Autonomic Nervous System and Muscle Physiology: Key Concepts and Receptors

Autonomic Nervous System

System of motor neurons that innervates smooth muscle, cardiac muscle, and glands.

Sympathetic Division

Fight-or-flight division; increases heart rate, breathing, glucose release, and directs blood to skeletal muscles.

Parasympathetic Division

Rest-and-digest division; promotes digestion, urination, and energy conservation.

Autonomic Ganglion

Bundle of neuron cell bodies that links involuntary organs to the CNS.

Preganglionic Neuron

Neuron originating in the CNS that releases acetylcholine onto ganglionic neurons.

Postganglionic Neuron

Neuron originating in a ganglion that releases acetylcholine or norepinephrine onto target organs.

Varicosity

Swelling along an axon that functions like an axon terminal.

Sympathetic Chain

Paired cord of sympathetic ganglia running alongside the vertebral column.

Collateral Ganglia

Sympathetic ganglia located anterior to the vertebral column near abdominal arteries.

Chromaffin Cells

Modified sympathetic neurons that release epinephrine into the bloodstream.

Acetylcholine

Neurotransmitter released by all preganglionic autonomic neurons and parasympathetic postganglionic neurons.

Norepinephrine

Main neurotransmitter released by sympathetic postganglionic neurons.

Nicotinic Cholinergic Receptors

Receptors that bind acetylcholine in autonomic ganglia and skeletal muscle.

Alpha Adrenergic Receptors

G-protein-coupled receptors that bind norepinephrine and epinephrine.

Alpha-1 Receptors

Cause vasoconstriction of blood vessels.

Alpha-2 Receptors

Inhibit further neurotransmitter release from presynaptic terminals.

Beta Adrenergic Receptors

G-protein-coupled receptors that bind norepinephrine and epinephrine.

Beta-1 Receptors

Increase heart rate and force of contraction.

Beta-2 Receptors

Cause bronchodilation and vasodilation in skeletal muscle blood vessels.

Vagus Nerve

Longest cranial nerve; carries about 75% of parasympathetic fibers.

Terminal Ganglia

Parasympathetic ganglia located near target organs.

Intramural Ganglia

Parasympathetic ganglia located within the walls of target organs.

Muscarinic Cholinergic Receptors

G-protein-coupled receptors that bind acetylcholine.

Atropine

Muscarinic receptor antagonist that blocks parasympathetic activity and increases heart rate.

Skeletal Muscle

Voluntary, striated muscle that moves and stabilizes the skeleton.

Smooth Muscle

Involuntary, nonstriated muscle found in hollow organs and blood vessels.

Cardiac Muscle

Involuntary, striated muscle found only in the heart.

Myofilaments

Protein filaments that make up sarcomeres.

Thick Filaments

Myosin-containing filaments.

Myosin

Motor protein that binds actin and produces contraction.

Thin Filaments

Actin-containing filaments.

Actin

Protein forming the backbone of thin filaments.

Troponin

Calcium-binding protein that moves tropomyosin away from binding sites.

Tropomyosin

Protein that blocks myosin-binding sites on actin during relaxation.

Titin

Elastic protein that anchors myosin and stabilizes sarcomeres.

Myofibril

Threadlike organelle responsible for muscle shortening.

Sarcomere

Functional contractile unit of muscle.

Crossbridge

Connection formed when myosin binds actin.

Muscle Fiber

A single muscle cell.

Sarcoplasm

Cytoplasm of a muscle fiber.

Sarcolemma

Cell membrane of a muscle fiber.

Sarcoplasmic Reticulum

Specialized ER that stores calcium.

Transverse Tubules (T-Tubules)

Invaginations of the sarcolemma that conduct action potentials deep into muscle fibers.

Myoglobin

Oxygen-binding protein in muscle cells.

Muscle Fascicle

Bundle of muscle fibers surrounded by connective tissue.

Sliding Filament Theory

Thin filaments slide past thick filaments to shorten muscles.

Motor Unit

A motor neuron and all muscle fibers it innervates.

Neuromuscular Junction

Synapse between a motor neuron and muscle fiber.

Motor End Plate

Postsynaptic membrane of the muscle fiber.

End Plate Potential

Local depolarization at the motor end plate.

DHP Receptors

Voltage-gated L-type calcium channels.

RyR Receptors

Calcium release channels of the sarcoplasmic reticulum.

Power Stroke

Myosin head pivots and pulls actin during contraction.

Calcium ATPase

Enzyme that pumps calcium back into the SR for relaxation.

Twitch

One contraction-relaxation cycle.

Latent Period

Time between muscle action potential and tension development.

Contraction Phase

Time from beginning of tension to peak contraction.

Relaxation Phase

Time from peak contraction back to baseline tension.

Phosphocreatine

Stored high-energy molecule used to rapidly regenerate ATP.

Central Fatigue

Perception of tiredness that can be overcome mentally.

Peripheral Fatigue

Physical inability of muscles to continue contracting effectively.

Type I Fibers

Slow-twitch oxidative fibers resistant to fatigue.

Type IIX Fibers

Fast-twitch glycolytic fibers that fatigue quickly.

Type IIA Fibers

Fast oxidative-glycolytic fibers with moderate fatigue resistance.

Summation

Adding muscle contractions together before relaxation is complete.

Incomplete Tetanus

Sustained contraction with brief relaxation periods.

Fused Tetanus

Continuous contraction with no relaxation.

Asynchronous Recruitment

Alternating motor units to maintain prolonged contractions.

Isotonic Contraction

Muscle changes length while moving a load.

Concentric Contraction

Muscle shortens while generating force.

Eccentric Contraction

Muscle lengthens while generating force.

Isometric Contraction

Muscle generates force without changing length.

Series Elastic Elements

Elastic structures that stretch while muscle develops tension.

Calmodulin

Calcium-binding protein that initiates smooth muscle contraction.

Myosin Light Chain Kinase

Enzyme that phosphorylates myosin to allow contraction.

Single Unit Smooth Muscle

Smooth muscle cells connected by gap junctions that contract together.

Multiunit Smooth Muscle

Smooth muscle cells that contract independently.

Reflex

Automatic response to a stimulus.

Monosynaptic Reflex

Reflex involving one afferent neuron and one efferent neuron.

Polysynaptic Reflex

Reflex involving one or more interneurons.

Autonomic Reflexes

Involuntary reflexes that help maintain homeostasis.

Stretch Reflex

Reflex that resists muscle stretching.

Proprioceptor

Receptor that monitors body position and movement.

Muscle Spindle

Stretch receptor found within skeletal muscles.

Alpha Motor Neuron

Neuron that stimulates extrafusal muscle fibers.

Gamma Motor Neuron

Neuron that stimulates intrafusal muscle fibers.

Extrafusal Muscle Fiber

Skeletal muscle fiber responsible for force generation.

Intrafusal Muscle Fiber

Muscle spindle fiber that detects stretch.

Patellar Reflex

Monosynaptic reflex causing knee extension when the patellar tendon is tapped.

Flexion/Crossed Extensor Reflex

Withdrawal reflex accompanied by extension of the opposite limb.

Special Senses

Vision, hearing, taste, smell, and equilibrium.

Somatic Senses

Touch, temperature, pain, itch, and proprioception.

Receptor Potential

A graded electrical signal produced when a sensory receptor detects a stimulus.

Modality

A specific type of sensory stimulus or sensation.

Phantom Limb Pain

Pain perceived in a limb that has been amputated due to continued CNS processing.

Lateral Inhibition

Process where the strongest sensory signal suppresses nearby weaker signals.

Tonic Receptors

Receptors that continue firing throughout the duration of a stimulus.

Phasic Receptors

Receptors that adapt and stop firing if the stimulus remains constant.

Perceptual Threshold

Minimum stimulus intensity needed for conscious awareness.

Habituation

Decreased perception of a repeated or constant stimulus.