The Cerebellum

In the past 25 years, scientists’ understanding of the cerebellum – one of the oldest parts of the vertebrate brain, having developed over 400 million years ago – has seen massive developments. It is well known that a large portion of the cerebellum’s function is involved with cognition – even though in the past, it was thought that the cerebellum largely contributed to movement. Since the 19^th century, scientists began researching the cerebellum based on this idea that the cerebellum was largely responsible for the planning and execution of motor activity, using animal models to study this. A large part of what contributed to this way of thinking was the cerebellum’s position in the body: It sits directly on top of the spinal cord, and, additionally, observations in patients who had neuronal deficits associated with the cerebellum led researchers to believe that the cerebellum was the “head ganglion of the proprioceptive system.” 

The cerebellum connects to the contralateral cerebrum via two circuits that pass through the pons, through the cerebellum, and outputs to the deep cerebellar nuclei, to the thalamus, and finally terminating in the cerebellar cortex. 

Regions of the Cerebellum

The cerebellum is made up of three distinct lobes, arranged by their individual functions. The fissures that anatomically separate these lobes are the primary fissure and posterior-lateral fissure. 

Flocculonodular lobe. This is believed to be important to the vestibulocerebellum which picks up information from the vestibular system. 

Posterior lobe. This lobe occupies the lateral hemispheres of the cerebellum and holds the dentate nucleus.

Anterior lobe. This is also known as the “spinocerebellum” and occupies the vermal zone (central to the cerebellum) and the paravermal zone, also known as the intermediate zone. The anterior lobe is responsible for processing sensory information with the homunculus of the cerebellum, taking in information from the upper and lower extremities. The structures nearer to the vermal zone particularly regulate sensory information received from the head, neck, and trunk of the body. 

The outer surface of the cerebellum is known as the arborvitae (which translates to the “tree of life”).

To help you remember more specific anatomical structures of the cerebellum, you can use the following pneumonic: Don’t Eat Greasy Food.

• D: Represents the dentate nucleus, located in the lateral hemisphere. This works with the cerebrocerebellum.
• E and G: Represents the components of the interposed nucleus, the emboliform nucleus, and the globose nucleus. The interposed nucleus occupies the vermal and paravermal areas of the cerebellum. These are associated with the spinocerebellum.
• F: Represents the fastigial nucleus, which is located within the flocculonodular lobe and the vermis (the structure that separates the vermal zone). This nucleus interacts with the inner ear.

These deep cerebellar nuclei are ordered laterally to medially – from the outer region of the body toward the inner region – and occurs on both sides of the brain (so, it would be arranged like this: D-E-G-F-F-G-E-D).   

Functions of the Cerebellum

The refocusing of research efforts into the cerebellum that strayed away from motor functionality began in the 1980s after a team of researchers summarized a large amount of evidence illustrating the human cerebellum contained significant neuronal links to the cerebrum. But this new focus was met with quite a lot of resistance – even scientists can be surprisingly resistant to change! 

This team of researchers gathered a collection of literature that accounted for the observation that the lateral output nucleus, belonging to the cerebellum shows similarities between apes and humans and is parallel with the expansion of the prefrontal cortex, adding to the argument that the cerebellum is related to cognitive functions. They carried out non-invasive research with human subjects and were ultimately able to make the argument that, based on cerebellar activity shown in the PET and IMRI tests, responses from the cerebellum, when compared between motor activities and cognitive activities, suggested that the cerebellum had a larger role in cognition than previously thought. 

The cerebellum is known to many as the “little brain” of the body because, although it only accounts for about 10% of the brain’s volume, it contains over 50% of all the brain’s neurons. The major functions of this structure are its involvement in balance (controlled via its connection to the inner ear), equilibrium, muscle tone, coordination, and motor learning:

• Control of balance and posture. The cerebellum is responsible for making adjustments to posture in order to maintain balance. With neuronal input from the vestibular receptors and proprioceptors, the cerebellum issues commands to motor neurons to compensate to changes in body position or to demands of muscles (i.e. lifting or putting down a heavy weight). In fact, these functions were one of the main reasons researchers believed that the cerebellum was largely responsible for motor control. Patients with damage to, or improper development of, the cerebellum suffer from balance disorders and adopt postural strategies, such as standing with a wide stance, to compensate for this. 
• Control of voluntary movements. The cerebellum coordinates the timing and force of various muscle groups in order to produce fluid movements of the whole body or individual limbs. 
• Role in motor learning. The cerebellum plays a major role in developing and fine-tuning motor plans to execute accurate movements, learning by trial-and-error (i.e. learning to ride a bike by falling off and getting back on again). 
• Participation in cognitive functions. Historically, the cerebellum is recognized as a part of the motor system, so its part in cognitive function is still being discovered. So far, scientists know that the cerebellum plays a role in memory and learning. 

The cerebellum controls muscle tone using its connections to proprioceptors (neurons that aid in the body’s awareness of the positioning of muscles, tendons, and body parts capable of independent movement). 

Types of Cells in the Cerebellum

Neurons in the cerebellar cortex send their processes in one of three directions (which is extremely conservative for the complexity of the nervous system!): 

• The dendrites of neurons in the cerebellar cortex grow in a tangential direction toward the pial surface (the boundary separating gray matter and cerebrospinal fluid) but also toward the excitatory fibers. 
• All excitatory fibers (i.e. the “parallel fibers”) run parallel to the pial surface. 
• Inhibitory fibers also grow parallel to the pial surface but tangentially to the direction of the excitatory fibers.

The cerebellar cortex is comprised of several types of five major nerve cells and three main neuronal fibers: the Purkinje cell, basket cell, stellate cell, Golgi cell, granular cell, mossy fibers, parallel fibers, climbing fibers.

The Purkinje cell is quite unique due to the incredibly high amount of parallel fiber synapses it is host to. The number of these synapses – approximately 200,000 – is approximately 10- to 20-fold more than that of other neurons. It receives impulses from granule cells specifically. The Purkinje cell is an inhibitory neuron, secreting neurotransmitters that bind to receptors to reduce or completely stop the activity of other neurons. (Fun fact Purkinje cells were the first neuronal cells ever identified!)

Interesting Theories Concerning the Cerebellum

Timing theory: When early scientists were researching the structure and function of the cerebellar cortex, they hypothesized that the parallel fibers could possibly function as “delay lines” since the cerebellum could produce time delays up to hundreds of milliseconds in order to precisely control the timing required for the activation of muscles during movement. Of course, we now know that this is not true and that the delays that are potentially produced by the parallel fibers are, in fact, much shorter than these earlier researchers presumed. (This delay actually varies between species, reaching their peak velocity at about 0.5m/s in vertebrates.)

Tidal wave theory: This is basically a logical extension of the timing theory, in that parallel fibers are still interpreted as delay lines but serve a slightly different purpose by producing even shorter delays. The parallel fibers transform action potentials received from sequential mossy fiber and synapse them onto Purkinje cells. If the granule cells along a “beam” of parallel fibers and are excited in sequence by mossy fiber synapses, several parallel fiber spikes would be elicited. The spikes would all lie in a single plane parallel to the plane of the dendritic trees of the Purkinje cells. This is dependent, however, on the sequence of synapses from the mossy fibers: if fired consecutively, the excitation of the parallel fibers would seemingly move in a direction parallel to the parallel fibers at a speed identical to the activation of the spikes. In essence, the action impulses create a wave, similar to that of people standing up and throwing up their hands in sequence at a football or basketball game. 

References

• Ninja Nerd Science. (2018, December 3). Neurology | Anatomy, and function of the cerebellum [Video file]. Retrieved from https://www.youtube.com/watch?v=NVsrexn3pT8
• Buckner, R. (2013). The cerebellum and cognitive function: 25 years of insight from anatomy and neuroimaging. Neuron, 80(3), 807-815. doi:10.1016/j.neuron.2013.10.044
• Knierim, J. (n.d.). Cerebellum (section 3, chapter 5) neuroscience online: An electronic textbook for the neurosciences | Department of neurobiology and anatomy – The University of Texas Medical School at Houston. Retrieved from https://nba.uth.tmc.edu/neuroscience/m/s3/chapter05.html

Image source:

• https://en.wikipedia.org/wiki/Anatomy_of_the_cerebellum
• https://nba.uth.tmc.edu/neuroscience/m/s3/chapter05.html