Neurophysiology – PNS Afferent and Sensory Physiology

Comprehensive practice flashcards covering peripheral neurophysiology, afferent pathways, reflex arcs, mechanoreception, thermoreception, pain neurobiology, olfaction, gustation, ocular anatomy, refractive optics, phototransduction, auditory processing, and equilibrioception.

Peripheral nervous system (PNS)

The division of the nervous system anatomically composed of the cranial nerves, spinal nerves, and sensory organs.

Afferent division of the PNS

The functional branch of the PNS that consists of sensory nerve fibers originating from sensory organs that convey impulses toward the CNS.

Efferent division of the PNS

The functional branch of the PNS consisting of somatic and autonomic nerve fibers that regulate skeletal muscle and subconsciously controlled organs, respectively.

Autonomic nervous system (ANS)

The involuntary efferent branch of the nervous system divided into the sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) that innervates cardiac muscle, smooth muscle, glands, and adipose, and modulates the enteric nervous system (ENS).

Enteric nervous system (ENS)

The intrinsic nervous network of the gastrointestinal tract that is innervated and modulated in its activity by the autonomic nervous system.

Exteroceptors

Sensory receptors that receive stimuli from the external environment and transmit that sensory information to the CNS.

Interoceptors

Sensory receptors that receive stimuli from the internal organs (visceral senses), sensing conditions such as stomach pain and angina.

Proprioceptors

Musculoskeletal receptors that provide information regarding joint angle, muscle length, and muscle tension to inform the CNS about the position of a limb in space.

General (somatic) senses

Subcategory of exteroceptive senses that respond to stimuli originating in the skin: pain, temperature, touch, pressure, vibration, and stretch.

Special senses

Subcategory of exteroceptive senses comprising taste (gustation), smell (olfaction), sight (vision), hearing (audition), and balance (equilibrioception) that enter the nervous system at the level of the brain via cranial nerves.

Enteroceptors

A special class of interoceptors located in the enteric nervous system that sense chemical and mechanical stimuli in the lumen of the gastrointestinal (GI) tract.

Cranial nerves

$12$ pairs of peripheral nerves originating from the brain; some are purely sensory, some are purely motor, and others are mixed.

Spinal nerves

$31$ pairs of mixed peripheral nerves formed by the convergence of the ventral and dorsal roots exiting through the intervertebral foramina.

Spinal stenosis

The narrowing of an intervertebral foramen resulting from intervertebral disc deterioration, thinning, bulging, or herniation that can lead to a pinched spinal nerve.

Intervertebral foramen

The anatomical opening between adjacent vertebrae through which spinal nerves exit the spinal cord.

Ventral root

The motor root of a spinal nerve composed of the axons of motor neurons exiting the spinal cord.

Dorsal root

The sensory root of a spinal nerve composed of the axons of sensory, unipolar neurons entering the spinal cord.

Dorsal root ganglion

An enlargement situated along the dorsal root that contains the cell bodies of sensory unipolar neurons.

Ventral ramus

The anterior branch or division of a spinal nerve.

Dorsal ramus

The posterior branch or division of a spinal nerve.

Sympathetic chain ganglia

Aggregations of autonomic nerve cell bodies that run longitudinally from the base of the skull to the coccyx as part of the autonomic nervous system.

Precentral gyrus

Cortical gyrus located in the frontal lobe that contains the primary motor cortex.

Postcentral gyrus

Cortical gyrus located in the parietal lobe that contains the primary somatosensory cortex.

Primary motor pathway

Pathway where motor neurons project from the primary motor cortex down descending spinal tracts, synapse in anterior gray matter with motor neurons, and exit through the ventral root to reach muscles.

Somatosensory pathway

Pathway where peripheral sensory input enters the spinal cord through the dorsal root, synapses with an interneuron in the posterior gray horn, and ascends via spinal tracts to the primary somatosensory cortex.

Primary motor homunculus

A brain map of the primary motor cortex where the size of representation for each body part is proportional to the degree of fine motor control and innervation it requires rather than physical size.

Primary somatosensory homunculus

A brain map of the primary somatosensory cortex where representation size is proportional to the density of sensory receptors and regional sensitivity rather than physical size.

Spinal reflex arc

A rapid, involuntary circuit involving an afferent sensory neuron, an interneuron in the spinal cord gray matter, and an efferent motor neuron that withdraws a muscle without initial regulation by the brain.

Interneuron (reflex arc)

A spinal neuron within the gray matter that receives sensory input from afferent neurons and transmits chemical signals via neurotransmitters across synaptic clefts to motor neurons.

Reflex pain perception

The physiological observation that conscious perception of pain in the brain occurs shortly after the hand or limb is reflexively withdrawn from a noxious stimulus.

Mechanoreceptors

Sensory receptors that respond to mechanical stimuli, including tactile receptors, baroreceptors, and proprioceptors.

Meissner's corpuscles

Cutaneous tactile mechanoreceptors specialized for sensing light touch.

Merkel cells

Cutaneous tactile mechanoreceptors specialized for sensing light touch.

Pacinian corpuscles

Cutaneous mechanoreceptors located deep in the skin specialized for sensing deep touch, pressure, and vibration.

Piezo channels

Mechanically-gated ion channels present in mechanoreceptors such as Merkel cells, Meissner's corpuscles, Pacinian corpuscles, Ruffini corpuscles, and hair follicle receptors.

Ruffini corpuscles

Cutaneous mechanoreceptors equipped with Piezo channels that detect skin stretch and pressure.

Hair follicle receptors

Mechanoreceptors wrapped around hair follicles that utilize Piezo channels to detect deflection of hair.

TRPM channels

Transient receptor potential channels found on cold thermoreceptors that are activated by cool temperatures, alcohol, and menthol.

TRPV channels

Transient receptor potential channels found on hot thermoreceptors that are activated by warmth, capsaicin, and camphor.

TRPA channels

Transient receptor potential channels activated by noxious cold stimuli (such as an ice cube) and chemical activators like wasabi.

Capsaicin

A chemical activator that stimulates TRPV channels to produce a perception of heat without altering actual skin temperature.

Wasabi

A chemical activator that stimulates TRPA channels, evoking a perception of noxious cold without altering skin temperature.

Menthol

A chemical activator that stimulates TRPM channels, producing a cooling perception without lowering skin temperature.

Camphor

A chemical activator that stimulates TRPV channels to produce a perception of warmth.

Vicks VapoRub sensory response

Topical product whose application to the chest simultaneously activates both TRPV and TRPM thermoreceptors.

Tonic receptors

Slowly adapting sensory receptors, such as nociceptors, that maintain action potential firing throughout the duration of a sustained stimulus.

Phasic receptors

Rapidly adapting sensory receptors, such as tactile receptors and odorant receptors, that quickly cease firing when a stimulus remains constant.

Nociceptors

Slowly adapting (tonic) sensory receptors responsible for detecting noxious, tissue-damaging stimuli and signaling pain.

Prostaglandins

Inflammatory chemical mediators that sensitize pain receptors, whose activity is reduced by cyclooxygenase (COX) inhibitors.

NSAIDs (Nonsteroidal anti-inflammatory drugs)

Cyclooxygenase (COX) inhibitors, such as ibuprofen and aspirin, that diminish prostaglandin synthesis to reduce peripheral pain and inflammation.

Acetaminophen (Tylenol)

A cyclooxygenase (COX) inhibitor that acts centrally within the CNS to reduce pain and fever.

Antihistamines

Medications such as Benadryl and Zyrtec taken to diminish the activity and inflammatory effects of histamine.

Substance P

A neuropeptide neurotransmitter involved in pain transmission whose activity can be depleted or reduced by capsaicin.

Opiates

Analgesic drugs such as morphine and heroin that induce profound analgesia in the CNS by chemically resembling endogenous endorphins.

Endorphins

Endogenous opioid peptides naturally produced in the CNS that act as natural pain inhibitors.

Suzetrigine (Journavx)

A non-addictive systemic pain therapeutic that selectively blocks voltage-gated sodium channels located specifically in peripheral nociceptors.

Lidocaine

A non-selective local anesthetic that non-specifically blocks voltage-gated sodium channels to inhibit action potentials.

Referred pain

The clinical phenomenon wherein visceral pain is felt in a somatic area due to neural crosstalk and convergence at the level of the spinal cord.

Dorsal horn crosstalk

The convergence of visceral and somatic sensory pain fibers onto the same secondary neurons within the dorsal horn gray matter of the spinal cord.

Bipolar olfactory neuron

The single primary sensory neuron type involved in the peripheral processing of olfactory information in the olfactory epithelium.

Dendrite membranes of olfactory neurons

The specific anatomical locations within the olfactory epithelium where odorant receptor proteins are embedded.

Olfactory basal cells

Stem cells located within the olfactory epithelium that divide and regenerate bipolar olfactory neurons.

Sustentacular cells

Glial-like support cells that surround, nourish, and insulate bipolar olfactory neurons in the olfactory epithelium.

Olfactory foramina

Tiny perforations in the floor of the cranium through which bipolar olfactory axons pass to synapse in the olfactory bulbs.

Thalamic bypass of olfaction

Anatomical organization whereby primary olfactory neurons enter olfactory bulbs directly through the cranial floor, bypassing an initial relay through the thalamus.

Bowman's glands

Olfactory glands that secrete serous fluid to dissolve odorants and wash them off receptors to reset the olfactory epithelium for novel stimuli.

Taste receptor cell

A non-neuronal epithelial sensory cell bearing tastant receptors that relays chemical gustatory input to bipolar sensory neurons.

Bitter taste tastants

Chemical compounds including coffee, beer, unsweetened cocoa, and earwax that stimulate bitter tastant receptors.

Sweet taste tastants

Carbohydrate molecules, specifically monosaccharides, that activate sweet tastant receptors.

Savory (umami) tastant

The amino acid glutamate, which serves as the specific tastant triggering savory or umami taste.

Sour taste tastant

Hydrogen ions or protons ($H^+$) that trigger sour taste receptor cells.

Salty taste tastant

Sodium ions ($Na^+$) that stimulate salty taste receptor cells.

Spiciness perception

A somatic thermal and pain sensation received by unipolar sensory neurons rather than bipolar gustatory neurons.

Flavor

A complex perceptual experience synthesized largely from the multimodal integration of both taste and smell.

Sclera

The tough, dense connective tissue outer shell (parietal layer) of the eyeball that preserves shape and is visible anteriorly as the 'white' of the eye.

Choroid

The vascular middle layer of the eyeball composed of loose connective tissue and blood vessels that extends anteriorly to form the ciliary body and iris.

Retina

The innermost sensory layer of the eyeball composed of photoreceptor cells (rods and cones), bipolar sensory neurons, and ganglion cells.

Ciliary body

An anterior vascular and muscular extension of the choroid that suspends the lens via suspensory ligaments and regulates its curvature.

Suspensory ligaments

Zonular fibers connecting the ciliary body to the lens capsule that transfer tension to change lens thickness.

Lens capsule

The elastic outer covering surrounding the ocular lens composed of type IV collagen.

Lens core

The central body of the lens composed of anucleate, organelle-lacking lens fiber cells packed with soluble crystallin proteins.

Crystallin proteins

Small, soluble proteins packed within lens fiber cells that provide the lens with its high refractive index and converging power.

Aqueous humor

Clear fluid in the anterior chamber of the eye that nourishes the cornea and lens with glucose and amino acids and maintains intraocular pressure.

Vitreous humor (vitreous body)

A gelatinous mass in the posterior chamber that allows light to reach the retina and functions as an internal hydrostatic skeleton to keep the eyeball spherical.

Glaucoma

A group of ocular diseases that cause progressive vision loss through permanent optic nerve damage, often due to high intraocular pressure from aqueous humor accumulation.

Visual axis

An imaginary straight line that passes directly through the center of the pupil and lands on the fovea centralis.

Fovea centralis

A central retinal pit within the macula lutea with displaced inner layers and a high density of cones ($30\%$ to $35\%$ of all cones) providing maximum visual acuity.

Macula lutea

A yellowish retinal region surrounding the fovea centralis that contains carotenoid pigments to absorb high-energy blue light and protect against photodamage.

Optic disc (blind spot)

The exit site for retinal ganglion cell axons forming the optic nerve, completely devoid of photoreceptors.

Refraction

The bending of light rays as they pass across interfaces between media with differing refractive indices.

Refractive index

A numerical value that indicates the degree to which a medium bends light rays, where higher numbers produce greater bending.

Refractive index of air

The optical refractive index of air, which has an exact baseline value of $1.00$.

Refractive index of water

The optical refractive index of water, which is equal to $1.33$.

Refractive index of the cornea

The optical refractive index of the cornea, which is equal to $1.38$.

Refractive index of the lens

The optical refractive index of the crystalline lens, which is equal to $1.41$.

Optical convergence

The inward bending of light rays toward the visual axis, which occurs when light passes from a lower to a higher refractive index medium.

Optical divergence

The outward bending of light rays away from the visual axis, which occurs when light passes from a higher to a lower refractive index medium.

Underwater vision blur

Visual blurring occurring underwater because the refractive indices of water ($1.33$) and cornea ($1.38$) are too close, preventing sufficient convergence and placing the focal point behind the retina.

Swimming goggles optical effect

The restoration of clear vision underwater by preserving an air-corneal interface that reinstates strong optical convergence at the cornea.

Convex lens

An outwardly curved lens, such as the human lens, that converges incoming light rays inward toward the visual axis.