Muscles and Muscle Tissue: Structure, Function, and Physiology

Muscle Tissue Overview

Introduction to Muscle Tissue

Muscle tissue is essential for movement, posture, and various physiological processes. It comprises nearly half of the body's mass and is specialized for converting chemical energy (ATP) into mechanical energy, enabling force generation and movement.

• Key prefixes: myo-, mys-, and sarco- refer to muscle structures (e.g., sarcoplasm = muscle cell cytoplasm).

• Three types of muscle tissue: Skeletal, Cardiac, Smooth.

Types of Muscle Tissue

• Skeletal Muscle: Attached to bones and skin, voluntary, striated, contracts rapidly but tires easily. Responsible for body movement and posture.

• Cardiac Muscle: Found only in the heart, involuntary, striated, contracts at a steady rate set by pacemaker cells but modulated by the nervous system.

• Smooth Muscle: Found in walls of hollow organs (e.g., stomach, bladder, airways), involuntary, non-striated, responsible for moving substances through internal body channels.

Characteristic	Skeletal	Cardiac	Smooth
Location	Attached to bones/skin	Heart walls	Walls of hollow organs
Control	Voluntary	Involuntary	Involuntary
Striations	Yes	Yes	No
Cell Shape	Long, cylindrical, multinucleate	Branching, uni/binucleate	Spindle-shaped, uninucleate

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 and Organization

Components of Skeletal Muscle

Skeletal muscle is an organ composed of muscle fibers, connective tissue, blood vessels, and nerves. It is organized into several hierarchical levels:

• Muscle (organ): Surrounded by epimysium.

• Fascicle: Bundle of muscle fibers, surrounded by perimysium.

• Muscle fiber (cell): Surrounded by endomysium.

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

Attachments

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

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

Microscopic Anatomy of Skeletal Muscle Fibers

Muscle Fiber Structure

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm containing glycosomes (glycogen storage) and myoglobin (O2 storage).

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

Sarcomere and Myofilaments

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

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

• Striations: Alternating A bands (dark) and I bands (light) due to arrangement of myofilaments.

Specialized Proteins

• Myosin: Thick filament with heads that form cross bridges during contraction.

• Actin: Thin filament with binding sites for myosin heads; includes regulatory proteins tropomyosin and troponin.

• Titin: Elastic filament that maintains sarcomere structure and elasticity.

• Dystrophin: Links thin filaments to sarcolemma proteins; mutations cause Duchenne muscular dystrophy.

Excitation-Contraction Coupling and Muscle Contraction

Sarcoplasmic Reticulum and T Tubules

• Sarcoplasmic Reticulum (SR): Stores and releases Ca2+ for muscle contraction.

• T Tubules: Invaginations of sarcolemma that transmit action potentials deep into muscle fiber.

• Triad: Structure formed by a T tubule and two terminal cisterns of the SR.

Sliding Filament Model of Contraction

Muscle contraction occurs when myosin heads bind to actin, forming cross bridges and pulling thin filaments toward the center of the sarcomere. This process shortens the muscle fiber without changing the length of the filaments.

• During contraction: I bands shorten, Z discs move closer, H zones disappear, A bands move closer together.

Neuromuscular Junction and Action Potential

• Motor Neurons: Stimulate muscle fibers via the neuromuscular junction (NMJ).

• Neurotransmitter: Acetylcholine (ACh) is released from the axon terminal, binds to receptors on the sarcolemma, and initiates an action potential.

• Ion Channels: Chemically gated (ACh) and voltage-gated channels regulate ion flow and membrane potential changes.

Steps in Muscle Fiber Excitation and Contraction

1. Action potential arrives at axon terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ enters neuron.

3. ACh is released into synaptic cleft.

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

5. Action potential propagates along sarcolemma and down T tubules.

6. Ca2+ is released from SR, initiating contraction.

Cross Bridge Cycle

• Requires Ca2+ and ATP.

• Four steps: Cross bridge formation, power stroke, cross bridge detachment, cocking of myosin head.

Whole Muscle Contraction

Motor Units

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

• Smaller motor units allow for fine control; larger units generate more force.

Muscle Twitch and Graded Responses

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

• Graded Responses: Varying strength of contraction by changing stimulus frequency (temporal summation) or strength (recruitment).

Types of Contractions

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

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

Energy for Muscle Contraction

ATP Regeneration Pathways

• Direct phosphorylation: Creatine phosphate donates phosphate to ADP to form ATP.

• Anaerobic pathway: Glycolysis and lactic acid formation (no oxygen required).

• Aerobic pathway: Glycolysis followed by aerobic respiration in mitochondria (requires oxygen).

Equation for direct phosphorylation:

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

Muscle Fatigue and Recovery

• Fatigue results from ionic imbalances, increased inorganic phosphate, decreased ATP, and other factors.

• Recovery involves replenishing oxygen, ATP, and glycogen stores (EPOC: excess postexercise oxygen consumption).

Muscle Fiber Types and Adaptations

Muscle Fiber Types

• Slow oxidative fibers: Endurance, fatigue-resistant, aerobic metabolism.

• Fast oxidative fibers: Intermediate properties, aerobic and some anaerobic capacity.

• Fast glycolytic fibers: Short-term, powerful movements, fatigue quickly, anaerobic metabolism.

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

Muscle Adaptation to Exercise

• Aerobic exercise: Increases capillaries, mitochondria, myoglobin; enhances endurance.

• Resistance exercise: Increases muscle size (hypertrophy), strength, and connective tissue.

• Disuse atrophy: Muscle wasting due to inactivity or loss of neural stimulation.

Smooth Muscle

Structure and Function

• Found in walls of hollow organs (except heart).

• Spindle-shaped, uninucleate, non-striated cells; organized in sheets.

• Contraction is involuntary, slow, and can be sustained for long periods.

Differences from Skeletal Muscle

• No sarcomeres, T tubules, or troponin; uses calmodulin for Ca2+ binding.

• Gap junctions allow for coordinated contraction.

• Contraction mechanism involves Ca2+ entry from extracellular space, activation of calmodulin, and phosphorylation of myosin.

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; fibers act independently.

Development and Aging of Muscle Tissue

• All muscle types develop from embryonic mesoderm (myoblasts).

• Skeletal muscle fibers form by fusion of myoblasts; cardiac and smooth muscle develop gap junctions.

• Muscle mass and function decline with age (sarcopenia), but regular exercise can slow this process.