Cellular Physiology of Muscle: Structure and Function of Muscle Tissue

Cellular Physiology of Muscle

Introduction

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

Common Features of Muscle Cells

General Characteristics

• Muscle fibers are elongated cells specialized for contraction.

• Terminology: Prefixes such as myo- and sarco- refer to muscle structures (e.g., myofibril, sarcoplasm).

• Muscle contraction depends on the interaction of actin and myosin myofilaments.

Comparison of Muscle Tissue Types

The three types of muscle tissue differ in structure, function, and control mechanisms.

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Location	Attached to skeleton	Heart	Walls of hollow, visceral organs
Striations	Striated	Striated	Nonstriated
Control	Voluntary	Involuntary (pacemaker sets rate; neural input can change rate)	Involuntary
Contraction	Rapid, tires easily, adaptable	No down time	Slow, sustained contractions

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 constantly work against gravity to keep the body upright.

• Stabilize joints: Muscles help stabilize joints, especially during movement (e.g., shoulders, knees).

• Generate heat: Muscle activity, especially in skeletal muscle, is crucial for maintaining body temperature (accounts for about 40% of body mass).

Functional Characteristics

• Excitability (Responsiveness): Ability to receive and respond to stimuli, usually chemical (neurotransmitter, hormone, pH). The response is 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.

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