Comprehensive Neural Function, Organization, and Sensory Physiology

Comprehensive vocabulary flashcards covering neural physiology, action potentials, synapses, CNS and PNS anatomy, meninges, CSF dynamics, sensory transduction, and special sensory systems.

Action Potential

A rapid, temporary reversal of electrical membrane potential across an excitable cell membrane, driven by the coordinated opening and closing of voltage-gated ion channels, used for long-distance communication.

Resting Membrane Potential (RMP)

The electrical charge difference across the plasma membrane when a neuron is at rest (typically $-70\,mV$), established predominantly by high membrane permeability to $K^+$ through leakage channels, impermeable intracellular protein anions, and maintained by the $Na^+/K^+$ ATPase pump.

$Na^+/K^+$ ATPase Pump

An electrogenic active transport pump that consumes one ATP molecule to export $3\,Na^+$ out of the cell and import $2\,K^+$ into the cell, maintaining essential chemical and electrical concentration gradients across the plasma membrane.

Threshold Potential

The critical membrane potential (typically approximately $-55\,mV$ in a neuron) that must be reached by depolarizing graded potentials to trigger the opening of voltage-gated $Na^+$ channels and initiate an action potential.

Depolarization Phase

The rapid upstroke of an action potential wherein voltage-gated $Na^+$ channels open in response to reaching threshold, allowing a massive influx of $Na^+$ down its electrical and concentration gradients to shift membrane potential toward $+30\,mV$.

Repolarization Phase

The downward phase of an action potential during which voltage-gated $Na^+$ channels inactivate and delayed voltage-gated $K^+$ channels open, permitting rapid $K^+$ efflux to restore negative intracellular potential.

Hyperpolarization (After-hyperpolarization)

A transient phase following repolarization where membrane potential dips below resting levels (approaching $-90\,mV$) due to prolonged opening of voltage-gated $K^+$ channels before they fully close.

All-or-None Principle

The neurophysiological law stating that if a stimulus depolarizes a neuron to threshold, a full-strength action potential is generated with constant amplitude; if threshold is not reached, no action potential occurs at all.

Frequency Coding

The mechanism by which the central nervous system decodes stimulus intensity through the frequency (firing rate) of action potentials rather than their amplitude.

Absolute Refractory Period

The period from the opening of voltage-gated $Na^+$ channels until their inactivation gates reset to the closed state, during which no subsequent action potential can be triggered regardless of stimulus strength.

Relative Refractory Period

The interval following the absolute refractory period where some $Na^+$ channels have reset but voltage-gated $K^+$ channels remain open, requiring an exceptionally strong (suprathreshold) stimulus to generate an action potential.

Saltatory Conduction

The rapid mode of action potential propagation in myelinated axons where electrical depolarization jumps from one node of Ranvier (myelin sheath gap) to the next.

Length Constant ($\lambda$)

A metric indicating how far an electrical signal travels along an axon without decaying, defined mathematically as $\lambda = \sqrt{\frac{R_m}{R_i}}$, where $R_m$ is membrane resistance and $R_i$ is axial resistance.

Anterograde Transport

Axonal transport powered by the molecular motor kinesin that carries synaptic components, cytoskeletal proteins, and ion channels away from the cell body (soma) toward the axon terminals along microtubules.

Retrograde Transport

Axonal transport powered by the molecular motor dynein that carries materials destined for degradation and neurotrophic signals from axon terminals back toward the cell body; notably hijacked by the rabies virus.

Astrocytes

The most abundant CNS neuroglia that provide metabolic support, nourish neurons, regulate extracellular potassium and neurotransmitter concentrations, form the perivascular feet of the blood-brain barrier, and create scar tissue after brain injury.

Oligodendrocytes

Glial cells of the central nervous system that wrap their cytoplasmic processes around segments of multiple axons to form the insulating myelin sheath in the brain and spinal cord.

Microglia

Specialized resident macrophages of the central nervous system that migrate to damaged areas and phagocytize pathogens, cellular debris, and dead neural tissue.

Ependymal Cells

Ciliated epithelial-like glial cells that line the fluid-filled ventricles of the brain and the central canal of the spinal cord, producing and circulating cerebrospinal fluid.

Schwann Cells

PNS neuroglial cells that form the neurilemma and myelin sheath around a single segment of a peripheral nerve fiber, facilitating axon regeneration following peripheral injury.

Synaptotagmin

A calcium-sensing protein on synaptic vesicle membranes that, upon binding calcium entering via voltage-gated $Ca^{2+}$ channels, triggers SNARE-mediated vesicle fusion and neurotransmitter exocytosis.

SNARE Proteins (v-SNARE and t-SNARE)

Complementary membrane-associated proteins located on synaptic vesicles (v-SNARE) and target presynaptic plasma membranes (t-SNARE) that twist together to form a fusion pore for neurotransmitter release.

Excitatory Postsynaptic Potential (EPSP)

A localized, graded depolarization of a postsynaptic membrane produced when excitatory neurotransmitters bind ionotropic receptors permeable to both $Na^+$ and $K^+$, causing net sodium influx.

Inhibitory Postsynaptic Potential (IPSP)

A localized, graded hyperpolarization of a postsynaptic membrane caused by neurotransmitter binding to receptors that increase permeability to either $Cl^-$ (influx) or $K^+$ (efflux), driving the membrane potential further below threshold.

Temporal Summation

The additive effect on postsynaptic membrane potential produced when a single presynaptic neuron fires multiple action potentials in rapid succession.

Spatial Summation

The algebraic addition of simultaneous postsynaptic potentials generated by multiple separate presynaptic axon terminals across different locations on the postsynaptic dendrites or soma.

Presynaptic Inhibition

A mechanism in which an axoaxonic synapse reduces or blocks neurotransmitter release from a target presynaptic terminal, typically by opening chloride or potassium channels or inhibiting calcium influx.

Diverging Circuit

A neuronal circuit configuration where one input neuron synapses with multiple postsynaptic neurons, amplifying the outgoing signal across broader pathways.

Reverberating Circuit

A neuronal circuit where incoming signals travel through a chain of neurons that loop back with collateral fibers to re-stimulate earlier neurons in the chain, generating sustained rhythmic outputs such as breathing.

Cervical and Lumbar Enlargements

Expanded regions of the spinal cord where motor and sensory neurons supplying the upper limbs (cervical enlargement) and lower limbs (lumbar enlargement) are concentrated.

Cauda Equina

A fan-like bundle of nerve roots arising from the inferior end of the spinal cord below the level of the conus medullaris ($L_1\text{--}L_2$), traversing the subarachnoid space within the lumbar cistern.

Dorsal Root Ganglion (DRG)

A nodular swelling located along each dorsal root of a spinal nerve that houses the cell bodies of pseudounipolar (unipolar) sensory neurons carrying afferent impulses toward the CNS.

Ventral Horn

The anterior column of gray matter in the spinal cord that contains the cell bodies of somatic motor neurons whose axons exit via the ventral roots to innervate skeletal muscle.

Dermatome

A specific cutaneous segment of skin innervated by sensory fibers arising from the cutaneous branches of a single pair of spinal nerves.

Blood-Brain Barrier (BBB)

A strictly regulated physiological barrier formed by capillary endothelial cells connected by tight junctions and surrounded by astrocyte perivascular feet, preventing the passage of harmful blood-borne substances into neural tissue.

Choroid Plexus

Spongy, vascularized networks of capillaries and pia mater covered by ependymal cells with tight junctions, situated in all four brain ventricles to produce cerebrospinal fluid.

Arachnoid Granulations

Berry-like projections of the arachnoid mater that penetrate the dura mater into dural venous sinuses (especially the superior sagittal sinus) to reabsorb cerebrospinal fluid into venous blood.

Medulla Oblongata

The most inferior brainstem region, immediately superior to the spinal cord, containing vital autonomic control centers for heart rate, blood pressure, respiration, swallowing, vomiting, coughing, and sneezing.

Substantia Nigra

A midbrain motor nucleus composed of dark, melanin-pigmented neurons that produce dopamine and project to the basal nuclei to suppress unwanted muscle movements; degenerates in Parkinson's disease.

Superior Colliculus

Paired visual reflex centers located on the dorsal surface of the midbrain tectum that coordinate head and eye movements to visually track moving objects.

Inferior Colliculus

Paired auditory reflex centers on the dorsal midbrain tectum that process sound localization and mediate the startle reflex to sudden loud noises.

Reticular Formation

A loosely organized web of gray matter extending throughout the brainstem that regulates alertness, somatic motor tone, posture, sleep-wake cycles, and habituation to repetitive, non-novel sensory stimuli.

Cerebellum

The large, highly folded posterior brain structure containing an inner branching white matter pattern (arbor vitae) that fine-tunes motor activity, calculates movement blueprints, and coordinates smooth muscle actions.

Thalamus

A paired oval mass of nuclei within the diencephalon that functions as the mandatory sensory gateway, sorting, editing, and relaying virtually all ascending sensory inputs (except olfaction) to the cerebral cortex.

Hypothalamus

The principal autonomic and endocrine control center of the diencephalon, vital for homeostatic regulation of body temperature, hunger, satiety, water balance, circadian cycles, and pituitary secretion.

Orexin

A hypothalamic neuropeptide that promotes wakefulness and alertness; defective signaling or destruction of orexin-producing neurons is directly implicated in narcolepsy.

Pineal Gland

An endocrine structure in the epithalamus that receives light-dark input indirectly from the hypothalamus and secretes the hormone melatonin in darkness to regulate circadian sleep rhythms.

Hippocampus

A curved limbic structure within the medial temporal lobe that processes immediate memory and coordinates memory consolidation to the cerebral cortex during sleep; bilateral destruction causes anterograde amnesia.

Association Fibers

Cerebral white matter tracts that interconnect different cortical regions within the same cerebral hemisphere.

Commissural Fibers

Transverse white matter tracts, prominently exemplified by the corpus callosum, that link corresponding gray matter regions between the right and left cerebral hemispheres.

Broca's Area

A specialized motor speech planning center located in the inferior frontal gyrus of the dominant (usually left) hemisphere; lesions cause expressive aphasia characterized by non-fluent speech despite intact language comprehension.

Wernicke's Area

A posterior language cortex region located near the junction of the temporal and parietal lobes of the dominant hemisphere; lesions cause fluent/receptive aphasia, where speech is effortless but nonsensical ('word salad').

Transduction

The physiological process by which a sensory receptor converts environmental stimulus energy (light, pressure, chemicals, temperature) into a graded receptor potential.

Law of Labeled Lines

The concept that each sensory modality and bodily receptive field is hard-wired via dedicated neuronal pathways to specific integrating centers in the cerebral cortex, dictating the perceived sensation regardless of how the pathway is stimulated.

Lateral Inhibition

A sensory mechanism where primary sensory pathways that receive the strongest stimulation activate inhibitory interneurons to suppress transmission from adjacent lateral pathways, thereby sharpening stimulus localization and acuity.

Two-Point Discrimination

A clinical test assessing tactile acuity based on the minimal distance at which two simultaneous skin contacts are perceived as distinct, which inversely correlates with receptive field size.

Sensory Adaptation

A physiological reduction in receptor sensitivity and action potential firing frequency that occurs when a receptor is subjected to a prolonged, constant, unchanging stimulus.

Dorsal Column-Medial Lemniscal Pathway

An ascending somatosensory tract carrying discriminative touch, vibration, and conscious proprioception; first-order axons ascend in the fasciculus gracilis or cuneatus to the medulla, where second-order neurons decussate in the medial lemniscus before terminating in the thalamus.

Spinothalamic Pathway

An ascending sensory tract transmitting pain and temperature sensations; first-order neurons synapse in the spinal cord dorsal horn, where second-order neurons decussate immediately in the cord and ascend via the lateral spinothalamic tract to the thalamus.

Spinocerebellar Pathway

An ascending 2-neuron pathway carrying unconscious proprioceptive signals from muscle spindles and tendon organs directly to the cerebellum without decussating, aiding subconscious motor coordination.

Sensory Homunculus

A somatotopic distortion map along the primary somatosensory cortex (postcentral gyrus) where cortical surface area is directly proportional to the receptor density and tactile sensitivity of each body part.

Fovea Centralis

A small retinal depression located in the center of the macula lutea containing a high density of cones and no rods, where overlying neural layers are parted to allow light direct access for maximal visual acuity.

Optic Disc

The circular site where retinal ganglion cell axons converge and exit the posterior eyeball to form the optic nerve; it completely lacks photoreceptors, producing the anatomical blind spot.

Phototransduction in Light

The cascade wherein retinal absorbs a photon, isomerizing from 11-cis to all-trans, activating transducin and phosphodiesterase (PDE), which degrades cGMP; cGMP-gated cation channels close, hyperpolarizing the photoreceptor and stopping glutamate release to disinhibit bipolar cells.

Dark Current

The steady influx of sodium and calcium ions through cGMP-gated cation channels into photoreceptor outer segments in darkness, maintaining continuous membrane depolarization and constant glutamate release.

Basilar Membrane Frequency Tuning

The mechanical tonotopic organization of the cochlea, where the narrow, stiff fibers at the cochlear base resonate to high-frequency sounds ($20{,}000\,Hz$), while the wide, flexible fibers at the apex resonate to low-frequency sounds ($20\,Hz$).

Stereocilia and Tip Links

Microvillar projections atop cochlear hair cells connected by extracellular filamentous tip links that tighten when deflected toward the tallest stereocilium, mechanically pulling open potassium channels.

Endolymph

An unusual extracellular fluid filling the cochlear duct and semicircular canals that possesses an exceptionally high potassium ($K^+$) concentration, causing potassium to rush inward to depolarize hair cells upon channel opening.

Cupula

A gelatinous dome-like structure situated within the ampulla of each semicircular canal that is deflected by moving endolymph during rotational head acceleration, bending the embedded hair cell stereocilia.

Otoliths

Dense calcium carbonate protein crystals embedded within the gelatinous membrane of the utricle and saccule maculae that shift with gravity and linear acceleration, deflecting hair cell stereocilia.

Gustducin

A G-protein coupled to taste receptors for sweet and umami tastants; upon activation, it stimulates adenylate cyclase to produce cAMP, which activates a protein kinase to phosphorylate and close potassium channels.

Fast Pain Pathway

Acute, sharp, localized nociceptive transmission carried by medium-diameter, myelinated A-delta ($A\delta$) fibers conducting at $6\text{--}30\,m/s$, utilizing glutamate as the primary neurotransmitter.

Slow Pain Pathway

Dull, aching, chronic, burning nociceptive transmission carried by small-diameter, unmyelinated C fibers conducting at $0.5\text{--}2\,m/s$, utilizing substance P as the primary neurotransmitter.

Gate Control Theory of Pain

A pain-modulation model wherein an enkephalin-releasing gate interneuron in the spinal cord dorsal horn provides presynaptic inhibition onto secondary pain transmission neurons; input from large non-painful tactile ($A\beta$) fibers stimulates the interneuron to close the gate, while painful C-fiber activity inhibits it to open the gate.

Referred Pain

Visceral pain erroneously perceived as originating from a superficial somatic site, caused by the convergence of visceral and somatic primary afferents onto the same ascending secondary projection neuron in the spinal cord.