# Lecture 6: Sensory Physiology ## Unit 1.6: Physiology Foundations --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the general principles of sensory transduction and coding 2. Explain receptor adaptation and its functional significance 3. Describe the physiology of somatosensory receptors 4. Explain pain pathways and pain modulation mechanisms 5. Describe the physiology of special senses (vision, hearing, balance, taste, smell) 6. Apply sensory physiology concepts to clinical scenarios --- ## General Principles of Sensory Systems Sensory systems enable organisms to detect and respond to their environment by converting diverse physical and chemical stimuli into the common language of neural signals. Every sensory system shares fundamental organizational principles while specialized adaptations optimize detection of particular stimulus modalities. Four components characterize all sensory systems. Receptors transduce stimulus energy into electrical signals (receptor potentials). Afferent pathways transmit these signals from receptors to the central nervous system. Central processing occurs in hierarchically organized brain regions that extract increasingly complex features from the incoming information. Perception emerges as the conscious experience of the stimulus, shaped by attention, memory, and context. Sensory transduction is the critical first step, converting stimulus energy into receptor potentials. Each receptor type responds optimally to its adequate stimulus—the particular form of energy to which it is most sensitive. Photoreceptors respond to electromagnetic radiation in the visible spectrum, mechanoreceptors respond to mechanical forces, chemoreceptors respond to specific molecules, and thermoreceptors respond to temperature changes. When stimulus intensity is sufficient, the receptor potential triggers action potentials in afferent neurons that carry information to the CNS. Receptors can be classified by their location or by the type of stimulus they detect. Exteroceptors detect stimuli from the external environment, including cutaneous receptors in skin and the special senses (vision, hearing, taste, smell). Interoceptors monitor the internal environment, detecting changes in blood pressure, blood chemistry, and visceral organ distension. Proprioceptors detect body position and movement through sensors in muscles, joints, and the vestibular apparatus. Panel A: General sensory pathway from stimulus through receptor transduction, afferent transmission, CNS processing, to perception. Panel B: Receptor classification by location including exteroceptors, interoceptors, and proprioceptors with examples. Panel C: Receptor classification by stimulus type including mechanoreceptors, thermoreceptors, nociceptors, chemoreceptors, and photoreceptors. Panel D: Receptor potential generation leading to action potential propagation when threshold is exceeded. --- ## Sensory Coding The nervous system must encode multiple aspects of each stimulus—what type it is, how intense it is, where it is located, and how long it lasts. Different coding mechanisms address each of these dimensions. Stimulus modality—whether the sensation is touch, temperature, pain, light, or sound—is encoded through the labeled line principle. Each receptor type connects to dedicated neural pathways projecting to specific brain regions. Activation of the pathway always produces the same type of sensation regardless of how it is stimulated. This explains why pressing on the eye produces the perception of light (phosphenes) rather than pressure—the visual pathway interprets any input as light. Stimulus intensity is encoded primarily through two mechanisms. Frequency coding relates stimulus strength to action potential firing rate: stronger stimuli produce more rapid firing. Population coding relates stimulus strength to the number of receptors activated: stronger stimuli recruit more receptors and activate more afferent neurons. These mechanisms operate together—a strong stimulus both increases firing rate in individual neurons and activates a larger population of neurons. Stimulus location is encoded through receptive fields and enhanced by lateral inhibition. A receptive field is the area of sensory surface that, when stimulated, changes the firing of a particular sensory neuron. Smaller receptive fields provide better spatial resolution. Lateral inhibition—where activated neurons inhibit their neighbors—sharpens the contrast between stimulated and unstimulated regions, enhancing the ability to localize stimuli precisely. Two-point discrimination, the minimum distance at which two stimuli can be perceived as separate, reflects receptive field size and is finest on the fingertips and lips, coarsest on the back. Stimulus duration is encoded through temporal patterns of receptor firing, influenced by receptor adaptation. Slowly adapting receptors continue firing throughout a sustained stimulus, providing ongoing information about stimulus presence. Rapidly adapting receptors fire mainly at stimulus onset and offset, signaling changes rather than steady states. Panel A: Labeled line principle showing different receptor types connecting to dedicated pathways producing specific sensations. Panel B: Frequency coding demonstrating stimulus intensity encoded as action potential firing rate. Panel C: Receptive fields on skin showing overlapping fields and two-point discrimination resolution. Panel D: Lateral inhibition enhancing spatial contrast through activated neurons inhibiting their neighbors. --- ## Receptor Adaptation Receptor adaptation refers to the decrease in receptor response during sustained stimulation. This ubiquitous phenomenon allows sensory systems to remain sensitive to changes while ignoring constant background stimuli. The rate and extent of adaptation vary among receptor types, reflecting their functional roles. Slowly adapting (tonic) receptors maintain their response throughout a sustained stimulus, providing continuous information about ongoing conditions. Muscle spindles must continuously report muscle length for postural control and cannot afford to adapt away that information. Merkel disc receptors in skin slowly adapt, enabling sustained perception of objects in contact with the skin. These receptors signal stimulus magnitude and duration. Rapidly adapting (phasic) receptors respond strongly at stimulus onset and offset but cease firing during sustained stimulation. Pacinian corpuscles respond to vibration because they adapt so rapidly that only oscillating stimuli can maintain their response. Meissner corpuscles adapt rapidly to light touch, making them ideal for detecting movement across the skin. These receptors signal stimulus change rather than steady state, drawing attention to new or dynamic events. Adaptation occurs through multiple mechanisms depending on the receptor type. Ion channel inactivation reduces transduction current even with maintained mechanical or chemical stimulation. Calcium-dependent processes can activate potassium channels that oppose depolarization. Mechanical properties of receptor structures can filter out sustained stimuli—the onion-like lamellae of Pacinian corpuscles absorb constant pressure but transmit vibrations. The functional significance of adaptation is profound. It prevents saturation of sensory pathways by constant stimuli, freeing neural processing capacity for novel or changing conditions. It explains common perceptual phenomena: we quickly become unaware of the pressure of clothing, cease to smell persistent odors, and don't notice the constant pressure of spectacles on the nose. Adaptation reflects the general principle that nervous systems prioritize change detection over static monitoring. Panel A: Slowly adapting receptor response showing sustained depolarization and continuous firing throughout stimulus duration. Panel B: Rapidly adapting receptor response showing brief depolarization and firing only at stimulus onset and offset. Panel C: Structural features of slowly adapting receptors like Merkel discs and muscle spindles. Panel D: Lamellar structure of Pacinian corpuscles as mechanical basis for rapid adaptation filtering sustained stimuli. --- ## Somatosensory System The somatosensory system provides information about stimuli contacting the body surface and about body position and movement. Cutaneous mechanoreceptors in the skin detect different aspects of touch and pressure, each optimized for particular stimulus features. Merkel disc receptors lie in the basal epidermis at the dermal-epidermal junction, particularly concentrated in fingertips. These slowly adapting receptors with small receptive fields respond to fine touch and texture, encoding the spatial details of objects in contact with the skin. Their sustained response allows continued perception of held objects. Meissner corpuscles occupy dermal papillae of glabrous (hairless) skin, particularly abundant in fingertips, lips, and other areas requiring fine tactile discrimination. These rapidly adapting receptors with small receptive fields detect light touch and low-frequency flutter (30-50 Hz), responding particularly to moving stimuli and edges. Their rapid adaptation makes them ideal for detecting texture changes as fingers scan across surfaces. Ruffini endings lie deep in the dermis and have large receptive fields. These slowly adapting receptors respond to skin stretch and sustained pressure, contributing to perception of hand and finger position. They continue firing during maintained stretch, providing proprioceptive information from the skin. Pacinian corpuscles are located deep in the dermis and subcutaneous tissue. Their distinctive layered capsule produces extreme rapid adaptation, making them exquisitely sensitive to vibration (200-300 Hz) and deep pressure. Each corpuscle has a large receptive field, limiting spatial resolution but maximizing sensitivity to transmitted vibrations. Thermoreceptors detect skin temperature through cold receptors that increase firing as temperature falls below approximately 35°C and warm receptors that increase firing as temperature rises above approximately 30°C. The overlapping ranges allow detection of absolute temperature through the pattern of cold and warm receptor activity. Static temperatures are encoded by the steady firing rate; temperature changes are encoded by transient rate changes during adaptation. Proprioceptors provide information about body position and movement. Muscle spindles detect muscle length and rate of length change, providing essential information for movement control. Golgi tendon organs detect muscle tension. Joint receptors contribute to joint position sense. These receptors enable the sense of proprioception—knowing where body parts are without looking—essential for coordinated movement. Two major ascending pathways carry somatosensory information to the brain. The dorsal column-medial lemniscal pathway carries fine touch, vibration, and proprioception; its first-order neurons have long axons that ascend ipsilaterally in the dorsal columns before synapsing in the medulla, where second-order neurons cross and ascend to the thalamus. The anterolateral (spinothalamic) pathway carries pain, temperature, and crude touch; its first-order neurons synapse in the spinal cord, and second-order neurons cross immediately before ascending to the thalamus. Panel A: Superficial mechanoreceptors including Merkel discs in epidermis and Meissner corpuscles in dermal papillae with small receptive fields. Panel B: Deep mechanoreceptors including Ruffini endings in dermis and Pacinian corpuscles in subcutis with larger receptive fields. Panel C: Adaptation rates and primary stimuli for each receptor type supporting different tactile functions. Panel D: Ascending somatosensory pathways including dorsal column-medial lemniscal and anterolateral spinothalamic tracts to thalamus and cortex. --- ## Pain Physiology Pain serves as a protective mechanism, alerting organisms to actual or potential tissue damage and motivating withdrawal and healing behaviors. Understanding pain pathways reveals opportunities for therapeutic intervention and explains clinical phenomena including referred pain and chronic pain syndromes. Nociceptors are the primary sensory receptors for pain, detecting noxious mechanical, thermal, and chemical stimuli. Aδ fibers are lightly myelinated axons conducting at 12-30 m/s, mediating fast pain—the immediate, sharp, well-localized sensation that triggers withdrawal reflexes. C fibers are unmyelinated axons conducting slowly at 0.5-2 m/s, mediating slow pain—the delayed, dull, diffuse, burning sensation that outlasts the stimulus and promotes guarding behavior. Polymodal nociceptors, the most common type, respond to mechanical, thermal, and chemical stimuli. The pain pathway follows a three-neuron sequence. First-order neurons have cell bodies in the dorsal root ganglia and synapse in the spinal cord dorsal horn. Second-order neurons cross to the contralateral side and ascend in the spinothalamic tract to the thalamus. Third-order neurons project from the thalamus to the somatosensory cortex, where pain is localized and its intensity perceived, and to the anterior cingulate cortex, where its emotional and motivational aspects are processed. Referred pain occurs when visceral pain is perceived at a somatic location. The mechanism involves convergence of visceral and somatic afferents onto shared second-order neurons in the spinal cord. Because somatic pain is more common in experience, the brain misattributes the source to the somatic territory. Classic examples include cardiac ischemia producing left arm and jaw pain, and gallbladder disease producing right shoulder pain. The gate control theory, proposed by Melzack and Wall, explains how touch can modulate pain perception. According to this model, large-diameter Aβ fibers carrying touch information activate inhibitory interneurons in the spinal cord dorsal horn that reduce transmission of pain signals from C fibers. This explains why rubbing an injured area reduces pain and provides the rationale for transcutaneous electrical nerve stimulation (TENS) therapy. Descending pain modulation originates from the periaqueductal gray (PAG) in the midbrain and projects through the rostral ventromedial medulla (including the raphe nuclei) to the spinal cord dorsal horn. These descending pathways release serotonin and norepinephrine that inhibit pain transmission, explaining why certain antidepressants provide analgesia. Endogenous opioid peptides (endorphins, enkephalins, dynorphins) act at multiple levels to reduce pain, and understanding their action guides opioid analgesic therapy. Sensitization can increase pain sensitivity following injury or inflammation. Peripheral sensitization occurs when inflammatory mediators (prostaglandins, bradykinin, histamine) lower the threshold of nociceptors, producing hyperalgesia (increased pain from normally painful stimuli) and allodynia (pain from normally innocuous stimuli). Central sensitization involves increased excitability of spinal cord neurons (wind-up phenomenon), contributing to chronic pain states. Panel A: Nociceptor fiber types including fast myelinated A-delta fibers for sharp pain and slow unmyelinated C fibers for dull pain. Panel B: Three-neuron pain pathway from peripheral receptor through dorsal horn and spinothalamic tract to thalamus and cortex. Panel C: Referred pain mechanism through convergence of visceral and somatic afferents on shared spinal neurons. Panel D: Gate control and descending modulation showing touch-activated inhibition and PAG-raphe-spinal cord opioid pathways. --- ## Vision The eye functions as an optical system focusing light onto the photoreceptor-containing retina. The cornea provides approximately two-thirds of the eye's refractive power through its curved surface separating air from the aqueous humor. The lens provides the remaining one-third and can change shape through accommodation to focus on near or distant objects. The iris controls pupil size, regulating light entry. Two types of photoreceptors in the retina—rods and cones—have distinct properties adapted for different lighting conditions. Rods, numbering approximately 120 million per retina, are exquisitely sensitive to low light levels and mediate scotopic (night) vision. They are concentrated in the peripheral retina and exhibit high convergence (many rods synapsing onto each ganglion cell), maximizing sensitivity at the cost of spatial resolution. Rods cannot distinguish colors—scotopic vision is achromatic. Cones, numbering approximately 6 million per retina, require higher light levels and mediate photopic (day) vision. They are concentrated in the fovea, the region of highest acuity, where each cone may connect to a dedicated ganglion cell (low convergence). Three types of cones (short-wavelength/blue, medium-wavelength/green, long-wavelength/red) enable color vision through comparison of their relative activation by different wavelengths. Phototransduction in rods and cones involves a unique cascade that produces hyperpolarization in response to light. In darkness, cyclic GMP (cGMP) keeps cation channels open, maintaining depolarization and continuous glutamate release. Light causes photoisomerization of retinal in the photopigment (rhodopsin in rods), activating the G-protein transducin. Activated transducin stimulates phosphodiesterase, which hydrolyzes cGMP, causing cGMP-gated channels to close. The resulting hyperpolarization reduces glutamate release, signaling light detection to postsynaptic bipolar cells. The visual pathway begins with retinal processing through bipolar and ganglion cells, then continues via the optic nerve. At the optic chiasm, fibers from the nasal hemiretinas cross to the contralateral side while temporal hemiretina fibers remain ipsilateral. This arrangement ensures that each hemisphere receives information from the contralateral visual field. The optic tracts project to the lateral geniculate nucleus (LGN) of the thalamus, from which the optic radiations carry information to the primary visual cortex (V1) in the occipital lobe. Panel A: Eye anatomy showing corneal refraction, lens accommodation, and retinal structure with fovea for high acuity vision. Panel B: Photoreceptor structure comparing rods for scotopic vision and cones for photopic color vision. Panel C: Phototransduction cascade from light-induced retinal isomerization through cGMP reduction to photoreceptor hyperpolarization. Panel D: Visual pathway from retina through optic chiasm to lateral geniculate nucleus and primary visual cortex with characteristic lesion patterns. --- ## Hearing Hearing enables detection and analysis of sound waves through an elegant mechanism of mechanical amplification and frequency analysis. Sound waves collected by the outer ear cause vibration of the tympanic membrane (eardrum), which transmits these vibrations through the three ossicles of the middle ear—malleus, incus, and stapes. This ossicular chain amplifies sound pressure approximately 20-fold through lever action and the area ratio between the large tympanic membrane and the small oval window. The cochlea, a snail-shaped structure in the inner ear, contains the sensory apparatus for hearing. Sound vibrations transmitted to the oval window create traveling waves in the cochlear fluids that propagate along the basilar membrane. The mechanical properties of the basilar membrane vary systematically along its length—stiff and narrow at the base, flexible and wide at the apex. This tonotopic organization means that high-frequency sounds produce maximal vibration near the base while low-frequency sounds produce maximal vibration near the apex. Hair cells are the mechanoreceptors of hearing, located in the organ of Corti atop the basilar membrane. When the basilar membrane vibrates, stereocilia projecting from the hair cell apex are deflected by contact with the overlying tectorial membrane. Deflection toward the tallest stereocilia stretches tip links connecting adjacent stereocilia, opening mechanically-gated potassium channels. Because the endolymph bathing the stereocilia has an unusually high potassium concentration (approximately 150 mM), potassium actually enters the cell down its electrochemical gradient, depolarizing the hair cell. This depolarization opens voltage-gated calcium channels, triggering glutamate release onto afferent fibers of the spiral ganglion. The auditory pathway is complex, with information crossing at multiple levels. The cochlear nerve projects to the cochlear nuclei in the brainstem. From there, fibers ascend through the superior olivary complex, where binaural comparison begins to enable sound localization. The pathway continues through the lateral lemniscus to the inferior colliculus, then to the medial geniculate nucleus (MGN) of the thalamus, and finally to the primary auditory cortex in the temporal lobe. Tonotopic organization is maintained throughout the pathway. Panel A: Sound transmission through outer ear, tympanic membrane, and ossicular chain amplification to the oval window. Panel B: Cochlear tonotopic organization with basilar membrane responding to high frequencies at base and low frequencies at apex. Panel C: Hair cell mechanotransduction through stereocilia deflection, tip link tension, and potassium channel opening causing depolarization. Panel D: Ascending auditory pathway through cochlear nuclei, superior olivary complex, inferior colliculus, and medial geniculate to auditory cortex. --- ## Vestibular System The vestibular system detects head motion and position relative to gravity, providing essential information for balance, postural control, and gaze stabilization during head movement. Located in the inner ear adjacent to the cochlea, the vestibular apparatus comprises two types of sensors: the semicircular canals that detect rotational (angular) acceleration and the otolith organs that detect linear acceleration and static head tilt. The three semicircular canals—horizontal (lateral), anterior, and posterior—lie in three roughly orthogonal planes, enabling detection of rotation about any axis. Each canal contains endolymph that moves relative to the canal wall during rotation due to inertia. This fluid movement deflects the cupula, a gelatinous structure spanning the ampulla at one end of each canal. Hair cells embedded in the cupula are deflected by this movement. Rotation toward the canal causes cupula deflection that depolarizes hair cells on one side while hyperpolarizing matched hair cells in the corresponding canal of the opposite ear. This push-pull arrangement doubles sensitivity and provides directional information. Hair cells in both vestibular and auditory systems share the same transduction mechanism: stereocilia deflection opens mechanically-gated channels. In vestibular hair cells, deflection toward the single kinocilium (the tallest process) produces depolarization, while deflection away produces hyperpolarization. This bidirectional response allows detection of both direction and magnitude of movement. The otolith organs—the utricle and saccule—detect linear acceleration and static head tilt. Hair cells in these organs are embedded in a gelatinous matrix topped by otoconia, small calcium carbonate crystals denser than the surrounding fluid. During linear acceleration or head tilt, the otoconia lag or shift under gravity's influence, deflecting the hair cells. The utricle primarily detects horizontal linear acceleration and head tilt in the sagittal plane, while the saccule primarily detects vertical linear acceleration and head tilt in the coronal plane. The vestibulo-ocular reflex (VOR) maintains stable gaze during head movement by producing compensatory eye movements in the opposite direction. When the head rotates right, the VOR rotates the eyes left at the same velocity, keeping the visual image stable on the retina. This reflex operates with extremely short latency (approximately 10 ms), ensuring that even rapid head movements do not blur vision. Damage to the vestibular system impairs the VOR, causing oscillopsia (visual blurring during head movement). The vestibulospinal reflexes contribute to postural control by adjusting limb and trunk muscle activity in response to detected body motion or tilt. These reflexes help maintain balance during standing and walking and support righting responses when balance is disturbed. Panel A: Vestibular apparatus structure with three semicircular canals in orthogonal planes plus utricle and saccule otolith organs. Panel B: Semicircular canal function showing endolymph flow deflecting the cupula and embedded hair cells during rotation. Panel C: Hair cell directional response with depolarization toward kinocilium and hyperpolarization away from it. Panel D: Otolith organs detecting linear acceleration and head tilt through otoconia displacement and vestibulo-ocular reflex maintaining stable gaze. --- ## Chemical Senses The chemical senses—taste and smell—detect molecular stimuli providing information about food quality and environmental chemicals. Despite their distinct receptor mechanisms and neural pathways, these systems interact closely in the perception of flavor. Taste (gustation) detects five primary qualities. Sweet taste, detected by T1R2/T1R3 G-protein coupled receptors, indicates energy-rich carbohydrates. Salty taste, detected by epithelial sodium channels (ENaC), signals sodium important for electrolyte balance. Sour taste, detected by hydrogen ion-sensitive channels, warns of spoilage or unripe fruit. Bitter taste, detected by the T2R family of approximately 25 different GPCRs, alerts to potential toxins—the large receptor family reflects the chemical diversity of plant alkaloids and other toxic compounds. Umami taste, detected by T1R1/T1R3 GPCRs, signals glutamate and other amino acids indicating protein content. Taste receptor cells reside in taste buds distributed across the tongue and palate. Different receptor types are expressed in different cells within each taste bud, not segregated by tongue region as earlier models suggested. When taste molecules bind receptors, intracellular signaling leads to ATP release that activates afferent nerve fibers. Cranial nerves VII (anterior two-thirds of tongue), IX (posterior one-third of tongue), and X (epiglottis and pharynx) carry taste information to the nucleus of the solitary tract in the brainstem, then to the ventral posteromedial (VPM) nucleus of the thalamus, and finally to the gustatory cortex in the insula. Smell (olfaction) detects volatile molecules and provides the richest information about environmental chemistry. Approximately 400 functional olfactory receptor types in humans (from a larger family of ~1,000 genes) enable discrimination of thousands of distinct odors through combinatorial coding—each odorant activates a unique pattern of receptor types. Olfactory receptor neurons in the olfactory epithelium are true neurons that generate action potentials. Their cilia extend into the nasal mucus, where odorant binding to GPCRs activates a dedicated G-protein (Golf) that stimulates adenylyl cyclase. The resulting cAMP increase opens cyclic nucleotide-gated (CNG) channels, causing depolarization. Olfactory receptor neurons are remarkable for their ongoing neurogenesis throughout life, with new neurons replacing those damaged by environmental exposure. The olfactory pathway is unique among sensory systems in not relaying through the thalamus before reaching cortex. Olfactory receptor neuron axons project through the cribriform plate to the olfactory bulb, where they synapse with mitral cells in structures called glomeruli. Each glomerulus receives input from neurons expressing the same receptor type. Mitral cell axons project in the olfactory tract directly to the piriform cortex (primary olfactory cortex) and to the amygdala and entorhinal cortex, explaining the strong emotional and memory associations of smell. Panel A: Taste bud structure with receptor cells for five basic tastes and their GPCR or ion channel mechanisms. Panel B: Taste pathway from tongue through cranial nerves VII, IX, X to nucleus solitarius, thalamus, and gustatory cortex. Panel C: Olfactory epithelium with receptor neurons, transduction cascade through Golf and cAMP to CNG channels. Panel D: Olfactory pathway from bulb to piriform cortex, amygdala, and entorhinal cortex bypassing thalamic relay. --- ## Clinical Applications Diabetic neuropathy exemplifies how metabolic disease produces sensory dysfunction. Chronic hyperglycemia damages peripheral nerves through multiple mechanisms including polyol accumulation, oxidative stress, and microvascular ischemia. Patients typically develop a "stocking-glove" distribution of sensory loss beginning in the toes and ascending, reflecting the vulnerability of long axons with their extensive metabolic demands. Loss of protective sensation predisposes to unnoticed trauma and the development of diabetic foot ulcers. Neuropathic pain—burning, shooting, or lancinating—may accompany or precede sensory loss, reflecting abnormal firing of damaged sensory neurons. Pain disorders present significant clinical challenges. Neuropathic pain results from damage to pain pathways themselves rather than activation by noxious stimuli, producing characteristic burning or shooting sensations that respond poorly to conventional analgesics. Phantom limb pain following amputation reflects reorganization of somatosensory cortex and spontaneous activity in remaining neurons. Complex regional pain syndrome (CRPS) involves severe pain, autonomic dysfunction, and trophic changes disproportionate to any initiating injury. Fibromyalgia features widespread pain with evidence of central sensitization, including hyperalgesia and allodynia. Visual field defects localize lesions along the visual pathway. Complete optic nerve lesions produce monocular blindness. Optic chiasm lesions affecting the crossing nasal fibers produce bitemporal hemianopia—loss of the temporal visual fields of both eyes. Optic tract lesions produce contralateral homonymous hemianopia—loss of the contralateral visual field of both eyes. More posterior lesions produce variations including quadrantanopias depending on which fibers of the optic radiations are affected. Hearing loss divides into two main categories distinguished by clinical testing. Conductive hearing loss results from outer or middle ear pathology that impairs sound transmission to the cochlea—causes include cerumen impaction, otitis media, and ossicular fixation. In conductive loss, the Weber test (tuning fork on vertex) lateralizes to the affected ear, and the Rinne test shows bone conduction better than air conduction on the affected side. Sensorineural hearing loss results from cochlear or neural pathology—causes include noise exposure, presbycusis (age-related), Meniere's disease, and acoustic neuroma. In sensorineural loss, Weber lateralizes to the unaffected ear, and Rinne shows air conduction better than bone conduction bilaterally. Panel A: Diabetic neuropathy showing stocking-glove sensory loss distribution and diabetic foot ulcer from loss of protective sensation. Panel B: Visual field defects corresponding to lesions at optic nerve, chiasm, tract, radiation, and cortex locations. Panel C: Weber and Rinne tuning fork tests distinguishing conductive from sensorineural hearing loss. Panel D: Dermatome map for sensory level determination in spinal cord lesion localization. --- ## Summary Sensory systems convert stimulus energy into neural signals through transduction at specialized receptors. The labeled line principle explains modality specificity, while frequency and population coding encode stimulus intensity. Receptive fields and lateral inhibition enable precise stimulus localization. Rapidly adapting receptors signal change while slowly adapting receptors provide continuous information. The somatosensory system employs cutaneous mechanoreceptors (Merkel, Meissner, Ruffini, Pacinian), thermoreceptors, and proprioceptors. Pain involves Aδ fibers (fast, sharp) and C fibers (slow, dull), with modulation by gate control mechanisms and descending pathways from the periaqueductal gray. Vision relies on rods for scotopic (dim light) vision and cones for photopic (color) vision, with phototransduction producing hyperpolarization through a cGMP cascade. The visual pathway includes the optic nerve, chiasm (nasal fibers crossing), tract, lateral geniculate nucleus, and primary visual cortex. Hearing depends on tonotopic organization of the basilar membrane and hair cell transduction through mechanically-gated potassium channels. The vestibular system uses semicircular canals for rotational acceleration detection and otolith organs for linear acceleration and head tilt. Taste recognizes five primary qualities through GPCRs (sweet, bitter, umami) and ion channels (salty, sour). Smell employs approximately 400 receptor types for combinatorial odor coding, with a unique pathway bypassing the thalamus. --- ## Key Terms | Term | Definition | |------|------------| | Sensory transduction | Conversion of stimulus energy into receptor potential and neural signals | | Receptive field | Area of sensory surface that influences firing of a particular sensory neuron | | Adaptation | Decreased receptor response to sustained stimulation | | Gate control theory | Model explaining how touch inhibits pain transmission via spinal cord interneurons | | Phototransduction | Light-induced cascade producing photoreceptor hyperpolarization through cGMP reduction | | Tonotopy | Spatial organization of auditory structures by sound frequency | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*