Afferent and efferent pathways of the cerebellum

Purkinje cells

These are some of the largest cells in the brain. Purkinje cells are essential to the cerebellar circuitry, and they emit action potentials even in the absence of extrinsic synaptic input. These have a dendritic shape with profuse branching that resembles a tree. These dendritesare also covered by a dense network of dendritic spines. They are however very flattened, and hence run perpendicular to the folds on the cerebellum. They create a net-like formation, and the parallel fibers pass through at right angles through that net. The cell bodies sit in a distinct Purkinje layer and travel into the deep cerebellar nuclei. These cells utilize GABA as their neurotransmitter and hence have an inhibitory effect.

Granule cells

In contrast to Purkinje cells, granule cells are some of the smallest cells in the brain. Granule cells use glutamate as their neurotransmitter and are hence excitatory in their effect on cells. Each cell has only a few dendrites, which form ‘dendritic claws’ at their ends. These claw sites receive excitatory input from mossy fibers (mentioned below) and inhibitory input from Golgi cells. The axons of these granule cells pass superiorly in order to enter the cortex, and split to form parallel fibers. These fibers pass through the Purkinje cell dendritic trees. All of the input received by granule cells is from mossy fibers.

Mossy fibers

Many of these fibers arise from the pontine nuclei, as well as vestibular nuclei and from the spinal cord. These fibers form excitatory connections with the deep cerebellar nuclei and the granule cells. Mossy fibers form connections with the dendrites of granule cells. Golgi cells and mossy fibers form ‘glomeruli’. These fibers enter the cerebellum from, and project to the deep nuclei. They also project to the deep nuclei via the granule cells, parallel fibers, and Purkinje cells.

Climbing fibers

These fibers arise from the inferior olivary nucleus and send collaterals to the deep nuclei before entering the cerebellar cortex. These fibers will reach Purkinje cells and will synapse over the proximal dendrites and cell bodies. The precise functions of these fibers are debated, with two theories. The predominant theory is that these fibers are responsible for signaling errors we make in our motor system.

Simplify the histology of the cerebellum by taking advantage of the illustrations and quizzes included in the following study unit:

Spinocerebellum

This is also known as the paleocerebellum. The spinocerebellum consists of the lobes near the midline. The primary function of this functional region is to monitor and fine-tune limb movements. This is achieved by proprioceptive input from the dorsal column pathway of the spinal cord, the cranial trigeminal nerve, the visual and auditory systems, as well as the spinocerebellar tract. This region sends its output to the deep cerebellar nuclei. These then project to the cerebral cortex, and brainstem. This enables the region to monitor and modify the activity of the descending motor pathways.

Cerebrocerebellum

This is also known as the neocerebellum, as it is the most recent region to develop in evolutionary terms. It consists mainly of the lateral parts of the cerebellar lobes. It receives information exclusively from the cerebral cortex (mainly the parietal lobe, the primary sensory lobe of the brain), with the pontine nuclei in between, and sends its output to the thalamus (the ventrolateral part to be precise), which will go on to connect with the premotor cortex and primary motor area. It will also send its output to the red nucleus. This region is primarily concerned with planning future movements, and also some purely cognitive functions such as matching verbs to nouns e.g. ‘run’ for ‘track.’

Vestibulocerebellum

This region is also known as the archicerebellum and is mainly focused on spatial awareness and balance. It mainly receives its input from the vestibular nuclei but also from the auditory and visual sensory input. If a patient has damaged this region, the result is disturbed balance and gait.

Middle cerebellar peduncle 

This is the largest peduncle and connects the cerebellum to the pons. It connects the contralateral pontine nucleus to the cerebellar cortex and also carries the input from the contralateral cerebral cortex. It is composed of three fasciculi including the superior, inferior and deep.

Superior cerebellar peduncle 

This is the major output of the brain and connects to the midbrain, via the cerebellothalamic tract (to the thalamus), and the cerebellorubral tract (to the red nucleus). It receives afferents from the locus coeruleus, and ventral spinocerebellar tract.

Inferior cerebellar peduncle

This connects the spinal cord and medulla to the cerebellum. The posterior spinocerebellar tract receives proprioceptive information from the body. The cuneocerebellar tract receives proprioceptive input from the upper limb and neck. The trigeminocerebellar tract sends proprioceptive input from the face. The juxtarestiform is an efferent system here.