
Key Terms & Definitions
- Sarcolemma: plasma membrane of a muscle fiber.
- Sarcoplasm: cytoplasm of a muscle fiber.
- Sarcomere: contractile unit of a myofibril, between two Z discs.
- Myofibril: cylindrical organelle in muscle fiber, made of myofilaments.
- T-tubule: invagination of sarcolemma for impulse conduction.
- Neuromuscular junction: synapse between a motor neuron and muscle fiber.
- Agonist: muscle that causes movement.
- Antagonist: muscle that opposes movement.
- ACh (acetylcholine): neurotransmitter for muscle stimulation.
- Connectin (titin): protein for elasticity and alignment.
- Satellite cells: assist in muscle repair and regeneration.
- Intercalated discs: specialized connections between cardiac muscle cells.
- Calmodulin: calcium-binding protein in smooth muscle.
Action Items / Next Steps
- Review the different patterns of muscle fiber architecture and their examples.
- Study the detailed steps of muscle contraction and relaxation, including the role of calcium and ATP.
- Memorize the structural and functional differences among muscle fiber types.
- Complete assigned readings or textbook exercises on muscle tissue organization and development.
- Practice identifying muscle names based on their naming criteria.