Cellular Physiology of Muscle: Structure, Function, and Mechanisms

Cellular Physiology of Muscle

Overview

This section explores the microscopic structure of skeletal muscle fibers and explains the cellular mechanisms underlying excitation-contraction coupling. It also compares skeletal, cardiac, and smooth muscle types, and outlines the major functions and characteristics of muscle tissue.

Comparison Features of Muscle Cells

Main Types of Muscle Tissue

Muscle cells, also known as muscle fibers, are specialized for contraction and movement. The terminology "myo-" and "sarco-" is commonly used to refer to muscle-related structures. Muscle contraction depends on the interaction of actin and myosin myofilaments.

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Location	Attached to skeleton	Structure of the heart	Walls of hollow, visceral organs
Cell Shape	Elongated fibers	Branched fibers	Spindle-shaped fibers
Striations	Striated	Striated	Nonstriated
Control	Voluntary	Involuntary (pacemaker & neural input)	Involuntary
Contraction Speed	Rapid, tires easily, adaptable	Moderate, rhythmic, no down time	Slow, sustained contractions
Example	Biceps brachii	Myocardium	Intestinal wall

Functions and Characteristics of Muscle Tissue

Major Functions

• Generate movement: Responsible for locomotion, manipulation, blood flow, and pressure regulation, as well as respiration and propulsion of substances (e.g., food, urine).

• Maintain posture: Muscles work continuously to counteract gravity and stabilize body position.

• Stabilize joints: Muscles help stabilize joints during movement, such as in the shoulders and knees.

• Generate heat: Muscle contractions produce heat, which is essential for maintaining body temperature (especially skeletal muscle, which constitutes about 40% of body mass).

Functional Characteristics

• Excitability (Responsiveness): Ability to receive and respond to a stimulus, usually a chemical such as a neurotransmitter, hormone, or change in pH. The response is the generation of an action potential along the sarcolemma, leading to muscle contraction.

• Contractility: Ability to shorten forcibly when adequately stimulated.

• Extensibility: Ability to be stretched or extended beyond resting length.

• Elasticity: Ability to return to resting length after being stretched.

Microscopic Anatomy of Skeletal Muscle Fiber

Structure and Terminology

• Skeletal muscle fibers are large, cylindrical cells with multiple oval nuclei located at the periphery (just beneath the sarcolemma).

• The muscle cell is a syncytium, meaning it is formed by the fusion of multiple cells and contains many nuclei.

• Diameter ranges from 10-100 μm; length can be several centimeters.

• Sarcoplasm (cytoplasm of muscle cell) contains abundant glycogen and myoglobin (an oxygen-binding protein).

• Each fiber contains hundreds to thousands of myofibrils, which make up about 80% of cell volume.

• Sarcoplasmic reticulum (SR) is an extensive network for calcium storage and release.

• T-tubules are invaginations of the sarcolemma that help transmit action potentials deep into the muscle fiber.

Connective Tissue Layers

• Endomysium: Thin connective tissue surrounding each muscle fiber.

• Perimysium: Connective tissue wrapping around bundles of muscle fibers (fascicles).

• Epimysium: Dense connective tissue surrounding the entire muscle.

• Tendon: Cord-like extension of connective tissue that attaches muscle to bone.

Myofibril and Sarcomere Structure

• Myofibrils are composed of repeating units called sarcomeres, the contractile units of muscle.

• Sarcomeres are defined by Z discs at each end.

• A bands (dark) and I bands (light) create the striated appearance.

• H zone: Central region of the A band with only thick filaments.

• M line: Center of the H zone, where thick filaments are linked.

• Titin: Elastic protein that anchors thick filaments and provides recoil after stretch.

Structure and Molecular Composition of Myofilaments

Thick Filaments (Myosin)

• Each thick filament is composed of about 200 myosin molecules.

• Myosin has a tail (two heavy chains) and two heads (ends of heavy chains plus four light chains).

• Myosin heads form cross bridges with actin during contraction.

• Each head contains an ATPase enzyme and an actin-binding site.

Thin Filaments (Actin, Tropomyosin, Troponin)

• Actin exists as G-actin (globular monomers) and F-actin (filamentous, two strands wound in a helix).

• Tropomyosin covers myosin-binding sites on actin in resting muscle.

• Troponin is a three-polypeptide complex that binds calcium and regulates tropomyosin's position.

Other Proteins

• Dystrophin: Links the cytoskeleton of muscle fibers to the extracellular matrix; mutations cause Duchenne muscular dystrophy.

Sliding Filament Mechanism of Contraction

Process Overview

• Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere.

• Filaments themselves do not change length; the degree of overlap increases.

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

Role of Calcium and ATP

• Contraction is triggered by a nerve impulse that increases intracellular calcium concentration.

• Calcium binds to troponin, causing a conformational change that moves tropomyosin and exposes myosin-binding sites on actin.

• Myosin heads bind to actin, perform a power stroke (using ATP), and pull thin filaments toward the center of the sarcomere.

• Only about 50% of myosin heads are actively pulling at any one time.

• Relaxation occurs when calcium is reclaimed by the sarcoplasmic reticulum, tropomyosin re-covers binding sites, and cross-bridge cycling stops.

Key Equations

$\text{Muscle Force} \propto \text{Number of Cross Bridges Formed}$ $\text{ATP} + \text{Myosin} \rightarrow \text{Myosin-ATP} \rightarrow \text{Myosin-ADP} + \text{P}_i$ $\text{Ca}^{2+} + \text{Troponin} \rightarrow \text{Conformational Change}$

Role of Sarcoplasmic Reticulum and T-Tubules

Calcium Regulation and Action Potential Transmission

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions during muscle contraction.

• T-tubules: Invaginations of the sarcolemma that conduct action potentials into the interior of the muscle fiber, ensuring synchronized contraction.

• Triad: A group of three structures: two terminal cisterns of SR and one T-tubule, found at each sarcomere.

Summary Table: Key Structures and Functions

Structure	Function
Endomysium	Connective tissue surrounding each muscle fiber
Perimysium	Connective tissue wrapping fascicles
Epimysium	Connective tissue surrounding entire muscle
Sarcolemma	Plasma membrane of muscle cell
Sarcoplasm	Cytoplasm of muscle cell, contains myoglobin and glycogen
Myofibril	Contractile organelle composed of sarcomeres
Sarcomere	Functional contractile unit of muscle
Tendon	Connects muscle to bone

Example: Duchenne Muscular Dystrophy (DMD)

• DMD is caused by mutations in the dystrophin gene, leading to progressive muscle weakness.

• Physical therapy is used to help manage symptoms and maintain mobility.

Additional info: Some details, such as the exact arrangement of myofilaments and the role of dystrophin, were expanded for academic completeness.