Memory tricks for eyes, ears, and beyond

The eyeball has three concentric layers. Fibrous tunic (outermost): sclera (white of eye) and cornea (transparent anterior portion). Vascular tunic (middle): choroid (posterior, nourishes retina), ciliary body (controls lens shape), and iris (controls pupil size). Retina (innermost): contains rods (low light, peripheral vision) and cones (color, central/detailed vision). The optic disc has no photoreceptors — this is the blind spot.

Rods: 120 million, distributed peripherally, sensitive to low light, only one photopigment (rhodopsin). No color vision — black and white only. Cones: 6 million, concentrated at the fovea centralis (macula), require bright light, three types (red, green, blue). The fovea has the highest visual acuity — contains only cones. Color blindness = deficiency in cone type — most commonly red-green.

Outer ear: auricle (pinna) and external auditory canal — channels sound to tympanic membrane. Middle ear: tympanic membrane and three ossicles (malleus, incus, stapes). Amplifies and transmits sound vibrations. Inner ear: cochlea (hearing) and vestibular apparatus (balance). The Eustachian tube connects middle ear to nasopharynx — equalizes pressure. Otitis media = middle ear infection.

The three ossicles form a mechanical chain that amplifies sound vibrations 20-fold and transmits them from the tympanic membrane to the oval window of the inner ear. The malleus (hammer) attaches to the tympanic membrane. The incus (anvil) connects malleus to stapes. The stapes (stirrup) is the smallest bone in the body — its footplate fits into the oval window of the cochlea.

Taste buds are located on papillae of the tongue, soft palate, and epiglottis. The five basic tastes are sweet, salty, sour, bitter, and umami (savory). Taste from the anterior 2/3 of the tongue is carried by CN VII (facial nerve). Taste from the posterior 1/3 is carried by CN IX (glossopharyngeal). The common belief that different tongue regions taste different flavors is a myth — all taste buds can detect all five tastes.

Olfactory receptor neurons in the nasal epithelium project through the cribriform plate of the ethmoid bone to the olfactory bulb. Uniquely, olfactory signals project directly to the limbic system (hippocampus and amygdala) without passing through the thalamus first — which is why smells trigger such powerful memories and emotional responses. Anosmia (loss of smell) can be an early sign of Parkinson's disease or COVID-19.

Three semicircular canals (anterior, posterior, lateral) detect rotational acceleration — each oriented in a different plane. The saccule detects vertical linear acceleration (elevator going up and down). The utricle detects horizontal linear acceleration (car moving forward and back). All use hair cells embedded in a gelatinous membrane — movement of fluid bends hair cells, generating nerve impulses sent via CN VIII.

Shine a light in one eye — both pupils should constrict (consensual reflex). The afferent limb uses CN II (optic nerve) to carry the light signal to the pretectal nucleus. The efferent limb uses CN III (oculomotor) to carry the constriction signal to the pupillary sphincter via the ciliary ganglion. No response in the illuminated eye = CN II lesion. No constriction in either eye when light shone in one eye = CN III lesion on that side.

Filiform papillae are the most numerous — cover the anterior tongue, give it the rough texture, but contain NO taste buds. Fungiform papillae (mushroom-shaped) are scattered among filiform — contain taste buds. Circumvallate/Vallate papillae form a V-shaped row at the posterior tongue — largest, contain the most taste buds. Foliate papillae are on the lateral edges — contain some taste buds.