Study Notes on Muscle Tissue and Organization
Chapter 10: Muscle Tissue and Organization
10.1 Properties of Muscle Tissue
Skeletal muscle fibers: Also referred to as muscle fibers. All muscle tissue is composed of muscle cells and exhibits common properties:
Excitability: Ability to respond to stimuli.
Conductivity: Ability to transmit electrical events along the cell membrane.
Contractility: Ability to generate tension and shorten cell length.
Elasticity: Ability to return to resting length after shortening or lengthening.
Extensibility: Ability to be stretched beyond resting length.
10.2 Characteristics of Skeletal Muscle Tissue
Skeletal muscle: Each skeletal muscle is considered an organ composed of the four tissue types.
General features:
Striated (marked by bands).
Usually attached to bones.
Muscles and muscle cells vary in shape and size.
10.2a Functions of Skeletal Muscle Tissue
Move the body: Skeletal muscles pull on bones to produce body movements.
Maintain posture: Continual contraction of specific muscles maintains body position.
Protect and support: Muscles are arranged in layers that protect abdominal organs and support their positions.
Regulate elimination of materials: Circular muscle bands called sphincters control the passage of materials through the orifices of the urinary and gastrointestinal tracts.
Produce heat: Muscle contraction generates heat, helping to maintain body temperature.
10.2b Gross Anatomy of Skeletal Muscle
Fascicle: A bundle of muscle fibers.
Myofibrils: Complex, cylindrical organelles within individual muscle fibers that are composed of myofilaments.
Structural Organization of Skeletal Muscle:
Includes components such as arteries, veins, nerves, deep fascia, and connective tissue coverings like epimysium, perimysium, and endomysium.
Connective Tissue Coverings of Muscle
Endomysium: Surrounds and electrically insulates each muscle fiber; consists of areolar connective tissue with reticular fibers.
Perimysium: Surrounds fascicles; consists of dense irregular connective tissue with blood vessels and nerves.
Epimysium: Surrounds the entire muscle; made of dense irregular connective tissue.
Deep fascia: Surrounds each muscle, binding muscles with similar functions.
Superficial fascia: Separates muscle from the skin; consists of areolar and adipose connective tissues.
Muscle Attachments
Tendons: Attach muscles to bones, skin, or other muscles; can form aponeurosis, a thin flattened sheet.
Origin: Less mobile attachment point of a muscle.
Insertion: More mobile attachment point of a muscle.
Muscle contractions cause one bone to move while another remains stationary.
Blood Supply and Innervation
Skeletal muscles are innervated by somatic motor neurons; each muscle fiber is controlled by one motor neuron, with axons passing through connective tissue layers to form junctions with individual muscle fibers.
10.2c Microscopic Anatomy of Skeletal Muscle
Cell Components: Similar to typical cells but named differently.
Sarcolemma: Plasma membrane of the muscle fiber.
Sarcoplasm: Cytoplasm of the muscle fiber.
Muscle fibers have abundant mitochondria to meet energy demands.
Unique Structures of Muscle Fibers
Transverse Tubules (T-tubules): Deep invaginations of the sarcolemma that extend into the sarcoplasm and carry impulses to stimulate contraction.
Sarcoplasmic Reticulum: Internal membrane complex similar to smooth endoplasmic reticulum, which stores calcium and includes terminal cisternae adjacent to T-tubules.
Triad: A T-tubule located between two terminal cisternae.
Development of Muscle Fibers
Skeletal muscle fibers develop from embryonic myoblasts that fuse into large multinucleated cells. Satellite cells remain as undifferentiated myoblasts, aiding in the regeneration of damaged muscles.
10.3 Contraction of Skeletal Muscle Fibers
Contraction leads to the generation of tension as sarcomeres shorten. Thick filaments attach to thin filaments and pull them toward the centers of the sarcomeres.
10.3a The Sliding Filament Theory
Sliding filament theory states during contraction:
Width of A bands remains constant.
H zone disappears.
I bands narrow.
Z discs in a sarcomere move closer together.
The overall length of the sarcomere shortens, while the lengths of filaments remain unchanged.
10.3b Neuromuscular Junctions
Skeletal muscle fibers are excited by axons of somatic motor neurons. The neuromuscular junction is formed when a motor neuron meets a muscle fiber.
Components include:
Synaptic knob: Expanded tip of a neuron axon.
Synaptic vesicles: Filled with acetylcholine (ACh).
Synaptic cleft: Space between the synaptic knob and the motor end plate.
Motor end plate: Folds of sarcolemma under the synaptic knob, enriched in ACh receptors.
Acetylcholinesterase (AChE): Enzyme that breaks down excess ACh to prevent continuous muscle stimulation.
10.3c Physiology of Muscle Contraction
A nerve impulse causes vesicles to release ACh into the synaptic cleft.
ACh binds to receptors on the motor end plate, starting a muscle impulse.
The impulse spreads down T-tubules, opening ion channels in terminal cisternae, allowing calcium ions to diffuse into the muscle fiber's sarcoplasm.
Calcium binds to troponin causing a shape change, which moves tropomyosin and exposes active sites on actin.
Myosin heads then bind to actin forming crossbridges, pulling actin towards the sarcomere's center, aided by ATP.
When stimulation ceases, calcium is pumped back into sarcoplasmic reticulum, tropomyosin re-blocks the active sites, and muscles relax.
10.3d Motor Units
Motor unit: A motor neuron and the muscle fibers it controls. When a motor unit is stimulated, all fibers contract.
Size and control of motor units are inversely related; smaller units provide more precise control.
Muscle tone refers to the resting tension in a muscle, achieved by the random stimulation of motor units.
10.4 Types of Skeletal Muscle Fibers
Muscle fibers are categorized into:
Slow oxidative (SO) fibers: Small and fatigue-resistant, also known as Type I fibers.
Fast oxidative (FO) fibers: Intermediate in size and fatigue-resistant, also known as Type IIa fibers.
Fast glycolytic (FG) fibers: Large, anaerobic, and fatigue quickly, also known as Type IIx fibers.
Distribution: SO fibers dominate postural muscles; FG fibers are prevalent in muscles requiring quick bursts of energy.
10.5 Skeletal Muscle Fiber Organization
Muscle fibers organized into fascicles can be arranged in four patterns:
Circular: Fascicles arranged around an opening.
Parallel: Fascicles run parallel to the muscle's long axis, increasing in diameter when contracting.
Convergent: Triangular muscle with a common attachment site.
Pennate: Fascicles arranged obliquely to a tendon:
Unipennate: Fibers on one side of a tendon.
Bipennate: Fibers on both sides of a tendon.
Multipennate: Tendon branches within the muscle.
10.6 Exercise and Skeletal Muscle
Muscle hypertrophy: An increase in muscle fiber size due to the addition of myofibrils, mitochondria, and glycogen from repetitive stimulation.
Muscle atrophy: A reduction in muscle fiber size from disuse or reduced stimulation, characterized by a decrease in muscle strength and tone.
10.7 Levers and Joint Biomechanics
Lever: A rigid object rotating about a fixed point (fulcrum).
Classes of levers:
First-class lever: Fulcrum between effort and resistance.
Second-class lever: Resistance is between the fulcrum and effort.
Third-class lever: Effort between fulcrum and resistance.
Mechanical Advantage: Dependent on the ratio of effort arm length to resistance arm length.
10.7a Actions of Skeletal Muscles
Muscles work in coordination to produce movements:
Agonist (Prime mover): Muscle that produces the primary action (e.g., triceps for forearm extension).
Antagonist: Muscle opposing the action of the agonist (e.g., biceps for forearm extension).
Synergist: Assists the agonist; includes stabilizing fixators.
10.8 Muscle Naming
Muscles are named based on:
Muscle action (e.g., pronator teres).
Specific body regions (e.g., tibialis anterior).
Muscle attachments (e.g., sternocleidomastoid).
Orientation of muscle fibers (e.g., rectus abdominis).
Muscle shape and size (e.g., trapezius, adductor magnus).
Muscle heads/proximal tendons (e.g., biceps brachii).
10.9 Cardiac Muscle
Properties of Cardiac Muscle: Striated, one or two nuclei per cell, rich in mitochondria. Arranged in thick bundles and join via intercalated discs made of gap junctions and desmosomes.
Autorhythmic: Generates impulses autonomously, under involuntary control from the autonomic nervous system.
10.10 Smooth Muscle
Located in the walls of viscera and blood vessels.
Characteristics include:
Short, fusiform shape with a centrally located nucleus.
No striations, dense bodies attached to thin filaments.
Calmodulin (not troponin) binds calcium, leading to smooth muscle contraction.
10.11 Aging and the Muscular System
Begins in mid-30s, accompanied by muscle fiber loss, decreased strength, and increased muscle recovery time. Replacement of muscle with dense connective tissue (fibrosis) affects elasticity.
Results from decreased satellite cells and reduced myofibrils and myofilaments.
10.12 Development of the Muscular System
Begins in the fourth week of embryonic development. Myoblasts fuse to form multinucleated myotubules derived from somites. Migrating myotomes form muscle masses corresponding to flexors and extensors.