Receptors

The absolute threshold is the lowest level of stimulus energy that an organism can consciously detect consistently.

While the lens provides adjustable focus (accommodation), the cornea has a fixed curvature and the largest difference in optical density compared to air, providing about 70-80% of the eye's total refractive power.

To focus on near objects (accommodation), the ciliary muscle contracts, acting like a sphincter. This releases tension on the suspensory ligaments, allowing the elastic lens to bulge and become more convex.

In the dark, photoreceptors are depolarized and steadily release glutamate. This glutamate binds to inhibitory metabotropic receptors on ON-center bipolar cells, keeping them hyperpolarized (inactive) in the dark.

Ruffini endings are deep, slowly adapting receptors that are highly sensitive to skin stretch, contributing significantly to kinesthetic sense and the control of finger position and grip.

Light activates phosphodiesterase, which aggressively breaks down cGMP. The drop in cGMP closes sodium channels, leading to the hyperpolarization of the rod cell.

The organ of Corti sits on the basilar membrane within the cochlear duct (scala media), which is filled with endolymph. The scala vestibuli and scala tympani contain perilymph.

Pulmonary stretch receptors are a type of mechanoreceptor located in the smooth muscle of the airways. They detect mechanical deformation (stretching) during inspiration (Hering-Breuer reflex).

The axons of primary olfactory sensory neurons travel through the porous cribriform plate of the ethmoid bone to synapse with mitral and tufted cells in the olfactory bulb.

The fingertips have extremely high densities of Meissner's corpuscles and Merkel discs with very small receptive fields, allowing for precise two-point discrimination and fine tactile resolution.

Lateral inhibition occurs when a strongly stimulated central neuron inhibits its less-stimulated neighbors via interneurons. This isolates the signal, improving stimulus localization and contrast.

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.

Unlike all other senses, the olfactory pathway routes directly from the olfactory bulb to the olfactory cortex (in the temporal lobe) without synapsing first in the thalamus.

The adequate stimulus is the specific form of energy (light, mechanical, chemical) to which a receptor is uniquely adapted and most sensitive, having the lowest threshold for activation. It is not the only energy that can activate it (e.g., a hard punch to the eye causes seeing "stars").

A generator potential is a graded depolarization in the sensory nerve ending itself. If it reaches threshold, it opens voltage-gated sodium channels at the first node of Ranvier, firing an action potential.

Rhodopsin consists of opsin protein bound to retinal. Retinal is a light-absorbing molecule synthesized directly from Vitamin A (retinol). Vitamin A deficiency leads to night blindness.