Somatosensory system
From Medical-Wiki
The somatosensory system carries general somatic sensory information to the central nervous system. For example, it carries pain or touch sensations from different parts of the body.
There are two basic pathways that carry different types of information from the body (that is, everything except the head). Light touch, proprioception and vibratory sensations are carried in the dorsal column medial lemniscus pathway. Pain and temperature information, on the other hand, is carried in the anterolateral system. Somatosensory information about the face is, for the most part, carried by the trigeminal nerve.
Types of sensory fibers
Fibers within a peripheral nerve are classified by conduction speed and diameter.
Fast fibers (A-alpha and A-beta fibers) are fibers of the DC-ML system, so carry light touch, proprioception and vibratory information. Specifically, A-alpha fibers carry proprioception while A-beta fibers carry fine touch.
Slow fibers (A-delta and C fibers) are fibers of the anterolateral system, so carry pain and temperature information. A-delta fibers are lightly myelinated, so conducted slowly. C-fibers are unmyelinated, so their conduction velocity is the slowest. In fact, you often perceive a painful stimulus in two stages: the first, rapid sensation is carried by A-delta fibers and can mediate a withdrawal reflex (imagine touching a hot pan and pulling away immediately), the second slower sensation is carried by C-fibers and produces the long-lasting painful sensation (the burning "ow" sensation that comes half a second after touching the hot pan).
Dorsal column - medial lemniscus system
Peripheral receptors
Pacinian corpuscles Pacinian corpuscles sit deep in the dermis and sense vibratory stimuli. They typically respond to the beginning and end of a constant stimulus, but very quickly adapt to constant stimuli. Because the respond to changes in pressure, rather than constant pressure, they respond well to vibrations. However, because they are found deep in the dermis Pacinian corpuscles do not have good spatial resolution.
Meissner's corpuscles Like the Pacinian corpuscles, the nerve endings of Meissner's corpuscles are embedded in an extracellular matrix that makes them more sensitive to vibratory stimuli than constant stimuli. However, because they are more superficial than Pacinian corpuscles, sitting near the epidermis-dermis border, they have better spatial resolution.
Merkel disk receptors Merkel disks consist of sensory receptor cells coupled, probably by a chemical synapse, to a sensory neuron. While the exact mechanism of mechano-sensory transduction is unknown, we can guess that deformation of the skin exerts pressure on the epithelial cell of the Merkel disk through the tight junctions it shares with neighboring epithelial cells. The cell surrounds its associated nerve ending and probably releases some sort of neurotransmitter onto the nerve, which can fire an action potential up.
Because the Merkel disk receptors sit superficially and are tightly coupled to basal epithelial cells, they are in a position to sense light touch with excellent spatial resolution. The density of this type of receptor is particularly high in parts of the body, like the fingertips, for which fine touch sensation is important.
The Merkel disk receptors adapt slowly, so they continue to signal constant stimuli much longer than either Pacinian or Meissner's corpuscles.
Ruffini endings Like Merkel disks, Ruffini endings signal constant stimuli well. However, because they are found deeper in dermis than Merkel disks, Ruffini endings have poor spatial resolution. Consistent with that, the density of Ruffini endings is almost always less than that of Merkel cells.
Receptor density varies by area
Our fingertips are much more sensitive to touch then is our back, and this is due primarily to differences in receptor density between the two areas. The density of all four types of receptors is much higher in the fingertips than in the back. The easiest way to appreciate this difference is to try the two-point discrimination test, for which you can use the backs of a pair of pins to test how far apart the pins need to be to tell if there are two or only one pin on different parts of the skin. At the fingertips, you can sense two separate points at distances as small as 2 mm. Up on the arm, you might not be able to distinguish two pins even if they are separated by 20 mm, and on the back the minimum distance can be greater.
Peripheral nerves innervate dermatomes
Because peripheral nerves enter the spinal cord through the dorsal root entry zone at each spinal level, each peripheral nerve carries information from a restricted band of skin. This band is called a dermatome, and mapping out sensory deficits by dermatome can help localize lesions.
Cell bodies of peripheral nerves in dorsal root ganglion
All the somatosensory neurons from the body have their cell bodies in the dorsal root ganglion, and extend an axon into the spinal cord through the dorsal root entry zone.
Dorsal column tracts
Fascicullus gracilis
The fasciculus gracilis carries fibers from the lower body, including the legs. As fibers enter, they are added to the lateral side of the dorsal funiculus, so the most medial fibers in the fasciculus gracilis come from the lowest dermatome. Because fibers are coursing rostrally, all levels of the spinal cord have a fasciculus gracilis.
Fascicullus cuneatus
The fasciculus cuneatus carries fibers from the upper half of the body, including the arms. It is the more lateral of the two fasciculi in the dorsal funiculus, and, because it carries fibers from the arm, is only found at cervical levels.
Nuclei and decussation in the medulla
Fibers from both fasciculus gracilis and cuneatus terminate and synapse within the medulla in nucleus gracilis and nucleus cuneatus, respectively. The second order neurons, whose dendrites sit in the nuclei, the decussate immediately through the internal arcuate fibers. Recall that when fibers decussate they typically cross both the midline (hence "decussation") and the dorsal-ventral boundary. So while the primary sensory neurons run in the dorsal half of the spinal cord (like good sensory systems should), the second-order decussated fibers move into the ventral portion of the medulla to run in the medial lemniscus.
Medial lemniscus carries fibers to thalamus
Medial lemniscus starts medial in medulla, then as it courses rostrally its position shifts laterally around the bottom of the tegmentum. Its final position is lateral as it enters the VPL of the thalamus. As it moves rostrally the medial lemniscus picks up fibers from the trigeminal nuclei, with the fibers adding to the medial/ventral end of the medial lemniscus.
The medial lemniscus is somatotopically organized, meaning that its organization can be mapped onto the shape of the body. Nearby parts of the body are represented by nearby locations in the medial lemniscus.
VPL is the primary thalamic relay nucleus for somatosensation from the body
The medial lemniscus terminates in the ventral-posterior-lateral nucleus of the dorsal thalamus. The is the primary relay nucleus of the somatosensory system for the body, receiving input from both the dorsal column-medial lemniscus system and the antero-lateral system.
The VPL then projects out to the primary somatosensory cortex through the internal capsule.
Anterolateral system
The anterolateral system carries pain and temperature information from the periphery to the VPL nucleus of the thalamus, and on up into the somatosensory cortex.
Peripheral receptors
Pain receptors (called nociceptors) are "free nerve endings," that is, there is no extra-cellular matrix capsule or epithelial cell receptor coupled to the neuron. These free nerve endings are effectively chemosensors, responding to cellular damage by detecting small molecules like ATP, bradykinin, prostaglandins, serotonin, protons (pH changes),and several other molecules released by injured cells.
In addition to initiating an action potential that propogates toward the spinal cord, the nociceptors amplify local inflammation by releasing a series of compounds to activate other local structures. For example, they release Substance P, which causes degranulation of local mast cells. They can also cause local vasodilation through the release of CGRP onto capillaries.
There are also temperature receptors - these will be covered in the section on the trigeminal nerve since these receptors, in addition to responding to different temperatures, are also sensitive to chemicals to which we often associate certain flavors (menthol and capsiacin).
Cell bodies in dorsal root ganglion
Just as the cell bodies of the DC-ML system sit in the dorsal root ganglion, so do the cell bodies of the anterolateral system. The endings of the anterolateral system are also distributed over dermatomes.
Synapse in substantia gelatinosa and decussate
The DC-ML and anterolateral pathways diverge immediately upon entering the spinal cord. Fibers of the anterolateral system enter the spinal cord then can go up or down a few spinal levels in Lissauer's tract. The axons then enter the dorsal horn and synapses within the Lamina II/III of the spinal cord gray matter. Lamina II/III in the dorsal horn is known as "substantia gelatinosa" and has a characteristic appearance that is also found in the spinal trigeminal nucleus.
The second-order fibers from the substantia gelatinosa decussate immediately and run rostrally in the tracts of the anterolateral system. At this point the spinal cord carries ipsilateral light touch, proprioception and vibration information, but contralateral pain and temperature information. Unilateral lesions of the spinal cord therefore produce contralateral pain and temperature deficits and ipsilateral touch, proprioception and vibratory sensory deficits.
Anterolateral tracts to VPL in thalamus
The anterolateral tracts run lateral and ventral through the spinal cord and most of the midbrain. The medial lemniscus joins the anterolateral tracts just before entering the thalamus. Both pathways synapse within the VPL of the thalamus.
Trigeminal system
Peripheral receptors
The trigeminal nerve carries information all the same type of peripheral receptors as both the DC-ML and anterolateral systems.
The trigeminal nerve innervates the face and mouth, and some of what we think of as taste comes from activation of the trigeminal nerve. In addition to sensing the texture of food, taste cues come from temperature and chemical sensitivity. In this regard, the temperature receptors do double duty. Temperature-sensitive ion channels were first identified using binding and sensitivity to the chemical capsiacin, which is the active ingredient in hot peppers. It turns out that foods that are spicy hot actually activate the same ion channels that detect temperature hot.
After the capsciacin receptor was discovered, the cold-temperature receptor was isolated based on its sensitivity to menthol. Both the capsciacin and menthol sensitive channels were from the TRP channel family. So the trigeminal nerve carries both temperature and chemical sensitivity that makes a small contribution to taste.
Sensory trigeminal nuclei
There are three trigeminal sensory nuclei, each nucleus having a comparable function to an analogous structure serving the body.
Spinal trigeminal nucleus (in the medulla) receives primarily pain and temperature inputs from the face, and has a similar structure to substantia gelatinosa, the recipient zone of pain and temperature information in the spinal cord. Note that this nucleus receives mostly fibers from the trigeminal nerve, but also somatosensory fibers from the vagus, glossopharyngeal and facial nerves.
The main sensory nucleus of the trigeminal nucleus receives light touch and conscious proprioception (note "conscious" proprioception differentiates its function from the trigeminal mesencephalic nucleus). This is most analogous to the dorsal column-medial lemniscus system.
The mesencephalic trigeminal nucleus is actually considered a "peripheral nerve" since it is part of a reflex system just like "unconscious" proprioceptive peripheral nerves in the rest of the body. While it resides in the CNS, the nucleus has no chemical synapses, and neurons are pseudo-unipolar receiving proprioceptive information from the jaw. Neurons in the nucleus project directly to the motor nucleus of the trigeminal nerve and mediate the jaw reflex (recall that the trigeminal nerve is also motor to muscles of mastication).
Central tracts
Post-synaptic fibers from spinal trigeminal nucleus and the main sensory trigeminal nucleus contribute to the anterolateral system and medial lemniscus respectively.
These fibers project to the VPM, which is the primary thalamic relay nucleus for somatosensation from the head.
Somatosensory cortical areas
VPM and VPL send projections up to primary somatosensory cortex, defined by Brodmann's areas 3, 1, and 2 sitting on the post-central gyrus. Because all the somatosensory tracts are crossed (remember the different decussation points for the DC-ML, anterolateral and trigeminal systems), the somatosensory cortex has a detailed representation of the CONTRALATERAL surface of the body.
While areas 3, 1 and 2 are all typically considered primary somatosensory areas, Area 3 receives the bulk of projections from the thalamus so is more properly consider THE primary somatosensory cortex (S1). Areas 1 and 2 receive input from S1, and the receptive fields within these higher areas are more complex than what is observed in S1.
Sensory homonculus
Just as there is a somatotopic map in the medial lemniscus and VP nuclei, the somatosensory cortex has a map of the body. In fact, it has multiple, mirror image maps, each responsive to different aspects of somatosensory inputs. The somatotopic map in S1 is called the homunculus (see figure) and is distorted by areas that have a high density of sensory inputs. For example, the back takes up a very small portion of S1, while the fingertips, face and mouth take up a disproportionately large area of cortex. This distorted representation is similar to that seen in visual cortex, where the fovea takes up a disproportionately large area of V1.
receptive field structure and lateral inhibition
Each neuron in the somatosensory area responds to a small part of the body, this corresponds to the spatial outline of the neuron’s receptive field. The receptive field, however, is also determined by the type of information it receives (eg. Pain, temperature, proprioception, etc.), the level in the processing system (higher cortical areas have progressively larger receptive fields that integrate across multiple modalities), and the type of interactions among neurons.
One prominent type of processing of somatosensory information is lateral inhibition. Lateral inhibition takes place at several levels, starting in the spinal cord. Like the effect of the center-surround structure of retinal ganglion cell receptive fields, lateral inhibition acts to increase the contrast sensitivity of somatosensory neurons. That is, nearby neurons inhibit each other. If a sensory surface (like your fingers) pass over a smooth surface all peripheral neurons would be activated equally, but all would inhibit each other in the spinal cord. If, however, a small group of sensory receptors are activated by a small bump (say the F and J markers on your keyboard) but surrounding sensors are only weakly activated by the smooth area surrounding (say the full F or J key), then the strongly activated receptors would inhibit the responses of the neighboring receptors (in the spinal cord and higher) so that the signal from the small bumps are progressively accentuated up the sensory system. Effectively, lateral inhibition makes our brain ignore smooth surfaces but become highly sensitive to changes in surfaces.
