How many senses do humans have? Five? You have known this since you were a kid, but, after reading this article, you might want to reconsider.
The somatosensory system detects, relays and processes somatosensory sensations, like touch, vibration, pressure, itch, nociception (information about painful stimuli), temperature, proprioception (information about the position and movement of our joints and muscles) and interoception (information about internal organs). So, without taking into account what other sensory systems do, the somatosensory system is already detecting more than five senses by itself.
The somatosensory system has been linked to the sense of self awareness as it provides an internal representation of the body. In pathologies affecting the somatosensory system, such as the phantom limb or the hemineglect syndrome, this awareness of the body is challenged.
This article will look at the basic anatomy and physiology of the somatosensory system, from receptors in the periphery to the cerebral cortex in the brain.
To explain the properties of the somatosensory receptors we are going to use two important concepts:
The somatosensory system is generally considered to have four modalities: mechanoreception, propioception, temperature and nociception. Each of these modalities has its specialised receptors, which will transmit their information through specific anatomical pathways to the brain.
Of the four modalities we will concentrate on touch.
Mechanoreceptors deal with mechanical stimuli, such as pressure and vibration. Most mechanoreceptors are found in the skin, although some are also found in the muscles, tendons and joints.
There are four types of skin mechanoreceptors:
Anatomical pathways are composed by populations of neurons. The speed by which information will reach the brain will depend on the type neurons that compose the anatomical pathway. To be more precise, by the size and myelination of the axons of these neurons. The bigger and the more myelinated* the axon is, the faster the signal will travel. Depending on these characteristics, axons are classified as:
* Myelin is a sheath that covers the axons of some neurons. The more myelinated an axon is, the faster the signal will travel through it, because the myelin sheath increases the speed of the electrical signal travelling through the axon by decreasing capacitance and increasing electrical resistance.
You fall on a pool (with water) belly first. What will you experience? First you will perceive the position of your body followed by the temperature of the water. Then you will have a quick acute pain in the stomach, the first area that contacted the water. Finally, a pain that starts a few miliseconds later, which makes your body curl. What happened? The A alpha fibers have transmitted information about the position of your body faster, then the A delta transmit the information of temperature and acute pain. Finally information carried through the C fibers reached the brain with the slow and more intense pain.
Axons from sensory receptors are segregated into parallel anatomical pathways that reach different processing areas in the brain. Each class of sensory receptor is capable of activating a corresponding cortical area. This is known as modality segregation, which allows the brain to reconstruct signals by time, modality and location. This is true for all sensory systems.
Modality specific information travels from the receptors to the brain through different anatomical pahtways:
This is a summary of the ascending pathways carrying somatosensory information to the brain, the descending pathways will not be covered here.
Information from the body:
Large-diameter afferents (A-alpha and A-beta fibres) synapse in the dorsal column nuclei of the medulla (gracile and cuneate), then cross the midline and ascend to the ventro posterolateral (VPL) thalamic nucleus, via the medial lemniscus. They then finally reach the somatosensory cortex. Information of discriminative touch and proprioception travels through fast conducting fibres. The afferents that compose this pathway cross the midline, which means that the right hemisphere of the brain will process the information regarding the left side of the body and viceversa.
Smaller afferents (A-delta and C fibres) synapse in the spinal cord, then cross midline and ascend via the spinal cord and brainstem to the VPL and other nuclei of the thalamus. Collaterals of these axons terminate in the reticular formation of the pons and medulla. Information about temperature, deep touch and nociception travels through slow conducting fibres. The anterolateral system is complex, it splits into three pathways, each of which ends in different brain areas with several relay synapses. Awareness of pain, temperature and deep touch is distributed through different brain areas. The afferents that compose this pathway also cross the midline, this means that the right hemisphere of the brain will process the information regarding the left side of the body and viceversa.
Information from the head:
Large, fast axons (A-alpha and A-beta) innervate the pars oralis and principal sensory nucleus. The second order axons cross the midline, ascend in the trigeminothalamic tract and terminate in the ventro posteromedial nucleus (VPM). From the thalamus information is conducted to the somatosensory cortex. The afferents that compose this pathway cross the midline, which means that the right hemisphere of the brain will process the information regarding the left side of the head and viceversa.
Small, slowly conducting axons (A-delta and C fibres) descend in the spinal trigeminal tract and terminate in the pars caudalis of the spinal nucleus of the brainstem. The second-order axons cross the midline and ascend to the VPM and intralaminar nuclei of the thalamus. From the thalamus information reaches the somatosensory cortex. The afferents that compose this pathway cross the midline, which means that the right hemisphere of the brain will process the information regarding the left side of the head and viceversa.
The thalamus is a structure with the shape of two attached beetles that belongs to the diencephalon. It is essential for gating, processing and transferring information to and from the cerebral cortex. The majority of sensory information coming from the periphery of the body is filtered in the thalamus before reaching the cortex.
The thalamus is divided in multiple specialised nuclei. The thalamic nuclei which process somatosensory information are the ventro posterolateral nucleus (VPL), which processes information from the body, and the ventro posteromedial nucleus (VPM), which processes information from the head. Somatosensory information also reaches the intralaminar nuclei in the thalamus which are inespecific: they process mixed type of information and they reach several different cortical areas.
In 1909 Korbinian Brodmann defined different areas of the cerebral cortex based on their cytoarchitecture (structure and shape of the cells). He discovered that different areas of the cortex have different types, density and organization of neurons and labelled each area with a number. The majority of the areas that Brodmann defined have been correlated with specific physiological functions. His terminology is still in current use today.
In the case of the somatosensory cortices, Brodmann’s areas 1,2 and 3 correspond to the primary somatosensory cortex (SI) and area 7 corresponds to the associative somatosensory cortex.
The somatosensory cortex is divided into:
All the somatosensory cortices are part of the neocortex. Like most of the neocortex, they too are divided in six layers, layer I being at the surface and layer VI the deepest one. Each cortical layer is characterised by different types of neurons and specific connectivity.
Thalamic input goes into layer IV and a minor input goes to layer VI. Information from layer IV is then transmitted to layer II and III where some connections travel horizontally, or intracortically, to neighbouring cortical areas. Apical dentrites from layer V cells contact layer III small pyramidal neurons and send outputs from the somatosensory cortex to subcortical areas. Finally information from layer VI that comes from layer V and the thalamus, sends projections back to the thalamus. This forms a loop of inputs and outputs of information coming into the cortex from deeper nuclei and being sent back to the thalamus and other neighbouring cortical areas.
The term somatotopy refers to the correspondence between a receptors in the body and the dedicated area of the cortex that is activated by it. In the case of the somatosensory system, this means that for every part of the body there is a corresponding area in the brain. A topographic map of the whole body can therefore be found in the somatosensory cortex. The sensory topographic map of our body is known as the sensory homunculus, and it represents the location and the amount of cortical area dedicated to a particular part of the body or the head.
As you can see from the figure, the representation of some parts of the body and head are distorted in the somatosensory cortex. Certain body parts, like the lips and the hands, have more cortical area dedicated to them than some others. This is because spatial resolution in the cortex is correlated with the innervation density of the skin, so the more sensitive a region of the body is, the more space it will take in the cortex. You could look at the sensory homunculus as a map of how sensitive our bodies are: hands and lips are clearly the most sensitive areas, whereas legs and torso are much less sensitive.
The sensory homunculus is different in each species. Rabbits have a broad cortical area dedicated to the representation of their face and especially their teeth. Cats have a great representation of their heads and their four paws. Most remarkably rats and mice have a large cortical area completely dedicated only to their whiskers, known as the barrel cortex. This is such a sophisticated example of cortical specialisation that it has become one of the principal models for the investigation of the somatosensory system.
Somatosensory maps are plastic and can be modified by training or use. They are shaped by experience during development and can be modified during adulthood. As reported by Mezernich in 1984, if a monkey looses a finger of the hand, the receptive field of the monkey's brain adjusts, so the somatosensory homunculus allocates more of its receptive field to the adjacent fingers.
Early reported cases:
Ambroise Pare (1510-1590) was a barber who became a French military surgeon. He observed that amputees reported strange sympthoms. In the 16th century he wrote: "...the patients who many months after the cutting away of the leg grievously complained that they yet felt exceeding great pain of the leg so cut off..."
Silas Weir Mitchell (1871) who was a surgeron in the American Civil War wrote: "...nearly every man who loses a limb, carries about with him a constant or inconstant phantom of the missing member, a sensory ghost of that much of himself and sometimes a most inconvenient presence faintly felt at times, but ready to be called up to his perceptios by a blow, touch or a wind of change...".
What we know today about the phantom limb
Phantom limbs occur in nearly every case of amputees who lose a limb.
The phantom is usually described as resembling the normal somatosensory experience of having the physical limb, the same way as it was before being amputated. It might move, the same way as before, with normal shape and size. As time passes after the amputation, the shape and size of the phantom can become distorted. This is termed as the telescopic phenomenon of the phantom limb. Phantom limbs can sometimes be painful and the sensation can range from itch to a strong muscular contraction to burning pain. Not only this, but pain to the missing limb can be elicited by stimulation of a different part of the body.
Why do amputees feel sensations on a limb that doesn't exist?
When a limb is amputated, changes occur both in the peripheral and the central nervous system:
The fibres belonging to the somatic receptors that innervated the amputated limb are cut, turning into free nerve endings that will eventually loose the myelination. This doesn't mean that they stop transmitting pulses, quite the opposite, the free nerve endings are activated and fire every time the stump is stimulated.
However, the brain codes information coming from those fibers as if they were still coming from the missing limb. So the brain constructs a percept with that sensation that corresponds to the phantom.
In the somatosensory cortex, the areas dedicated to the missing limb do not receive as much stimulation anymore. When an area in the brain stops receiving information it eventually degenerates. As explained earlier, adjacent areas in the homunculus take over the unused area to make the most of the cortical space. It can therefore happen that stimulation of one body part can provoke sensations on the phantom limb.
If you would like to find out more about the phantom limb, we recommend the book by V.S. Ramanchandran and S Blakeslee: "Phantoms in the brain".
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