Special Senses and Neurophysiology

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Special Senses and Neurophysiology — Key Facts for FMGE Core concept: Vision, hearing, balance, taste, and smell involve specialized receptor cells that convert specific stimuli into neural signals High-yield point: The retina uses specialized photoreceptor cells (rods for dim light, cones for color and detail); the cochlea contains the organ of Corti for hearing ⚡ Exam tip: Know the pathway of visual processing from retina to visual cortex; understand how the semicircular canals detect angular acceleration for balance

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Special Senses and Neurophysiology — FMGE Study Guide

Vision (Visual System)

Eye Structure

Layers of the eye:

• Fibrous tunic: Cornea (transparent, avascular) + Sclera (white, protective)
• Vascular tunic (uvea): Choroid (pigmented, vascular), Ciliary body (muscles, produces aqueous humor), Iris (muscles, regulates pupil)
• Neural tunic: Retina (photoreceptor layer)

Refractive media: Cornea, aqueous humor, lens, vitreous body

Lacrimal apparatus: Lacrimal gland (produces tears), puncta, canaliculi, lacrimal sac, nasolacrimal duct

Photoreceptors

Rods:

• Function: Low light (scotopic) vision; detect motion
• Distribution: Mostly peripheral retina; absent at fovea
• Pigment: Rhodopsin (visual purple); derived from vitamin A
• Sensitivity: Very sensitive; single photon can activate
• Response: Single type (no color); slow response

Cones:

• Function: Bright light (photopic) vision; color vision; detail
• Distribution: Mostly at fovea; S, M, L cones
• Pigments: Opsins with different spectral sensitivities (blue, green, red)
• Sensitivity: Less sensitive; require more light
• Response: Fast; high spatial acuity

Visual Pathway

1. Retina: Photoreceptors → bipolar cells → ganglion cells
2. Optic nerve: Ganglion cell axons
3. Optic chiasm: Nasal retinal fibers cross; temporal fibers don’t
4. Optic tract: To lateral geniculate nucleus (LGN) of thalamus
5. Optic radiations: To primary visual cortex (V1, Brodmann area 17)
6. Visual cortex: Processes and interprets visual information

Visual field defects:

• Right homonymous hemianopia: Left optic tract/radiation/cortex lesion
• Bitemporal hemianopia: Optic chiasm lesion (pituitary tumor)
• Right nasal hemianopia: Left optic disc/retinal lesion

Pupillary Reflex

• Direct reflex: Light in right eye → right pupil constricts
• Consensual reflex: Light in right eye → left pupil also constricts
• Afferent: Optic nerve; Efferent: Oculomotor nerve (CN III)
• Light-near dissociation: Argyll Robertson pupil (bilateral small irregular pupils that accommodate but don’t react to light - neurosyphilis)

Accommodation

• Ciliary muscle contraction → zonular fibers relax → lens becomes more convex → near focus
• Parasympathetic (CN III)

Refractive Errors

• Myopia: Too long eye or too strong lens → focal point in front of retina
• Hyperopia: Too short eye or too weak lens → focal point behind retina
• Astigmatism: Unequal curvature of cornea/lens
• Presbyopia: Loss of accommodation with age (lens stiffens) → need reading glasses

Hearing and Balance (Vestibular System)

External Ear

• Pinna: Collects sound waves
• External auditory canal: Conducts sound to tympanic membrane

Middle Ear

• Tympanic membrane: Vibrates with sound waves
• Ossicles: Malleus → incus → stapes (amplifies and transmits vibration to oval window)
• Muscles: Tensor tympani (m. tensor) and stapedius (protect from loud sounds)

Inner Ear (Cochlea)

Structure:

• Scala vestibuli: Filled with perilymph; receives vibrations from oval window
• Scala media: Filled with endolymph (high K, low Na); contains organ of Corti
• Scala tympani: Perilymph; leads to round window

Organ of Corti:

• Hair cells: Inner (1 row) and outer (3 rows); stereocilia on apical surface
• Tectorial membrane: Overlies hair cells
• Basilar membrane: Vibrates with sound; different frequencies stimulate different locations (tonotopy)

Sound transduction:

• Vibration at oval window → perilymph movement → basilar membrane vibration
• Hair cell stereocilia bend → mechanically-gated K channels open → depolarization
• Inner hair cells: Transduce sound → signal to brain
• Outer hair cells: Amplify low-level sounds (otoacoustic emissions)

Hearing Pathway

• Hair cells → Auditory (VIII) nerve → Cochlear nuclei → Superior olivary nucleus (bilateral) → Lateral lemniscus → Inferior colliculus → Medial geniculate body → Auditory cortex (temporal lobe)

Pitch Discrimination

• Place theory: Different frequencies activate different locations on basilar membrane (high frequency near base, low frequency near apex)
• Temporal theory: Frequency encoded by firing rate

Balance (Vestibular Apparatus)

Semicircular canals (3):

• Detect angular acceleration (rotation)
• Ampulla: Contains cupula (gelatinous) and hair cells
• Endolymph: Moves with head rotation → deflects cupula → bends hair cell stereocilia → signal

Otolith organs (utricle and saccule):

• Detect linear acceleration and head position relative to gravity
• Otoconial membrane: Contains calcium carbonate crystals (otoconia)
• Hair cells: Embedded in macula

Nystagmus:

• Physiological: Jerk nystagmus after spinning (slow in direction of rotation, fast return)
• Pathological: May indicate vestibular or cerebellar lesions

Vestibular Pathways

• VIII nerve → Vestibular nuclei (medulla) → cerebellum, eye movement nuclei (CN III, IV, VI), spinal cord (vestibulospinal tracts for posture)
• Connect to cerebellum: For coordination of movement and balance

Taste (Gustation)

Taste Buds

• Located: Tongue (circumvallate, fungiform, filiform papillae), soft palate, epiglottis, esophagus
• Taste cells: Modified epithelial cells with microvilli
• Age: Taste buds diminish with age (especially salt and bitter)

Taste Modalities

Sweet: Tip of tongue; T1R2 + T1R3 receptors; G-protein gustducin Sour: Sides of tongue; H⁺ blocking K channels Salty: Front of tongue; Na⁺ channels Bitter: Back of tongue; T2R receptors (many - detection of toxins) Umami: T1R1 + T1R3; glutamate (savory)

Taste Pathway

• Chorda tympani (VII) → anterior 2/3 of tongue
• Glossopharyngeal (IX) → posterior 1/3
• Vagus (X) → epiglottis
• All → nucleus of solitary tract → thalamus → gustatory cortex (insula, frontal operculum)

Smell (Olfaction)

Olfactory Epithelium

• Location: Superior nasal cavity
• Olfactory receptor neurons: Bipolar neurons with cilia
• G-protein coupled receptors (GPCR) - 400 different types
• ** regeneration**: Neurons replaced every 30-60 days (unique among neurons)

Olfactory Pathway

• Olfactory receptor neurons → cribriform plate → olfactory bulb (glomeruli)
• Mitral/tufted cells → lateral olfactory tract → piriform cortex, amygdala, entorhinal cortex → orbitofrontal cortex
• Direct connection to limbic system (explains smell-memory association)

Somatosensation

Sensory Receptors

Mechanoreceptors:

• Merkel discs: Light touch, texture; slowly adapting
• Meissner corpuscles: Light touch, grip; rapidly adapting
• Pacinian corpuscles: Deep pressure, vibration; rapidly adapting
• Ruffini endings: Skin stretch; slowly adapting

Proprioceptors:

• Muscle spindles: Detect muscle length; intrafusal fibers
• Golgi tendon organs: Detect tension
• Joint receptors: Position sense

Nociceptors: Pain (thermal, mechanical, chemical) Thermoreceptors: Temperature (warmth and cold)

Somatosensory Pathways

Dorsal column-medial lemniscal pathway:

• Fine touch, vibration, pressure, proprioception
• Receptors: Mechanoreceptors
• Pathway: Peripheral nerve → dorsal root ganglion → dorsal columns (fasciculus gracilis for lower body, fasciculus cuneatus for upper body) → medulla (gracile/cuneate nuclei) → decussate (internal arcuate fibers) → medial lemniscus → VPL thalamus → primary somatosensory cortex

Anterolateral system (spinothalamic):

• Pain, temperature, crude touch
• Pathway: Peripheral nerve → dorsal root ganglion → dorsal horn (substantia gelatinosa) → cross → anterior spinothalamic tract (crude touch) or lateral spinothalamic tract (pain/temperature) → VPL thalamus → cortex

Dermatomes

• Maps of skin innervation by specific spinal nerves
• Important for localizing lesions

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