Special Senses – Vision & Related Anatomy

Eyeball: Layer-by-Layer (Superficial ➜ Deep)

• Three concentric “tunics” (think onion-skins):
– Fibrous Tunic (protective C.T.)
– Vascular Tunic (pigment, blood, smooth muscle)
– Sensory Tunic / Retina (nervous, photoreceptors)

Fibrous Tunic

• Components & functions
– Sclera: opaque “whites”; irregular dense C.T.; \approx\frac56 of surface; attachment site for 6 extrinsic eye mm.
– Cornea: anterior, transparent; same collagen re-laid in perfectly regular sheets (≈ 40 layers) → clarity; first major site of refraction (light-bending).
• Clinical tie-in: LASIK = lift corneal flap, ablate a few layers, change curvature → change refraction.

Vascular Tunic (aka "Uvea")

• Choroid: pigmented, highly vascular; absorbs stray light & nourishes retina.
• Ciliary Body: circular smooth muscle + suspensory ligaments (zonular fibers); pulls/relaxes to change lens shape (accommodation).
• Iris: pigmented diaphragm; central opening = pupil (just an absence of tissue).
– Parasympathetic ("rest & digest") → sphincter pupillae → constrict.
– Sympathetic ("fight or flight" / scared) → dilator pupillae → dilate.
• Lens: biconvex, elastic; at rest wants to be spherical; ciliary tension flattens it; second site of controlled refraction (fine focusing).

Sensory Tunic (Retina)

• Layers: pigmented layer (absorbs stray light) + neural layer (photoreceptors → bipolar cells → ganglion cells).
• Photoreceptors (rods & cones) sit deepest, abutting choroid.
• Axons of ganglion cells converge → optic nerve (CN II).

Internal Cavities & Humors

• Lens divides eye into two unequal chambers:
– Anterior Segment (anterior to lens) → aqueous humor (watery, continuously produced by ciliary processes, drains via canal of Schlemm; provides O₂ & nutrients to cornea/iris).
– Posterior Segment (posterior to lens) → vitreous humor (gelatinous, formed embryologically, not replaced; stabilizes retina, supports lens, shock absorber).

Extrinsic Eye Muscles (6) & Innervation Mnemonic

LR6\,,\;SO4\,,\;AO3
(= Lateral Rectus – CN VI; Superior Oblique – CN IV; All Others – CN III)

• Real-life exam: ophthalmologist draws large “H” to isolate each muscle individually.
• Eye-roll extremes can recruit all 6 simultaneously; fine gaze control = agonist–antagonist balance.

Lacrimal (Tear) Apparatus

• Lacrimal gland (superolateral orbit): parasympathetic supply via CN VII (facial) → constant tear film.
• Tears flow across cornea → lacrimal canaliculi (medial canthus) → nasolacrimal duct → inferior nasal meatus (=> runny nose when crying).
• External corneal layers obtain O₂ directly from atmospheric O₂ dissolved in tears (frog analogy).

Visual Pathway & Field Mapping

• Each eye captures portions of both visual fields; information sorted at optic chiasm.
• Flow (Left eye example):

1. Photoreceptors (rods & cones)

2. Optic Nerve (carries both visual fields from one eye)

3. Optic Chiasm – only nasal (peripheral) fibers cross

4. Optic Tract (now carries one complete visual field; e.g. right tract = left visual field)

5. Thalamus (lateral geniculate nucleus – relay)

6. Optic radiations → Primary Visual Cortex (occipital lobe).
   • Lesion patterns:
   – Left optic nerve → total blindness in left eye.
   – Left optic tract → right homonymous hemianopia (lose entire right field).
   – Chiasm (e.g. pituitary tumor) → bitemporal hemianopia (tunnel vision).

General vs. Special Senses – Receptor Types

• General (skin-like): thermo-, nociceptors (pain), baro- (pressure/stretch), proprioceptors (position).
• Special:
– Vision → photoreceptors.
– Taste & Smell → chemoreceptors.
– Hearing & Equilibrium → mechanoreceptors.

Gustation (Taste)

• Tongue regions: anterior \frac23 = body; posterior \frac13 = root.
• Surface elevations = papillae (numerous); each houses multiple taste buds (individual chemoreceptors).
• Five primary flavors: sweet, salty, sour, bitter, umami (all sensed across tongue; classic “tongue map” is obsolete).
• Cranial-nerve supply:
– Special Sense (taste)
• Anterior \frac23 → CN VII (facial).
• Posterior \frac13 → CN IX (glossopharyngeal).
– General Sensation (touch, temp, pain)
• Anterior \frac23 → mandibular division of CN V (V₃).
• Posterior \frac13 → CN IX (same as taste).

Tongue Musculature (Extrinsic)

• All motor innervation via CN XII (hypoglossal).
• Key muscles & actions:
– Styloglossus (styloid process → tongue): elevate & retract.
– Hyoglossus (hyoid → tongue): depress.
– Genioglossus (genial tubercle/mandible → tongue bulk): protract ("genie comes out of the bottle").
• Tongue = muscular hydrostat (no internal skeleton; octopus-arm analogy) → intrinsic fibers provide fine shaping for speech & swallowing.

Olfaction (Smell)

• Receptors = bipolar chemoreceptor neurons within olfactory epithelium (superior nasal cavity) – these neurons are CN I.
• Their axons pass through cribriform plate (ethmoid) → synapse in olfactory bulb → olfactory tract → primary olfactory cortex (temporal lobe, bypasses thalamus).
• Unique: neurons regenerate; only sense with direct limbic connections (emotion, memory).

Quick Reference: Cranial Nerves Mentioned

• CN I – Olfactory (smell)
• CN II – Optic (vision)
• CN III – Oculomotor (motor to 4/6 extrinsic eye mm.; parasymp. pupil constriction)
• CN IV – Trochlear (motor to superior oblique)
• CN V (V₃) – Trigeminal mandibular (general sense ant. tongue; muscles of mastication)
• CN VI – Abducens (motor to lateral rectus)
• CN VII – Facial (taste ant. \frac23 tongue; parasymp. lacrimal gland; facial expression mm.)
• CN IX – Glossopharyngeal (taste & general sense post. \frac13 tongue)
• CN XII – Hypoglossal (motor to all intrinsic & extrinsic tongue mm.)

Study/Exam Tips

• Redraw eye muscles from multiple viewpoints; label actions & nerves until automatic.
• Re-sketch visual pathway & predict field losses for lesions at nerve, chiasm, tract.
• Make a two-column chart for tongue: region / sense type / cranial nerve.
• Practice parasymp. vs sympathetic effects on pupil & lacrimation.
• Remember mnemonic LR6\,,SO4\,,AO3.