Sensory Receptors, Special Senses, and Vision

Comprehensive vocabulary review flashcards covering sensory receptors, cutaneous sensations, gustation, olfaction, equilibrium, audition, and the visual system.

Sensory Transduction

The process by which sensory receptors convert environmental energy into electrical signals (nerve impulses), with sensation modality determined by the specific neural pathways and synaptic connections stimulated.

Adequate Stimulus

The normal stimulus that activates a specific sensory receptor with the least amount of energy.

Proprioceptors

Sensory receptors located in muscles, tendons, and joints that provide information regarding body position and fine muscle control.

Phasic and Tonic Receptors

Phasic receptors adapt quickly by displaying a strong initial response that decreases to detect changes and provide on/off information, whereas tonic receptors adapt slowly or very little, firing continuously to provide ongoing information such as pain.

Law of Specific Nerve Energies

The principle stating that each sensory nerve produces only one type of perceived sensation regardless of how it is stimulated, because the brain identifies sensation by the specific neural pathway carrying it.

Receptor (Generator) Potential

A local, graded depolarization in a sensory receptor stimulated by an adequate stimulus; once threshold is reached, all-or-none action potentials fire at a frequency proportional to stimulus strength.

Cutaneous Receptors

Sensory receptors in the skin consisting of naked sensory dendrites for pain, cold, and heat, alongside specialized structures: Merkel's disks (sustained touch, pressure), Meissner's corpuscles (texture, vibrations), Pacinian corpuscles (deep pressure, vibrations), and Ruffini corpuscles (sustained pressure).

Cold and Warm Receptors

Thermoreceptors where cold receptors outnumber warm receptors, reside near the epidermis, are activated by cooling, inhibited by warming, and respond to menthol; warm receptors reside deeper in the dermis, are activated by warming, and inhibited by cooling.

Capsaicin Receptor

A specialized nociceptor that detects painful heat, acts as a $Na^+/Ca^{2+}$ ion channel, and responds to capsaicin, the chemical found in chili peppers.

Nociceptors

Sensory receptors depolarized by tissue damage, chemicals (such as ATP), pH changes, or mechanical force that release glutamate and substance P, transmitting sudden sharp pain via myelinated neurons and dull persistent pain via unmyelinated neurons.

Acute and Chronic Itch

Itch sensations transmitted via unmyelinated sensory axons to the spinal cord, where acute itch is triggered by histamine released by mast cells and basophils, and chronic itch is caused by other chemicals and does not respond to antihistamines.

Interoceptors and Exteroceptors

Sensory receptors classified by location: interoceptors detect internal chemical changes inside the body, whereas exteroceptors detect external chemicals, such as dissolved food/drink molecules for taste and airborne molecules for smell.

Taste Buds

Barrel-shaped structures containing $50\text{ to }100$ specialized epithelial cells with microvilli extending through taste pores, located on tongue papillae including fungiform (front/anterior), circumvallate (back/posterior), and foliate (sides).

Neuroepithelial Taste Cells

Specialized epithelial cells within taste buds that behave like neurons by depolarizing, producing action potentials, and releasing neurotransmitters to sensory neurons when chemicals contact their microvilli.

Five Primary Tastes

The five taste modalities: salty ($Na^+$ enters), sour ($H^+$ enters), sweet (sugar activates G-protein/second messenger to close $K^+$ channels and open $Ca^{2+}$ channels), umami (amino acids glutamate/aspartate act via G-protein/second messenger to close $K^+$ channels), and bitter (quinine acts via G-protein/second messenger to release $Ca^{2+}$ from the endoplasmic reticulum).

Olfactory Receptors

Bipolar neurons located in the olfactory epithelium of the nasal cavity that have dendrites with cilia bearing odor-binding proteins and axons that convey signals toward the olfactory bulb, with one odorant molecule stimulating one receptor protein.

Olfactory Transduction

The signaling pathway where odorant binding activates a G-protein, causing adenylate cyclase to convert ATP to cAMP, which opens $Na^+/Ca^{2+}$ channels to generate a graded depolarization that triggers action potentials.

Glomeruli (Olfactory)

Synaptic structures in the olfactory bulb where unmyelinated olfactory neurons connect based on receptor type; complex odors activate distinct spatial patterns across multiple glomeruli to identify specific scents.

Vestibular Apparatus

The inner ear equilibrium structure consisting of the otolith organs (utricle for horizontal linear acceleration and saccule for vertical linear acceleration) and semicircular canals for rotational (angular) acceleration.

Perilymph and Endolymph

Fluids of the inner ear: perilymph is situated between the outer bony labyrinth and inner membranous labyrinth, while endolymph fills the membranous labyrinth and has a high concentration of $K^+$ to drive hair cell depolarization.

Vestibular Hair Cells

Modified epithelial sensory cells featuring $20\text{ to }50$ stereocilia and one true kinocilium; deflection toward the kinocilium opens $K^+$ channels causing depolarization and increased CN VIII firing, while bending away causes hyperpolarization and decreased firing.

Otolith Organs

The utricle and saccule, which detect linear acceleration through a sensory macula containing hair cells whose stereocilia project into a gelatinous otolithic membrane embedded with calcium carbonate crystals called otoliths.

Semicircular Canals

Three fluid-filled canals aligned in different spatial planes that detect rotational acceleration when inertial endolymph movement deflects a gelatinous cupula within the ampulla, bending the stereocilia of hair cells in the crista ampullaris.

Cerebellar Balance Function

The cerebellum's role in processing sensory inputs regarding body position and movement to coordinate skeletal muscle contractions, motor responses, and maintenance of equilibrium.

Triad of Balance Inputs

The three sensory sources that project to the cerebellum to maintain balance: the vestibular apparatus (head position/movement), proprioceptors (muscle, tendon, joint position), and vision (body position relative to the environment).

Pitch and Loudness

Auditory properties where frequency in Hz determines pitch (human detection range: $20\text{ to }20{,}000\text{ Hz}$), and amplitude/intensity in dB determines perceived loudness (optimal listening range: $0\text{ to }80\text{ dB}$).

Auditory Ossicles

Three middle ear bones—the malleus (attached to the tympanic membrane), incus, and stapes (attached to the oval window)—that mechanically transmit and amplify sound vibrations across the middle ear cavity to the cochlea.

Eustachian (Auditory) Tube

A passage connecting the middle ear cavity to the nasopharynx (throat) that equalizes air pressure across the tympanic membrane and assists in draining middle ear fluids.

Middle Ear Pressure Equalization

The process by which swallowing, yawning, or chewing forces open the Eustachian tube, relieving altitude-induced pressure differentials between the middle ear cavity and ambient atmosphere.

Cochlear Chambers

The three fluid-filled compartments of the cochlea: the upper scala vestibuli (bony labyrinth, perilymph), middle cochlear duct (membranous labyrinth, endolymph), and lower scala tympani (bony labyrinth, perilymph).

Cochlear Sound Vibration Pathway

The physical route of vibrations: oval window $\rightarrow$ perilymph in scala vestibuli $\rightarrow$ vestibular membrane $\rightarrow$ endolymph in cochlear duct $\rightarrow$ basilar membrane $\rightarrow$ perilymph in scala tympani $\rightarrow$ round window.

Spiral Organ (Organ of Corti)

The sensory structure for hearing seated on the basilar membrane within the cochlear duct, containing inner hair cells with stereocilia that associate with the overlying gelatinous tectorial membrane.

Auditory Hair Cell Transduction

Sound vibrations move the basilar and tectorial membranes, bending stereocilia to open $K^+$ channels; $K^+$ enters from the endolymph to depolarize the hair cell, releasing glutamate to stimulate action potentials in CN VIII sensory neurons.

Place Theory of Pitch

The auditory principle that pitch discrimination depends on the location of basilar membrane displacement: high-pitch sounds stimulate the narrow base near the oval window, whereas low-pitch sounds stimulate the wider apex.

Conduction vs. Sensorineural Deafness

Conduction deafness results from impaired sound transmission through the outer or middle ear (caused by earwax, fluid, ruptured eardrum, or otosclerosis), while sensorineural deafness involves hair cell or auditory pathway impairment (caused by loud sounds, aging, or drugs).

Pathway of Light Through the Eye

The physical route of light: cornea $\rightarrow$ anterior chamber $\rightarrow$ pupil $\rightarrow$ lens $\rightarrow$ posterior chamber $\rightarrow$ vitreous chamber $\rightarrow$ retina $\rightarrow$ absorption by the pigmented choroid.

Iris Pupillary Control

Regulation of pupil diameter by smooth muscles: parasympathetic stimulation contracts circular muscles to constrict the pupil, while sympathetic stimulation contracts radial muscles to dilate the pupil.

Crystalline Lens

An avascular, transparent optical structure consisting of layers of living epithelial cells with low anaerobic metabolism, held by suspensory ligaments to ciliary bodies to change shape for focusing light.

Aqueous Humor

A clear, watery fluid secreted by ciliary bodies into the anterior and posterior chambers that nourishes the avascular cornea and lens; impaired drainage causes elevated intraocular pressure resulting in glaucoma.

Refraction (Ocular)

The bending of light rays as they transition between media of differing optical densities, occurring most significantly at the air-cornea interface; a more convex lens increases refraction, forming an inverted and laterally reversed retinal image.

Accommodation

The process where the lens changes shape to focus objects on the retina: near vision involves ciliary muscle contraction, suspensory ligament relaxation, and a round/thick lens, while far vision involves ciliary muscle relaxation, ligament tightening, and a flat/thin lens.

Retinal Cellular Organization

The neural circuitry of the retina (a forward brain extension) transmitting signals from photoreceptors to bipolar cells and then to ganglion cells, whose axons exit at the optic disc (blind spot) to form the optic nerve (CN II).

Rods and Cones

The two classes of retinal photoreceptors: rods mediate black-and-white vision, provide high sensitivity in dim light, contain rhodopsin, and show high neural convergence; cones mediate color vision, operate in bright light, have high visual acuity, contain photopsins, and concentrate in the fovea.

Rhodopsin Bleaching

The photo-dissociation event in rods where incoming light induces 11-cis retinal to isomerize into all-trans retinal, detaching it from opsin and causing the purple photopigment to lose its color.

Dark Adaptation and Visual Cycle

The visual cycle where all-trans retinal is transported to the retinal pigment epithelium to be converted back into 11-cis retinal and recombined with opsin, gradually restoring maximal rod light sensitivity over approximately $20\text{ minutes}$ in darkness.

Dark Current

The steady influx of $Na^+$ through cGMP-gated channels in darkness, keeping photoreceptors depolarized and continuously releasing inhibitory neurotransmitter that suppresses bipolar cells.

Photoreceptor Light Transduction

The biochemical cascade where light isomerizes retinal to activate transducin (G-protein), triggering phosphodiesterase to convert cGMP to GMP; the resulting closure of $Na^+$ channels causes hyperpolarization, stopping inhibitory neurotransmitter release to disinhibit bipolar cells.

Trichromatic Theory of Color Vision

The theory that color perception is mediated by the brain comparing signals from three distinct cone types: S cones (blue, $\sim 420\text{ nm}$), M cones (green, $\sim 530\text{ nm}$), and L cones (red, $\sim 562\text{ nm}$).

Visual Acuity vs. Sensitivity

Visual acuity (greatest at the fovea centralis) provides high resolution via an approximate $1:1$ cone-to-ganglion cell ratio with displaced outer retinal layers, whereas high light sensitivity in the peripheral retina results from the convergence of many rods (up to $105:1$) onto single ganglion cells.