Cerebellum

Definitions & Key takeaways

The cerebellum is a part of the brain that lies at the back of the head, beneath the cerebrum. It plays an important role in controlling movement and balance. Damage to the cerebellum can lead to problems with movement and balance. The cerebellum contains many Purkinje cells, which are responsible for processing information related to movement and balance from other parts of the brain. Damage to these cells can lead to problems with movement and balance. The cerebellum also contains deep nuclei, which are responsible for processing information from other parts of the brain related to movement and balance. These nuclei include the dentate nucleus, the interposed nuclei, which comprise the globose and emboliform nuclei, and the fastigial nucleus. Damage to these nuclei can lead to problems with movement and balance.

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Introduction0:00–0:21

Cerebellum literally means little brain - and it looks like a miniature version of the brain, or cerebrum. The cerebellum coordinates movements, controls posture, balance and fine motor movement, and is involved in motor learning - like learning how to ride a bicycle.

Anatomy0:21–2:55

The cerebellum sits in the posterior part of the skull called the posterior cranial fossa. Above it are the occipital and temporal lobes of the brain.
It’s separated from the brain by a fibrous membrane called the tentorium cerebelli - a fold of dura matter which is one of the layers called meninges that covers the brain and spinal cord.
The cerebellum lies posterior to the brainstem and is attached to it by a stalk of tissue divided into three parts - the superior, middle, and inferior peduncles.
These peduncles contain nerve axons going back and forth between the cerebellum and the brain, the internal ear, and the spinal cord via the brainstem.
The cerebellum consists of two hemispheres separated by a narrow, ridge in the middle called the vermis. Now if we look at a cross-section, we can see three lobes.
We have the anterior lobe superiorly, and it is separated from the posterior lobe by the primary fissure. At the tip of the posterior lobe is a very tiny lobe called the flocculonodular lobe and these two are separated by the posterolateral fissure.
The outer layer of the cerebellum is called the cortex and it’s folded into many tiny wrinkles called folia. These are much smaller than the wrinkles found on the cerebrum, and this allows it to have a larger surface area when unfolded even though it occupies only 10% of the brain volume.
The cortex of the cerebellum consists of three layers: The innermost, granular layer which contains the cell bodies of the granular cells, the Purkinje layer which contains the cell bodies of the Purkinje cells, and the molecular layer were various neurons synapse with each other.
Under the cortex, lies the white matter, also referred to as the arbor vitae, which means the tree of life, because in cross section it looks like a tree.
This white matter is made of neural axons which carry information to and from the cerebellum. Deep within the white matter, there are four deep cerebellar nuclei – these grey clusters are made of neuronal cell bodies.
From lateral to medial we have the dentate nucleus, the interposed nuclei, which are actually made up of the globose and emboliform nuclei, and the fastigial nucleus.

Mossy Fibers2:55–4:39

Input from the cerebrum and the spinal cord comes into the cerebellum through two types of axons, which are also referred to as fibers; the mossy fibers named so because of their axonal dendrites which look like moss in a pond, and the climbing fibers that ascend from the inferior olive in the medulla oblongata to enter the cerebellum.
Some mossy fibers come from the vestibular nuclei located in the pons and medulla oblongata, while others arise from the cerebrum, and pass through the pontine nuclei to reach the cerebellum.
Inside the cerebellum, mossy fibers travel through the white matter and give off a branch that synapses with a neuron in one of the deep cerebellar nuclei.
The rest of the mossy fibers travel to the cerebellar cortex and branch out in the granular layer and synapse with multiple granular cells.
Think of it like a moss that stays low and spreads wide. The granular cells then send telephone pole like axons all the way to the molecular layer, which then give off 2 parallel fibers that are like telephone lines.
Now the Purkinje cells also send their branching dendrites into the molecular layer. Each parallel axon synapses with and excites multiple Purkinje cells, and each Purkinje cell also synapses with multiple parallel axons.
Now, the Purkinje cell also sends out an axon that travels down to synapse with one of the deep cerebellar nuclei, but unlike the mossy fibers, they have an inhibitory effect.

Climbing Fibers4:39–5:28

Next are the climbing fibers which originate from the inferior olivary nucleus in the medulla oblongata, travel through the cerebellar white matter and give off a branch that synapses with one of the deep cerebellar nuclei.
The rest of the climbing fibers travel to the cerebellar cortex and wrap around the axon of the Purkinje cell like an ivy climbing up a tree until they reach the molecular layer where they branch into dendrites and synapse with the Purkinje cell.
Each climbing fiber only synapses with one Purkinje cell, unlike the mossy fiber which affects thousands of Purkinje cells through the granular cells.
But, even here, the Purkinje cells send their axons down to the deep cerebellar nuclei to inhibit them. So, let’s say we want to shoot a basketball.

Motor plan5:28–6:12

The cerebellum receives input from the motor cortex telling it to make a plan for that movement, and it also receives proprioceptive information from the limbs and information concerning balance from the inner ear.
Based on all of that input, the cerebellum then comes up with a motor plan for the timing and initiation of the movement, speed, direction, precision, and figures out what muscles groups need to work together.
It’s like the ultimate event planner. This motor plan is then sent back to the cerebrum or motor nuclei in the brainstem, which sends it down the spinal cord to activate the muscles needed to carry out the action.

Functional zones6:12–6:40

Now the cerebellum is divided into several functional zones, where each zone has different inputs and outputs. There’s the spinocerebellum which is the medial portion made of the vermis and adjacent area of the cerebellar hemispheres, the cerebrocerebellum which consists of the lateral portions of the cerebellar hemispheres, and lastly the vestibulocerebellum which mainly consists of the flocculonodular lobe.

Spinocrebellum6:40–9:11

The spinocerebellum is in charge of coordinating the muscles in the trunk and the limbs. It receives sensory input about the position of your body parts like your stance, how flex are your knees, elbows, and wrists joints.
All this information is collected from proprioceptors like the muscle spindles and Golgi tendon organs in the muscles and joints.
This information is sent to the spinal cord, and then travels up through the dorsal and ventral spinocerebellar tracts. The dorsal tract stays in the ipsilateral spinal cord and enters the cerebellum through the inferior peduncles.
The ventral tract crosses over to the contralateral side of the spinal cord before ascending, and enters the cerebellum through the superior peduncle.
Once in the cerebellum, the axons for the ventral tract cross over once again, so for both tracts, the sensory information from one side of the body ends up in the ipsilateral cerebellar hemisphere.
In other words, the fibers cross over twice which is almost as if the didn’t cross at all - and makes the cerebellum quite different from other parts of the brain.
For example, if a patient has a stroke in the right cerebellar hemisphere, they will get dysmetria, and dysdiadochokinesia on the right arm and leg, and the may fall or lean to the right when walking due to poor balance and coordination.
Most of the axons synapse with the fastigial nucleus and the interposed nuclei. The fastigial nucleus sends axons bilaterally through the inferior peduncle to the reticular formation in the brainstem, which send axons down the reticulospinal tract that controls muscles in the trunk and proximal limbs.
The fastigial nucleus also controls saccades, which are tiny jerky movements of the eye when you’re shifting your gaze from one fixed point to another.
Without saccades, you couldn’t follow fast-moving visual stimuli, and you’d end up feeling dizzy and missing every shot you try to take.
The interposed nuclei send axons through the superior peduncle which then cross over to the contralateral red nucleus in the midbrain.
The axons from the red nucleus cross back over and become the rubrospinal tract which controls the flexor muscles in the upper limb.

Cerebrocerebellum9:11–10:10

The cerebrocerebellum is in charge of initiating movements and coordinates fine motor control like typing on a keyboard, or moving your tongue and lips to produce speech.
It’s also involved in motor learning as well, which is pretty similar to how you might miss many of your first shot, but through repetitive adjustments of the muscles, proper positioning of your joints and better visualization the basket rim, you improve your shooting techniques and progressively make more and more of them.
In other words, practice makes perfect. The cerebrocerebellum receives all of its neuronal input from the cerebral cortex, which pass through the thalamus to reach the cerebellum via middle cerebellar peduncle.
Most of the axons cross over to synapse with contralateral dentate nucleus, and from here arise axons which go to the contralateral cerebral hemisphere through the superior peduncle.
Finally, to make a that shot you also need to keep balance, and that’s achieved by the vestibulocerebellum. It helps us maintain balance by controlling the muscles of the neck and back, because it receives sensory information concerning movement and position of the head relative to gravity from receptor cells in the semicircular canals and the vestibule located in the inner ear.

Vestibulocerebellum10:10–11:17

Neurons carrying sensory input travel to the vestibular nucleus in the pons and medulla which sends out axons through the inferior peduncle to the ipsilateral vestibulocerebellum.
These axons don’t synapse with a deep cerebellar nucleus. Instead, the cerebellar Purkinje cells send their descending axons through the inferior cerebellar peduncle and back out to the vestibular nucleus.
So, from the vestibular nucleus, motor output is sent out through the vestibulospinal tract in the spinal cord to the axial muscles as well as extensors of the leg to help us maintain balance.

Review11:17–11:50

Alright, as a quick recap, the cerebellum plays an important role in coordinating and planning movement as well as maintaining balance.
The spinocerebellum receives proprioceptive input from the spinal cord and coordinates the muscles in the trunk and the limbs.
The cerebrocerebellum receive input from the cerebrum and is in charge of initiating movements and fine motor control. The vestibulocerebellum receives sensory information from the vestibule and helps us maintain balance.