Chapter 3

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Description and Tags

A vocabulary set of key terms and their concise definitions based on the lecture notes.

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58 Terms

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Neuron

Basic structural unit of the nervous system; has a cell body (soma), dendrites, and an axon; the same basic structure is found throughout the body.

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Soma (cell body)

Contains the nucleus; from it radiate dendrites and the axon.

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Dendrites

Receivers of impulses; carry impulses toward the cell body.

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Axon

conducts impulses away from the cell body; begins at the axon hillock and ends at axon terminals with neurotransmitters.

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Axon hillock

Region where the axon begins and where action potentials are typically initiated.

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Resting membrane potential (RMP)

Difference in electrical charge across the cell membrane, about −70 mV; polarized due to ion distribution.

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Sodium (Na+) and potassium (K+) gradients

High Na+ outside, high K+ inside; Na+ channels closed at rest, K+ channels open, contributing to RMP.

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Sodium-potassium pump (Na+/K+ pump)

Maintains gradients by moving 3 Na+ out and 2 K+ in, contributing to resting membrane potential.

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Depolarization

Inside of the cell becomes less negative (−70 mV toward 0 mV); Na+ influx initiates the process.

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Hyperpolarization

Inside of the cell becomes more negative (toward −90 mV); K+ efflux makes it harder to fire another impulse.

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Threshold

Critical membrane potential (~−55 mV) that must be reached to trigger an action potential.

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Graded potentials

Localized changes in membrane potential that can be excitatory (depolarizing) or inhibitory (hyperpolarizing) and influence whether an AP occurs.

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Action potential (AP)

Rapid, large depolarization that travels along the axon; lasts about 1 ms; follows the all-or-none rule.

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Absolute refractory period

During depolarization when Na+ channels are inactivated; neuron cannot fire another AP.

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Relative refractory period

During repolarization when a stronger stimulus is needed; Na+ channels may reopen and K+ channels remain open.

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Myelin

Fatty sheath around axons (produced by Schwann cells) that speeds conduction; segmented by nodes of Ranvier.

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Saltatory conduction

Propagation of an AP by jumping from node to node across myelinated segments.

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Nodes of Ranvier

Gaps in the myelin sheath where the axon is exposed; sites of rapid AP regeneration.

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Propagation speed

Increases with larger axon diameter and with myelination.

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Synapse

Junction between neurons where the AP is transmitted; transmission can be electrical to chemical to electrical.

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Presynaptic neuron

Neuron sending the signal; contains synaptic vesicles with neurotransmitters.

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Postsynaptic neuron

Neuron receiving the signal; has receptors for neurotransmitters.

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Synaptic cleft

The gap between the presynaptic and postsynaptic membranes.

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Neurotransmitters

Chemical messengers; >50 known; include small-molecule transmitters and neuropeptides; include acetylcholine and norepinephrine.

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Acetylcholine (ACh)

Neurotransmitter at the neuromuscular junction; excitatory and mediates parasympathetic effects.

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Neuromuscular junction (NMJ)

Specialized synapse between a motor neuron and a muscle; ACh is released and causes muscle depolarization.

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Motor end plate

Postsynaptic region of the muscle fiber at the NMJ where ACh receptors are located.

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Excitatory postsynaptic potential (EPSP)

Depolarizing postsynaptic potential that makes an AP more likely; summation can trigger an AP.

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Inhibitory postsynaptic potential (IPSP)

Hyperpolarizing postsynaptic potential that makes an AP less likely; summation can prevent an AP.

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Sensory division (afferent)

Transmits information from the periphery to the brain; major receptor types include mechanoreceptors, thermoreceptors, nociceptors, photoreceptors, chemoreceptors.

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Mechanoreceptors

Receptors sensitive to physical forces such as touch, pressure, and stretch.

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Thermoreceptors

Receptors sensitive to temperature changes.

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Nociceptors

Pain receptors.

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Photoreceptors

Receptors sensitive to light (in the visual system).

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Chemoreceptors

Receptors sensitive to chemical stimuli (e.g., changes in blood or tissue chemistry).

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Muscle spindle

Intrafusal fibers sense muscle length and rate of length change; innervated by gamma motor neurons; trigger reflexes and contribute to force production.

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Golgi tendon organ

Tendon-tension receptor; inhibits agonists and excites antagonists to prevent injury during high tension.

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Kinesthetic receptors (joint receptors)

Joint receptors sensitive to joint angles and the rate of angle change; sense joint position.

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Gamma motor neurons

Innervate intrafusal fibers to adjust spindle sensitivity.

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Alpha motor neurons

Innervate extrafusal muscle fibers to generate contraction.

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Motor unit

A motor neuron and all the muscle fibers it innervates; fiber numbers per unit affect fine versus gross motor control; fibers within a unit are of the same type.

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Primary motor cortex

Frontal lobe region responsible for conscious control of skeletal muscle movement; corticospinal tract involvement.

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Basal ganglia

Deep cerebral structures that help initiate and regulate sustained or repetitive movements and posture.

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Primary sensory cortex

Parietal lobe area responsible for processing somatosensory information.

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Thalamus

Major sensory relay center; contributes to conscious awareness of sensory input.

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Hypothalamus

Maintains homeostasis; regulates visceral functions; controls neuroendocrine activity (appetite, thirst, sleep, BP, HR, breathing, temperature).

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Cerebellum

Coordinates rapid, complex movements; refines timing, sequence, and motor corrections; integrates input from motor cortex.

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Brain stem

Midbrain, pons, and medulla oblongata; houses reticular formation for motor tone and autonomic control; contains analgesia system.

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Spinal cord

Continues from the medulla; contains ascending (sensory) and descending (motor) tracts for two-way conduction.

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Central nervous system (CNS)

Brain and spinal cord.

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Peripheral nervous system (PNS)

All nerves outside the CNS; connects CNS to limbs and organs; includes sensory and motor divisions.

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Somatic nervous system

Voluntary nervous system; controls skeletal muscles.

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Autonomic nervous system

Involuntary nervous system; regulates visceral functions; has sympathetic and parasympathetic divisions.

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Sympathetic nervous system

Fight-or-flight response; increases heart rate and force, blood flow to muscles, bronchodilation, metabolic rate, and mental activity.

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Parasympathetic nervous system

Rest-and-digest actions; promotes digestion and energy conservation; generally decreases heart rate and may constrict bronchi.

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PGC-1α (peroxisome proliferator–activated receptor-γ coactivator 1α)

A transcriptional coactivator upregulated with exercise; associated with NMJ adaptations such as increased presynaptic branching and acetylcholine receptor density.

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Reflex activity

A motor reflex is an instant, preprogrammed response to a stimulus; occurs without conscious awareness and is mediated at lower, simple levels of the CNS.

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Afferent feedback (Group III/IV)

Group III: thinly myelinated mechanosensitive fibers; Group IV: unmyelinated, chemically sensitive; convey signals to CNS to aid performance and fatigue management.