Muscle Tissue: Structure, Function, and Physiology Study Guide

Muscle Tissue: Structure, Function, and Physiology

Characteristics of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement in the body. There are four universal characteristics that define muscle tissue:

• Excitability (Responsiveness): The ability of muscle tissue to respond to stimuli, usually from the nervous system.

• Contractility: The ability to shorten and generate force.

• Extensibility: The ability to stretch without being damaged.

• Elasticity: The ability to return to its original shape after being stretched.

Muscle Structure and Organization

Muscles are organized into bundles and layers, each with specific structures and functions.

• Blood vessel: Supplies oxygen and nutrients to muscle tissue.

• Endomysium: Connective tissue surrounding individual muscle fibers (cells).

• Epimysium: Connective tissue surrounding the entire muscle.

• Muscle fiber (muscle cell): The basic cellular unit of muscle tissue, specialized for contraction.

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

• Sarcoplasm: The cytoplasm of a muscle fiber, containing organelles and myofibrils.

• Tendon: Connects muscle to bone, transmitting the force of contraction.

• Bone: The rigid structure to which muscles attach, enabling movement.

Key Muscle Terminology

• Muscle: A bundle of fibrous tissue that contracts to produce movement.

• Fascicle: A bundle of muscle fibers arranged in parallel.

• Muscle fiber (cell): Specialized cell responsible for contraction.

• Myofibril: Long protein cords within the sarcoplasm; composed of repeating units called sarcomeres.

• Sarcomere: The smallest functional unit of striated muscle tissue; segment of a myofibril.

• Myofilament: Protein filaments (actin and myosin) responsible for muscle contraction.

Types of Myofilaments

There are two primary types of myofilaments in muscle fibers:

• Actin (Thin) Filaments: Composed mainly of the protein actin; involved in the sliding filament mechanism of contraction.

• Myosin (Thick) Filaments: Composed of the protein myosin; myosin heads bind to actin to generate contraction.

Structure of the Sarcomere

The sarcomere is the basic contractile unit of muscle fiber, defined by Z discs. It contains:

• Thin and thick filaments

• Z discs

• A bands (dark, containing thick filaments)

• I bands (light, containing thin filaments only)

Muscle Contraction: Sliding Filament Theory

Muscle contraction occurs when actin (thin) filaments slide over myosin (thick) filaments, shortening the sarcomere.

• During contraction, the thin filaments slide past the thick filaments, bringing Z discs closer together.

• The length of the filaments does not change; only their relative position changes.

Neuromuscular Junction and Excitation-Contraction Coupling

The neuromuscular junction is where a motor neuron communicates with a muscle fiber to initiate contraction.

• Acetylcholine (ACh): Neurotransmitter released from the neuron, binds to receptors on the sarcolemma (muscle cell membrane).

• Sarcolemma: The plasma membrane of a muscle fiber.

• Troponin and Tropomyosin: Regulatory proteins on the thin filament. Calcium ions bind to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

• Cross-bridge formation: Myosin heads bind to actin, forming cross-bridges and pulling the actin filament toward the center of the sarcomere.

• Result: Muscle contraction and shortening of the muscle fiber.

Motor Units

A motor unit consists of a single motor neuron and all the muscle fibers it innervates. Motor units are distributed throughout the muscle, allowing for coordinated contraction.

• Motor units can be small (for fine control) or large (for powerful contractions).

Muscle Paralysis and Toxins

• Botulinum toxin (Botox): Produced by Clostridium botulinum, this toxin blocks the release of acetylcholine at the neuromuscular junction, preventing muscle contraction and causing paralysis.

Muscle Fatigue

Muscle fatigue is the decline in ability of a muscle to generate force. It can be caused by:

• Depletion of ATP

• Ionic imbalances (e.g., potassium, sodium)

• Decreased glycogen stores

• Accumulation of lactic acid

Post-Exercise Oxygen Consumption (EPOC)

Excess Post-Exercise Oxygen Consumption (EPOC) is the increased rate of oxygen intake following strenuous activity. It helps restore oxygen reserves, replenish ATP and creatine phosphate, and remove lactic acid.

Muscle Fiber Types

Muscle fibers can be classified based on their contraction speed and metabolic properties:

Type	Properties	Best for
Slow oxidative muscle fibers	Slow contraction, high fatigue resistance, aerobic ATP synthesis, high myoglobin content	Endurance activities (e.g., marathon running)
Fast oxidative muscle fibers	Fast contraction, moderate fatigue resistance, aerobic and some anaerobic ATP synthesis	Intermediate activities
Fast glycolytic muscle fibers	Fast contraction, low fatigue resistance, anaerobic ATP synthesis, low myoglobin content	Short, intense activities (e.g., sprinting)

Effects of Exercise on Muscle

• Aerobic exercise: Increases capillary density, mitochondria, and myoglobin content in muscle fibers.

• Resistance exercise: Causes hypertrophy (increase in muscle fiber size) and increased strength.

Cardiac and Smooth Muscle

• Cardiac muscle: Involuntary, striated, branched cells connected by intercalated discs (contain gap junctions and desmosomes). Found only in the heart.

• Smooth muscle: Involuntary, non-striated, spindle-shaped cells. Found in walls of hollow organs (e.g., intestines, blood vessels).

• Skeletal muscle: Voluntary, striated, cylindrical, multinucleated cells. Attached to bones for movement.

Vocabulary Table: Key Muscle Terms

Vocab term	Use in a sentence	Synonym	Reminds me of...
Striations	Muscle cells have striations which look like light and dark bands.	Stripes	Patterns in fabric
Muscle fiber	Muscle fibers are muscle cells and are very long and skinny.	Muscle cell	Long threads in fabric
Sarcomere	The sarcomere is the basic contractile unit of muscle tissue.	Muscle segment	Accordion stretched out
Thin filament	Actin filaments slide over myosin to allow muscle contraction.	Actin filament	Thread
Thick filament	Myosin heads bind to actin and pull to contract the muscle.	Myosin filament	Golf clubs
Motor unit	A motor unit consists of a motor neuron and the muscle fibers it controls.	Connection	Transmission of a motor

Key Equations and Concepts

• ATP hydrolysis: The energy for muscle contraction comes from the hydrolysis of ATP.

• Sliding filament equation:

$\text{Muscle Contraction} = \text{Actin Filament Sliding} + \text{Myosin Cross-Bridge Cycling}$

• Oxygen debt (EPOC):

$\text{EPOC} = \text{Oxygen Consumed Post-Exercise} - \text{Oxygen Consumed at Rest}$

Summary

• Muscle tissue is essential for movement, posture, and heat production.

• Understanding the structure and function of muscle fibers, motor units, and the mechanisms of contraction is fundamental in anatomy and physiology.

• Different types of muscle fibers and exercise have distinct effects on muscle performance and adaptation.