What is the specific role of the cerebellum when it comes to 'coordinating movement'?

Question

In elementary biology (high school level in the UK - A levels), we are told that the cerebellum is the part of the brain that 'coordinates movement'. Literally nobody takes the time to explain what the word 'coordinates' encompasses. Hence, I do not know the specific role of the cerebellum in coordinating movement.

For example, one very daunting thing that we are told is that the frontal lobe contains the 'primary motor cortex', which supposedly contains motor neurons that connect to the spinal cord and brain stem and can send nerve impulses that enable movement in the body. Where is the distinction between such a role and the role of the cerebellum in 'coordinating movement'?

Answer 1

The principal function of the cerebellum, which was detected years ago, is to calibrate detailed movements rather than initiating movements or deciding which movements to execute (Ghez et al, 1985). This was concluded by observing the changes which occurred after damaging cerebellum. Animals and humans with cerebellar dysfunction show, above all, problems with motor control, on the same side of the body as the damaged part of the cerebellum. They continue to be able to generate motor activity, but it loses precision, producing erratic, uncoordinated, or incorrectly timed movements. Thus, cerebellum helps in coordinating fine-tuned movements and inhibits involuntary movements. Apart from this, fMRI studies have indicated that more than half of the cerebellum is intertwined with association areas (Buckner et al, 2011). But since it is out-of-scope here, I'll leave it at that much.

See it like this:

For a more detailed mechanism and circuitry of the cerebellum, you can have a look at this interactive page from the University of Texas. @FilipeRocha have also come up with some fine details in their answer.

References:

1. Ghez C, Fahn S (1985). "The cerebellum". In Kandel ER, Schwartz JH. Principles of Neural Science, 2nd edition. New York: Elsevier. pp. 502–522.

2. Buckner RL, Krienen FM, Castellanos A, Diaz JC, Yeo BT (2011). "The organization of the human cerebellum estimated by intrinsic functional connectivity". J. Neurophysiol. 106 (5): 2322–2345. doi:10.1152/jn.00339.2011.

Answer 2

When attempting to execute a movement, a signal is not only sent from the motor cortex through the spinal chord and to the muscles, but also one is sent from the same motor cortex to the cerebellum, containing the information of what is the intended movement (this happens via the cortico-pontocerebellar tract, and enters the cerebellum through the middle cerebellar peduncle, in case you are familiar with some neuroanatomy).

At the same time, the cerebellum is receiving information about the current position and forces of the body, called proprioception, through signals that come from proprioceptors located on bones, muscles and joints, which travel up the spinal chord (through the spinocerebellar pathways) and enter the cerebellum (through the inferior cerebellar peduncle). The cerebellum then processes both pieces information - intended movement and current mechanical state of the body - and sends out a signal back to the motor cortex (via the superior cerebellar peduncle) containing information that will improve the overall execution of the movement, by correcting and smoothing out with how much force, how much time and to what extent each component of the movement should be done.

The cerebellum is also implied in the learning process, not only movement-wise (learning to ride a bike or swim), but also cognitively.

To learn more:

http://www.brainfacts.org/brain-basics/neuroanatomy/articles/2012/the-cerebellum/

https://www.youtube.com/watch?v=mpO4lTDr1to&index=21&list=PLp9HSlEm97VXyQ32Uwjfz3dpmQ8nl63zJ