Receptors

At the optic chiasm, fibers from the medial (nasal) half of each retina cross over (decussate) to the opposite side of the brain, while lateral (temporal) fibers stay on the same side.

The dark pigment epithelium absorbs stray light (preventing reflection/blurring) and plays a critical biochemical role in storing Vitamin A and reconverting all-trans-retinal back to 11-cis-retinal for the photoreceptors.

Conduction velocity increases with axon diameter and myelination. A-alpha fibers (carrying proprioceptive information from muscle spindles) are the largest and most heavily myelinated, allowing the fastest signal transmission.

Severed afferent nerve pathways in the stump can generate spontaneous action potentials. The brain, relying on the labeled line code, interprets these signals as originating from the receptors that used to be in the amputated limb.

The otolithic membrane is the gelatinous layer studded with heavy otoliths. When the head tilts, gravity pulls this membrane, bending the hair cells of the macula. The cupula is in the semicircular canals, and the tectorial membrane is in the cochlea.

Cold receptors are most active between 25°C and 30°C. Below 10°C, cold receptors become less active and act as a local anesthetic, while temperatures above 45°C activate pain receptors (nociceptors) rather than warm receptors.

Sound waves vibrate the tympanic membrane, which moves the malleus, then the incus, and finally the stapes. The footplate of the stapes pushes directly into the oval window, transmitting pressure waves into the cochlear fluid.

The organ of Corti sits atop the basilar membrane. Sound waves displace the basilar membrane, causing the hair cells to push against the overlying stationary tectorial membrane, resulting in stereocilia deflection.

Unlike typical extracellular fluids, endolymph is rich in potassium and low in sodium. This high K+ concentration provides the electrochemical gradient that drives K+ into hair cells when mechanically gated channels open.

Nearsightedness (myopia) occurs when the eyeball is too long or the lens is too strong, causing images of distant objects to focus in front of the retina, resulting in blurred distance vision.

Taste receptor cells are subjected to friction and heat, surviving only about 10-14 days. Basal cells at the periphery of the taste bud divide and differentiate to constantly replace them.

Cranial nerves carrying taste information (VII, IX, X) project first to the gustatory nucleus within the nucleus of the solitary tract in the medulla oblongata before relaying to the thalamus.

Sweet, bitter, and umami tastants bind to specific G-protein coupled receptors (GPCRs), activating secondary messenger cascades (like IP3/DAG) that eventually lead to cell depolarization. Salty and sour use direct ion channels.

The cerebellum receives continuous input from proprioceptors, the vestibular apparatus, and the eyes, integrating this information to coordinate smooth movements, posture, and equilibrium.

The gate control theory suggests that non-painful tactile stimuli (rubbing) carried by fast, myelinated A-beta fibers activate inhibitory interneurons in the spinal cord, "closing the gate" to pain signals carried by slower C fibers.

Calcium influx through voltage-gated calcium channels is absolutely essential for the exocytosis of neurotransmitter vesicles. Low extracellular calcium directly impairs this release mechanism.

Visual information from the retinas travels via the optic nerve, optic chiasm, and optic tracts to the lateral geniculate nucleus of the thalamus, and finally projects to the primary visual cortex in the occipital lobe.

The sensory homunculus is a distorted topographical map of the body in the postcentral gyrus. Areas with high receptor density (lips, fingers) are represented by much larger areas of the cortex than regions with low density (back).

Peripheral chemoreceptors located in the aortic arch and carotid bodies are highly sensitive to drops in arterial PO2​, triggering an increase in respiratory rate. Central chemoreceptors monitor CO2​ and pH, not O2​.