Senses begin as specialized receptors tiling your world
The sensory nervous system processes incoming information via receptor cells, pathways, and brain regions. Each organ's receptive field is the slice of body or environment it can report—an eye's view, a fingertip's patch. Buddha and Aristotle popularized five traditional human senses, though neurologists still argue counts.
Every sensation begins with a receptor cell reacting to some physical stimulus and converting it into an electrical impulse, which afferent nerves then carry to a dedicated patch of brain. Physiologists group these receptors into four families: chemoreceptors, photoreceptors, mechanoreceptors and thermoreceptors. How many senses humans have depends on definitions, but the traditional five of touch, taste, smell, sight and hearing go back to the Buddha and Aristotle, and pain, balance and temperature are widely accepted additions.
Chemical sensing comes in two forms. Distance receptors in the nose and the vomeronasal organ pick up airborne molecules, while direct receptors respond to substances in liquid, as taste buds do and as sensors in the aortic bodies do when oxygen levels shift. In the eye, three kinds of cone respond chiefly to blue, green and yellow-red light, and rods handle dim conditions. People have around twenty rods per cone; in a nocturnal hunter like the tawny owl the ratio approaches a thousand to one. Of the retina's roughly 1.3 million ganglion cells, 1 to 2 percent are themselves light-sensitive.
Touch relies on four kinds of skin mechanoreceptor. Two adapt slowly: one with a small field that reads shape and roughness, and one with a broad field that responds to stretch. Rapidly adapting receptors detect slipping, and Pacinian receptors pick up fast vibration. Temperature is split between the end-bulb of Krause, which fires above body heat, and Ruffini's end organ, below it, with the ion channels TRPV1 and TRPM8 acting as molecular thermometers for warmth and cold. Nociceptors signal harm, responding to extreme heat or cold, crushing pressure, or chemicals, including some spices.
Each stream has its own cortex. Touch and body position land in the parietal lobe's primary somatosensory cortex, Brodmann areas 1 to 3, with area 3 receiving most input from the thalamus. Vision starts in V1, area 17, in the occipital lobe, while hearing and smell are handled in the temporal lobe. Smell is also unusual in not crossing sides: the right olfactory bulb feeds the right hemisphere.
Source: Sensory nervous system
