Where is the cerebellum?
The cerebellum is hard to miss when you're looking at a brain. Cerebellum is Latin for "little brain", and indeed the cerebellum looks a bit like a smaller version of our brain. It protrudes from the back and bottom of the cerebral cortex. You can see it when looking at a side view of a brain as the most posterior and inferior portion of the brain (see picture to the right).
What is the cerebellum and what does it do?
Although the cerebellum is involved in a number of brain functions, it is best known for its part in the modulation of movement. When we make a voluntary movement, the signal to initiate that movement originates in the motor cortices. Before the signal is sent to our muscles, however, it is sent to the cerebellum.
In addition to receiving information about the planned movement from the cortex, the cerebellum also gets information about the position of the body from the spinal cord. It uses this information to coordinate the movement and allow us to make it in a smooth manner, while maintaining our balance and equilibrium.
For example, imagine you are standing up and there is a piece of cake on the table in front of you. A plan to reach for the cake originates in the motor cortex. But, in order for that plan to result in a fluid movement, a number of things have to happen. For example, the movement must be executed with the current position of the body in mind. If you are standing stably on two feet, the action would require a different motor plan than if you were trying to balance on one foot. Also, muscles that oppose the movement must be inhibited; otherwise your attempt to extend your arm might be negated by the muscles whose role is to flex your arm. When the cerebellum receives information about the motor plan from the motor cortex, it incorporates what it knows about the position of the body and muscles; then it sends the plan back to the cortex to put it into action.
As you reach for the cake, however, there will also be a number of small corrections that must be made along the way. Although to us our movements seem like they are made up of large components (e.g. reaching, grasping), in the brain they are actually made of up very small increments. The plan for each increment is developed based on the results of the previous one. In other words, when you reach for the cake your brain is constantly getting data about the position of your hand and arm in real-time and using that data to make minor adjustments to the movement as it occurs.
Thus, the movement of our arm is made up of a number of smaller movements that involve slight deviations and then a return to the originally designated course. It is similar to the approach an airplane uses to get from, for example, New York to San Francisco. Although the course between the two cities seems to be straightforward, due to variables like wind and weather the route will never be exactly the same. When any particular trip is examined closely, one will see that the plane frequently deviated and then returned to its path. Similarly, your brain must get information about where your arm is in space and, if it is not on the mark in reaching for the cake, the path must be corrected.
These corrections are happening on the order of milliseconds, and so we are not aware of them, but the cerebellum is essential to making them happen. When the cerebellum detects any potential deviations from the route originally planned, it uses that information to send a modified plan back to the motor cortex. This corrected plan is what is used to generate the next increment of movement in reaching for the cake.
In this way, the cerebellum provides an error detection and correction mechanism. This allows our movements to appear smooth, precise, and coordinated. The importance of the cerebellum in facilitating smooth movement can especially be seen in someone who has experienced cerebellar damage. They may develop cerebellar ataxia, which involves problems not in the initiation, but in the execution, of movement. Their movements may be abnormally timed, jerky, and riddled with tremors.
The cerebellum also seems to be important to the learning of motor movements. When we repeat a motor movement over and over again, we gradually learn to execute it more smoothly and precisely. This learning process is based in part on the strengthening of synapses in the cerebellum.
Someone who experiences cerebellar damage (e.g. through a stroke) may also display cognitive and emotional disturbances or deficits. Thus, there appears to be more to the cerebellum than just its role in movement. However, due to the distinctive movement disorders that appear when someone experiences cerebellar damage, its capacity to promote smooth and coordinated movement is what the cerebellum is best known for.