Skeletal Muscle Tissue: Structure, Function, and Physiology Study Guide

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Skeletal Muscle Tissue

Functions of Skeletal Muscle Tissue

Skeletal muscle tissue is essential for voluntary movement, posture, and various physiological processes. Its functions are accomplished through specialized cellular and molecular mechanisms.

Movement: Skeletal muscles contract to produce movement of body parts by pulling on bones.
Posture Maintenance: Continuous muscle contractions help maintain posture and stabilize joints.
Support of Soft Tissues: Muscles support and protect internal organs.
Guarding Entrances and Exits: Sphincter muscles control openings of the digestive and urinary tracts.
Heat Production: Muscle contractions generate heat, helping maintain body temperature.
Storage of Nutrients: Muscle tissue stores glycogen, which can be converted to glucose for energy.

Organization of Skeletal Muscle Tissue

Skeletal muscle tissue is organized into hierarchical structures, from the whole muscle down to the molecular level.

Muscle (Organ): Composed of bundles of fascicles.
Fascicle: A bundle of muscle fibers (cells).
Muscle Fiber: A single muscle cell, multinucleated and elongated.
Myofibril: Contractile organelle within muscle fibers, composed of sarcomeres.
Sarcomere: The functional unit of contraction, made of actin and myosin filaments.

Microscopic Anatomy of Skeletal Muscle

Connective Tissue Components

Connective tissue layers organize and protect muscle fibers.

Structure	Description
Endomysium	Surrounds individual muscle fibers
Perimysium	Encloses bundles of muscle fibers (fascicles)
Epimysium	Envelops the entire muscle

Muscle Cell Structures

Term	Description
Sarcolemma	Plasma membrane of a muscle cell
Sarcoplasm	Cytoplasm of a muscle cell
Myofibril	Long, banded organelle for contraction
Sarcomere	Contractile unit within myofibril
Actin	Thin filament protein
Myosin	Thick filament protein
Tendon	Connects muscle to bone

Relationship: Muscle, Myofibrils, Myofilaments, Sarcomeres

Muscle contains fascicles.
Fascicles contain muscle fibers.
Muscle fibers contain myofibrils.
Myofibrils are made of repeating sarcomeres.
Sarcomeres contain myofilaments (actin and myosin).

Muscle Contraction: Molecular Mechanisms

Cross-Bridge Formation

Muscle contraction occurs when myosin heads bind to actin filaments, forming cross-bridges.

Actin: Thin filament; binding site for myosin.
Myosin: Thick filament; forms cross-bridges with actin.

Sarcomere Structure and Function

The sarcomere is the basic contractile unit of muscle, defined by Z discs.

Z disc: Boundary of sarcomere; anchors actin filaments.
A band: Region containing thick filaments (myosin).
I band: Region containing only thin filaments (actin).
H zone: Center of A band; only myosin present.
M line: Center of sarcomere; holds myosin filaments together.

Relaxed vs. Contracted Sarcomeres

Relaxed: Sarcomere is at maximum length; minimal overlap of actin and myosin.
Contracted: Sarcomere shortens; actin and myosin overlap increases, I band and H zone decrease.

Neuromuscular Junction and Muscle Excitation

Steps in Neuromuscular Transmission

Action potential arrives at axon terminal of motor neuron.
Acetylcholine (ACh) is released into synaptic cleft.
ACh binds to receptors on sarcolemma, opening sodium channels.
Action potential spreads along sarcolemma and T-tubules.
Calcium ions released from sarcoplasmic reticulum.
Calcium binds to troponin, exposing actin binding sites.
Myosin binds to actin, initiating contraction.
Calcium is reabsorbed, muscle relaxes.

Resting Membrane Potential and Action Potential

Resting Membrane Potential: The electrical charge difference across the plasma membrane, typically -70 mV. Maintained by sodium-potassium pump ( ATPase).
Action Potential: Rapid depolarization and repolarization of the membrane, leading to muscle contraction.

Equation:

ATPase: 3 out, 2 in per ATP hydrolyzed

Muscle Twitch, Tension, and Summation

Phases of Muscle Twitch

Latent Period: Time between stimulus and contraction onset.
Contraction Period: Muscle fibers shorten.
Relaxation Period: Muscle returns to resting length.

Length-Tension Relationship

The force a muscle can generate depends on its length at the time of stimulation.

Optimal Length: Maximum overlap of actin and myosin for greatest tension.
Too Short/Too Long: Reduced tension due to less overlap or overstretching.

Wave Summation and Tetanus

Wave Summation: Increased muscle tension due to rapid, repeated stimuli.
Incomplete Tetanus: Partial relaxation between stimuli.
Complete Tetanus: No relaxation; sustained contraction.

Types of Muscle Contraction

Isometric, Isotonic, Concentric, Eccentric

Type	Description	Example
Isometric	Muscle tension without length change	Holding a weight steady
Isotonic	Muscle changes length while tension remains constant	Lifting a weight
Concentric	Muscle shortens during contraction	Biceps curl upward
Eccentric	Muscle lengthens while contracting	Lowering a weight

Muscle Fatigue and Energy Sources

Types of Muscle Fatigue

Central Fatigue: Reduced neural drive from the central nervous system.
Peripheral Fatigue: Depletion of energy stores or accumulation of metabolic byproducts in muscle.

ATP Synthesis and Energy Sources

Creatine Phosphate: Immediate source, lasts ~15 seconds.
Anaerobic Glycolysis: Produces ATP without oxygen, lasts ~1-2 minutes.
Aerobic Respiration: Uses oxygen, supports prolonged activity.

ATP Yield:

Anaerobic: 2 ATP per glucose
Aerobic: up to 36 ATP per glucose

Muscle Fiber Types

Slow vs. Fast Fibers

Type	Characteristics	Example Activity
Slow (Type I)	High endurance, rich in mitochondria, fatigue-resistant	Long-distance running
Fast (Type II)	Quick, powerful contractions, fatigue quickly	Sprinting

Muscle Hypertrophy and Atrophy

Definitions

Hypertrophy: Increase in muscle size due to increased synthesis of contractile proteins.
Atrophy: Decrease in muscle size due to disuse or disease.

Additional info:

Examples of gated ion channels: Voltage-gated sodium channels (for action potentials), Ligand-gated channels (for neurotransmitter binding).
Stimulus terminology: Threshold stimulus (minimum required for contraction), Maximal stimulus (stimulates all fibers).
Staircase effect (treppe): Gradual increase in contraction strength with repeated stimulation.