The central processes of trigeminal ganglion cells enter the brainstem at the midpontine level, which also corresponds to the level of the chief sensory trigeminal nucleus (nucleus of cranial nerve V). Some axons terminate in this nucleus (primarily large-caliber axons carrying the information needed for fine discriminative touch), whereas others (intermediateand small-caliber axons that carry information about touch as well as pain and temperature) form the descending trigeminal tract, which descends through the medulla just lateral to the descending trigeminal nucleus. As the tract descends, axons peel off and synapse in this nucleus.
Proprioceptive information is also conveyed via the trigeminal nerve; however, in this unique case the cell bodies of the first-order fibers are located within the CNS in the mesencephalic portion of the trigeminal nucleus. The central processes of these neurons terminate in the motor trigeminal nucleus (to subserve segmental reflexes equivalent to the segmental spinal cord reflexes [see ]), the reticular formation, and the chief sensory trigeminal nucleus.
As may already be clear, information related to the different somatosensory submodalities travels to a large extent into the CNS via distinct sets of axons and targets different structures in the spinal cord and brainstem. Within the CNS this segregation continues as the information travels via separate pathways up the spinal cord and brainstem. For example, from the body, fine discriminatory touch information is conveyed by the dorsal column–medial lemniscus pathway, whereas pain, temperature, and crude touch information is conveyed by the anterolateral system.
Proprioceptive information is transmitted by yet another route that partially overlaps with the dorsal column–medial lemniscal pathway. Note, however, that this functional segregation is not absolute, so, for example, there can be some recovery of discriminative touch ability after a lesion of the dorsal columns. The anterolateral system will be discussed in the section on pain because it is the critical pathway for that information. Here, the central pathways
CHAPTER 7 The Somatosensory System for discriminatory touch and proprioception are considered in detail.
This pathway is shown in its entirety in
Fig. 7.1A.
The dorsal columns are formed by ascending branches of the large myelinated axons of dorsal root ganglion cells (the first-order neurons). These axons enter at each spinal segmental level and travel rostrally up to the caudal medulla to synapse in one of the dorsal column nuclei: the nucleus gracilis, which receives information from the lower part of the body and leg, and the nucleus cuneatus, which receives information from the upper part of the body and arm. Note that across the dorsal columns and across the dorsal column nuclei there is a somatotopic representation of the body, with the legs represented most medially, followed by the trunk and then the upper limb. This somatotopy is a consequence of newly entering afferents being added to the lateral border of the dorsal funiculus as the spinal cord is ascended. Such somatotopic maps are present at all levels in the somatosensory system, at least through the primary sensory cortices.
The dorsal column nuclei are located in the medulla and contain the second-order neurons of the pathway for discriminatory touch sensation. These cells respond similarly to the primary afferent fibers that synapse on them (see the earlier description of afferent types). The main differences between the responses of dorsal column neurons and primary afferent neurons are as follows: (1) dorsal column neurons have larger receptive fields because multiple primary afferent fibers synapse on a given dorsal column neuron, (2) dorsal column neurons sometimes respond to more than one class of sensory receptor because of the convergence of several different types of primary afferent fibers on the second-order neurons, and (3) dorsal column neurons often have inhibitory receptive fields that are mediated through local interneurons.