by fibers from a large group of cells in the medulla, the inferior olivary complex. Fibers enter the cerebellum through the inferior peduncle and wrap around the dendrites of the Purkinje cell. Because of their vinelike appearance, they are called climbing fibers. Sensory information from the spinal cord, brain stem, and cerebrum is funneled through the inferior olivary complex and provides strong excitatory drive through synapses of the climbing fibers with Purkinje cells. This synapse is unusual in that it is considered to be one of the strongest, most powerful synaptic connections in the CNS, as it elicits an excitatory postsynaptic potential (EPSP) with every firing of the climbing fiber; however, one climbing fiber innervates one or very few Purkinje cells, and climbing fibers fire at a low frequency of about 1 Hz. This arrangement is not capable of providing the overall excitatory drive of the cerebellum; rather, it functions to modulate the more pervasive influence of a second Purkinje cell input coming from mossy fibers.

Nuclei in the pons send extensive projections, called mossy fibers, to the cerebellum, where they synapse not on Purkinje cells directly but on granule cells, which constitute the largest cell population in the brain. Excitatory input to granule cells is then relayed through their processes, called parallel fibers, which make excitatory contact with Purkinje cells, each of which receives as many as 200,000 parallel fiber inputs. This input generates small EPSPs that must summate temporally and spatially in order to produce an action potential in the Purkinje cell. Granule cells, because of their extensive input, produce a constant, high-frequency firing of Purkinje cells of about 50 to 100 Hz. The two Purkinje cell inputs are interactive, so that climbing fiber input modulates the effect of mossy fibers on the Purkinje cell. Long-term modulation and interaction between the two inputs are thought to be involved in motor learning.

Three neurons serve as local inhibitory neurons to counterbalance the excitatory pathways within the cerebellum. Stellate and basket cells, excited by collaterals of parallel fibers from the granule cell, send inhibitory synapses to Purkinje cells in a feedforward loop. Golgi cells, also excited by parallel fibers, send inhibitory synapses to granule cells in a feedback loop. Output of the cerebellar cortex is through Purkinje cells that inhibit tonically active cells within the deep cerebellar nuclei.

The unique contribution of the cerebellum to the execution of motor commands is evident in patients with cerebellar lesions. Loss of cerebellar function does not produce paralysis. Instead, affected individuals exhibit abnormal movements with lack of coordination, generally referred to as cerebellar ataxia. One aspect of the disorder is characterized by a delay in initiating movements and errors in the rate and regularity of movements. The standard clinical test to demonstrate cerebellar deficits is to have the patient attempt to perform rapid alternating movements, such as tapping with one hand while alternating between the back and the palm of the hand. If cerebellar function is altered, the patient cannot sustain a regular rhythm or steady force. Luckily, the symptoms of cerebellar disease can improve gradually with time, perhaps reflecting the ''plasticity'' of cerebellar circuits.

Suggested Readings

Flament D, Hore J. Movement and electromyographic disorders associated with cerebellar dysmetria. Neurophysiology 1986; 55:1221-1233.

Holmes G. The cerebellum of man. Brain 1939; 62:1-30. Keele SW, Ivry R. Does the cerebellum provide a common computation for diverse tasks? A timing hypothesis. Ann NY Acad Sci 1990; 608:179-211.

FIGURE 4 Cellular anatomy of the cerebellar cortex. (A) Lateral view of the cerebellum and brain stem shows the major input nuclei (pontine, inferior olive, and vestibular), as well as the major output target nuclei (deep cerebellar, vestibular, and red nuclei); the orientation of the Purkinje cell dendritic trees is shown in blue along one folium. (B) A wedge of a cerebellar folium shows both cross and longitudinal sections; inhibitory neurons are shown in blue and excitatory neurons are black. Note that the Purkinje cells and all of the interneurons are inhibitory and use GABA as their neurotransmitter. Purkinje cells have a flattened dendritic tree covered with spines. (C) Input neurons include climbing fibers and the mossy fiber/granule cell/parallel fiber relay. The climbing fiber has a configuration similar to the Purkinje cell, except it is not as extensive and lacks spines. Parallel fibers are organized perpendicular to the plane of Purkinje cell dendrites, and they provide excitatory input to all Purkinje cells within a 1-mm distance. (D) Three types of interneurons inhibit Purkinje and granules cells. (E) Purkinje cells are the sole output neurons of the cerebellar cortex. They project to the deep cerebellar nuclei and vestibular nuclei.

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