Intracerebellar circuitry

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The cerebellum is being presented from the dorsal perspective (as in Figure 9A). The third ventricle is situated between the two diencephala; the pineal gland is seen attached to the posterior aspect of the thalamus. Below are the colliculi, superior and inferior. On the right side of the illustration, the cerebellar hemisphere has been cut away, revealing the "interior" on this side.

The cerebellum is organized with cortical tissue on the outside, the cerebellar cortex. The cortex consists of three layers, and all areas of the cerebellum are histolog-ically alike. The most important cell of the cortex is the Purkinje neuron, which forms a layer of cells; their massive dendrites receive the input to the cerebellum. Various interneurons are also located in the cortex. The axon of the Purkinje neuron is the only axonal system to leave the cerebellar cortex.

Deep within the cerebellum are the intracerebellar nuclei or the deep cerebellar nuclei, now shown from the posterior view (see Figure 56A).

Overall, the circuitry is as follows: All (excitatory) afferents to the cerebellum go to both the deep cerebellar nuclei (via collaterals) and the cerebellar cortex. After processing in the cortex, the Purkinje neuron sends its axon on to the neurons of the deep cerebellar nuclei — all Purkinje neurons are inhibitory. Their influence modulates the activity of the deep cerebellar neurons, which are tonically active (described in more detail below). The output of the deep cerebellar neurons, which is excitatory, influences neurons in the brainstem and cerebral cotex via the thalamus (discussed with the next illustration).

The connections of the cortical areas with the intrac-erebellar nuclei follow the functional divisions of the cerebellum:

• The vestibulocerebellum is connected to the fastigial nucleus, as well as to the lateral vestibular nucleus.

• The spinocerebellum connects with the interposed nucleus (the globose and emboliform).

• The neocerebellum connects to the dentate nucleus.

Axons from the deep nuclei neurons project from the cerebellum to many areas of the CNS, including brainstem motor nuclei (e.g., vestibular, reticular formation) and thalamus (to motor cortex). In this way, the cerebellum exerts its influence on motor performance. This will be discussed with the next illustration.

Details of Cerebellar Circuitry

The cerebellum receives information from many parts of the nervous system, including the spinal cord, the vestibular system, the brainstem, and the cerebral cortex. Most of this input is related to motor function, but some is also sensory. These afferents are excitatory in nature and influence the ongoing activity of the neurons in the intracere-bellar nuclei, as well as projecting to the cerebellar cortex.

The incoming information to the cerebellar cortex is processed by various interneurons of the cerebellar cortex and eventually influences the Purkinje neuron. This will lead to either increased or decreased firing of this neuron. Its axon is the only one to leave the cerebellar cortex, and these axons project, in an organized manner, to the deep cerebellar nuclei.

The Purkinje neurons are inhibitory, and their influence modulates the activity of the deep cerebellar nuclei. Increased firing of the Purkinje neuron increases the ongoing inhibition onto these deep cerebellar nuclei, while decreased Purkinje cell firing results in a decrease in the inhibitory effect on the deep cerebellar cells, i.e., this results in the increased firing of the deep cerebellar neurons (called disinhibition).

It is interesting to note that the cerebellar cortex projects fibers directly to the lateral vestibular nucleus (see Figure 50, not illustrated). As would be anticipated, these are inhibitory. The lateral vestibular nucleus could therefore, in some sense, be considered one of the intracere-bellar nuclei. This nucleus also receives input from the vestibular system, and then projects to the spinal cord (see Figure 50 and Figure 51A).

Thalamus

Pineal

Optic tract -

Lateral geniculate body

Primary fissure

Fastigial n. Globose n. Emboliform n.

Dentate n.

Lateral vestibular n.

Horizontal fissure

Tonsil

Medial geniculate body

Brachium of the inferior colliculus

Medial geniculate body

Brachium of the inferior colliculus

Thalamus

Pineal

Optic tract -

Lateral geniculate body

Primary fissure

Fastigial n. Globose n. Emboliform n.

Dentate n.

Lateral vestibular n.

Horizontal fissure

Tonsil

Fastigial Nucleus

An = Anterior lobe Fl = Flocculonodular lobe

3 = 3rd ventricle SC = Superior colliculus IC = Inferior colliculus

An = Anterior lobe Fl = Flocculonodular lobe

FIGURE 56B: Cerebellum 4 — Intracerebellar Circuitry

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