Histology and Physiology of Muscles

Contractility

Ability to shorten forcibly during contraction.

Excitability

Ability to receive and respond to stimuli.

Extensibility

Ability to be stretched or extended.

Elasticity

Ability to recoil to original length.

Skeletal Muscle

Responsible for body movements and posture.

Smooth Muscle

Found in hollow organs; moves substances.

Cardiac Muscle

Found in the heart; pumps blood.

Fascia

Connective tissue sheets surrounding muscles.

Epimysium

Dense connective tissue surrounding entire muscle.

Perimysium

Surrounds groups of muscle fibers (fascicles).

Endomysium

Connective tissue surrounding individual muscle fibers.

Sarcolemma

Plasma membrane of a muscle cell.

Sarcoplasm

Cytoplasm within a muscle cell.

Myofibrils

Rod-like contractile elements in muscle fibers.

Actin Myofilaments

Thin filaments involved in muscle contraction.

Myosin Myofilaments

Thick filaments with ATPase activity.

Sarcomere

Smallest contractile unit of muscle.

Z Disk

Anchors thin filaments in sarcomeres.

A Band

Region of overlapping thick and thin filaments.

I Band

Region of thin filaments only.

H Zone

Area where thin filaments do not overlap.

Sliding Filament Model

Explains muscle contraction via filament sliding.

Action Potential

Electrical signal triggering muscle contraction.

Depolarization

Inside of membrane becomes positive.

Repolarization

Return to resting membrane potential.

Ligand-gated Channels

Open in response to specific molecules.

Voltage-gated Channels

Open in response to membrane potential changes.

Threshold

Minimum stimulus to trigger action potential.

Na+ Channels

Allow sodium ions to enter during depolarization.

K+ Channels

Allow potassium ions to exit during repolarization.

All-or-None Principle

Action potentials occur fully or not at all.

Threshold Stimulus

Minimum stimulus required to generate action potential.

Motor Neurons

Nerve cells stimulating skeletal muscle fibers.

Neuromuscular Junction

Connection between motor neuron and muscle fiber.

Synaptic Vesicles

Membranous sacs containing neurotransmitter ACh.

Acetylcholine (ACh)

Neurotransmitter released at neuromuscular junction.

Motor End Plate

Specialized region of muscle sarcolemma with ACh receptors.

Synaptic Cleft

Space between axonal endings and muscle fibers.

Voltage-Gated Ca2+ Channels

Open in response to action potential, allowing Ca2+ influx.

Exocytosis

Process of ACh release into synaptic cleft.

Ligand-Gated Na+ Channels

Channels that open when ACh binds, allowing Na+ entry.

Acetylcholinesterase

Enzyme breaking down ACh in synaptic cleft.

Sarcoplasmic Reticulum (SR)

Smooth ER regulating intracellular calcium levels.

T Tubules

Invaginations of sarcolemma facilitating action potential spread.

Triad

T tubule and two terminal cisternae structure.

Calcium Ions (Ca2+)

Trigger for muscle contraction after release from SR.

Troponin

Protein that binds Ca2+, initiating muscle contraction.

Tropomyosin

Covers active sites on G actin in relaxed muscle.

Cross-Bridges

Connections formed between myosin heads and actin.

Intracellular Ca2+ Levels

Essential for muscle contraction initiation.

Acetic Acid

Product of ACh breakdown by acetylcholinesterase.

Choline

Recycled component of ACh, taken back into presynaptic terminal.

Somatic Nervous System

Part of nervous system controlling voluntary muscle movements.

Presynaptic Terminal

End of motor neuron where ACh is released.

Postsynaptic Membrane

Muscle fiber membrane receiving neurotransmitter signal.

G Actin

Globular protein forming the backbone of thin filaments.

Excitation-Contraction Coupling

Process linking muscle action potentials to contraction.

Muscle Twitch

Single contraction-response cycle of muscle fibers.

Lag Phase

Initial delay before muscle contraction begins.

Contraction Phase

Period when muscle fibers actively shorten.

Relaxation Phase

Return to resting state after contraction.

Multiple Motor Unit Summation

Increased force by recruiting more motor units.

Incomplete Tetanus

Partial relaxation between muscle contractions.

Complete Tetanus

No relaxation between muscle contractions.

Treppe

Progressive increase in contraction strength with repeated stimuli.

Isometric Contraction

Muscle tension increases without changing length.

Isotonic Contraction

Muscle changes length while maintaining tension.

Concentric Contraction

Muscle shortens while generating force.

Eccentric Contraction

Muscle lengthens while under tension.

Muscle Tone

Sustained tension in muscles at rest.

Fatigue

Reduced ability to perform work due to exertion.

Anaerobic Respiration

ATP production without oxygen, rapid but inefficient.

Aerobic Respiration

Oxygen-dependent ATP production for sustained activity.

Slow-Twitch Fibers

Fatigue-resistant fibers for endurance activities.

Fast-Twitch Fibers

Fibers for quick, powerful bursts of activity.

Muscle Hypertrophy

Increase in muscle size due to fiber growth.

Muscle Atrophy

Decrease in muscle size due to disuse.

Visceral Smooth Muscle

Contracts as a single unit, found in organs.

Multiunit Smooth Muscle

Functions independently, found in specific tissues.

Calmodulin

Calcium-binding protein activating smooth muscle contraction.

Intercalated Disks

Connections between cardiac muscle cells for synchronized contraction.

Autorhythmicity

Ability of cardiac muscle to generate its own rhythm.