Afferent and Efferent Pathways of the Cerebellum
The word cerebellum derives its name from the Latin word for ‘little brain’, which is exactly what it looks like. Its main function is in motor control, where it enables smooth, well timed, proportional responses. However the cerebellum has many other cortical functions including speech, emotions, as well as pleasure and fear. This article will discuss the anatomy of the cerebellum, as well as its functions and clinical relevance. We will also discuss the cell types, fiber types, cerebellar nuclei and functional consequences of cerebellar damage.
Neural development is one of the first to begin. The cerebellum developed from the metencephalon secondary vesicle, which in turn developded from the roof of the hind-brain. Anatomically, the cerebellum consists of the anterior lobe (anterior to the primary fissure), the posterior lobe (posterior to the primary fissure), and the flocculonodular lobe (below the posterior fissure).
The cerebellum is located in the posterior cranial fossa of the cranial vault. Above it lies the tentorium cerebelli (a fold of dura mater), that separates it from the cerebrum above. The surface of the cerebellum, like the cerebral cortex, is marked by many indentations and crevasses as well as elevations. In the brain the crevasses are called sulci and the elevations gyri. In the cerebellum they are called sulci and folia respectively. A worm like structure called the vermis, wraps sagittally around its centre, and marks the midline. Despite its much smaller size, the cerebellum has almost four times as many neurons as the cerebral cortex. The surface of the cerebellum is grey, and beneath this we have the myelinated white matter communicating with the cerebellar cortex. Cutting the cerebellum in cross section will reveal a tree like appearance (arbor vitae or ‘tree of life’), marked by four deep grey cerebellar nuclei.
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 dendrites are 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 utilise GABA as their neurotransmitter, and hence have an inhibitory effect.
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.
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.
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.
The cerebellum consists of three functional subdivisions. These are the vestibulocerebellum, the spinocerebellum and the cerebrocerebellum.
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.
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.’
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.
These are structures that connect the cerebellum to the brainstem.
Middle cerebellar peduncle
This is the largest, 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.
The deep cerebellar nuclei
These are the dentate, emboliform, fastigial and globose. They receive projections from mossy fibers and climbing fibers. The nuclei receive GABAergic (inhibitory) input from the Purkinje cells, as well as excitatory input from the mossy and climbing fibres mentioned above. The majority of the output from the cerebellum arises from these nuclei. They communicate together as well as with the cerebral cortex and the rest of the brain. The flocculonodular lobe will send its output to the vestibular nuclei and hence is an exception.
The dentate nuclei are located deep within the lateral hemispheres and receives the majority of its input from the lateral hemispheres. The fastigial nuclei are found within the vermis, and therefore receive afferents from the vermis. The interposed nuclei (globose and emboliform) are located within the intermedial paravermal zone, and receive the majority of their inputs from the paravermis.
Olivocerebellar: Fibers arise from the olivary nucleus and decussate to reach the fibers of the opposite Raphe nucleus. From here they pass onwards as internal arcuate fibers, through the inferior peduncle and to the opposite cerebellar hemisphere.
Vestibulocerebellar: This is a pathway that joins the pontine tegmentum to the cerebellar cortex.
Reticulocerebellar: These fibers originate at various levels of the reticular formation and mainly terminate in the vermis (which lies in the midline).
Corticopontocerebellar tract: This connects the premotor areas to the contralateral cerebellar hemisphere via the pontocerebellar tract.
Trigeminocerebellar fibres: These ascend via the inferior cerebellar peduncles and transmit proprioceptive information from the face, to the cerebellum.
Cerebellovestibular tract: This is an output from the cerebellum to the extensor muscles of the axial muscles which coordinate muscle tone adjustment.
Cerebelloreticular tract: This tract sends information to the motor circuits of the brain stems.
Corticonuclear tract: This connects the cerebral cortex to the brainstem and is functions for the motor function of the oculomotor nerve.
Cerebellothalamic tract: This arises from the superior cerebellar peduncle, arises from the cerebellar nuclei and decussates to terminate in the ventral anterior nucleus of the thalamus.
Cerebellorubral tract: Thus sends information from the cerebellum to motor systems of the brainstem.