Somatosensory system

The somatosensory system, or somatic sensory system, is a subset of the sensory nervous system. The main functions of the somatosensory system are the perception of external stimuli, the perception of internal stimuli, and the regulation of body position and balance. It is believed to act as a pathway between the different sensory modalities within the body.
As of 2024 debate continued on the underlying mechanisms, correctness and validity of the somatosensory system model, and whether it impacts emotions in the body.
The somatosensory system has been thought of as having two subdivisions;

one for the detection of mechanosensory information related to touch. Mechanosensory information includes that of light touch, vibration, pressure and tension in the skin. Much of this information belongs to the sense of touch which is a general somatic sense in contrast to the special senses of sight, smell, taste, hearing, and balance.
one for the nociception detection of pain and temperature. Nociceptory information is that received from pain and temperature that is deemed as harmful. Thermoreceptors relay temperature information in normal circumstances. Nociceptors are specialised receptors for signals of pain.

The sense of touch in perceiving the environment uses special sensory receptors in the skin called cutaneous receptors. They include mechanoreceptors such as tactile corpuscles that relay information about pressure and vibration; nociceptors, and thermoreceptors for temperature perception.
Stimulation of the receptors activate peripheral sensory neurons that convey signals to the spinal cord that may drive a responsive reflex, and may also be conveyed to the brain for conscious perception. Somatosensory information from the face and head enter the brain via cranial nerves such as the trigeminal nerve.
The neural pathways that go to the brain are structured such that information about the location of the physical stimulus is preserved. In this way, neighboring neurons in the somatosensory cortex represent nearby locations on the skin or in the body, creating a map or sensory homunculus.

Touch communication

Tactile signing

is a common means of communication used by people with deafblindness. It is based on a sign language or another system of manual communication.

Emotion communication

Humans can communicate specific emotions through touch alone including anger, fear, disgust, love, gratitude, and sympathy via touch at much-better-than-chance levels.

Overview

Sensory receptors

The two different types of mechanoreceptor in the skin are termed low-threshold mechanoreceptors, and high threshold mechanoreceptors.
The four mechanoreceptors in glabrous skin are low-threshold that respond to harmless stimuli. They are innervated by four different afferent fibers. High-threshold mechanoreceptors, respond to harmful stimuli.
Merkel cell nerve endings are found in the basal epidermis and hair follicles; they react to low vibrations and deep static touch such as shapes and edges. Due to having a small receptive field, they are used in areas like fingertips the most; they are not covered and thus respond to pressures over long periods.
Tactile corpuscles react to moderate vibration and light touch. They are located in the dermal papillae; due to their reactivity, they are primarily located in fingertips and lips. They respond in quick action potentials, unlike Merkel nerve endings. They are responsible for the ability to read Braille and feel gentle stimuli.
Pacinian corpuscles determine gross touch and distinguish rough and soft substances. They react in quick action potentials, especially to vibrations around 250 Hz. They are the most sensitive to vibrations and have large receptor fields. Pacinian corpuscles react only to sudden stimuli so pressures like clothes that are always compressing their shape are quickly ignored. They have also been implicated in detecting the location of touch sensations on handheld tools.
Bulbous corpuscles react slowly and respond to sustained skin stretch. They are responsible for the feeling of object slippage and play a major role in the kinesthetic sense and control of finger position and movement. Merkel and bulbous cells - slow-response - are myelinated; the rest - fast-response - are not. All of these receptors are activated upon pressures that distort their shape causing an action potential.

Somatosensory cortex

The postcentral gyrus is in the parietal lobe and its cortex is the primary somatosensory cortex collectively referred to as S1.
BA3 receives the densest projections from the thalamus. BA3a is involved with the sense of relative position of neighboring body parts and amount of effort being used during movement. BA3b is responsible for distributing somatosensory information, it projects texture information to BA1 and shape and size information to BA2.
Region S2 divides into Area S2 and parietal ventral area. Area S2 is involved with specific touch perception and is thus integrally linked with the amygdala and hippocampus to encode and reinforce memories.
Parietal ventral area is the somatosensory relay to the premotor cortex and somatosensory memory hub, BA5.
BA5 is the topographically organized somato memory field and association area.
BA1 processes texture info while BA2 processes size and shape information.
Area S2 processes light touch, pain, visceral sensation, and tactile attention.
S1 processes the remaining info.
BA7 integrates visual and proprioceptive info to locate objects in space.
The insular cortex plays a role in the sense of bodily-ownership, bodily self-awareness, and perception. Insula also plays a role in conveying info about sensual touch, pain, temperature, itch, and local oxygen status. Insula is a highly connected relay and thus is involved in numerous functions.

Structure

The somatosensory system is spread through all major parts of the vertebrate body. It consists both of sensory receptors and sensory neurons in the periphery, to deeper neurons within the central nervous system.

General somatosensory pathway

All afferent touch/vibration information ascends the spinal cord via the dorsal column-medial lemniscus pathway via gracilis or cuneatus. Cuneatus sends signals to the cochlear nucleus indirectly via spinal grey matter, this info is used in determining if a perceived sound is just villi noise/irritation. All fibers cross in the medulla.
A somatosensory pathway will typically have three neurons: first-order, second-order, and third-order.

The first-order neuron is a type of pseudounipolar neuron and always has its cell body in the dorsal root ganglion of the spinal nerve with a peripheral axon innervating touch mechanoreceptors and a central axon synapsing on the second-order neuron. If the somatosensory pathway is in parts of the head or neck not covered by the cervical nerves, the first-order neuron will be the trigeminal nerve ganglia or the ganglia of other sensory cranial nerves).
The second-order neuron has its cell body either in the spinal cord or in the brainstem. This neuron's ascending axons will cross to the opposite side either in the spinal cord or in the brainstem.
In the case of touch and certain types of pain, the third-order neuron has its cell body in the ventral posterior nucleus of the thalamus and ends in the postcentral gyrus of the parietal lobe in the primary somatosensory cortex.

Photoreceptors, similar to those found in the retina of the eye, detect potentially damaging ultraviolet radiation, inducing increased production of melanin by melanocytes. Thus tanning potentially offers the skin rapid protection from DNA damage and sunburn caused by ultraviolet radiation. However, whether this offers protection is debatable, because the amount of melanin released by this process is modest in comparison to the amounts released in response to DNA damage caused by ultraviolet B radiation.

Tactile feedback

The tactile feedback from proprioception is derived from the proprioceptors in the skin, muscles, and joints.

Balance

The receptor for the sense of balance resides in the vestibular system in the ear. Balance is also mediated by the kinesthetic reflex fed by proprioception. In addition, proprioception estimates the location of objects which are sensed by the visual system, as input to the mechanical reflexes of the body.

Fine touch and crude touch

Fine touch is a sensory modality that allows a subject to sense and localize touch. The form of touch where localization is not possible is known as crude touch. The dorsal column–medial lemniscus pathway is the pathway responsible for the sending of fine touch information to the cerebral cortex of the brain.
Crude touch is a sensory modality that allows the subject to sense that something has touched them, without being able to localize where they were touched. Its fibres are carried in the spinothalamic tract, unlike the fine touch, which is carried in the dorsal column.
As fine touch normally works in parallel to crude touch, a person will be able to localize touch until fibres carrying fine touch have been disrupted. Then the subject will feel the touch, but be unable to identify where they were touched.

Neural processing of social touch

The somatosensory cortex encodes incoming sensory information from receptors all over the body. Affective touch is a type of sensory information that elicits an emotional reaction and is usually social in nature, such as a physical human touch. This type of information is actually coded differently than other sensory information. Intensity of affective touch is still encoded in the primary somatosensory cortex and is processed in a similar way to emotions invoked by sight and sound, as exemplified by the increase of adrenaline caused by the social touch of a loved one, as opposed to the physical inability to touch someone you do not love.
Meanwhile, the feeling of pleasantness associated with affective touch activates the anterior cingulate cortex more than the primary somatosensory cortex. Functional magnetic resonance imaging data shows that increased blood-oxygen-level contrast signal in the anterior cingulate cortex as well as the prefrontal cortex is highly correlated with pleasantness scores of an affective touch. Inhibitory transcranial magnetic stimulation of the primary somatosensory cortex inhibits the perception of affective touch intensity, but not affective touch pleasantness. Therefore, the S1 is not directly involved in processing socially affective touch pleasantness, but still plays a role in discriminating touch location and intensity.
Tactile interaction is important amongst some animals. Usually, tactile contact between two animals occurs through stroking, licking, or grooming. These behaviours are essential for the individual's social healthcare, as in the hypothalamus they induce the release of oxytocin, a hormone that decreases stress and anxiety and increases social bonding between animals.
More precisely, the consistency of oxytocin neuron activation in rats stroked by humans has been observed, especially in the caudal paraventricular nucleus. It was found that this affiliative relationship induced by tactile contact is common no matter the relationship between the two individuals. It has also been discovered that the level of oxytocin release through this behaviour correlates with the time course of social interaction as longer stroking induced a greater release of the hormone.
The importance of somatosensory stimulation in social animals such as primates has also been observed. Grooming is part of the social interaction primates exert on their conspecifics. This interaction is required between individuals to maintain the affiliative relationship within the group, avoid internal conflict and increase group bonding. However, such social interaction requires the recognition of every member in the group. As such, it has been observed that the size of the neocortex is positively correlated with the size of the group, reflecting a limit to the number of recognizable members amongst which grooming can occur. Furthermore, the time course of grooming is related to vulnerability due to predation to which animals are exposed to whilst performing such social interaction. The relationship between tactile interaction, stress reduction and social bonding depends on the evaluation of risks that occur during conducting such behaviours in the wild life, and further research is required to unveil the connection between tactile caring and fitness level.
Studies show a correlation between touching a soft or hard object and how a person thinks or even makes decisions. Further, between the firmness of a touch and the evoking of gender stereotyping.
Tactile memories as part of haptic memory, are organized somatotopically, following the organization of the somatosensory cortex.