Cerebellum in the Control of Action

The Role of the Cerebellum in the Control of Action

The cerebellum is one of the main parts of the hindbrain. It is located behind the pons, near the brainstem, beneath the temporal and occipital lobes of the cerebral cortex (Figure 1). Also known as the ‘little brain’; the cerebellum receives  signals from the sensory systems i.e. the spinal cord and various parts of the brain playing a major role in motor movements. It coordinates voluntary movements from speech to posture and muscular coordination. The cerebellum produces fluid limb and body movements. The cerebellum’s role in movement and its downstream pathways have been well studied however there have been incidences where individuals were born without a cerebellum or attained damage to it resulting in cerebellar dysfunction. This essay will look at the role of a healthy cerebellum in movement control followed by cerebellar dysfunctions and its clinical manifestations. Understanding the cerebellum fully is very important as it has functions beyond just posture and balance including cognitive functions such as short-term memory, decision making, and learning motor behaviours.

(Figure 1: TeachMeAnatomy, 2020)

In the cerebellum there are three lobes that are anatomically separated from each other; flocculonodular lobe, the posterior lobe, and the anterior lobe all of which are found inside the outer grey matter. These are the three areas which carry out the functions of the cerebellum. In brief, connections from the inner ear coordinate balance and movement, proprioceptors from the body are involved in muscle tone control and upper motor planning areas send descending signals to the cerebellum coordinating movement planning. Taking a posterior view gives a much better insight into the functional zones of the cerebellum (Figure 2). The anterior lobe (spinocerebellum) which occupies the vermal and the paravermal areas serves to pick up sensory information. Just as there is a homunculus in the cerebrum, there are a number of homunculi in the cerebellum. Major inputs come from the spinocerebellar tract with outputs projecting to the vestibulospinal, rubrospinal and reticulospinal tracts. Adaptive motor coordination is produced through integration of motor commands and sensory inputs.   The posterior lobe (cerebrocerebellum) comprising dentate nuclei and lateral hemispheres functions in timing and planning of movement. It also has an influence on cognitive functions of the cerebellum. Finally, the flocculonodular lobe (vestibulocerebellum) consisting of the lateral vestibular nuclei and the flocculonodular lobe is responsible for vestibular reflexes with the main function of postural stabilisation. The spinocerebellum receives input from proprioceptors, the cerebrocerebellum receives higher input from the cerebrum and the vestibulocerebellum receives input from the inner ear.

(Figure 2: MedLibre, 2020)

Looking further into a healthy cerebellum, a sliced view reveals key nuclei involved in the functional processes. The first nucleus is the dentate nucleus. It is associated with the lateral hemisphere. The next nucleus is the interposed nucleus consisting of the emboliform and globose nuclei associated with the vermal and perivermal areas. The fastigial nucleus is the final nucleus and is heavily associated with the flocculonodular lobe. The dentate nucleus projects to the red nucleus and thalamic nucleus. The interposed nuclei project to the origin of the rubrospinal tract and finally the fastigial nucleus projects to the vestibular nuclei and the reticular formation. Lastly it is important to look at the connectivity of the cerebellum before observing potential damages. The cerebellar cortex covers the cerebellum. Any information received at the cerebellar cortex is integrated by the cerebellum and refined into accurate and well-coordinated movements. There are three layers of the cerebellar cortex; the innermost granule cell layer, the middle purkinje cell layer and the outer molecular layer. The granule cells receive inputs from the mossy fibers and project to the purkinje cells. The purkinje cells create inhibitory effects. How does it connect to the cerebellum? Cerebellar input occurs through the mossy fibers which originate in the vestibular nuclei, spinal cord, and brainstem. Their projections are excitatory. Similarly, the climbing fibers, which also connect to the cerebellum, originate from the inferior olive project excitatory signals. A combination of these excitatory and inhibitory inputs is measured and refined allowing for fluid limb and body movements.

All of the above describes some of the anatomical and functional aspects of a normal functioning cerebellum but the question is; what occurs if the cerebellum is damaged or completely missing? There are many studies looking at this and the and also the consequent clinical manifestations. First, let us look at cerebellar agenesis. This is an extremely rare condition in which an individual is born without a cerebellum. The etiology is heterogeneous and in most instances the disease occurs sporadically. Mutations in the PTF1A gene is a potential cause of the disease however as of 1998, only ten cases have been reported with the first case being reported in 1831 with genetic mutation being the cause. Interestingly, some individuals with the disease show only mild symptoms. This can be due to compensation albeit partial from other areas of the brain. In the vast majority of cases, there are profound abnormalities in motor skills which can often result in severe disability. Other clinical symptoms include hypotonia (low muscle tone), psychomotor delays (e.g. beginning to walk  at 6-7 years old), dysarthria(issues with speech), nystagmus (rapid, involuntary movements of the eyes), and ataxia (cannot coordinate voluntary movements).

The above symptoms are found in many different forms of cerebellar dysfunction. A clinician usually carries out a number of special tests to assess for signs of cerebellar dysfunction. One such test is the rapid alternating movements test which can be observed in any number of ways; ask the patient to tap your hand with the ball of each foot. Take note of the smoothness, speed, and rhythm of each movement. Patients with cerebellar dysfunction have trouble performing one movement followed closely by another similar movement. A Romberg’s test is also commonly used. The patient is asked to stand upright with their feet together and eyes open. Then the patient is asked to close their eyes for a short period of time usually a minute or two and the clinician checks for swaying. Someone with cerebellar ataxia for example will have trouble standing with both feet closed whether or not the eyes are open. Point to point movements can also reveal signs of cerebellar function. The patient is asked to touch their nose followed by the clinician’s index finger repeatedly. The distance and location of the finger is moved. Things like tremors, overshoot, accuracy, and smoothness are watched out for. In cerebellar dysfunction, point to point movements lack smoothness, are clumsy and are often accompanied by deviation to one side. Speech disturbances manifest as slow or explosive speech, nystagmus is observed as the patient’s inability to focus on stationary or moving objects (watch for visual acuity and ocular movements), the gait of an individual with cerebellar dysfunction manifests in poor posture and balance and finally tremors may also occur in these individuals. Tremors may be in the form of abnormal head tilt, nodding from side to side, myoclonic jerks (involuntary movements) and tremors seem to worsen when reaching for an object. These tests/symptoms serve as both evidence for cerebellar dysfunction and a means to diagnose an individual. These are accompanied by neuroimaging, MRI’s, and other scans.

The following is an overview of some of the conditions associated with cerebellar dysfunction. Cerebellar Hypoplasia results from reduced cerebellar volume. It is usually considered a congenital malformation though it can occur later in life. It has been associated with many neurodegenerative diseases that occur early in life such as ataxia telangiectasia. Later in life, the symptoms seem to be milder; clumsiness, dizziness whereas in younger people conditions are more severe; issues with balance and walking, seizures, poor muscle tone and speech delays (Senanarong et al., 2019). Dysdiadochokinesia describes difficulties performing quick and alternating tasks. This is usually by alternating muscle groups such as the biceps and triceps. This condition is also caused by MS and primarily affects muscles in the limbs. Subjects show issues in balance, walking, coordination, rigidity, and speech. Friedrich’s ataxia and ataxic dysarthria may also lead to Dysdiadochokinesia. Adiadochokinesia is a more severe form of the same condition and carries the same symptoms. It is a manifestation of more advanced stages of cerebellar dysfunction. The umbrella term for many muscle control conditions is ataxia. Persistent ataxia is often immediately attributed to issues with the cerebellum. Causes of ataxia range from tumours, strokes, and MS to defective genes. Unfortunately, there are few drug treatments for ataxia and it often depends on the individual case and underlying cause. In most cases, physical therapy, occupational therapy, and adaptive devices are provided.  Below is a short summary.

In conclusion, the role of the cerebellum is very clear and has been established over decades with scientific research. It operates alongside the cerebral cortex to refine, filter and measure incoming signals and to produce normal movement. Its functions range from postural stabilisation through connections with vestibular nuclei, balance control, muscle tone control, smooth movements, and controlled speech. Cerebellar dysfunctions are varied and well-studied however treatments are limited. All forms of cerebellar dysfunction have clear connection with impaired movement – be it through speech, balance, gait, muscle tone or refinement. Furthermore, cerebellar dysfunction has a strong connection with cognitive function – especially short term memory, which may affect walking at a very young age. The importance of the cerebellum is clear, and it is a very interesting area of further research with respect to motor impairments.