Anatomy of the cerebellum

The word cerebellum translates to little brain. Not because it’s the brain of a tiny animal or baby, but rather because of the fact that the cerebellum looks like a smaller version of the human cerebrum.

Very simply, the cerebellum assists with coordinating and adjusting voluntary movement. It plays a major role in posture, balance, maintenance of muscle tone and coordinating skilled voluntary motor activities - things like riding a bicycle, or for the more adventurous, walking a tightrope!

In order for the cerebellum to undertake these functions, it has to be in constant communication with the cerebral cortex. It also receives and sends signals to many other structures in the central and peripheral nervous systems, processing information about current movement and positional states in order to help refine, correct and improve the motion.

Now, the cerebellum sits in the posterior part of the cranium, called the posterior cranial fossa, and it is covered by the tentorium cerebelli, which separates the cerebellum from the occipital and temporal lobes of the brain. Anterior to the cerebellum lies the fourth ventricle, pons, and medulla oblongata.

Just like the cerebrum, the cerebellum consists of two hemispheres. These two hemispheres are connected by a narrow ridge in the middle called the vermis. From an inferior view, parallel to the vermis, there are two distinguishable lobules called the cerebellar tonsils.

The cerebellum can be divided into three lobes; the anterior lobe, the posterior lobe, and the flocculonodular lobe. From a superior view, we can identify the anterior lobe, functionally referred to as the spinocerebellum, which is responsible for the regulation of muscle tone and adjusting on-going movements. Posterior to the anterior lobe is the V shaped primary fissure.

From a superior view, and posterior to this primary fissure, is the posterior lobe, functionally referred to as the cerebrocerebellum, or pontocerebellum, which contains the horizontal fissure separating the superior and inferior surface of the cerebellum. The cerebrocerebellum is the largest part of the cerebellum, and is responsible for assisting in planning and programming of skilled or fine motor movements.

Looking at the cerebellum from an anterior view, the posterior lobe is bounded by the posterolateral fissure. This fissure separates the posterior lobe from the third lobe of the cerebellum, called the flocculonodular lobe, or functionally referred to as the vestibulocerebellum.

The flocculonodular lobe is responsible for the maintenance of posture and balance. The flocculonodular lobe is named as such because it contains a central part of the vermis called the nodule as well as two lateral flocculi.

If we continue to view the cerebellum from the anterior aspect, we can see bundles of dense white matter that attach the cerebellum to the brainstem. These white matter stalks are called cerebellar peduncles and consist of superior, middle and inferior divisions. They contain efferent and afferent axons that signal back and forth between the cerebellum and the central nervous system.

The superior cerebellar peduncle connects the cerebellum with the midbrain, the middle cerebellar peduncle connects with the pons, and the inferior cerebellar peduncle attaches to the medulla oblongata.

Afferent fibers to the cerebellum can be found within all three cerebellar peduncles, but the majority of afferent signals use the inferior and middle peduncles for passage. Efferent signals from the cerebellum, however, travel mainly through the superior peduncle.

On a sagittal section, the cerebellum looks similar to the cerebrum, in that the cortex is folded, creating ridges with small sulci in between. The difference, however, is that in the cerebellum the cortical ridges are thinner, smaller and organized in more parallel layers, which are called folia.

These folia not only increase the surface area, but enable the large area of cortex to fit into a smaller space, just like the cerebrum. The folia contain an external gray matter layer, called the cerebellar cortex, and a subcortical white matter region deep to the external gray matter.

As we see, the shape of this white matter within the folia creates a ‘tree like’ arrangement, or branching pattern, referred to as arbor vitae, or ‘tree of life’.

On a transverse section of the cerebellum, we can see four clusters of deep gray matter nuclei buried deep within the subcortical white matter. These deep cerebellar nuclei, or intracerebellar nuclei, contain multipolar neurons that receive signals from the cerebellar cortex and other parts of the nervous system, and their axons contribute to the formation of the three cerebellar peduncles.

From lateral to medial, these deep cerebellar nuclei consist of the dentate, emboliform, globose and fastigial nuclei. To remember these, remind yourself that in order to have a healthy cerebellum, you Don’t Eat Greasy Foods.

In addition to having anatomical divisions, the cerebellar cortex can also be divided into three functional regions that are positioned longitudinally. The most lateral, and largest functional region, is the lateral zone. The lateral zone sends signals to the dentate nucleus, the largest of the deep cerebellar nuclei and together they assist in planning and programming movements.

Medial to the lateral zone, is the intermediate zone, also known as the paramedian or paravermal zone. The intermediate zone sends signals to the emboliform and globose nuclei. Collectively, these two nuclei are known as the interposed nuclei and are usually referred to together as they both work with the intermediate zone.

Finally, most medial and occupying the cortex of the vermis is the third functional zone, the median, or vermal zone. The median zone will send signals to the fastigial nucleus, which is the most medial of the deep cerebellar nuclei, located within the vermis and next to the roof of the fourth ventricle.

The intermediate and median zones along with their deep cerebellar nuclei are involved in modulating motor execution of lateral and medial descending motor pathways, respectively.

Let’s take a quick break and see if you can identify the lobes of the cerebellum, as well as the functional zones and deep cerebellar nuclei.

Now let’s have a look at the afferent pathways, which bring information to the cerebellum to be processed, and the efferent pathways, which leave the cerebellum to help coordinate motor activity.

Afferent pathways generally originate from the spinal cord and brainstem, the cerebral cortex and the vestibular system. Starting with the afferent pathways from the spinal cord to the cerebellum, let’s look at the ventral, or anterior, spinocerebellar pathway first. It carries proprioceptive information from muscle spindles, golgi tendon organs, and joint receptors of the lower extremities.

Then, the afferent fibers enter the spinal cord, where they synapse with spinal border cells located in Lamina VII of the spinal cord gray matter. From here, the majority of these axons cross to the contralateral side of the spinal cord and form the ventral spinocerebellar pathway which ascends in the white matter of the spinal cord to the brainstem.

Here, the axons cross back over and enter the cerebellum through the superior cerebellar peduncle to reach the cerebellar cortex. The signals on the ventral spinocerebellar pathway cross over the neural axis and then cross back, so it is often referred to as a “double-crosser”!

Next is the dorsal, or posterior, spinocerebellar pathway. This pathway contains fibers that receive proprioceptive information from muscle spindles, golgi tendon organs, and joint receptors mainly from the trunk and lower extremities. This information enters the spinal cord from peripheral nerves and the signal synapses on Clarke’s nucleus, or Clarke’s column, also known as nucleus dorsalis.

Instead of crossing over after they synapse, the axons ascend in the ipsilateral white matter of the spinal cord to the brainstem where they then enter the cerebellum through the inferior cerebellar peduncle to reach the cerebellar cortex.

The final afferent pathway that carries proprioceptive information from the extremities is called the cuneocerebellar pathway. The axons in this pathway receive proprioceptive information from muscle spindles, golgi tendon organs, and joint receptors within the upper limb and upper thorax.