Muscle Tissue: Structure, Function, and Physiology

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Muscle Tissue

Overview of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and various bodily functions. There are three main types of muscle tissue in the human body: skeletal muscle, cardiac muscle, and smooth muscle.

Skeletal Muscle: Voluntary, striated muscle attached to bones; responsible for body movement.
Cardiac Muscle: Involuntary, striated muscle found only in the heart; responsible for pumping blood.
Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs; responsible for movements such as peristalsis.

Characteristics of Muscle Tissue

Excitability: Ability to respond to stimuli by generating electrical changes (membrane potential).
Contractility: Ability to shorten forcibly when stimulated.
Extensibility: Ability to be stretched or extended.
Elasticity: Ability to return to original length after contraction or extension.

Functions of Skeletal Muscle

Movement: Facilitates body movement, breathing, and facial expressions.
Protection and Support: Arranged in layers to protect internal organs and support their position.
Posture: Maintains body position and stabilizes joints.
Temperature Control: Generates heat to help maintain body temperature.

Muscle Structure and Organization

Layers to Reach Muscle Tissue

Skin: Epidermis and dermis.
Hypodermis: Contains adipose tissue and superficial fascia.
Deep Fascia: Dense fibrous connective tissue surrounding entire muscle.

Muscle Terminology and Hierarchy

Muscle (Organ): Composed of bundles called fascicles.
Fascicle: Bundle of muscle fibers (cells).
Muscle Fiber: Single muscle cell, contains many myofibrils.
Myofibril: Composed of myofilaments (actin and myosin).
Myofilaments: Thin (actin) and thick (myosin) filaments responsible for contraction.

Connective Tissue Layers

Layer	Location	Function
Epimysium	Around whole muscle	Dense irregular connective tissue sheath
Perimysium	Around fascicle	Contains blood vessels and nerves
Endomysium	Around individual fiber	Areolar connective tissue

Muscle Cell (Fiber) Structure

Sarcolemma: Plasma membrane of muscle fiber; contains T-tubules for conducting action potentials.
Sarcoplasm: Cytoplasm of muscle fiber.
Sarcoplasmic Reticulum (SR): Smooth ER; stores and releases Ca2+ for contraction.
Triad: Two terminal cisternae of SR and one T-tubule.

Sarcomere and Myofilaments

Sarcomere Structure

The sarcomere is the functional unit of muscle contraction, defined as the region between two Z lines. Striations are due to the arrangement of myofilaments within sarcomeres.

A band (dark): Contains thick (myosin) filaments and overlapping thin (actin) filaments.
I band (light): Contains only thin (actin) filaments.
Z line: Center of I band; anchors actin filaments.
H-zone: Lighter region in center of A band; only myosin present.

Myofilament Proteins

Myosin (Thick Filament): Has two heads (bind to actin and ATP) and two intertwined tails. Myosin heads hydrolyze ATP and form cross-bridges with actin.
Actin (Thin Filament): Contains binding sites for myosin heads.
Tropomyosin: Wraps around actin, covering myosin binding sites at rest.
Troponin: Binds to tropomyosin and Ca2+; when Ca2+ binds, it causes a conformational change exposing myosin binding sites.

Muscle Contraction Mechanism

Sliding Filament Theory

Muscle contraction occurs as myosin heads bind to actin, pulling the thin filaments toward the center of the sarcomere, shortening the muscle.

Action potential travels down sarcolemma and T-tubules.
Ca2+ is released from SR.
Ca2+ binds to troponin, shifting tropomyosin and exposing binding sites on actin.
Myosin heads bind to actin, forming cross-bridges.
Myosin heads pivot (power stroke), pulling actin filaments inward.
ATP binds to myosin, causing detachment from actin; ATP hydrolysis "cocks" the myosin head for another cycle.

Equation for ATP hydrolysis:

Neuromuscular Junction (NMJ)

Synaptic knob: Contains vesicles of acetylcholine (ACh).
Synaptic cleft: Space between neuron and muscle; contains acetylcholinesterase (AChE).
Motor end plate: Region of sarcolemma with ACh receptors.

When ACh is released, it binds to receptors, triggering an action potential in the muscle fiber.

Relaxation

Ca2+ is pumped back into SR.
Tropomyosin covers binding sites on actin.
Muscle fiber relaxes; if ATP is depleted, muscle becomes fatigued.

Motor Units and Muscle Control

Motor Unit

A motor unit consists of one motor neuron and all the muscle fibers it innervates.

Small motor units: Few fibers; precise control (e.g., fingers, eyes).
Large motor units: Many fibers; gross movements (e.g., posture).

All-or-None Principle

When a neuron fires, all muscle fibers in its motor unit contract maximally. The degree of contraction is varied by recruitment—increasing the number of active motor units.

Muscle Tone

Continuous, partial contraction of muscles.
Hypertonic: Increased tone (spasticity; upper motor neuron damage).
Hypotonic: Decreased tone (flaccid; lower motor neuron damage).

Muscle Growth and Atrophy

Hypertrophy

Increase in muscle cell size and number of myofibrils.
Common in bodybuilders (more fast glycolytic fibers).

Atrophy

Decrease in muscle mass due to disuse, denervation, or loss of myofibrils.
Can be reversible (with exercise) or irreversible (muscle replaced by collagen).

Muscle Metabolism and Energy Supply

ATP Requirements

Needed for exocytosis, Ca2+ pumps, Na+/K+ pumps, and myosin head detachment/cocking.

Sources of ATP

Creatine Phosphate: Transfers phosphate to ADP to form ATP (1 ATP per creatine phosphate).
Anaerobic Glycolysis: Glucose → 2 pyruvic acid → 2 lactic acid + 2 ATP (no oxygen required; short duration).
Aerobic Respiration: Glucose + O2 → CO2 + H2O + 36-38 ATP (long duration; requires oxygen).

Equation for aerobic respiration:

Muscle Fiber Types

Fiber Type	Features	Metabolism	Example
Slow Oxidative (SO)	Small diameter, lots of myoglobin, many mitochondria	Aerobic	Postural muscles, marathon runners
Fast Glycolytic (FG)	Large diameter, little/no myoglobin, few mitochondria	Anaerobic	Sprinters, bodybuilders

Other Energy Concepts

Muscle fatigue: Reduced ability to generate tension.
Oxygen debt: Extra oxygen needed post-exercise to restore conditions.
Heat production: Byproduct of muscle activity.

Muscle Contraction Patterns

Muscle Twitch and Myogram

Latent Period: Delay between stimulus and contraction (no tension change).
Contraction Period: Tension increases; muscle shortens.
Relaxation Period: Ca2+ reabsorbed; muscle relaxes.
Refractory Period: Muscle cannot be restimulated.

Summation, Tetanus, and Treppe

Wave Summation: Successive stimuli add together, increasing force.
Tetanus: High-frequency stimulation; no relaxation; sustained contraction.
Treppe: Gradual increase in contraction strength with repeated stimulation ("staircase effect").

Types of Muscle Contraction

Isometric: Tension increases, muscle length does not change (e.g., holding a weight steady).
Isotonic: Tension constant, muscle changes length (moves load).
- Concentric: Muscle shortens as it contracts (lifting a weight).
- Eccentric: Muscle lengthens as it contracts (lowering a weight).

Cardiac Muscle

Structure and Function

Striated, branched cells with sarcomeres (shorter and thicker than skeletal muscle).
No motor neuron innervation; self-excitable (autorhythmic).
Controlled by autonomic nervous system.
Abundant mitochondria; relies on aerobic metabolism.
Intercalated discs with gap junctions and desmosomes for electrical and mechanical connection.
Long refractory period; no tetanus possible.
Poor regeneration capacity.

Smooth Muscle

Structure and Function

Found in walls of hollow organs (digestive, urinary, cardiovascular, respiratory, reproductive tracts).
No sarcomeres; thick and thin filaments not aligned.
Innervated by autonomic nervous system.
Ca2+ enters from extracellular fluid and binds to calmodulin (no troponin).
Slow, sustained, wave-like contractions (peristalsis).
Stress-relaxation response: can stretch and adapt to new length while retaining contractility.
Latchbridge mechanism: myosin remains attached to actin for extended periods without extra ATP use.
Capable of regeneration.

Muscle Tissue Comparison Table

Type	Sarcomere	Motor Neuron	Regeneration
Skeletal	Yes	Yes	Not well
Cardiac	Yes	No (SA node)	No
Smooth	No	Sometimes	Yes

Example Applications

Skeletal muscle: Biceps brachii contracting to lift a dumbbell.
Cardiac muscle: Heart pumping blood throughout the body.
Smooth muscle: Peristalsis moving food through the digestive tract.