Somatosensory system
The somatosensory system is a part of the sensory nervous system. The somatosensory system is a complex system of sensory neurons and neural pathways that responds to changes at the surface or inside the body. The axons of sensory neurons connect with, or respond to, various receptor cells. These sensory receptor cells are activated by different stimuli such as heat and nociception, giving a functional name to the responding sensory neuron, such as a thermoreceptor which carries information about temperature changes. Other types include mechanoreceptors, chemoreceptors, and nociceptors which send signals along a sensory nerve to the spinal cord where they may be processed by other sensory neurons and then relayed to the brain for further processing. Sensory receptors are found all over the body including the skin, epithelial tissues, muscles, bones and joints, internal organs, and the cardiovascular system.
Somatic senses are sometimes referred to as somesthetic senses, with the understanding that somesthesis includes the sense of touch, proprioception, and haptic perception.
The mapping of the body surfaces in the brain is called somatotopy. In the cortex, it is also referred to as the cortical homunculus. This brain-surface map is not immutable, however. Dramatic shifts can occur in response to stroke or injury.
System overview
Sensory receptors
The four mechanoreceptors in the skin each respond to different stimuli for short or long periods.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 squish their shape causing an action potential.
Somatosensory cortex
The postcentral gyrus includes 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 + shape info.
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 afferent neurons in the periphery, to deeper neurons within the central nervous system.General somatosensory pathway
All afferent touch/vibration info ascends the spinal cord via the posterior 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.
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 posterior 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 don't 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.