Muscle Tissue: Structure, Function, and Physiology

Muscle Tissue

Types of Muscle Tissue

The human body contains three main types of muscle tissue, each with distinct structure and function:

• Skeletal Muscle: Voluntary, striated muscle attached to bones; responsible for movement, posture, and heat production.

• Cardiac Muscle: Involuntary, striated muscle found only in the heart; specialized for rhythmic contraction.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of internal organs; responsible for slow, sustained contractions.

Characteristics of Muscle Tissue

• Excitability: Ability to respond to stimuli by generating electrical impulses (membrane potential).

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to stretch beyond resting length.

• Elasticity: Ability to return to original length after contraction or stretching.

Functions of Skeletal Muscle

• Movement: Facilitates locomotion, breathing, communication, and facial expressions.

• Protection and Support: Arranged in layers to protect internal organs and maintain organ position.

• Posture: Maintains body position and stabilizes joints.

• Temperature Control: Generates heat during contraction to maintain body temperature.

Muscle Structure

Layers Surrounding Muscle

• Skin: Epidermis and dermis.

• Hypodermis: Adipose tissue and superficial fascia.

• Deep Fascia: Dense fibrous connective tissue surrounding entire muscle.

Muscle Terminology

• Muscle (Organ): Composed of fascicles.

• Fascicle: Bundle of muscle fibers (cells).

• Muscle Fiber: Individual muscle cell, contains myofibrils.

• Myofibril: Composed of myofilaments (actin and myosin).

• Myofilaments: Actin (thin) and myosin (thick) filaments.

Connective Tissue Layers

Layer	Description
Epimysium	Dense irregular connective tissue around whole muscle
Perimysium	Connective tissue around fascicle; contains blood vessels and nerves
Endomysium	Areolar connective tissue encasing individual muscle fiber

Muscle Cell Terminology

• 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 Structure and Function

Sarcomere Organization

The sarcomere is the functional unit of muscle contraction, defined as the region between two Z lines.

• A Band: Dark band; contains thick myosin filaments and overlapping actin.

• I Band: Light band; contains only thin actin filaments.

• Z Line: Anchors actin filaments; located in the middle of I band.

• H Zone: Light region in center of A band; contains only myosin.

Myofilaments

• Myosin (Thick Filament): Two heads and two intertwined tails; heads bind actin and ATP, perform power stroke.

• Actin (Thin Filament): Contains myosin binding sites; associated with tropomyosin and troponin.

• Tropomyosin: Covers myosin binding sites on actin at rest.

• Troponin: Binds calcium and tropomyosin; regulates exposure of binding sites.

Sliding Filament Theory

Muscle contraction occurs via the sliding filament mechanism:

1. Action potential travels down sarcolemma.

2. Ca2+ released from SR.

3. Ca2+ binds to troponin, causing tropomyosin to move and expose myosin binding sites.

4. Myosin binds to actin, forming cross-bridges.

5. Myosin heads pivot (power stroke), shortening sarcomere.

6. ATP binds to myosin, causing detachment; hydrolysis resets myosin head.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

Example: During muscle contraction, the Z lines move closer together, I band shortens, A band remains constant.

Neuromuscular Junction (NMJ)

Structure and Function

The NMJ is the site where a motor neuron communicates with a muscle fiber:

• Synaptic Knob: Contains vesicles with acetylcholine (ACh).

• Synaptic Cleft: Fluid-filled gap; contains acetylcholinesterase enzyme.

• Motor End Plate: Region of sarcolemma with ACh receptors.

When ACh is released, it binds to receptors, triggering muscle contraction.

Motor Units and Muscle Contraction

Motor Unit

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

• Small Motor Unit: Few muscle fibers; fine, precise movements (e.g., fingers, eyes).

• Large Motor Unit: Many muscle fibers; gross movements (e.g., posture).

All-or-None Principle

When a neuron fires, all its muscle fibers contract maximally. The degree of contraction is varied by recruitment (activating more motor units).

Muscle Tone

• Muscle Tone: Continuous, passive partial contraction; motor units switch on and off.

• Hypertonic: Increased tone; spasticity from upper motor neuron damage.

• Hypotonic: Decreased tone; flaccidity from lower motor neuron damage.

Muscle Growth and Atrophy

Hypertrophy

Muscle cells enlarge by increasing myofibrils; common in bodybuilders.

Atrophy

Muscle mass decreases with disuse, lack of innervation, or loss of myofibrils. Can be reversible or irreversible (muscle replaced by collagen).

Muscle Metabolism and Energy Supply

ATP Requirements

• Needed for exocytosis, calcium pump, Na/K pump, myosin head detachment and cocking.

Sources of ATP

• Phosphate Creatine: Creatine phosphate donates phosphate to ADP to form ATP; quick, short-term energy.

• Anaerobic Glycolysis: Glucose converted to lactic acid; produces 2 ATP per glucose; used for rapid, short bursts.

• Aerobic Respiration: Glucose and oxygen produce CO2, water, and up to 38 ATP; used for sustained activity.

Equations:

$\text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

$\text{Glucose} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + 38~\text{ATP} + \text{heat}$

$\text{Glucose} \rightarrow 2~\text{Pyruvic~acid} \rightarrow 2~\text{Lactic~acid} + 2~\text{ATP}$

Muscle Fiber Types

Fiber Type	Characteristics	Example
Slow Oxidative (SO)	Small diameter, lots of myoglobin, many mitochondria, aerobic ATP production, resistant to fatigue	Postural muscles, marathon runners
Fast Glycolytic (FG)	Large diameter, little/no myoglobin, few mitochondria, anaerobic ATP production, rapid fatigue	Sprinting, bodybuilders

Muscle Fatigue and Recovery

• Muscle Fatigue: Reduced ability to produce tension.

• Oxygen Debt: Extra oxygen needed after exercise to restore pre-exercise conditions.

• Heat Production: Byproduct of muscle activity; helps maintain body temperature.

Muscle Twitch and Contraction Types

Muscle Twitch

A muscle twitch is the response of a single motor unit to an action potential.

• Latent Period: Delay between stimulus and contraction; action potential propagates.

• Contraction Period: Tension increases; sarcomeres shorten.

• Relaxation Period: Ca2+ reabsorbed; tension decreases.

• Refractory Period: Muscle cannot be stimulated again.

Myogram (EMG)

Graphical representation of muscle contraction phases.

Summation and Tetanus

• Wave Summation: Successive stimuli increase contraction force.

• Tetanus: High-frequency stimuli eliminate relaxation phase; continuous contraction.

• Treppe: Graded increase in tension with repeated stimulation.

Types of Muscle Contraction

• Isometric: Tension increases, muscle does not shorten; e.g., holding a weight steady.

• Isotonic: Tension constant, muscle changes length; load is moved.

• Concentric: Muscle shortens as it contracts; e.g., lifting a dumbbell.

• Eccentric: Muscle lengthens as it contracts; e.g., lowering a dumbbell.

Cardiac Muscle

Structure and Function

• Sarcomeres: Present, but shorter and thicker than in skeletal muscle.

• Motor Neurons: Not innervated by motor neurons; self-excitation; rate controlled by autonomic nervous system.

• Energy Source: Abundant mitochondria; relies on aerobic respiration; vulnerable to oxygen supply interruptions.

• Intercalated Disks: Gap junctions and desmosomes for electrical and mechanical connectivity.

• Calcium: T-tubules larger; Ca2+ can enter from extracellular fluid; SR less developed.

• Long Refractory Period: Prevents tetanus.

• Regeneration: No regeneration capability.

Smooth Muscle

Structure and Function

• Locations: Digestive, urinary, cardiovascular, respiratory, reproductive systems.

• Sarcomeres: Absent; thick and thin filaments not aligned.

• Motor Neurons: Not innervated by motor neurons; controlled by autonomic nervous system.

• Calcium: Enters from extracellular fluid; binds to calmodulin (no troponin).

• Contraction: Slow, low force, wave-like, continuous.

• Stress-Relaxation Response: Can stretch and adapt to new length; retains contractility.

• Latchbridge Mechanism: Myosin remains attached to actin for extended time without extra ATP.

Muscle Comparison Table

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

Summary

Muscle tissue is essential for movement, posture, and homeostasis. Its structure, function, and energy metabolism are specialized according to the type of muscle. Understanding these concepts is fundamental for anatomy and physiology students.