Somatosensory System
The somatosensory system is the
part of the nervous system that is
involved in the processes of touch, pressure, proprioception (or joint position
sense; see also Chapter 36), pain and temperature perception (see Chapters 32
and 33).
Sensory receptors
The receptors for touch are
specialized nerve endings located in the skin with their cell bodies in the
dorsal root ganglia. They are found at particularly high density in the
fingertips, while those for proprioception are found not only in the skin but
also in the muscle and joints (see Chapter 36).
Skin receptors can best be characterized
by their structure, location, receptive fields and speed of adaptation.
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Type I
receptors with very small, sharply demarcated receptive fields (Meissner’s
corpuscles and Merkel’s discs) are packed in high density at the
fingertips. In particular, Meissner’s corpuscles convey information about
objects slipping or moving across the skin, while Merkel’s discs are more
involved with fine touch (i.e. sensory detail).
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In
contrast, the rapidly adapting (RA) Pacinian corpuscles convey vibration
perception as they quickly stop firing to continuous sensory stimulus.
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The more
slowly adapting (SA) Ruffini endings sense the magnitude, direction and
rate of change of tension in the skin and deeper tissues (i.e. skin stretch).
Dorsal column–medial lemniscal
pathway The sensory receptors
are specialized nerve endings and the fast conducting, large diameter axons
associated with them are found in peripheral nerves and project into the dorsal
horn of the spinal cord. The trigeminal sensory system for the face
has a similar organization.
Each class of receptor has a
specific pattern of passage through the dorsal horn, but all ultimately end up
in the dorsal column (with the exception of the trigeminal system),
where they are organized according to receptor type and body location
(somatotopy; see Chapter 9). They then project ipsilaterally up to the dorsal
column nuclei at the cervicomedullary junction (consisting of the gracile and
cuneate nuclei), where they make their first synapse, although it should be
understood that many dorsal column axons synapse at other spinal sites.
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The dorsal
column nuclei (DCN) are a complex series of structures that lie at
the cervicomedullary junction and send axons which immediately decussate to
form the medial lemniscus, which projects to the thalamus. The DCN also
project to other brainstem structures, as well as receiving input from the
primary somatosensory cortex (SmI).
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The medial
lemniscus projects to the ventroposterior (VP) nucleus of the
thalamus, connecting with the trigeminal system as it ascends. This latter
projection synapses in the medial part of the VP nucleus (VPM) with the
remainder of the tract terminating in the lateral nucleus (VPL). This medial
lemniscal termination is in the form of an anteroposterior thalamic rod, where
all the cells within the rod have a similar modality and peripheral location
(e.g. index finger, RA type I
receptors). The thalamic rod subsequently projects to layer IV of the SmI and forms the basis of the
cortical column (see also Chapter 10).
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The SmI
consists of four different areas (Brodmann’s areas 3a, 3b, 1 and 2), each
of which has a separate representation of the contralateral body surface, with
the tongue being represented laterally and the feet medially. The cortical
representation is proportional to the receptor density in the skin so, for
example, the hand has a much greater representation than the trunk (the sensory
homunculus).
Primary and secondary sensory
cortices Each cortical area
within SmI has slightly different response properties with respect to the
neurones found in these areas. As one moves towards the posterior parietal
cortex the response properties of the neurones become more complex, implying a
higher level of cortical analysis. SmI projects not only back to the dorsal
column nuclei but to the posterior parietal cortex and second
somatosensory area (SmII). This latter area is found in the lateral
wall of the Sylvian sulcus and is important in tactile object recognition,
while the posterior parietal cortex input from SmI is important in the
attribution of significance to a sensory stimulus (see Chapter 34).
The primary somatosensory pathway
has developed during evolution with the corticospinal tract (CoST), which has a
selective role in the control of fine finger movements (see Chapters 35–39).
These two systems act together in the process of ‘active touch’ by which we
explore our environment. Both systems display a degree of plasticity even in
adult life (see Chapters 39 and 49). This is in part made possible by
somatotopic organization of the sensory pathway: adjacent areas of skin are
represented in neighbouring parts of the sensory system, at least as far as
SmI.
Clinical disorders of the
somatosensory system
Damage to the receptors and their
afferent fibres can occur in a large number of peripheral neuropathies.
Patients typically complain of both paraesthesiae and numbness, often in
association with alterations in proprioception especially if the dorsal root
ganglion is involved (see Chapter 54).
Damage to the somatosensory pathway
above the level of the DCN produces a contralateral sensory loss that will
involve the face if the lesion lies at or above the level of the upper
brainstem. Lesions to the dorsal columns in the spinal cord are described in
Chapter 54.
