Neuroscience: Sensory Systems, Autonomic Control, Motor Pathways, and Neurological Disorders

Comprehensive vocabulary flashcards reviewing sensory physiology, auditory and visual phototransduction, autonomic signaling pathways, sensorimotor cortical hierarchies, basal ganglia circuitry, and major neuropathologies.

Sensory Transduction

The biological process in which sensory cells convert physical or chemical stimuli into changes in membrane voltage via specialized receptor proteins, enabling neuronal signaling.

Ionotropic Receptors

Fast sensory receptor proteins that directly regulate ion channels, opening or closing them in response to a stimulus without involving G-protein-coupled receptors (GPCRs).

Metabotropic Receptors

Slower sensory receptor proteins involving G-protein-coupled receptors (GPCRs) that activate intracellular second-messenger cascades to indirectly regulate ion channels and alter membrane potential.

Receptor Potential

A graded change in membrane voltage (depolarization or hyperpolarization) in a sensory cell produced by a stimulus, resembling postsynaptic potentials but initiated by sensory stimulation rather than neurotransmitter binding.

Rate Coding (Sensory)

The encoding principle where stronger stimuli produce higher frequencies of action potentials rather than altering the amplitude or size of individual action potentials.

Sensory Adaptation

A phenomenon in which a sensory response diminishes over time despite the continuous, unchanging presence of a stimulus.

Proprioception

The sensory modality that provides continuous internal information regarding limb positions, joint angles, and bodily movement.

Intrafusal Fibers

Small, specialized muscle fibers located inside an encapsulated muscle spindle that are innervated by gamma motor neurons and wrapped by sensory endings to detect muscle length and stretch velocity.

Extrafusal Fibers

Large, force-generating skeletal muscle fibers innervated and controlled directly by alpha motor neurons.

Gamma Motor Neurons

Motor neurons that contract the contractile poles of intrafusal fibers to maintain tension in the central sensory region, tuning the sensitivity of muscle spindles.

Basal Cells (Olfactory Epithelium)

Stem cells situated within the olfactory epithelium that divide to replenish supporting cells and olfactory receptor neurons.

Mitral Cells

Principal neurons in the olfactory bulb that receive input from olfactory sensory neurons within glomeruli and relay odor information directly to the uncus and olfactory cortex.

Uncus

A structure located in the temporal lobe that houses the primary olfactory cortex and receives axonal projections from the olfactory tract.

Organ of Corti

The auditory sensory organ located within the cochlear scala media, composed of stereocilia-bearing hair cells supported on the basilar membrane.

Tonotopic Coding

The spatial frequency mapping of the basilar membrane, where the stiff base near the oval window responds maximally to high-frequency sounds ($2{,}000\,\text{Hz}$) and the flexible apex responds to low-frequency sounds ($400\,\text{Hz}$).

Fovea Centralis

A cone-packed specialized region within the retinal macula that exhibits minimal neural convergence, supporting high visual acuity and sharp central vision.

Rhodopsin

A light-sensitive photopigment embedded in the disc membranes of rod photoreceptors, composed of the G-protein-coupled receptor protein opsin covalently bound to retinal.

Dark Current

The continuous inward flow of positive cations ($Na^+$ and $Ca^{2+}$) through open cGMP-gated channels in darkness, maintaining photoreceptors at a depolarized potential of approximately $-35\,\text{mV}$ and driving continuous glutamate release.

ON-Center Bipolar Cells

Retinal bipolar neurons expressing inhibitory metabotropic glutamate receptors; they hyperpolarize in darkness when photoreceptor glutamate is high and depolarize in light when glutamate release falls.

OFF-Center Bipolar Cells

Retinal bipolar neurons expressing excitatory ionotropic ligand-gated ion channels; they depolarize in darkness when photoreceptor glutamate is high and hyperpolarize in light.

Sympathetic Chain Ganglia

A paired bilateral chain of ganglia running along the spinal cord where preganglionic sympathetic axons from thoracic and lumbar segments synapse onto postganglionic neurons using acetylcholine ($\text{ACh}$) on nicotinic receptors.

Chromaffin Cells

Modified neuroendocrine cells inside the adrenal medulla that receive direct sympathetic preganglionic $\text{ACh}$ stimulation and secrete epinephrine and norepinephrine into adjacent capillary beds.

Muscarinic ACh Receptors (Cardiac)

G-protein-coupled receptors ($G_i$-coupled) on cardiac tissue that bind acetylcholine ($\text{ACh}$) released by parasympathetic postganglionic fibers (such as the vagus nerve) to decrease heart rate.

Posterior Parietal Association Cortex

An association cortex area where the planning of motor behavior begins, integrating multimodal sensory inputs and projecting to the secondary motor cortex and dorsolateral prefrontal cortex.

Dorsolateral Prefrontal Association Cortex

A frontal cortical area identified by fMRI as activating first during the initiation of voluntary movements, projecting directly to the secondary (premotor) cortex.

Neuromuscular Junction (NMJ)

The synapse between an alpha motor neuron terminal and a skeletal muscle fiber, where $\text{ACh}$ release activates nicotinic receptors, generating an end-plate potential that triggers muscle action potentials.

Motor Unit

A single motor neuron and all individual extrafusal skeletal muscle fibers innervated by its terminal axon branches.

Basal Ganglia

A group of subcortical forebrain nuclei consisting of the caudate, putamen, and globus pallidus, embedded in white matter and central to the initiation and regulation of voluntary movement.

Direct Pathway (Basal Ganglia)

The motor-initiating basal ganglia circuit where cortical activation drives striatal neurons expressing dopamine $D_1$ ($G_s$) receptors to inhibit the internal globus pallidus (GPi), disinhibiting the thalamus to promote movement.

Indirect Pathway (Basal Ganglia)

The motor-suppressing basal ganglia circuit where striatal neurons expressing dopamine $D_2$ ($G_i$) receptors inhibit the external globus pallidus (GPe), disinhibiting the subthalamic nucleus (STN) to excite the GPi and inhibit thalamocortical drive ('Do Not Move').

Substantia Nigra

A midbrain dopaminergic nucleus whose degeneration deprives the striatum of dopamine, leading to insufficient thalamic disinhibition and the movement deficits of Parkinson's disease.

Penumbra

The zone of underperfused, structurally damaged yet viable neural tissue surrounding the ischemic infarct core that can potentially recover with timely intervention.

Excitotoxicity

A pathological cascade in ischemic brain injury where depleted $\text{ATP}$ halts $Na^+/K^+$ pumps, inducing sustained depolarization, massive synaptic glutamate efflux, and destructive intracellular $Ca^{2+}$ and zinc influx leading to apoptosis.

Potassium-Chloride ($K^+/Cl^-$) Pump Cotransporter

A membrane transport protein responsible for exporting $Cl^-$ and $K^+$ out of neurons to maintain a steep concentration gradient; its genetic impairment causes intracellular $Cl^-$ accumulation, blunted GABA-mediated IPSPs, and runaway epileptic seizures.

Huntington's Disease

An autosomal dominant neurodegenerative genetic disorder caused by abnormal polyglutamine expansions in the huntingtin protein, producing chorea and dementia typically emerging around age 40.

Amyotrophic Lateral Sclerosis (ALS)

A fatal progressive neurodegenerative disease characterized by the degeneration of upper motor neurons in the primary motor cortex and lower motor neurons in the spinal cord ventral horn, eventually halting breathing when the diaphragm loses innervation.