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).
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.
• 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).
• In contrast, the rapidly adapting (RA) Pacinian corpuscles convey vibration perception as they quickly stop firing to continuous sensory stimulus.
• 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.
• 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).
• 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).
• 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.