Muscle Tissue and Physiology: Structure, Function, and Mechanisms

Muscle Tissue and Physiology

Overview of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, joint stabilization, heat generation, and regulation of material flow through hollow organs. Muscle cells, or myocytes, are surrounded by an extracellular matrix called the endomysium, which helps transmit tension between cells.

• Muscle Tension: The force generated by muscle tissue.

• Main Types: Skeletal, Cardiac, and Smooth muscle tissue.

Types of Muscle Tissue

There are three main types of muscle tissue, each with distinct structure, location, control, and function.

Type	Structure	Location	Voluntary?	Function
Striated (Skeletal)	Long, cylindrical, multinucleated, striated cells	Mostly attached to skeleton	Voluntary	Movement of body
Cardiac	Short, branched, striated, single nucleus, intercalated discs	Heart	Involuntary	Pumps blood
Smooth	Spindle-shaped, single nucleus, non-striated	Walls of hollow organs, eyes, skin, glands	Involuntary	Moves materials, regulates diameter

Properties of Muscle Cells

• Contractility: Ability to shorten and generate force.

• Excitability: Ability to respond to stimuli.

• Conductivity: Ability to conduct electrical signals.

• Distensibility: Ability to stretch without rupture.

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

Structure of Muscle Cells

Muscle Cell Anatomy

Muscle cells share organelles with other cells but have specialized structures for contraction.

• Sarcoplasm: Cytoplasm of muscle cell.

• Sarcolemma: Plasma membrane of muscle cell.

• Myofibrils: Bundles of contractile proteins, making up 50–80% of cell volume.

• Sarcoplasmic Reticulum (SR): Modified smooth ER, stores and releases calcium ions.

Skeletal Muscle Fiber Structure

• Long, cylindrical, multinucleated cells (up to 30 cm long).

• Striated appearance due to arrangement of myofilaments.

• Formed by fusion of embryonic myoblasts.

Specialized Structures

• T-tubules: Inward extensions of sarcolemma, surround myofibrils, filled with extracellular fluid.

• Terminal Cisternae: Enlarged SR regions flanking T-tubules.

• Triad: T-tubule plus two terminal cisternae.

Structure of the Myofibril

Myofilaments

Myofibrils are composed of three types of myofilaments:

• Thick Filaments: Made of myosin; each myosin has two heads and a tail.

• Thin Filaments: Made of actin (with active sites), tropomyosin (covers active sites), troponin (binds calcium and controls tropomyosin), and nebulin (regulates length).

• Elastic Filaments: Made of titin, provide elasticity and resist overstretching.

Myofilament Arrangement and the Sarcomere

The sarcomere is the functional unit of muscle contraction, defined by the arrangement of myofilaments:

• I band: Light region, only thin filaments.

• A band: Dark region, thick and thin filaments overlap.

• H zone: Middle of A band, only thick filaments.

• M line: Middle of A band, holds thick filaments in place.

• Z-disc: Anchors thin filaments, attachment for elastic filaments, connects myofibrils.

Structure	Description	Mnemonic
A band	Dark band, thick and thin filaments	dArk band
I band	Light band, only thin filaments	lIght band
H zone	Middle of A band, only thick filaments	"H" in A band
M line	Middle of A band, holds thick filaments	"M" for middle/myosin
Z-disc	Bisects I band, anchors filaments	"Z" shape

Levels of Organization in Skeletal Muscle

Skeletal muscle is organized from the whole muscle down to the molecular level:

• Muscle → Fascicle → Muscle Fiber → Myofibril → Myofilament

The Sliding-Filament Mechanism of Contraction

Mechanism

Muscle contraction occurs when thin filaments slide past thick filaments, shortening the sarcomere without changing the length of the filaments themselves.

• I bands and H zone narrow as myosin heads pull thin filaments toward the M line.

• Z-discs move closer together, shortening the sarcomere and the muscle fiber.

Membrane Potential in Muscle Cells

Electrophysiology and Resting Membrane Potential

Muscle fibers maintain a resting membrane potential due to the distribution of ions across the sarcolemma, primarily sodium (Na+) and potassium (K+).

• Resting membrane potential: About −90 mV in muscle fibers.

• Sodium-potassium pump: Moves 3 Na+ out and 2 K+ in, maintaining gradients.

• Leak channels: Allow passive movement of ions, especially K+.

Action Potentials

An action potential is a rapid, temporary change in membrane potential that propagates along the sarcolemma, enabling muscle contraction.

• Depolarization: Na+ channels open, Na+ enters, membrane becomes positive.

• Repolarization: K+ channels open, K+ leaves, membrane returns to negative.

The Neuromuscular Junction (NMJ)

Structure and Function

The NMJ is the synapse between a motor neuron and a skeletal muscle fiber, where nerve impulses are transmitted to initiate muscle contraction.

• Axon terminal: Contains synaptic vesicles with acetylcholine (ACh).

• Synaptic cleft: Space between neuron and muscle fiber.

• Motor end plate: Specialized sarcolemma with ACh receptors.

Skeletal Muscle Contraction

Phases of Contraction

• Excitation: Motor neuron releases ACh, generating an end-plate potential in the muscle fiber.

• Excitation-Contraction Coupling: Action potential travels along sarcolemma and T-tubules, triggering Ca2+ release from SR.

• Contraction (Crossbridge Cycle): Ca2+ binds troponin, tropomyosin shifts, myosin binds actin, and power strokes occur using ATP.

• Relaxation: ACh is degraded, Ca2+ pumped back into SR, muscle returns to resting state.

Rigor Mortis

After death, ATP production ceases, Ca2+ remains in the cytosol, and myosin heads cannot detach from actin, causing muscles to stiffen until proteins degrade.

Energy Sources for Skeletal Muscle

ATP Regeneration Pathways

• Immediate: Creatine phosphate donates phosphate to ADP (via creatine kinase) for rapid ATP regeneration (10 seconds).

• Glycolytic (Anaerobic): Glycolysis in cytosol produces ATP and pyruvate (30–40 seconds).

• Oxidative (Aerobic): Mitochondrial reactions use glucose, fatty acids, or amino acids with oxygen for sustained ATP production (minutes to hours).

Muscle Tension at the Fiber Level

Muscle Twitch and Myogram

A muscle twitch is the response of a muscle fiber to a single stimulus, recorded as a myogram with three phases: latent, contraction, and relaxation.

• Latent period: Action potential spreads, crossbridge cycle begins.

• Contraction period: Tension rises as crossbridges form.

• Relaxation period: Tension falls as Ca2+ is pumped back into SR.

Wave Summation and Tetanus

• Wave summation: Repeated stimuli increase tension.

• Unfused tetanus: Partial relaxation between stimuli.

• Fused tetanus: No relaxation, maximal tension.

Length-Tension Relationship

The amount of tension a muscle fiber can produce depends on the initial length of the sarcomere. Optimal overlap of thick and thin filaments allows maximal crossbridge formation and tension.

Classes of Skeletal Muscle Fibers

Fiber Types

• Slow-twitch (Type I): Slow, fatigue-resistant, oxidative metabolism, high myoglobin (red).

• Fast-twitch (Type II): Fast, powerful, fatigue quickly, glycolytic metabolism, low myoglobin (white).

• Subtypes: Type IIa (fast oxidative glycolytic), Type IIx (fast glycolytic).

Motor Units and Muscle Contractions

Motor Units

A motor unit consists of a single motor neuron and all the muscle fibers it innervates. Recruitment of more motor units increases force production.

Types of Muscle Contractions

• Isotonic Concentric: Muscle shortens, force > load.

• Isotonic Eccentric: Muscle lengthens, force < load.

• Isometric: Muscle length unchanged, force = load.

Adaptations to Physical Training

Endurance vs. Resistance Training

• Endurance training: Increases oxidative enzymes, mitochondria, blood supply; improves fatigue resistance.

• Resistance training: Increases myofibril number and fiber diameter (hypertrophy); increases strength but may reduce endurance.

• Disuse (atrophy): Decreases fiber diameter, myofibrils, and oxidative capacity.

Smooth and Cardiac Muscle

Smooth Muscle

• Structure: Non-striated, spindle-shaped, single nucleus, dense bodies anchor actin.

• Function: Peristalsis, sphincter formation, regulation of vessel diameter.

• Contraction: Triggered by Ca2+ binding to calmodulin, activating myosin light-chain kinase.

• Types: Single-unit (visceral) and multi-unit smooth muscle.

Cardiac Muscle

• Structure: Striated, branched, one or two nuclei, intercalated discs with gap junctions and desmosomes.

• Function: Pumps blood, autorhythmic due to pacemaker cells.

Additional info: This summary covers the essential concepts of muscle tissue and physiology, including structure, function, contraction mechanisms, energy sources, and adaptations to training, as well as the unique features of smooth and cardiac muscle.