The Cerebellum

The Cerebellum

The preceding chapters, which describe the physiology of movement, discuss the function of lower motor neurons through which the central nervous system (CNS) can initiate and control movement by initiating contraction of skeletal muscle. The corticospinal system and the descending brainstem motor system are described in those previous chapters as major subgroups of upper motor neurons that influence the lower motor neurons. More medial portions of those systems coursing through the spinal cord are primarily responsible for the control of axial and proximal antigravity extensor muscles. The more lateral portions primarily control more skilled, learned, voluntary movements caused by contraction of distal flexor muscles. This chapter describes the function of the cerebellum, part of another subgrouping of upper motor neurons critical for proper movement.

The cerebellum (Latin, “little brain”) is caudal to the cerebral cortex and dorsal to the brainstem (Figure 12-1). Although it constitutes only about 10% of the gross brain volume because of its highly folded structure, the cerebellum contains more than half of all CNS neurons. The outer layer of cerebellar gray matter, the cerebellar cortex, has a highly regular, three-layered, histological appearance, which suggests that all cerebellar regions may perform a common underlying task. Like the cerebral cortex, the particular inputs to a given region of cerebellar cortex, and the particular output targets that it influences, in large part account for the functional differences between cerebellar regions. In addition to the cerebellar cortex, and the cerebellar white matter axons entering and leaving the cortex, a group of deep cerebellar nuclei is embedded within the cerebellar white matter (Figure 12-2). The cells of these nuclei are a principal origin of the axons leaving the cerebellum. Two large pairs of white matter stalks, the rostral and middle cerebellar peduncles, respectively carry axons out from and into the cerebellum. A third, smaller pair of cerebellar peduncles, the caudal cerebellar peduncles, carry axons both into and out from the cerebellum.

The cerebellum is not necessary for the initiation of movement. Muscle strength remains largely intact with complete destruction of the cerebellum. However, the cerebellum plays a crucial role in the timing and coordination of movement initiated by the parts of the motor system hierarchy discussed in Chapter 10. It does so by adjusting and modulating the output of the motor cortices, corticospinal tract, descending brainstem motor pathways, and spinal cord. Lesions of the cerebellum lead to major clinical deficits in the precision and grace with which movement is accomplished.

The Cerebellum Constantly Compares the Intended Movement with the Actual Movement and Makes Appropriate Adjustments

In performing the essential role of adjusting the timing and coordination of movement, the cerebellum first receives information from components of the motor system hierarchy about the movement it has commanded. It also receives information from muscle spindles, the vestibular and visual systems, and other sensory receptors about the movement the body is actually performing. When the intended movement and the actual movement are not the same, the cerebellum’s job is to perform the adjustments necessary to make them the same. For example, if a cat’s brain intends that its mouth move to a piece of food in a dish, but sensory receptors inform the cerebellum that the trajectory of the head will cause the mouth to miss the dish, the cerebellum makes appropriate adjustments in the components of the motor system hierarchy to correct the head’s trajectory. The correction can be made to the movement in progress and to the plan for subsequent movement.

Cerebellar Histology and Phylogeny Give Clues to Cerebellar Function

The cortex throughout the cerebellum is quite uniform and consists of three layers and only five types of neurons: stellate, basket, Golgi, granule, and Purkinje cells (Figure 12-3). The outermost layer is the molecular layer and consists primarily of granule cell axons, known as parallel fibers (Figure 12-4); dendrites of neurons located in deeper layers; and scattered inhibitory interneurons, the stellate and basket cells.

The middle Purkinje cell layer of cerebellar cortex consists of the large cell bodies of Purkinje neurons, which have a flat but extremely expansive dendritic field that extends into the molecular layer (see Figures 12-3 and 12-4). This dendritic field is oriented at right angles to the parallel fibers. Therefore, a Purkinje cell is contacted by an expansive array of parallel fiber axons of granule cells, and an individual parallel fiber contacts the dendrites of many Purkinje cells. The stellate and basket cell inhibitory interneurons, noted above, can act to refine, or prune, this extensive spatial pattern of Purkinje cell activation by parallel fibers.

The innermost granule cell layer of cerebellar cortex contains the vast number of granule cell somas that give rise to the parallel fibers (see Figures 12-3 and 12-4). This layer also contains occasional Golgi cell bodies. These are inhibitory interneurons that can regulate the overall level of excitation of the Purkinje cells by the granule cell parallel fibers.

Axons of the Purkinje neurons go to the deep cerebellar nuclei, located outside of the cerebellar cortex, embedded in the cerebellar white matter (see Figure 12-2). The Purkinje cells are the only output neurons of the cerebellar cortex and are all inhibitory. They can inhibit the spontaneously active neurons of the deep cerebellar nuclei, whose axons leave the cerebellum. This selective inhibition represents a sensitive temporal refinement of cerebellar processing that supplements the spatial refinement, and the excitation level control, noted above. The cerebellar output neurons participate in regulating the activity of brainstem motor pathways and motor cortices involved in the execution and planning of movement.

The two primary groups of input axons to the cerebellum are the mossy fiber and climbing fiber axons (see Figure 12-3). Both are excitatory; they cause excitatory postsynaptic potentials (EPSPs) within the cerebellar cortex and, through collateral axons, within the deep cerebellar nuclei (Figure 12-5). The mossy and climbing fibers collectively carry information from components of the motor system hierarchy and from peripheral sensory receptors regarding the planning, initiation, and execution of the movement. The shorter input/output circuit of the cerebellum consists of the climbing and mossy fiber stimulation to the deep cerebellar nuclei, whose output in turn leaves the cerebellum to modify components of the motor system hierarchy. However, the output of the deep cerebellar nuclei is itself modified by inhibition from Purkinje cell axons that originate in cerebellar cortex. The Purkinje cell inhibition of deep cerebellar nuclei is based on the cerebellar cortex’s own integration of mossy and climbing fiber inputs. In other words, the same information coming into the cerebellum that drives the cerebellar nuclei is also processed by cerebellar cortex, whose resulting Purkinje cell output refines or “sculpts” the output of the cerebellar nuclei that project to components of the motor system. Within the cerebellar cortex, inhibitory interneurons help to refine or “sculpt” the Purkinje cell output of cerebellar cortex.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on The Cerebellum
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