Somatosensory, Sensorimotor, Visual, and Circadian Systems

Vocabulary practice flashcards covering somatosensory receptors, pain pathways, motor systems, visual pathways, retinal mechanics, and the neurobiology of circadian rhythms and sleep stages.

Sensory Transduction

The process of converting electrical energy from a physical stimulus into a change in membrane potential in a receptor cell.

Generator Potential

A local, graded change in membrane potential produced in a receptor cell in response to a stimulus.

Labeled Lines

The concept that the brain recognizes distinct senses because action potentials travel along separate, dedicated nerve tracts.

Receptive Field

The specific region of sensory space in which an appropriate stimulus alters the firing rate of a given neuron.

Central Modulation of Sensory Information

The process by which higher brain centers actively suppress some sensory inputs and amplify others.

Pacinian Corpuscle

A encapsulated skin receptor (lamellated corpuscle) that detects vibration, pressure, and textures by opening sodium channels when mechanical force stretches its membrane.

Meissner's Corpuscles

Tactile skin receptors located in the dermis that respond to changes in stimuli and light touch, contributing to form perception.

Merkel's Discs

Skin receptors that respond to isolated points and fine touch, enabling precise perception of object form.

Ruffini Corpuscles

Skin receptors that detect stretching of the skin when digits or limbs move.

Phasic Receptors

Receptors that display rapid sensory adaptation by decreasing their frequency of action potentials during sustained stimulation.

Tonic Receptors

Receptors that show little or no decline in action potential frequency during continuous stimulation.

Dorsal Column System

An ascending somatosensory pathway delivering touch information via the dorsal spinal cord to brainstem nuclei, where axons cross the midline before projecting to the thalamus.

Dermatome

A strip of skin innervated by the sensory fibers of a single, specific spinal nerve.

Primary Somatosensory Cortex (S1)

The region located in the postcentral gyrus of the parietal cortex that receives touch information from the contralateral side of the body.

Sensory Homunculus

A cortical map depicting the disproportionate representation of the body surface across the primary somatosensory cortex based on receptor density.

Synesthesia

A condition in which stimulation of one sensory modality evokes an involuntary perceptual experience in an entirely different modality.

Nociceptors

Peripheral receptor endings on free nerve terminals that detect painful or damaging stimuli.

TRPV1

Transient Receptor Potential Vanilloid Type 1; a receptor located on thin, unmyelinated C fibers that binds capsaicin and detects painful heat.

TRP2

Transient Receptor Potential 2; a receptor located on large myelinated $A\delta$ fibers that detects very high temperatures but does not respond to capsaicin.

Anterolateral System

The spinothalamic pathway that transmits pain and temperature signals by synapsing in the dorsal horn of the spinal cord, crossing the midline, and ascending to the brain.

Mirror Box Illusion

A visual technique that uses a mirror reflection of an intact limb to provide the visual illusion of two symmetrical limbs, relieving clenching sensations in phantom limb pain.

Gate Control Theory

A model stating that spinal cord circuits can modulate ascending pain signals because non-painful touch input carried on large fibers blocks pain transmission on small fibers.

Transcutaneous Electrical Nerve Stimulation (TENS)

A pain-relief method that delivers electrical stimulation to large cutaneous fibers near a painful area to close spinal pain gates, an effect blocked by naloxone.

Motor Plan

A programmed set of muscle commands established before an action occurs, also known as a motor program.

Motor Unit

A single motoneuron and all of the individual skeletal muscle fibers that its axon innervates.

Muscle Spindle

A capsule-encased proprioceptor containing intrafusal fibers that detects changes in muscle length and rate of stretch.

Golgi Tendon Organ

A proprioceptor situated in muscle tendons that monitors contraction force and muscle tension, triggering a relaxing reflex under mechanical overload.

Stretch Reflex

A monosynaptic reflex contraction of a muscle elicited when incoming sensory feedback from muscle spindles activates motor neurons while inhibiting antagonist muscles.

Pyramidal System

A motor pathway composed of cerebral cortex neurons whose axons pass through the medullary pyramids, decussate, and form the corticospinal tracts.

Motor Homunculus

A mapped representation of the human body along the precentral gyrus (primary motor cortex, M1) illustrating disproportionate motor control devoted to the hands, face, and lips.

Supplementary Motor Area (SMA)

A region on the medial surface of the hemisphere critical for initiating internally generated or preplanned motor sequences.

Premotor Cortex

A motor cortical area located anterior to M1 that orchestrates motor sequences guided primarily by external events and sensory cues.

Apraxia

The inability to execute purposeful, complex motor commands in the absence of primary muscle paralysis or weakness, caused by damage to nonmotor zones.

Mirror Neurons

Neurons in subregion F5 of the premotor cortex that fire both when an individual executes a specific movement and when they observe another perform the same action.

Basal Ganglia

A group of interconnected subcortical nuclei (caudate nucleus, putamen, and globus pallidus) that control the amplitude, direction, and initiation of movements via the thalamus.

Ataxia

A loss of coordination and balance in voluntary movements, particularly gait and posture, commonly resulting from cerebellar damage.

Decomposition of Movement

A motor deficit caused by cerebellar lesions in which fluid gestures are broken down into disjointed, segmented steps.

Accommodation

The optical process in which ciliary muscles contract to alter the curvature and shape of the lens, bringing images at various distances into sharp focus on the retina.

Bipolar Cells

Retinal interneurons that receive graded inputs from photoreceptors and synapse onto retinal ganglion cells.

Scotopic System

A rod-based visual system characterized by high sensitivity to dim light, low visual acuity, lack of color discrimination, and high neural convergence onto ganglion cells.

Photopic System

A cone-based visual system characterized by low sensitivity requiring bright illumination, high visual acuity, and three opsin photopigments enabling color vision.

Rhodopsin

The photopigment contained within the stacked membrane discs of rod photoreceptors that captures photons of light.

Fovea

The central retinal region packed with densely clustered cones, free from overlying cells and blood vessels, providing maximum visual acuity.

Optic Disc

The anatomical exit point on the retina where ganglion cell axons join to form the optic nerve and retinal blood vessels pass, creating a physiological blind spot devoid of photoreceptors.

Lateral Geniculate Nucleus (LGN)

The visual relay thalamic nucleus that receives axons from the optic tract and projects optic radiations to the primary visual cortex.

Striate Cortex (V1)

The primary visual cortex of the occipital lobe, exhibiting visible stripes in cross-sections corresponding to binocular input from the lateral geniculate nucleus.

Retinotopic Mapping

The orderly, point-to-point topographical preservation of retinal spatial relations throughout central visual structures, such as LGN and V1.

Blindsight

A clinical phenomenon where patients with visual cortex lesions discriminate visual stimuli without conscious perceptual awareness of seeing them.

Illusory Contours

Perceptual boundaries that are not physically present in the optical image but are generated by cortical processing in areas such as V2.

Area V5 (MT)

The medial temporal extrastriate cortical region containing neurons specialized for processing motion and directional stimuli.

Dorsal Visual Stream

The parietal visual pathway originating in V1 that processes spatial location and movement ('where') to guide motor behavior.

Ventral Visual Stream

The temporal visual pathway originating in V1 that processes object features, faces, and form ('what') for conscious recognition.

Optic Ataxia

A motor-spatial deficit resulting from dorsal stream damage where a person has difficulty using visual cues to accurately reach for and grasp objects.

Prosopagnosia

A specialized visual agnosia caused by bilateral ventral stream lesions, leaving individuals unable to recognize specific individual faces.

Capgras Syndrome

A rare delusion characterized by the belief that familiar loved ones have been replaced by identical imposters, caused by a disconnection between the ventral visual stream and the limbic system.

Fusiform Face Area (FFA)

A region on the boundary of the temporal and occipital lobes that exhibits increased neural activation during facial recognition.

PRIMA System

Photovoltaic Retina Implant Microarray; a subretinal device designed to restore central visual acuity in geographic atrophy by converting projected near-infrared light into electrical signals.

Circadian Rhythm

A regular biological fluctuation in behavioral, physiological, or biochemical processes that repeats with an approximate period of $24\text{ hours}$.

Ultradian Rhythm

A biological cycle that repeats more frequently than once every $24\text{ hours}$, such as episodic feeding or hormone secretion.

Infradian Rhythm

A biological cycle that repeats less frequently than once a day, such as seasonal or reproductive cycles.

Free-Running

The condition of an organism maintaining its endogenous circadian cycle in the complete absence of external environmental time cues.

Zeitgeber

An external environmental cue, such as the light-dark cycle, used by an organism to synchronize (entrain) its internal biological clock.

Phase Shift

A shift in the timing of an activity cycle or rhythm induced by a synchronizing stimulus like an altered light-dark onset.

Suprachiasmatic Nucleus (SCN)

A hypothalamic nucleus situated above the optic chiasm that serves as the master biological clock generating mammalian circadian rhythms.

Melanopsin

A specialized photopigment in retinal ganglion cells that transmits light signals to the SCN via the retinohypothalamic tract.

Clock and Bmal1

Two transcription factors in mammalian SCN cells that dimerize to promote the transcription of the Period (Per) and Cryptochrome (Cry) genes.

Beta Activity

A pattern of desynchronized, low-amplitude, high-frequency ($15\text{--}20\text{ Hz}$) EEG activity characteristic of an alert, awake individual.

Alpha Rhythm

A regular, synchronized EEG oscillation ranging between $8\text{--}12\text{ Hz}$ that emerges over the cortex during relaxed wakefulness.

Vertex Spikes

Sharp, distinct EEG waveforms that appear during Stage 1 non-REM sleep as alpha rhythms decline.

Sleep Spindles

Periodic bursts of $12\text{--}14\text{ Hz}$ waves characteristic of Stage 2 non-REM sleep.

K-Complexes

Sharp negative electrical potentials on an EEG record that uniquely define Stage 2 sleep.

Delta Waves

Large-amplitude, slow EEG oscillations occurring at roughly $1\text{ per second}$ that define Stage 3 slow-wave sleep (SWS).

Rapid-Eye-Movement (REM) Sleep

A sleep stage also called paradoxical sleep, marked by fast, desynchronized awake-like EEG waves, rapid eye twitches, and flaccid postural muscle tone.

Night Terrors

Sudden episodes of panic and intense autonomic arousal arising abruptly out of Stage 3 slow-wave sleep.

The Nightmare

A 1781 artwork by Henri Fuseli depicting an incubus demon sitting atop a sleeping woman, illustrating the historical association of sleep paralysis and terrifying dreams.

Sleep Recovery

The rebound process of experiencing elevated amounts of Stage 3 SWS (on Night 1) and intense REM sleep following a duration of sleep deprivation.

Fatal Familial Insomnia (FFI)

An inherited autosomal dominant prion disease caused by mutations in the PRNP gene, causing severe thalamic degeneration, total insomnia, dementia, and death within $12\text{--}18\text{ months}$.

Pontine System

A brainstem sleep center located in the pons near the locus coeruleus that triggers REM sleep and actively inhibits spinal motor neurons to produce muscle atonia.

Hypocretin

A hypothalamic neuropeptide (also called orexin) that orchestrates transitions between wakefulness, non-REM, and REM sleep; its loss underlies narcolepsy.

Cataplexy

A sudden, paroxysmal loss of muscle tone triggered by strong emotional reactions, causing sudden collapse while consciousness is retained.

Sleep Paralysis

A brief, alarming inability to move voluntarily occurring right at sleep onset or awakening, caused by the pontine center prolonging motor inhibition.

Sleep Apnea

A sleep disorder characterized by frequent pauses or slow respiration caused by excessive throat relaxation or impaired brainstem pacemaker activity.

Somnambulism

Sleepwalking behavior that occurs specifically during Stage 3 slow-wave sleep.

Sudden Infant Death Syndrome (SIDS)

The sudden, unexpected death of a sleeping infant attributed to respiratory failure or arousal failure from sleep apnea, minimized by back-sleeping posture.