The Cerebellum’s Gross Anatomy
There are two halves to the cerebellum. The deep nuclei (or cerebellar nuclei) of the cerebellum are the cerebellum’s only output structures. The cerebellar cortex encases these nuclei in a highly convoluted layer of tissue called the cerebellar cortex, which includes almost all of the neurons in the cerebellum. A cross-section of the cerebellum reveals the complex network of folds and fissures that characterise the cerebellar cortex. Cerebellar gyri, like the cerebral cortex, have been identified and named because they are reproducible among individuals. Only a few of the larger divisions of the cerebellar cortex will be discussed.
There are several sections to the cerebellum. Two large fissures that run mediolaterally split the cerebellar cortex into three primary subdivisions. The posterolateral fissure separates the flocculonodular lobe from the corpus cerebelli, while the major fissure divides the corpus cerebelli into two lobes. The cerebellum is organised into three zones that run from medial to lateral in sagittal plane. The vermis (worm in Latin) is a midsagittal plane component of the cerebellum. The vermis is directly lateral to the intermediate zone. Finally, on the other side of the intermediate zone are the lateral hemispheres (there are no clear morphological borders between the intermediate zone and the lateral hemisphere that are visible from a gross specimen).
Cerebellar nuclei are the cerebellum’s nuclei. All cerebellar outputs originate in the deep nuclei of the cerebellum. As a result, a cerebellar nuclei lesion has the same effect as a whole cerebellar lesion. The inputs, outputs, and structural connections between the various cerebellar nuclei and cerebellum subdivisions must all be understood.
The cerebellar nucleus with the most medial placement is the fastigial nucleus. It receives input from the vermis and cerebellar afferents transmitting vestibular, proximal somatosensory, auditory, and visual information. Its targets are the vestibular nuclei and the reticular formation.
The intervening nuclei are the emboliform nucleus and the globose nucleus. They can be discovered on the fastigial nucleus’s side.
They collect data from the intermediate zone and cerebellar afferents carrying spinal, proximal somatosensory, auditory, and visual information. On the other side, they project to the red nucleus (the origin of the rubrospinal tract).
The dentate nucleus is the largest of the cerebellar nuclei, located lateral to the intervening nuclei. It receives information from the cerebral cortex as well as input from the lateral hemisphere via cerebellar afferents (via the pontine nuclei). It connects to the contralateral red nucleus and the ventrolateral (VL) thalamic nucleus.
The vestibular nuclei are located outside the cerebellum and inside the medulla. They are not exactly cerebellar nuclei as a result, but their connection patterns are identical to those of the cerebellar nuclei, hence they are deemed functionally equal. The vestibular nuclei receive information from the flocculonodular lobe and the vestibular labyrinth. They give rise to the vestibulospinal tracts and project to a number of motor centres.
In addition to these inputs, the inferior olive of the medulla sends specialised signals to all of the cerebellar nuclei and regions (discussed below).
It’s useful to remember that the physical placements of the cerebellar nuclei correspond to the regions of the cerebellar cortex from which they receive input. The medial fastigial nucleus is fed by the medial vermis, the slightly lateral interposed nuclei by the slightly lateral intermediate zone, and the most lateral dentate nucleus by the lateral hemispheres.
The cerebellum’s peduncles. Three fibre bundles carry the cerebellum’s input and output.
The inferior cerebellar peduncle contains afferent axons from the medulla as well as efferents to the vestibular nuclei (also known as the restiform body).
The pontine nuclei afferents are usually found in the middle cerebellar peduncle (also known as the brachium pontis).
Efferent fibres from the cerebellar nuclei and afferents from the spinocerebellar tract are housed in the superior cerebellar peduncle (also known as the brachium conjunctivum).
The lower and middle cerebellar peduncles generally convey inputs to the cerebellum, while the superior cerebellar peduncle mostly transmits outputs. The inputs are provided by the ipsilateral side of the body, while the outputs are received by the ipsilateral side of the body. The outputs to the contralateral red nucleus are similarly affected. The rubrospinal tract crosses the midline as soon as the fibres exit the red nucleus, as you recall from the chapter on descending motor pathways. As a result, the red nucleus receives cerebellar output via a double-crossed channel that affects the ipsilateral side of the body. Because the cerebellum, unlike the cerebral cortex, receives input from and controls output to the ipsilateral side of the body, damage to the cerebellum results in ipsilateral side impairments.