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.
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.
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.
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. 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.