Muscular System: Structure, Function, and Contraction Mechanics

Functions of the muscular system

Movement, posture, joint stabilization, heat production, support of soft tissues, guarding body openings.

Characteristics of skeletal muscle

Striated, voluntary, multinucleated, attached to bones.

Special characteristics of muscle tissue

Excitability, conductivity, contractility, extensibility, elasticity.

Endomysium

Connective tissue surrounding each individual muscle fiber.

Perimysium

Connective tissue surrounding a fascicle (bundle of muscle fibers).

Epimysium

Connective tissue surrounding the entire muscle.

Sarcolemma

Cell membrane of a muscle fiber.

Sarcoplasm

Cytoplasm of a muscle fiber.

Sarcoplasmic reticulum (SR)

Stores and releases calcium for muscle contraction.

T tubules

Carry action potentials deep into the muscle fiber.

Myofibrils

Long contractile organelles composed of repeating sarcomeres.

Glycosomes

Store glycogen as an energy reserve.

Myoglobin

Oxygen-binding protein that stores oxygen in muscle fibers.

Sarcomere

Functional contractile unit of skeletal muscle between two Z discs.

Z disc

Boundary of each sarcomere; anchors thin filaments.

A band

Length of thick filaments; remains constant during contraction.

I band

Thin filaments only; shortens during contraction.

H zone

Thick filaments only; decreases or disappears during contraction.

M line

Holds thick filaments together in the center of the sarcomere.

Thick filament

Composed primarily of myosin.

Thin filament

Composed of actin, troponin, and tropomyosin.

Myosin

Thick filament protein with heads that form cross-bridges with actin.

Actin

Main protein of thin filaments.

Tropomyosin

Covers myosin-binding sites on actin when muscle is relaxed.

Troponin

Binds calcium and moves tropomyosin to expose binding sites.

Role of calcium in contraction

Binds troponin, exposing actin binding sites so myosin can attach.

Requirements for skeletal muscle contraction

ATP, calcium, intact nerve supply, acetylcholine, functional muscle fibers.

Excitation-contraction coupling

Process linking muscle action potential to contraction through calcium release.

Cross-bridge cycle

Myosin binds actin → power stroke → ATP binds → myosin detaches → ATP hydrolysis recocks head.

Power stroke

Myosin head pivots, pulling thin filament toward center of sarcomere.

ATP function in muscle

Detaches myosin from actin and provides energy for recocking.

Neuromuscular junction (NMJ)

Synapse between a motor neuron and a skeletal muscle fiber.

Motor unit

One motor neuron and all muscle fibers it innervates.

Synaptic transmission

Communication across a synapse via neurotransmitters.

Neurotransmitter at NMJ

Acetylcholine (ACh).

Muscle metabolism order

Stored ATP → creatine phosphate → anaerobic glycolysis → aerobic respiration.

Creatine phosphate function

Rapidly regenerates ATP during short bursts of activity.

Oxygen debt

Extra oxygen needed after exercise to restore muscles to resting state.

Muscle fatigue

Reduced ability to contract due to ATP depletion, ion imbalance, or metabolite buildup.

Red muscle fibers (Type I)

Slow-twitch, aerobic, fatigue resistant, high myoglobin.

White muscle fibers (Type IIb)

Fast-twitch, anaerobic, fatigue quickly, low myoglobin.

Intermediate fibers (Type IIa)

Fast oxidative-glycolytic; moderate fatigue resistance.

Aerobic exercise effects

Increases endurance, mitochondria, capillaries, and myoglobin.

Resistance exercise effects

Increases muscle size and strength through hypertrophy.

Sodium-potassium pump

Moves 3 Na+ out and 2 K+ into the cell using ATP.

Leakage channels

Maintain resting membrane potential by allowing passive ion movement.

Resting membrane potential (RMP)

Approximately -70 mV.

Synapse

Connection between two neurons or between neuron and muscle.

Leak channels

Always open; help establish resting membrane potential.

Chemically gated channels

Open when neurotransmitters bind.

Voltage-gated channels

Open in response to changes in membrane voltage.

Voltage

Electrical potential difference.

Current

Flow of charged particles.

Resistance

Opposition to current flow.

EPSP

Excitatory postsynaptic potential; depolarizes membrane.

IPSP

Inhibitory postsynaptic potential; hyperpolarizes membrane.

Depolarization

Voltage-gated Na+ channels open; Na+ enters cell.

Repolarization

Voltage-gated K+ channels open; K+ leaves cell.

Hyperpolarization

Excess K+ leaves the cell, making membrane more negative.

Absolute refractory period

No second action potential can occur.

Relative refractory period

Another action potential is possible with a stronger stimulus.

Threshold

Minimum stimulus required to trigger an action potential.

Strong vs weak stimulus

Determined by frequency of action potentials, not their size.

Functions of the nervous system

Sensory input, integration, motor output.

Central nervous system (CNS)

Brain and spinal cord; integration and control.

Peripheral nervous system (PNS)

Nerves outside CNS; communication between CNS and body.

Sensory (afferent) division

Carries information toward CNS.

Motor (efferent) division

Carries commands away from CNS.

Somatic nervous system

Controls voluntary skeletal muscle.

Autonomic nervous system

Controls involuntary smooth muscle, cardiac muscle, and glands.

Neuron

Basic functional unit of the nervous system.

Dendrites

Receive incoming signals.

Cell body (soma)

Contains nucleus and integrates information.

Axon

Conducts nerve impulses away from the cell body.

Axon terminals

Release neurotransmitters.

Astrocytes

Support neurons and help maintain the blood-brain barrier.

Microglia

Immune defense cells of the CNS.

Ependymal cells

Produce and circulate cerebrospinal fluid.

Oligodendrocytes

Form myelin in the CNS.

Schwann cells

Form myelin in the PNS.

Satellite cells

Support neuron cell bodies in PNS ganglia.

Myelin

Insulates axons and increases conduction speed.

Saltatory conduction

Action potentials jump between nodes of Ranvier.

Graded potential

Local, variable signal that decreases with distance.

Action potential

All-or-none electrical signal traveling along an axon.

Factors increasing conduction velocity

Myelination and larger axon diameter.

White matter

Myelinated axons.

Gray matter

Neuron cell bodies, dendrites, and unmyelinated axons.

Sensory neuron

Carries information to the CNS.

Motor neuron

Carries commands from the CNS.

Interneuron

Connects neurons within the CNS.

Sensory nerve

Contains only sensory fibers.

Motor nerve

Contains only motor fibers.

Mixed nerve

Contains both sensory and motor fibers.

Brain white matter

Deep inside the brain.

Brain gray matter

Outer cerebral cortex.

Spinal cord white matter

Outside.

Spinal cord gray matter

Central butterfly-shaped region.

Sympathetic nervous system

Fight-or-flight responses.

Parasympathetic nervous system

Rest-and-digest responses.