BIO NMDCAT

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​.

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.

Many rod cells converge onto a single bipolar cell (and many bipolar cells onto a ganglion cell), increasing light sensitivity through spatial summation but sacrificing visual acuity. Cones have a 1:1 ratio in the fovea.

Otoliths (or otoconia) are heavy calcium carbonate crystals in the maculae of the utricle and saccule. Gravity pulls on them, shifting the gelatinous membrane and bending the underlying hair cells.

Bitter taste evolved as a protective mechanism against ingesting plant toxins, which are frequently alkaloids (like quinine, caffeine, and nicotine) or other nitrogenous compounds.

Referred pain happens when sensory fibers from an organ (e.g., heart) and a somatic region (e.g., left arm) synapse on the same second-order neurons. The brain interprets the pain as coming from the more frequently stimulated somatic area.