Other Senses

Olfaction: chemoreceptors in the olfactory epithelium (cilia) → olfactory nerve → olfactory bulb; strong direct connection to the limbic system (emotion/memory link). Pheromones affect many animal species; human effects unconfirmed.
Gustation: taste buds on the tongue, each cell tuned to one of 5 tastes — sweet, salty, sour, bitter, umami (savory, glutamate). "Flavor" = taste + smell combined (why food is bland with a stuffy nose).
5 somatosensory receptors: Meissner (light touch, rapidly adapting), Pacinian (deep pressure/vibration, rapidly adapting), Merkel (sustained pressure/texture, slowly adapting), Ruffini (stretch, slowly adapting), free nerve endings (pain via nociceptors, temperature via thermoreceptors).
Two-point threshold: smaller (more sensitive) at fingertips/lips (high receptor density), larger at the back (low density).
Physiological zero = skin's baseline temperature; perception tracks deviation from it, not absolute value.
Gate theory of pain (Melzack & Wall, 1965): touch signals can compete with/suppress pain signals in the spinal cord's dorsal horn (substantia gelatinosa) — why rubbing an injury helps.
Proprioception (kinesthetic sense): proprioceptors in muscles/tendons/joints monitor body position without needing to look.

Olfactory epithelium: Tissue in the nasal cavity containing chemoreceptor neurons for smell.
Umami: Savory taste, produced primarily by amino acids like glutamate.
Two-point threshold: How close two touch points can be before feeling like one; smaller where receptor density is high.
Physiological zero: The skin's normal baseline temperature; perception tracks deviations from it.
Gate theory of pain: Spinal-cord-level competition (dorsal horn/substantia gelatinosa) where touch signals can reduce pain transmission to the brain.
Proprioceptors: Receptors in muscles, tendons, and joints monitoring stretch, tension, and joint position.

Receptor	Detects / Adaptation Speed
Meissner corpuscles	Light touch — rapidly adapting
Pacinian corpuscles	Deep pressure, high-frequency vibration — rapidly adapting
Merkel cells	Sustained pressure, texture, fine detail — slowly adapting
Ruffini endings	Skin stretch — slowly adapting
Free nerve endings	Pain (nociceptors) and temperature (thermoreceptors)

Pheromone research is well-documented in animals but NOT conclusively established as behaviorally significant in humans — don't assume the animal research generalizes.
Rapidly adapting receptors (Meissner, Pacinian) fire at stimulus onset then stop; slowly adapting receptors (Merkel, Ruffini) keep firing while the stimulus is present.
Gate theory operates at the SPINAL CORD level (substantia gelatinosa), not in the brain — a common misattribution.
Proprioception is an internal/body-position sense, distinct from skin-surface somatosensation.

Why does food taste bland during a cold even though taste receptors work normally?

Which somatosensory receptor detects deep pressure and high-frequency vibration?

Which body regions have the smallest two-point threshold, and why?

Where does the "gate" in gate theory of pain physically sit?

What structures detect proprioceptive information?

Biology chapters 1–3 are free. Sign up to unlock every chapter.