![Tactile markings stairs for visually impaired](https://upload.wikimedia.org/wikipedia/commons/0/09/Tactile_markings_stairs_for_visually_impaired.jpg)
In
physiology
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
, the somatosensory system is the network of
neural structures in the brain and body that produce the perception of touch (
haptic perception
Haptic perception ( gr, haptόs, script=latn, italics=yes "palpable", ''haptikόs'' "suitable for touch") means literally the ability "to grasp something". Perception in this case is achieved through the active exploration of surfaces and objects b ...
), as well as temperature (
thermoception
Thermoception or thermoreception is the sensation and perception of temperature, or more accurately, temperature differences inferred from heat flux. It deals with a series of events and processes required for an organism to receive a temperature s ...
), body position (
proprioception
Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense".
Proprioception is mediated by proprioceptors, mechanosensory neurons ...
), and
pain
Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...
. It is a subset of the
sensory nervous system
The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neural pathways, and parts of the brain involved i ...
, which also represents
visual
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the ...
,
auditory,
olfactory
The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.
In humans, it ...
, and
gustatory
The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
stimuli.
Somatosensation begins when
mechano- and
thermosensitive structures in the skin or internal organs sense physical stimuli such as pressure on the skin (see
mechanotransduction
In cellular biology, mechanotransduction ('' mechano'' + '' transduction'') is any of various mechanisms by which cells convert mechanical stimulus into electrochemical activity. This form of sensory transduction is responsible for a number of ...
,
nociception
Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, co ...
). Activation of these structures, or receptors, leads to activation of
peripheral sensory neurons that convey signals to the
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...
as patterns of
action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
s. Sensory information is then processed
locally in the spinal cord to drive reflexes, and is also
conveyed to the brain for conscious perception of touch and proprioception. Note, somatosensory information from the face and head enters the brain through peripheral sensory neurons in the
cranial nerve
Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and ...
s, such as the
trigeminal nerve
In neuroanatomy, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and chewin ...
.
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 cerebral cortex in the brain represent nearby locations on the skin or in the body, creating a map, also called a
cortical homunculus
A cortical homunculus () is a distorted representation of the human body, based on a neurological "map" of the areas and proportions of the human brain dedicated to processing motor functions, or sensory functions, for different parts of the bo ...
.
System overview
Sensory receptors
The four
mechanoreceptor
A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, ...
s in the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
each respond to different stimuli for short or long periods.
Merkel cell
Merkel cells, also known as Merkel-Ranvier cells or tactile epithelial cells, are oval-shaped mechanoreceptors essential for light touch sensation and found in the skin of vertebrates. They are abundant in highly sensitive skin like that of the f ...
nerve endings are found in the basal
epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...
and
hair follicles
The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...
; they react to low vibrations (5–15
Hz) and deep static touch such as shapes and edges. Due to having a small receptive field (extremely detailed information), they are used in areas like fingertips the most; they are not covered (shelled) and thus respond to pressures over long periods.
Tactile corpuscle
Tactile corpuscles or Meissner's corpuscles are a type of mechanoreceptor discovered by anatomist Georg Meissner (1829–1905) and Rudolf Wagner. This corpuscle is a type of nerve ending in the skin that is responsible for sensitivity to pressure. ...
s react to moderate vibration (10–50 Hz) and light touch. They are located in the
dermal papillae
The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...
; due to their reactivity, they are primarily located in fingertips and lips. They respond in quick
action potentials
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, c ...
, unlike Merkel nerve endings. They are responsible for the ability to read Braille and feel gentle stimuli.
Pacinian corpuscle
Pacinian corpuscle or lamellar corpuscle or Vater-Pacini corpuscle; is one of the four major types of mechanoreceptors (specialized nerve ending with adventitious tissue for mechanical sensation) found in mammalian skin. This type of mechanorece ...
s determine gross touch and distinguish rough and soft substances. They react in quick action potentials, especially to vibrations around 250 Hz (even up to centimeters away). 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 corpuscle
The Bulbous corpuscle or Ruffini ending or Ruffini corpuscle is a slowly adapting mechanoreceptor located in the cutaneous tissue between the dermal papillae and the hypodermis. It is named after Angelo Ruffini
Angelo Ruffini (Pretare of Arquata ...
s 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
Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense".
Proprioception is mediated by proprioceptors, mechanosensory neurons ...
sense and control of finger position and movement. Merkel and bulbous cells - slow-response - are
myelinated
Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be l ...
; the rest - fast-response - are not. All of these receptors are activated upon pressures that squish their shape causing an action potential.
Somatosensory cortex
![Gray759](https://upload.wikimedia.org/wikipedia/commons/6/66/Gray759.png)
The postcentral gyrus includes the
primary somatosensory cortex
In neuroanatomy, the primary somatosensory cortex is located in the postcentral gyrus of the brain's parietal lobe, and is part of the somatosensory system. It was initially defined from surface stimulation studies of Wilder Penfield, and paralle ...
(
Brodmann area
A Brodmann area is a region of the cerebral cortex, in the human or other primate brain, defined by its cytoarchitecture, or histological structure and organization of cells.
History
Brodmann areas were originally defined and numbered by the ...
s
3, 2 and 1) collectively referred to as S1.
BA3 receives the densest projections from the
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
. 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 (
secondary somatosensory cortex
The human secondary somatosensory cortex (S2, SII) is a region of cortex in the parietal operculum on the ceiling of the lateral sulcus.
Region S2 was first described by Adrian in 1940, who found that feeling in cats' feet was not only represente ...
) 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 (crude touch, pain, temperature).
BA7 integrates visual and proprioceptive info to locate objects in space.
The
insular cortex
The insular cortex (also insula and insular lobe) is a portion of the cerebral cortex folded deep within the lateral sulcus (the fissure separating the temporal lobe from the parietal and frontal lobes) within each hemisphere of the mammalian bra ...
(insula) 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
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
body. It consists both of
sensory receptor
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
s and
sensory neuron
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
s in the periphery (skin, muscle and organs for example), to deeper neurons within the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
.
General somatosensory pathway
All afferent touch/vibration info ascends the spinal cord via the
dorsal column-medial lemniscus pathway
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
* Dorsal co ...
via gracilis (T7 and below) or cuneatus (T6 and above). 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 (left becomes right) 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
A pseudounipolar neuron is a type of neuron which has one extension from its cell body. This type of neuron contains an axon that has split into two branches. A single process arises from the cell body and then divides into an axon and a dendrite ...
and always has its
cell body
The soma (pl. ''somata'' or ''somas''), perikaryon (pl. ''perikarya''), neurocyton, or cell body is the bulbous, non-process portion of a neuron or other brain cell type, containing the cell nucleus. The word 'soma' comes from the Greek '' σῶμ ...
in the
dorsal root ganglion
A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons (a ganglion) in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the dorsa ...
of the
spinal nerve
A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into th ...
with a peripheral
axon
An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...
innervating touch
mechanoreceptor
A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, ...
s 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
Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...
).
# The second-order neuron has its
cell body
The soma (pl. ''somata'' or ''somas''), perikaryon (pl. ''perikarya''), neurocyton, or cell body is the bulbous, non-process portion of a neuron or other brain cell type, containing the cell nucleus. The word 'soma' comes from the Greek '' σῶμ ...
either in the spinal cord or in the brainstem. This neuron's ascending
axons
An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, th ...
will cross (
decussate
Decussation is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' (), ). In Latin anatomical terms, the form is used, e.g. .
Similarly, the anatomical term chiasma is named aft ...
) to the opposite side either in the
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...
or in the
brainstem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...
.
# In the case of touch and certain types of pain, the third-order neuron has its
cell body
The soma (pl. ''somata'' or ''somas''), perikaryon (pl. ''perikarya''), neurocyton, or cell body is the bulbous, non-process portion of a neuron or other brain cell type, containing the cell nucleus. The word 'soma' comes from the Greek '' σῶμ ...
in the
ventral posterior nucleus
The ventral posterior nucleus is the somato-sensory relay nucleus in thalamus of the brain.
Input and output
The ventral posterior nucleus receives neuronal input from the medial lemniscus, spinothalamic tracts, and trigeminothalamic tract. It p ...
of the thalamus and ends in the
postcentral gyrus
In neuroanatomy, the postcentral gyrus is a prominent gyrus in the lateral parietal lobe of the human brain. It is the location of the primary somatosensory cortex, the main sensory receptive area for the somatosensory system, sense of touch. Lik ...
of the
parietal lobe
The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus.
The parietal lobe integrates sensory informa ...
in the
primary somatosensory cortex
In neuroanatomy, the primary somatosensory cortex is located in the postcentral gyrus of the brain's parietal lobe, and is part of the somatosensory system. It was initially defined from surface stimulation studies of Wilder Penfield, and paralle ...
(or S1).
![Tickle](https://upload.wikimedia.org/wikipedia/commons/1/19/Tickle.jpg)
Photoreceptors, similar to those found in the
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
of the
eye
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...
, detect potentially damaging
ultraviolet radiation
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
(
ultraviolet A
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
specifically), inducing increased production of
melanin
Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
by
melanocytes
Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea),
the inner ear,
vaginal epithelium, meninges,
bones,
and heart.
...
.
Thus tanning potentially offers the skin rapid protection from DNA damage and sunburn caused by
ultraviolet radiation
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
(DNA damage caused by
ultraviolet B
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
). 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
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
radiation.
Tactile feedback
The tactile feedback from
proprioception
Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense".
Proprioception is mediated by proprioceptors, mechanosensory neurons ...
is derived from the proprioceptors in the skin, muscles, and joints.
Balance
The receptor for the sense of balance resides in the
vestibular system
The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitutes ...
in the ear (for the three-dimensional orientation of the head, and by inference, the rest of the body). Balance is also mediated by the kinesthetic reflex fed by
proprioception
Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense".
Proprioception is mediated by proprioceptors, mechanosensory neurons ...
(which senses the relative location of the rest of the body to the head).
In addition, proprioception estimates the location of objects which are sensed by the
visual system
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...
(which provides confirmation of the place of those objects relative to the body), as input to the mechanical reflexes of the body.
Fine touch and crude touch
![Sensory Homunculus](https://upload.wikimedia.org/wikipedia/commons/1/13/Sensory_Homunculus.png)
Fine touch (or discriminative 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
Posterior may refer to:
* Posterior (anatomy), the end of an organism opposite to its head
** Buttocks, as a euphemism
* Posterior horn (disambiguation)
* Posterior probability
The posterior probability is a type of conditional probability that r ...
is the pathway responsible for the sending of fine touch information to the
cerebral cortex
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of ...
of the brain.
Crude touch (or non-discriminative 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 (contrasting "fine touch"). Its fibres are carried in the
spinothalamic tract
The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cor ...
, 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 (
Posterior column–medial lemniscus pathway
Posterior may refer to:
* Posterior (anatomy), the end of an organism opposite to its head
** Buttocks, as a euphemism
* Posterior horn (disambiguation)
* Posterior probability
The posterior probability is a type of conditional probability that r ...
) 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
Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area o ...
(fMRI) data shows that increased blood-oxygen-level contrast (BOLD) 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
Transcranial magnetic stimulation (TMS) is a noninvasive form of brain stimulation in which a changing magnetic field is used to induce an electric current at a specific area of the brain through electromagnetic induction. An electric pulse gener ...
(TMS) 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 most 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 (mother-infant, male-female, human-animal). 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.
Individual variation
A variety of studies have measured and investigated the causes for differences between individuals in the sense of fine touch. One well-studied area is passive tactile spatial acuity, the ability to resolve the fine spatial details of an object pressed against the stationary skin. A variety of methods have been used to measure passive tactile spatial acuity, perhaps the most rigorous being the grating orientation task. In this task subjects identify the orientation of a grooved surface presented in two different orientations, which can be applied manually or with automated equipment. Many studies have shown a decline in passive tactile spatial acuity with age;
the reasons for this decline are unknown, but may include loss of tactile receptors during normal aging. Remarkably, index finger passive tactile spatial acuity is better among adults with smaller index fingertips;
this effect of finger size has been shown to underlie the better passive tactile spatial acuity of women, on average, compared to men.
The density of
tactile corpuscle
Tactile corpuscles or Meissner's corpuscles are a type of mechanoreceptor discovered by anatomist Georg Meissner (1829–1905) and Rudolf Wagner. This corpuscle is a type of nerve ending in the skin that is responsible for sensitivity to pressure. ...
s, a type of mechanoreceptor that detects low-frequency vibrations, is greater in smaller fingers; the same may hold for
Merkel cells
Merkel cells, also known as Merkel-Ranvier cells or tactile epithelial cells, are oval-shaped mechanoreceptors essential for light touch sensation and found in the skin of vertebrates. They are abundant in highly sensitive skin like that of the f ...
, which detect the static indentations important for fine spatial acuity.
Among children of the same age, those with smaller fingers also tend to have better tactile acuity. Many studies have shown that passive tactile spatial acuity is enhanced among blind individuals compared to sighted individuals of the same age,
possibly because of
cross modal plasticity
Cross modal plasticity is the adaptive reorganization of neurons to integrate the function of two or more sensory systems. Cross modal plasticity is a type of neuroplasticity and often occurs after sensory deprivation due to disease or brain dama ...
in the cerebral cortex of blind individuals. Perhaps also due to cortical plasticity, individuals who have been blind since birth reportedly consolidate tactile information more rapidly than sighted people.
Clinical significance
A somatosensory deficiency may be caused by a
peripheral neuropathy
Peripheral neuropathy, often shortened to neuropathy, is a general term describing disease affecting the peripheral nerves, meaning nerves beyond the brain and spinal cord. Damage to peripheral nerves may impair sensation, movement, gland, or or ...
involving peripheral nerves of the somatosensory system. This may present as
numbness
Hypoesthesia or numbness is a common side effect of various medical conditions that manifests as a reduced sense of touch or sensation, or a partial loss of sensitivity to sensory stimuli. In everyday speech this is generally referred to as num ...
or
paresthesia
Paresthesia is an abnormal sensation of the skin (tingling, pricking, chilling, burning, numbness) with no apparent physical cause. Paresthesia may be transient or chronic, and may have any of dozens of possible underlying causes. Paresthesias ar ...
.
Society and culture
Haptic technology
Haptic technology (also kinaesthetic communication or 3D touch) is technology that can create an experience of touch by applying forces, vibrations, or motions to the user. These technologies can be used to create virtual objects in a computer ...
can provide touch sensation in virtual and real environments. In the field of
speech therapy
Speech is a human vocal communication using language. Each language uses phonetic combinations of vowel and consonant sounds that form the sound of its words (that is, all English words sound different from all French words, even if they are th ...
, tactile feedback can be used to treat
speech disorders
Speech disorders or speech impairments are a type of communication disorder in which normal speech is disrupted. This can mean stuttering, lisps, etc. Someone who is unable to speak due to a speech disorder is considered mute. Speech skills ar ...
.
Affectionate touch is present in everyday life and can take multiple forms. These actions, however, seem to carry specific functions even though the evolutionary benefit from such a wide range of behaviours is not entirely understood. Researchers investigated the expression patterns and characteristics of 8 different affectionate touch actions - embracing, holding, kissing, leaning, petting, squeezing, stroking, and tickling - in a self-report study. It was found that the affectionate touch has distinct target areas on the body, different associated affect, comfort-value, and expression frequency based on the type of touch action that is performed.
Besides the rather obvious sensory consequences of touch, it can also affect higher-level aspects of cognition such as social judgements and decision-making. This effect might arise due to a physical-to-mental scaffolding process in early development, whereby sensorimotor experiences are linked to the emergence of conceptual knowledge. Such links might be maintained throughout life, and so touching an object may cue the physical sensation to its related conceptual processing. Indeed, it was found that different physical properties - weight, texture, and hardness - of a touched object can influence social judgement and decision-making.
For example, participants described a passage of a social interaction to be harsher when they touched a hard wooden block instead of a soft blanket prior to the task. Building on these findings, the ability of touch to have an unconscious influence on such higher-order thoughts may provide a novel tool for marketing and communication strategies.
See also
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Allochiria
Allochiria is a neurological disorder in which the patient responds to stimuli presented to one side of their body as if the stimuli had been presented at the opposite side. It is associated with spatial transpositions, usually symmetrical, of s ...
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Cell signalling
In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...
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Golgi tendon organ
The Golgi tendon organ (GTO) (also called Golgi organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a musc ...
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Haptic communication
Haptic communication is a branch of nonverbal communication that refers to the ways in which people and animals communicate and interact via the sense of touch. Touch is the most sophisticated and intimate of the five senses. Touch or haptics, f ...
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Haptic perception
Haptic perception ( gr, haptόs, script=latn, italics=yes "palpable", ''haptikόs'' "suitable for touch") means literally the ability "to grasp something". Perception in this case is achieved through the active exploration of surfaces and objects b ...
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Muscle spindle
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...
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Molecular cellular cognition Molecular cellular cognition (MCC) is a branch of neuroscience that involves the study of cognitive processes with approaches that integrate molecular, cellular and behavioral mechanisms. Key goals of MCC studies include the derivation of molecular ...
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Phantom limb
A phantom limb is the sensation that an amputated or missing limb is still attached. Approximately 80 to 100% of individuals with an amputation experience sensations in their amputated limb. However, only a small percentage will experience painf ...
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Physical intimacy
Physical intimacy is sensual proximity or touching. It is an act or reaction, such as an expression of feelings (including close friendship, platonic love, romantic love or sexual attraction), between people. Examples of physical intimacy i ...
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Sensory maps
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Special senses
In medicine and anatomy, the special senses are the senses that have specialized organs devoted to them:
* vision (the eye)
* hearing and balance (the ear, which includes the auditory system and vestibular system)
* smell (the nose)
* taste (th ...
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Supramarginal gyrus
The supramarginal gyrus is a portion of the parietal lobe. This area of the brain is also known as Brodmann area 40 based on the brain map created by Korbinian Brodmann to define the structures in the cerebral cortex. It is probably involved wit ...
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Tactile illusion A tactile illusion is an illusion that affects the sense of touch. Some tactile illusions require active touch (e.g., movement of the fingers or hands), whereas others can be evoked passively (e.g., with external stimuli that press against the skin ...
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Vibratese
Vibratese is a method of communication through touch. It was developed by F. A. Geldard, 1957.Adventures in tactile literacy. Geldard, Frank A. ''American Psychologist''. 12(3), Mar 1957, 115–124. It is a tactile system based on both practical ...
, method of communication through touch
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Tactile imaging
Elastography is any of a class of medical imaging modalities that map the elastic properties and stiffness of soft tissue.Sarvazyan A, Hall TJ, Urban MW, Fatemi M, Aglyamov SR, Garra BSOverview of elastography–an emerging branch of medical im ...
Notes
References
Further reading
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* Flanagan, J.R., Lederman, S.J
Neurobiology: Feeling bumps and holes News and Views, Nature, 2001 July 26;412(6845):389-91.
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* Grunwald, M. (Ed.) ''Human Haptic Perception – Basics and Applications.'' Boston/Basel/Berlin: Birkhäuser, 2008,
Encyclopedia of TouchScholarpedia
''Scholarpedia'' is an English-language wiki-based online encyclopedia with features commonly associated with open-access online academic journals, which aims to have quality content in science and medicine.
''Scholarpedia'' articles are written ...
Expert articles
External links
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Anatomy of Touch Factual documentary series by
BBC Radio 4
BBC Radio 4 is a British national radio station owned and operated by the BBC that replaced the BBC Home Service in 1967. It broadcasts a wide variety of spoken-word programmes, including news, drama, comedy, science and history from the BBC' ...
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