Muscle Tissue Overview and Structure

Muscle Tissue Overview

• Most of the body's muscle tissue is skeletal muscle
• Remaining muscle tissue is smooth muscle (found in organ walls) and cardiac muscle (in the heart)
• Skeletal muscle is voluntary, striated (striped appearance), and multi-nucleated (many nuclei per cell)
• Smooth and cardiac muscles are involuntary and move substances within the body
• Skeletal muscle fibers are relatively large (10-100 µm)
• Hundreds of embryonic cells fuse to form a muscle cell

Skeletal Muscle Fiber Structure

• Muscle fibers are made up of myofibrils
• Myofibrils contain myofilaments (smaller structures)
• Myofilaments are made of proteins (actin and myosin)
• Sacromeres are functional contractile units (organized arrangement of filaments)
• Each thick filament is surrounded by six thin filaments
• Each thin filament is surrounded by three thick filaments

Muscle Fiber Organization

• Muscle fibers are bundled together with connective tissue to form fascicles
• Fascicles are grouped together further to form muscles
• Endomysium: Connective tissue sheath surrounding individual muscle fibers
• Perimysium: Connective tissue surrounding fascicles
• Epimysium: Surrounds entire muscle
• Tendons/Aponeurosis: Connect muscles to bones

Neuromuscular Junction

• Neuromuscular junction describes the point where a motor neuron connects to a muscle fiber
• Terminal branches of motor neurons connect to muscle fibers
• Axon terminals are at the end of each branch, and contain acetylcholine (ACh) vesicles
• ACh is a neurotransmitter that carries signals between nerve and muscle cells
• Synaptic cleft is the small space between the axon terminal and muscle fiber

Muscle Contraction Steps

• Electrical Excitation of Muscle Fiber: Nerve impulse triggers depolarization
• Excitation-Contraction Coupling: Electrical signal causes calcium release
• Shortening of Muscle Fiber: Calcium ions allow muscle contraction

Resting Membrane Potential

• Electrical difference across the cell membrane at rest (negative inside) due to ion concentration differences (more K+ inside, more Na+ outside).
• The membrane is more permeable to potassium, allowing it to leave the cell and increasing negativity
• Important for creating electrical signals in muscles and nerves.

Muscle Contraction (Detail)

• Action Potential: Nerve impulse triggers depolarization of sarcolemma (muscle cell membrane)
• Calcium Release: Action potential travels down T-tubules, triggering calcium release from sarcoplasmic reticulum
• Triggering Contraction: Calcium binds to troponin, allowing actin and myosin filaments to slide past each other
• Relaxation: Calcium pumped back into sarcoplasmic reticulum, stopping contraction

All-or-None Principle

• Muscle fiber either contracts fully or not at all
• For strong contractions, multiple motor units must be activated.

Muscle Fatigue

• Loss of muscle contraction ability due to insufficient energy (ATP)
• Causes includes oxygen debt, lactic acid buildup, depletion of ATP and accumulation of metabolic byproducts
• Symptoms include pain and cramping, muscle weakness

Muscle Tissues and Their Regulation

• Muscle is stimulated by motor units (neuron and its muscle cells)
• The frequency of impulses influences the strength of contraction.
• The brain controls how many motor units are activated and the rate of impulses to control strength, speed, and duration of contraction
• Tonus is the constant, slight tension in muscles to stay ready (for example, maintaining posture)

Types of Contraction

• Isometric: Muscles contract but do not change length (e.g., holding a weight).
• Isotonic: Muscles contract and change length (e.g., lifting a weight).

Muscle Activity

• Twitch: A single, brief contraction.
• Treppe (staircase effect): Repeated stimuli lead to stronger contractions.
• Wave summation: Muscle contracts before fully relaxing, leading to stronger contractions.
• Tetanus: A sustained, forceful contraction without relaxation.