The cornea does most fixed refraction; the lens accommodates. Rods (rhodopsin) handle dim, peripheral, colorless vision; three cone types give color and acuity, packed in the fovea.
Phototransduction is a hyperpolarizing exception: in the dark, high cGMP keeps Na+ channels open and glutamate flowing; light isomerizes 11-cis-retinal, transducin activates phosphodiesterase, cGMP falls, channels close, and the cell hyperpolarizes.
Pathway: retina→ Nasal fibers cross, so a midline chiasm lesion (pituitary tumor) causes bitemporal hemianopia. Trichromatic theory explains the cone stage; opponent-process theory explains afterimages.
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optic chiasm
→
LGN (thalamus)→
V1
Hearing and Balance
Sound: tympanic membrane to ossicles (impedance matching, about 20-fold gain) to oval window to basilar membrane to organ of Corti hair cells to CN VIII. Tonotopy: the stiff cochlear base encodes high frequencies, the apex low ones.
Bending stereocilia pulls tip links open, potassium enters from endolymph, and glutamate is released. Semicircular canals sense angular acceleration; utricle and saccule sense linear acceleration and tilt.
Chemoreception
Salty and sour tastes use ion channels; sweet, bitter, and umami use GPCRs. Smell alone bypasses the thalamus, wiring directly to the limbic system.
Key Takeaways
Photoreceptors depolarize in the dark and hyperpolarize to light.