Previous chapters described the ways in which the different somatosensory receptors respond to specific types of somatosensory stimuli and that the receptors, by virtue of their selective sensitivities, extract specific information about the somatosensory stimulus. The specificity of the receptors forms the basis for a parsing (i.e., a sorting) of somatosensory experience into separate “information channels” or pathways. For example, sharp-pricking pain is mediated in the neospinothalamic (information channel) pathway, whereas proprioception is mediated in the medial lemniscus pathway. Recall that the receptor's extraction of somatosensory information is very specific (e.g., during limb movement, muscle spindles respond to muscle stretch, whereas Golgi tendon organs respond to muscle contraction) and the processing of this extracted information is kept separate along most of the ascending pathway. In addition to this parsing of stimulus information, the somatosensory system is also organized to provide a somatotopic representation of the body surface and parts. The resulting spatial maps provide the anatomical basis for our ability to localize somatosensory stimuli and for our sense of a 'body image".
As described above, the nervous system reduces somatosensory experience into parallel streams of neural activity - a decomposition of the experience into stimulus fragments spread over body pieces. So how does one have a sense of "oneness" of the body and how does one identify an object by handling it? One can do so because somatosensory information converges in the parietal lobe of the cerebral cortex to provide a cohesive perception of the body and of somatosensory stimuli.
The first part of this chapter will present additional details about the general organization of the somatosensory system and how somatosensory information is represented and processed in the parietal cortex. This understanding of the general organization of the somatosensory pathways will be used in the clinical assessments of somatosensory function.
Sensory Pathways Decussate before Reaching the Thalamus
In each of the somatosensory pathways covered thus far, afferent axons decussate (cross the midline) once on their course to the thalamus (Figure 5.1).
Above the level of decussation, the neurons in a somatosensory pathway represent the contralateral (i.e., opposite) side of the body or face. It is important to learn the decussation site, as it will aid in clinical diagnosis. When an afferent pathway is damaged somewhere below the site of decussation, the sensory loss will be on the side ipsilateral to the lesion (i.e., the loss is on the same side as the lesion or ipsilesional). When an afferent pathway is damaged somewhere above the site of decussation, the sensory loss will be on the side contralateral to the lesion (i.e., the loss is on the side opposite the lesion or contralesional).
In the medial lemniscal pathway, the axons of the gracile and cuneate nuclei decussate in the medulla. The decussation in the neospinothalamic pathway is in the spinal cord and involves the axons of the posterior marginal nucleus. The spinal trigeminal nucleus axons decussate upon leaving the nucleus in the medulla and lower pons, whereas the main sensory trigeminal nucleus axons decussate at mid pons levels immediately upon leaving the nucleus.
Modality Specificity is Maintained up to the Cortex
The sensory information necessary for discriminative touch, proprioception, pain and thermal sensations are kept separate within the somatosensory pathways (Figure 5.2).
To ensure the fidelity of stimulus representation in the discriminative touch-proprioceptive pathways, there is minimal convergence of modality specific information along the pathways. That is, within the medial lemniscal and main sensory trigeminal pathways, there is little mixing of information from different receptor types and from afferents with different adaptive properties up to the level of the cerebral cortex. For example, a cuneate nucleus neuron (2° medial lemniscal afferent) will synapse only with one type of posterior root neuron (e.g., 1° afferents with small “touch” receptive fields and rapidly adapting discharges). Within VPL and VPM, the medial lemniscal and ventral trigeminal lemniscal fibers terminate in different regions based on the sensory information they are carrying. Fibers carrying cutaneous information terminate within the core of the nucleus, whereas those carrying proprioceptive information terminate in the surrounding, peripheral shell of the nucleus.
Within the cerebral cortex, there is a convergence of modality specific information and the response properties of cortical neurons become more complex. The primary somatosensory cortex is responsible for the first stage of cortical processing. Within the primary sensory cortex, discriminative touch and proprioceptive information from overlapping areas may be combined such that a cortical neuron may respond to both cutaneous and proprioceptive stimulation of a digit.
Somatosensory Neurons have Receptive Fields
Each subcortical somatosensory neuron responds to modality-specific stimuli applied to a specific region of the body or face.
For example, an axon in the medial lemniscus (i.e., the fiber tract) that responds to tactile stimulation of the right index finger pad will not respond to tactile stimulation of any other area in the hand, body or face. The stimulated area producing the response is called the neuron’s receptive field (Figure 5.3). The neuron’s receptive field can also be defined anatomically as that area of the sense organ (i.e., skin, muscles or joints) innervated directly or indirectly by the neuron. Consequently, a somatosensory neuron can be described to channel information about stimulus location - as well as stimulus modality. Furthermore, the size of a neuron’s receptive field is related to the body area innervated/represented. The receptive fields of neurons innervating/representing the finger pads, lips, and tongue are the smallest, whereas those of neurons innervating/representing the shoulders, back and legs are the largest. For greater accuracy in locating the point of stimulus contact or movement, smaller cutaneous receptive fields are required. For fine motor control, as in playing the piano or speaking, small proprioceptive receptive fields are required.
Spatial Information is Topographically Mapped in Sensory Pathways
Within each somatosensory structure, neurons are organized to provide a spatial representation of the body and face called the somatotopic map (Figure 5.4). That is, within the spinal cord, brain stem, thalamus and postcentral gyrus, the location of a neuron is related to its receptive field. Consequently, body and face (i.e., the receptive fields) are represented spatially (topographically) within nuclei and cortex such that, neurons with contiguous receptive fields are located adjacent to one another within a given structure. For example, adjoining areas of the body are represented in adjoining areas of the cortex (Figure 5.4). The resulting neural maps of the body and face are not isomorphic representations and appear distorted because of the disproportionate representation of the hand and face areas (Figure 5.4). That is, as the neurons representing the hand and face have small receptive fields (Figure 5.3), a greater number of neurons are required to represent the hand and face. Because somatosensory neurons represent specific stimulus features and specific areas of the body or face, electrical stimulation of a restricted area of the postcentral gyrus (e.g., the area representing the tongue) will produce a somatic (and not gustatory) sensation that is perceived as arising from the specific region of the body (i.e., the tongue).
