As described in the previous chapter, thinly myelinated or unmyelinated fibers, subserving mainly pain sensibility, but some sensitive to touch and pressure, enter the cord on the lateral aspect of the dorsal horn and synapse with dorsal horn cells, mainly within a segment or two of their point of entry into the cord. The dorsal horn cells, in turn, give rise to secondary sensory fibers, some of which may ascend ipsilaterally but most of which decussate and ascend in the spinothalamic tracts (see Figs. 7-1 and 7-2). Observations based on surgical interruption of the anterolateral funiculus indicate that fibers mediating touch and deep pressure occupy the ventromedial part (anterior spinothalamic tract). Also as remarked in Chap. 7, an ascending tract of secondary sensory axons lies in or medial to the descending corticospinal system.
After the posterior columns terminate in the gracile and cuneate nuclei of the rostral cervical cord and medulla, synapses are made with fibers that cross the midline and ascend to form the medial lemniscal tracts in the brainstem. The lemniscal system is situated in a paramedian position, changing orientation slightly at different levels of the brainstem, and joining the spinothalamic system in the rostral midbrain to terminate in the posterior thalamic nuclei (Fig. 8-5 and also Fig. 7-2).
Comparable to the peripheral nerves, the sensory cell bodies for touch, pressure, and pain lie in the gasserian ganglion. The pathways mediating cutaneous sensation from the face and head—especially touch, pain, and temperature—are conveyed to the brainstem by the trigeminal nerve. After entering the pons, the fibers subserving touch synapse in the principal trigeminal sensory nucleus and the secondary neurons cross and join the medial lemniscus on its transit to the thalamus. A specialized system for proprioception of mandibular (masseter) movement originates in muscle spindles but has cell bodies in the mesencephalic nucleus of the fifth nerve.
As discussed in the preceding chapter, the pain and temperature fibers of the trigeminal nerve turn caudally and run through the ipsilateral medulla as the descending spinal trigeminal tract, synapsing with the long, vertically oriented spinal trigeminal nucleus that lies beside the tract. Axons from the neurons of this nucleus cross the midline and ascend as the trigeminal quintothalamic tract (also termed, somewhat imprecisely, trigeminal lemniscus) along the medial side of the spinothalamic tract (see Figs. 7-2 and 8-5), of which it is the equivalent. There is a somatotopic organization of fibers for pain and temperature, but not touch, in the trigeminal nucleus, so that the central part of the face is represented most rostrally, and the peripheral portion, most caudally, giving rise to an “onion skin” pattern of sensory loss (see also Chap. 44).
The ventral posterior thalamic nucleus receives fibers from the medial lemniscal, spinothalamic, and trigeminal (fibers from the principal sensory and spinal trigeminal nuclei) tracts, and projects mainly to two somatosensory cortical areas. The first area (S1) corresponds to the post-central cortex or Brodmann areas 3, 1, and 2 (see Fig. 3-3). S1 afferents are derived primarily from the ventral posterolateral nucleus (VPL, the terminus of medial lemniscal and spinothalamic fibers) and the ventral posteromedial nucleus (VPM, the terminus of trigeminal fibers) and are distributed somatotopically, with the leg represented superomedially and the face inferolaterally (face and hand are juxtaposed). Figure 8-6 (“sensory homunculus”) shows the cortical representation of sensory information in the postcentral gyrus. As in the case of the motor representation (see also Fig. 3-4), a disproportionate area is devoted to localization in the fingers, lips, and face.