Sensory nervous system

The sensory nervous system handles sensory information. It includes sensory neurons (with their receptor cells), neural pathways, and brain regions that manage perception and interoception. Sense organs act as transducers, turning physical data from the outside world into mental interpretations, forming each person's perception of their surroundings. A receptive field is the area of the body or environment that a receptor organ or its cells respond to—for example, what an eye can see is its receptive field, and the light a single rod or cone detects is its receptive field. Receptive fields are defined for the visual, auditory, and somatosensory systems.

There is debate among neurologists about the exact number of senses, but Gautama Buddha and Aristotle listed five traditional human senses—touch, taste, smell, vision, and hearing—which are widely accepted. Other senses well-accepted in most mammals, including humans, are pain, balance, and temperature. Some nonhuman animals have additional senses, such as magnetoreception and electroreception.

Sensation begins when a specific receptor responds to a physical stimulus. Receptors are grouped into four categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors. All receive distinct stimuli and convert the signal into an electrical action potential, which travels along afferent neurons to specific brain regions for processing and interpretation.

Chemoreceptors detect chemical stimuli and transduce them into action potentials. There are two main types: distance chemoreceptors, which receive stimuli in gases via the olfactory system (using olfactory receptor neurons and vomeronasal organ neurons), and direct chemoreceptors, which detect stimuli in liquids—these include taste buds in the gustatory system and receptors in the aortic bodies that sense oxygen concentration changes.

Photoreceptors are specialized neuron cells that initiate vision. They are light-sensitive and capture different light wavelengths, responding to color and converting the light into electrical signals through phototransduction, which turns electromagnetic radiation into a membrane potential. These cells have five compartments: the outer segment (captures and transduces light), the inner segment (contains organelles for metabolism and biosynthesis, like mitochondria, Golgi apparatus, and endoplasmic reticulum), the connecting cilium (links the outer and inner segments for protein trafficking), the nuclear region (contains the nucleus and continues from the inner segment), and the synaptic region (the final terminal, with synaptic vesicles that release the neurotransmitter glutamate to secondary neurons). The three primary types are cones (respond to color; in humans, three types correspond to short, medium, and long wavelengths—blue, green, and yellow/red), rods (very sensitive to light intensity, enabling dim-light vision; their ratio to cones relates to whether an animal is diurnal or nocturnal—in humans, rods outnumber cones about 20:1, while in nocturnal animals like the tawny owl, the ratio is near 1000:1), and ganglion cells (found in the adrenal medulla and retina, involved in the sympathetic response; of about 1.3 million ganglion cells in the retina, 1–2% are photosensitive, playing a role in conscious vision for some animals and likely in humans as well).

Mechanoreceptors respond to mechanical forces like pressure or distortion. They are present in hair cells for the vestibular and auditory systems, but most are cutaneous and fall into four categories: slowly adapting type 1 (small receptive fields, respond to static stimulation, used for form and roughness), slowly adapting type 2 (large receptive fields, respond to stretch, produce sustained responses to continued stimuli), rapidly adapting receptors (small receptive fields, underlie slip perception), and Pacinian receptors (large receptive fields, are the predominant receptors).