Muscles and Muscle Tissue: Structure, Function, and Physiology

Muscle Tissue Overview

Types of Muscle Tissue

Muscle tissue is essential for movement and comprises nearly half of the body's mass. It transforms chemical energy (ATP) into mechanical energy, enabling force generation. There are three main types of muscle tissue, each with distinct characteristics and functions:

• Skeletal Muscle: Attached to bones and skin, responsible for voluntary movements, striated, and contracts rapidly but tires easily.

• Cardiac Muscle: Found only in the heart, striated, involuntary, and contracts at a steady rate due to the heart's pacemaker.

• Smooth Muscle: Located in the walls of hollow organs (e.g., stomach, bladder), non-striated, involuntary, and contracts slowly.

Characteristic	Skeletal	Cardiac	Smooth
Body Location	Attached to bones/skin	Heart walls	Walls of hollow organs
Cell Shape	Long, cylindrical, multinucleate, striated	Branching, uni/binucleate, striated	Spindle-shaped, uninucleate, non-striated
Control	Voluntary	Involuntary	Involuntary

Characteristics of Muscle Tissue

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length.

Functions of Muscle Tissue

• Produce movement (locomotion, manipulation, pumping blood, digestion)

• Maintain posture and body position

• Stabilize joints

• Generate heat during contraction

Skeletal Muscle Structure

Organization and Connective Tissue Sheaths

Skeletal muscle is an organ composed of muscle fibers, nerves, blood vessels, and connective tissue sheaths. These sheaths support and reinforce the muscle:

• Epimysium: Surrounds the entire muscle (dense irregular connective tissue).

• Perimysium: Surrounds fascicles (bundles of muscle fibers).

• Endomysium: Surrounds each individual muscle fiber (areolar connective tissue).

Muscle Attachments

• Direct (fleshy) attachment: Epimysium fused to periosteum of bone or perichondrium of cartilage.

• Indirect attachment: Connective tissue wrappings extend beyond muscle as a tendon or aponeurosis.

Levels of Skeletal Muscle Organization

Level	Description	Connective Tissue
Muscle (organ)	Hundreds to thousands of muscle cells, blood vessels, nerves	Epimysium
Fascicle	Bundle of muscle cells	Perimysium
Muscle fiber (cell)	Elongated, multinucleate cell, striated	Endomysium

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Components

• Sarcolemma: Plasma membrane of muscle fiber

• Sarcoplasm: Cytoplasm of muscle fiber, contains glycosomes (glycogen storage) and myoglobin (O2 storage)

• Myofibrils: Densely packed, rodlike elements responsible for muscle contraction

Myofibril Structure

• Striations: Alternating dark (A bands) and light (I bands) regions

• Sarcomere: Smallest contractile unit, extends from Z disc to Z disc

• Myofilaments: Thick (myosin) and thin (actin) filaments arranged in a hexagonal pattern

Key Proteins in Myofibrils

• Myosin: Forms thick filaments, has heads for cross-bridge formation

• Actin: Forms thin filaments, contains binding sites for myosin

• Tropomyosin and Troponin: Regulatory proteins on thin filaments

• Titin: Provides elasticity and stabilizes thick filaments

• Dystrophin: Links thin filaments to sarcolemma

Skeletal Muscle Physiology

Sarcoplasmic Reticulum (SR) and T Tubules

The SR is a network of smooth endoplasmic reticulum that stores and releases Ca2+. T tubules are invaginations of the sarcolemma that transmit action potentials deep into the muscle fiber, ensuring coordinated contraction.

Sliding Filament Model of Contraction

During contraction, thin filaments slide past thick filaments, increasing overlap. Myosin heads bind to actin, forming cross-bridges, and pull the thin filaments toward the center of the sarcomere. This process shortens the muscle fiber without changing the length of the filaments.

Neuromuscular Junction and Muscle Excitation

The neuromuscular junction (NMJ) is the site where a motor neuron stimulates a muscle fiber. The process involves:

1. Action potential arrives at axon terminal.

2. Voltage-gated Ca2+ channels open, Ca 2+ enters neuron.

3. Ca 2+ triggers release of acetylcholine (ACh) into synaptic cleft.

4. ACh binds to receptors on sarcolemma, opening Na+ channels and generating an end plate potential.

5. Acetylcholinesterase degrades ACh, ending the signal.

Action Potential and Excitation-Contraction Coupling

An action potential (AP) is generated and propagated along the sarcolemma and down T tubules, triggering Ca2+ release from the SR. This Ca2+ binds to troponin, moving tropomyosin and allowing myosin to bind actin, initiating contraction.

Cross Bridge Cycle

1. Cross bridge formation: Myosin head attaches to actin.

2. Power stroke: Myosin head pivots, pulling actin filament.

3. Cross bridge detachment: ATP binds to myosin, causing detachment.

4. Cocking of myosin head: ATP hydrolysis re-energizes the myosin head.

Whole Muscle Contraction

Motor Units and Muscle Twitch

• Motor Unit: A motor neuron and all the muscle fibers it innervates. Smaller units allow fine control.

• Muscle Twitch: Response of a muscle to a single stimulus, consisting of latent, contraction, and relaxation phases.

Graded Muscle Responses

• Strength and duration of contraction can be varied by changing the frequency and strength of stimulation.

• Temporal (wave) summation: Increased frequency of stimulation increases tension.

• Recruitment: Increasing stimulus strength recruits more motor units.

Types of Contractions

• Isotonic: Muscle changes length (concentric: shortens; eccentric: lengthens).

• Isometric: Muscle tension increases, but length does not change.

Energy for Muscle Contraction

ATP Regeneration Pathways

• Direct phosphorylation: Creatine phosphate donates phosphate to ADP to form ATP (short duration, ~15 seconds).

• Anaerobic pathway: Glycolysis and lactic acid formation (no oxygen required, short-term energy).

• Aerobic pathway: Aerobic respiration in mitochondria (requires oxygen, long-term energy).

Equation for direct phosphorylation:

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

Muscle Fatigue and Recovery

• Muscle fatigue: Inability to contract despite continued stimulation, often due to ionic imbalances, increased inorganic phosphate, or decreased ATP/glycogen.

• Excess postexercise oxygen consumption (EPOC): Oxygen required to restore muscle to pre-exercise state.

Factors Affecting Muscle Contraction

• Frequency of stimulation

• Number of muscle fibers recruited

• Size of muscle fibers

• Degree of muscle stretch (length-tension relationship)

Muscle Fiber Types

Type	Speed	ATP Pathway	Myoglobin	Fatigue	Best Suited For
Slow Oxidative	Slow	Aerobic	High	Resistant	Endurance, posture
Fast Oxidative	Fast	Aerobic/Some Anaerobic	High	Intermediate	Sprinting, walking
Fast Glycolytic	Fast	Anaerobic	Low	Fatigable	Short, intense movements

Smooth Muscle

Structure and Function

• Found in walls of hollow organs (except heart)

• Spindle-shaped, uninucleate, non-striated

• Arranged in longitudinal and circular layers

• Contracts via sliding filament mechanism, but regulated by calmodulin (not troponin)

Types of Smooth Muscle

• Unitary (visceral): Most common, found in hollow organs, electrically coupled by gap junctions

• Multi unit: Found in large airways, arteries, arrector pili, and iris; few gap junctions, independent fibers

Development and Aging of Muscle

• All muscle tissues develop from myoblasts (embryonic mesoderm cells)

• Cardiac and smooth muscle can regenerate to some extent; skeletal muscle has limited regeneration

• With age, muscle mass decreases (sarcopenia), but regular exercise can slow this process