Cellular Physiology of Muscle: Structure and Function of Muscle Tissue

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

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

Elongated cells are called muscle fibers.
Terminology: prefixes myo- and sarco- refer to muscle.
Muscle contraction depends on actin and myosin myofilaments.

Comparison of Muscle Tissue Types

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

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)	Involuntary
Contraction	Rapid, tires easily, adaptable	Pacemaker sets rate; neural input can change rate; no down time	Slow, sustained contractions

Functions and Characteristics of Muscle Tissue

Functions of Muscle

Generate movement: locomotion, manipulation, blood flow, pressure regulation, respiration, propulsion of food, urine, etc.
Maintain posture: muscles work constantly against gravity.
Stabilize joints: especially important in shoulders and knees during movement.
Generate heat: helps maintain body temperature (skeletal muscle is about 40% of body mass).

Functional Characteristics of Muscle

Excitability (Responsiveness): ability to receive and respond to a stimulus (usually a chemical such as a neurotransmitter, hormone, or pH change). 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.

Microscopic Anatomy of Skeletal Muscle Fiber

Structure of a Skeletal Muscle Cell

Large, cylindrical cells with multiple oval nuclei located at the periphery.
Muscle cells are a syncytium (formed by the fusion of many cells).
Diameter: 10–100 μm; Length: up to several centimeters.
Cytoplasm (sarcoplasm) contains abundant glycogen and myoglobin (an oxygen-binding protein).
Contains hundreds to thousands of myofibrils (about 80% of cell volume).
Extensive sarcoplasmic reticulum (SR) and T-tubules (transverse tubules).

Connective Tissue Sheaths

Endomysium: thin connective tissue surrounding each muscle fiber.
Perimysium: connective tissue surrounding a fascicle (bundle of muscle fibers).
Epimysium: dense connective tissue surrounding the entire muscle.
Tendon: cordlike extension of connective tissue attaching muscle to bone.

Myofibril and Sarcomere Organization

Each muscle fiber contains parallel myofibrils, which are composed of repeating units called sarcomeres (the contractile unit of muscle).
Sarcomeres are defined by Z discs at each end.
Within sarcomeres, myofilaments (actin and myosin) are arranged in a precise pattern, giving rise to striations.
A bands: dark bands containing thick (myosin) filaments.
I bands: light bands containing thin (actin) filaments.
H zone: central region of A band with only thick filaments.
M line: line of proteins that holds thick filaments together in the H zone.
Titin: an elastic protein that anchors thick filaments and helps muscle recoil after stretch.

Ultrastructure and Molecular Composition of Myofilaments

Thick Filaments (Myosin)

Composed of myosin molecules, each with a tail (2 heavy chains) and 2 heads (ends of heavy chains) with 4 light chains.
Myosin heads form cross-bridges that bind to actin during contraction.
Heads contain ATPase activity and actin-binding sites.

Thin Filaments (Actin, Tropomyosin, Troponin)

Composed of actin subunits arranged as two intertwined strands (F-actin).
G-actin: globular actin monomers; F-actin: filamentous actin polymer.
Tropomyosin: rod-shaped protein that blocks myosin-binding sites on actin in a relaxed muscle.
Troponin: complex of three proteins that binds Ca2+ and moves tropomyosin away from binding sites during contraction.

Other Proteins

Dystrophin: links the cytoskeleton of a muscle fiber to the extracellular matrix; mutations cause Duchenne muscular dystrophy.

Sliding Filament Mechanism of Contraction

Overview

Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere.
During contraction, the distance between Z discs decreases, I bands and H zones shorten, but A bands remain the same length.

Role of Calcium and ATP

Contraction is triggered by an increase in intracellular Ca2+ concentration.
Ca2+ 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 energy from ATP hydrolysis), and pull thin filaments toward the center of the sarcomere.
ATP is required for both the power stroke and the detachment of myosin from actin.

Muscle Relaxation

Occurs when Ca2+ is actively transported back into the sarcoplasmic reticulum (SR).
Troponin returns to its original shape, tropomyosin covers the binding sites, and the muscle fiber relaxes.

Excitation-Contraction Coupling

Role of the Sarcoplasmic Reticulum and T-Tubules

The sarcoplasmic reticulum (SR) is a specialized endoplasmic reticulum that stores and releases Ca2+.
T-tubules are invaginations of the sarcolemma that conduct action potentials deep into the muscle fiber.
A triad consists of a T-tubule flanked by two terminal cisternae of the SR.

Sequence of Events

An action potential travels along the sarcolemma and down the T-tubules.
This triggers the release of Ca2+ from the SR into the cytosol.
Ca2+ binds to troponin, initiating contraction as described above.
When stimulation ceases, Ca2+ is pumped back into the SR, and the muscle relaxes.

Key Equations

ATP hydrolysis by myosin ATPase:

Example: Rigor Mortis

After death, ATP production ceases, Ca2+ leaks into the cytosol, and myosin heads remain bound to actin, causing muscle stiffness (rigor mortis).

Additional info: For more details, see Chapter 9 of your textbook and the A&P Flix on the Cross Bridge Cycle.