Muscular System

Excitability

Ability of muscle to respond to a stimulus

Contractility

Ability of a muscle to shorten with force and produce tension

Extensibility

Ability of muscle to stretch when pulled

Elasticity

Ability of muscle to recoil to its original resting length after being stretched

Skeletal muscle

Striated, voluntary muscle attached to the skeleton that moves the body

Cardiac muscle

Striated, involuntary muscle found in the wall of the heart that pumps blood

Smooth muscle

Non-striated, involuntary muscle found in the walls of organs and blood vessels

Voluntary muscle

Muscle under conscious control

Involuntary muscle

Muscle not under conscious control

Epimysium

Connective tissue that surrounds the entire muscle

Perimysium

Connective tissue that surrounds fascicles (bundles of muscle fibers)

Endomysium

Connective tissue that surrounds each individual muscle fiber

Sarcolemma

Specialized cell membrane of a muscle fiber

Sarcoplasm

Specialized cytoplasm inside a muscle fiber

Sarcoplasmic reticulum

Specialized endoplasmic reticulum that stores calcium ions

Calcium ions (Ca²⁺)

Ions released from the sarcoplasmic reticulum that allow muscle contraction

Myofibrils

Bundles of myofilaments responsible for muscle contraction

Myofilaments

Protein filaments (actin and myosin) inside myofibrils

Actin

Thin filament with active sites for myosin binding

Myosin

Thick filament with heads that bind to actin

Sarcomere

Smallest functional unit of a muscle fiber

Z line

Boundary of a sarcomere

M line

Center of the sarcomere where thick filaments are connected

A band

Dark band containing both thick and thin filaments

I band

Light band containing only thin (actin) filaments

Neuromuscular junction (NMJ)

Specialized connection between a motor neuron and a skeletal muscle fiber

Synaptic knob

End of the motor neuron axon that releases acetylcholine

Synaptic cleft

Space between the neuron and the muscle fiber

Motor end plate

Portion of the sarcolemma with acetylcholine receptors

Acetylcholine (ACh)

Neurotransmitter that stimulates muscle contraction

Acetylcholinesterase

Enzyme that breaks down acetylcholine

Action potential

Electrical signal that travels along the sarcolemma

Isotonic contraction

Muscle shortens and movement occurs

Isometric contraction

Muscle contracts but does not shorten

Muscle twitch

Response of a muscle fiber to one stimulus

Latent phase

Action potential occurs; calcium released; no tension produced

Contraction phase

Cross-bridges form and sarcomeres shorten

Relaxation phase

Calcium pumped back into the sarcoplasmic reticulum and cross-bridges detach

Incomplete tetanus

Partial relaxation with increasing tension

Complete tetanus

No relaxation and maximum sustained tension

All-or-none principle

Individual muscle fibers either fully contract or do not contract at all

Sliding filament theory

Actin and myosin slide past each other, shortening the sarcomere

ATP

Immediate energy source required for muscle contraction

Creatine phosphate (CP)

High-energy compound that recharges ADP into ATP

Aerobic respiration

Uses oxygen and produces large amounts of ATP in the mitochondria

Anaerobic respiration

Does not use oxygen and produces small amounts of ATP

Lactic acid fermentation

Process that occurs when oxygen is limited and lactic acid is produced

Slow oxidative fibers

Red fibers that use aerobic respiration and resist fatigue

Fast oxidative-glycolytic fibers

Fibers that use both aerobic respiration and glycolysis

Fast glycolytic fibers

White fibers that use anaerobic respiration and fatigue quickly

Red muscle fibers

Muscle fibers with high myoglobin and many blood capillaries

White muscle fibers

Muscle fibers with low myoglobin and few blood capillaries