Brainstem Structures Associated with Posture and Balance
Vestibular System. The cerebellum receives signals from the otolith organ (i.e., macula of saccule and utricle) and semicircular canals of the vestibular system. Fibers arising from the vestibular apparatus may enter the cerebellar cortex via a monosynaptic or disynaptic pathway. The monosynaptic pathway (called the juxtarestiform body) involves first-order vestibular neurons that terminate within the ipsilateral flocculonodular lobe. The second route involves primary vestibular fibers that synapse in the vestibular nuclei and second-order neurons that project chiefly from the inferior and medial vestibular nuclei to the same region of cerebellar cortex (Fig. 21-5). In this manner, the cerebellum receives important information concerning the position of the head in space at any given point in time as well as the status of those ves-tibular neurons that regulate extensor motor neurons (via the vestibulospinal and reticulospinal tracts). There is a further differentiation of function within the cerebellar cortex in that the flocculonodular lobe represents the specific receiving area for vestibular inputs, and the anterior lobe is the primary receiving area for spinal cord afferents.
Reticular Formation. The reticular formation also powerfully modulates spinal reflexes, acting, in part, on the gamma motor system (principally of extensor muscles). This important component of motor systems provides a key input to the cerebellum that presumably relates to the manner in which reticular neurons regulate extensor motor tone at any given point in time. The fibers that project to the cerebellar cortex from the reticular formation arise from the lateral and paramedian reticular nuclei of the medulla and from the reticulotegmental nucleus of the pons. These fibers are distributed mainly to the vermal region of the anterior and posterior lobes. Much of the information transmitted from the reticular formation is not modality specific. However, it is believed that the lateral reticular nucleus may transmit tactile impulses from the anterolateral funiculus to the cerebellar cortex. For this reason, the reticulocerebellar projection may contribute to the spinocerebellar system. Moreover, because the lateral reticular nucleus receives inputs from the spinal cord and cerebral cortex, in effect, it is also capable of sending an integrated signal to the cerebellar hemispheres (through the mossy fiber system) in a manner parallel to that of the inferior olivary nucleus (Fig. 21-5).
It should also be noted that several of the nuclei of the reticular formation that project to the cerebellum also receive significant inputs from the sensorimotor cortex. This indicates that the reticular formation also serves as a relay for cerebral cortical inputs to the vermal and par-avermal regions of the anterior and posterior lobes of the cerebellar cortex. This circuit thus provides the cerebellum with the neural substrate for both coordination of movements and movements that govern equilibrium and maintenance of an erect posture.
Cerebral Cortex
The cerebral cortex is concerned with the coordination, planning, and execution of movements. As indicated earlier, there are several ways by which the cerebral cortex can transmit such information to the cerebellar cortex. One way involves a multisynaptic pathway whose initial link is from the cerebral cortex to the red nucleus, the second limb is from the red nucleus to the ipsilateral inferior olivary nucleus, and the final limb includes a projection from the inferior olivary nucleus to the contralateral cere-bellar hemisphere (Fig. 21-6). The other ways include projections to relay nuclei located in the basilar part of the pons and reticular formation.
Whereas the major inputs to the cerebellar cortex from the reticular formation are directed to the vermal and par-avermal regions, the projections from the red nucleus (via the inferior olivary nucleus) and deep pontine nuclei to the cerebellar cortex are directed mainly to the cerebellar hemispheres. One point to be noted is that there appear to be "patches" of somatotopically organized regions of cerebellar cortex restricted mainly to the vermal and par-avermal regions. Specifically, evidence supports the view that the axial musculature is represented mainly in the vermal region, whereas the distal musculature is represented more laterally in the paravermal regions of the hemispheres. Overall, the pathways from the cerebral cortex to the deep pontine nuclei and red nucleus represent the substrates by which the cerebral cortex can coordinate movements associated mainly with the distal musculature.
Red Nucleus
We have indicated that the red nucleus serves, in part, as a relay from the sensorimotor cortex to the spinal cord (via the rubrospinal tract) that activates the flexor motor system. In a similar manner, the sensorimotor cortex can provide signals to the cerebellar cortex through relays in the red nucleus and inferior olivary nucleus (Fig. 21-5). Through this circuit, the status of neurons in the red nucleus (and, indirectly, those of the motor regions of the cerebral cortex) is transmitted in a somatotopic manner to the cerebellar cortex. The red nucleus communicates with the cerebellar cortex through the following pathway: red nucleus ^ inferior olivary nucleus (crossed olivocerebellar fibers) ^ contralateral anterior and posterior lobes of cer-ebellar cortex.
Deep Pontine Nuclei
The primary route by which the cerebral cortex communicates with the cerebellar cortex is via a relay in the basi-lar (ventral) pons. Fibers arising from all regions of the cerebral cortex project through the internal capsule and crus cerebri, making synaptic connections upon deep pontine nuclei. The deep pontine nuclei give rise to axons called transverse pontine fibers that enter the contralateral middle cerebellar peduncle and are distributed to the anterior and posterior lobes of the cerebellum (Fig. 21-6).
The largest component of the projection to the cerebel-lar cortex arises from the frontal lobe. This provides the primary substrate by which motor regions of the cerebral cortex can communicate with the cerebellar cortex. However, sensory regions of the cerebral cortex also contribute fibers to the cerebellar cortex. These include parietal, temporal, and visual cortices. The posterior parietal cortex provides the cerebellum with information concerning the planning or programming signals that are transmitted to the motor regions of the cerebral cortex.
FIGURE 21-6 The disynaptic pathway from the cerebral cortex to the cerebellar cortex with a synapse in the deep pontine nuclei. Corticopontine fibers are shown in red, and pontocerebellar fibers are shown in blue.
FIGURE 21-7 Homunculi illustrating the somatotopic organization of the cerebellar cortex. Note that the body is represented on the cerebellar cortex more than once and that the axial musculature is represented in a medial position, whereas the distal musculature is represented more laterally. The neuronal regions associated with regulation of the distal musculature are shown in green, and the regions associated with the axial musculature are shown in red.
Temporal and occipital cortices provide the cerebellar cortex with signals associated with auditory and visual functions. In particular, the connection from the visual cortex may signal such events as moving objects in the visual field. Visual and auditory signals may also reach the cerebellar cortex from the tectum (see "Tectum" section). Somatosensory signals also reach the cerebellar cortex from the cerebral cortex. Evidence suggests that fibers from the sensorimotor cortex are somatotopically arranged within the vermal and par-avermal regions of the cerebellar cortex in a manner that corresponds to the somatotopic organization associated with spinal cord inputs. However, there is little evidence that the lateral aspects of the hemisphere of the posterior lobe are somatotopically organized.
Other Inputs to the Cerebellar Cortex
Tectum
In addition to the inputs from the occipital and temporal neocortices, fibers arising from both the superior colliculus and inferior colliculus of the tectum also provide visual and auditory information, respectively, to the cerebellar cortex (Fig. 21-7). They do so by projecting to the pontine nuclei, which, in turn, project through the middle cerebellar peduncle to the cerebellar cortex.
Trigeminal System
Secondary proprioceptive fibers associated with muscle spindle activity of muscles of the face and jaw reach the cerebellum chiefly from the mesencephalic trigeminal nucleus. Other (tactile) sensory fibers arising from trigeminal nuclei have been shown to project to mainly the ver-mal and paravermal regions of the cerebellar cortex. Their distribution to the cerebellar cortex is directed to the head regions depicted by the homunculus Figure 21-7.
Monoaminergic Systems
Brainstem neurons contribute noradrenergic and seroton-ergic fibers to wide areas of the cerebellar cortex. The noradrenergic fibers arise mainly from the locus ceruleus, and the serotonin fibers arise from the raphe complex. Both groups of fibers function as modulators of the activities of cerebellar cortical neurons.
The Anatomical and Functional Organization of the Cerebellar Cortex
Mossy and Climbing Fibers
There are two kinds of afferent fibers that convey impulses to the cerebellar cortex. They are identified on the basis of their morphology and are referred to as mossy and climbing fibers (Fig. 21-8).
Mossy Fibers
Mossy fibers are found widely throughout the cerebellum. As they course through the granular layer, they give rise to many branches in this layer. These branches terminate by forming mossy fiber rosettes, which are held by claw-like dendrites of the granule cells (Fig. 21-9). Thus, impulses traveling along a single mossy fiber can activate many granule cells. The mossy fiber rosettes form the focus of a cerebellar glomerulus, which consists of the synaptic relationships between mossy fiber axons and both granule cell dendrites and Golgi cell axon terminals. Specifically, the cerebellar glomerulus consists of: (1) a mossy fiber rosette, (2) dendrites of many granule cells, (3) the proximal aspect of Golgi cell dendrites, and (4) terminals of Golgi cell axons. Mossy fibers are excitatory and arise from all regions of the CNS that project to the cerebellar cortex with the exception of the inferior olivary nucleus. The neurotrans-mitter released from the endings of mossy fibers is believed to be glutamate.
FIGURE 21-8 Diagrammatic representation of a folium of the cerebellar cortex illustrating the different types of cells and fibers found within the three cell layers of the cortex. Climbing fibers (cerebellar afferent fiber) are shown in blue; mossy fibers (cerebellar afferent fiber) and granule cells and their parallel fibers are shown in black; Purkinje cells and their axons (output of cerebellar cortex) are shown in red; and output of the cerebellum to other regions of the brain from deep cerebellar nuclei are shown in green.
FIGURE 21-9 Diagram of a cerebellar glomerulus. It consists of one mossy fiber rosette (light pink), granule cell dendrites (purple), and Golgi cell axons and their dendrites (green). Mossy fiber axon terminals make numerous synaptic contacts with granule cell den-drites as well as with dendrites and axons of Golgi cells.
FIGURE 21-10 Diagram illustrating the most significant connections within the cerebellar cortex. Cells shown in red are inhibitory neurons. All other neurons shown in this figure are excitatory. Arrows indicate direction of transmission along the axon. The dotted appearance of the Purkinje cell in the left side of the figure indicates that it is present outside the plane of the section. (+) = excitatory synapse; (-) = inhibitory synapse.
Climbing Fibers
Climbing fibers arise from the inferior olivary nucleus and ascend through the granular and Purkinje cell layers to reach the molecular layer. Within the molecular layer, they make synapse by climbing up the branches of the den-drites of the Purkinje cells (Fig. 21-8). In contrast to the arrangement of a single mossy fiber that can excite many granule cells, the climbing fiber maintains a one-to-one relationship with the Purkinje cell (i.e., one climbing fiber excites a single Purkinje cell). The neurotransmitter released from the endings of climbing fibers is believed to be aspartate.
An important fact to remember is that both mossy and climbing fibers, which excite their target neurons in the cerebellar cortex, also provide excitatory inputs from collaterals to the deep cerebellar nuclei (Fig. 21-10).
