Sensory Systems

Vision

• 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: $\text{retina} \to \text{optic chiasm} \to \text{LGN (thalamus)} \to V1$ Nasal fibers cross, so a midline chiasm lesion (pituitary tumor) causes bitemporal hemianopia. Trichromatic theory explains the cone stage; opponent-process theory explains afterimages.

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.
• Chiasm lesions produce bitemporal field loss.
• Cochlear place coding maps frequency to position.
• Olfaction's thalamic bypass explains smell-memory links.