Fig. 7.1B . These pathways carry both cutaneous and proprioceptive information to the cerebellum. For the trunk and lower limb, the pathway starts with dorsal root ganglion cells whose axons synapse in Clarke’s column (nucleus dorsalis). The cells of Clarke’s column send their axons into the ipsilateral lateral funiculus to form the dorsal spinocerebellar tract, which enters the cerebellum via the inferior cerebellar peduncle. The ventral spinocerebellar tract also provides somatosensory input from the lower limb to the cerebellum. Note the double decussation of the ventral spinocerebellar pathway (one decussation at the spinal cord levels and a second one in the cerebellar white matter). This double crossing highlights the general rule that each half of the cerebellum is functionally related to the ipsilateral side of the body.
To provide proprioceptive information from the lower limb to the cerebral cortex, the main axons of the dorsal spinocerebellar tract give off a branch in the medulla that terminates in nucleus z, which is just rostral to the nucleus gracilis. The axons of cells from nucleus z then form part of the internal arcuate fibers and medial lemniscus and ascend to the VPL nucleus of the thalamus.
The ascending somatosensory pathways to the cerebellum for the upper limb are simpler than those from the lower limb (see
Fig. 7.1B ). The route to the cerebellum starts with dorsal root ganglion fibers from the cervical spinal levels that ascend in the cuneate fasciculus to the external cuneate nucleus. The axons of the external cuneate nucleus then form the cuneocerebellar tract, which enters the cerebellum via its inferior peduncle.
The route to the cerebral cortex for proprioceptive information from the upper limb is identical to that for discriminative touch: the dorsal column–medial lemniscal pathway, with a synapse in the cuneate nucleus and then in the VPL nucleus of the thalamus.
For the head, proprioceptive input is carried by cells of the mesencephalic nucleus of the trigeminal nerve. Recall that the neurons in this nucleus are actually the cell bodies of the primary afferents that innervate stretch receptors in the muscles of mastication (muscles that move the jaw) and in other muscles of the head. The central processes of these neurons project to the trigeminal motor nucleus for local reflexes or to the nearby reticular formation. Axons from these reticular formation neurons join the trigeminothalamic tract, which terminates in the VPM of the thalamus. There are also trigeminocerebellar pathways for conveying somatosensory (tactile and proprioceptive) information from the head to the cerebellum.
The ventroposterior nuclear complex of the thalamus represents the main termination site for ascending somatosensory information in the diencephalon. It consists of two major nuclei, the VPL and VPM, and a smaller nucleus called the ventral posterior inferior (VPI) (see
Fig. 7.5 ). The medial lemniscus forms the main input to the VPL nucleus, and the equivalent trigeminothalamic tract from the main sensory nucleus of the trigeminal nerve forms the main input to the VPM nucleus. These nuclei also receive input conveying pain and temperature information from the spinothalamic or equivalent trigeminothalamic tracts, respectively. The VPI nucleus receives input from the spinothalamic tract. In addition, the spinothalamic tract terminates in parts of the posterior nuclear complex and several other thalamic nuclei.