Functional Aspects of the Cerebellum

The cerebellum | Main Anatomy Index | The basal ganglia

• There are characteristic motor disorders and no significant sensory deficit following cerebellar damage.

• The major neural circuit is the great loop from several areas of the cerebral cortex to the cerebellum and back to the motor and premotor cortex.

• The hypothesis about the function of the lateral hemisphere-dentate nucleus portion of the cerebellum is that it participates in the planning and programming of voluntary movements.
• This is especially in learned, skilful movements that become more rapid, precise, and automatic with practice.

• The major inputs to the paravermal cortex are superimposed, somatotopically arranged projections from the motor cortex and spinal cord.
• The major output is via the interposed nucleus to the red nucleus and also back to the motor cortex (VL/VA of the thalamus).

• Thus the paravermal cerebellum can influence spinal cord motor neurons through the corticospinal tract and also through the rubrospinal pathway.

• The hypothesis is that the paravermal zone of the cerebellum compares the commands from motor cortex (via pontine nuclei) with the actual position and velocity of the moving part (via spinocerebellar and similar tracts).
• It then, by way of the interposed nucleus, issues correcting signals.

Cerebral cortex (via pontine nuclei) & proprioceptive information (spinocerebellar and etc) --> paravermal cerebellar cortex --> interposed nucleus --> SCP --> decussation of SCP (cross midline) -->

1. Red nucleus --> rubrospinal tract (cross midline) --> spinal cord;
2. VA/VL --> motor cortex (limb areas).

• The vermis includes the representation of the trunk conveyed by the spinocerebellar tracts.
• Its major outputs reach the vestibular nuclei and the reticular formation.
• This is through both the fastigial nucleus and through direct projections.
• The vestibulospinal and reticulospinal tracts then influence spinal motor neurons.

• This part of the cerebellum seems to be involved in the regulation of posture and of stereotyped movements that are programmed in the brainstem and spinal cord.

• The principal connections of the flocculonodular lobe are with the vestibular nerve and nuclei.
• This is involved in the maintenance of equilibrium.

• In addition, the flocculus seems to have a special role in the co-ordination of slow eye movements.

1. Vestibular nuclei --> vestibulospinal tract;
2. Fastigial nucleus --> vestibular nuclei or reticular formation --> reticulospinal tract.

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• Syndromes referable to individual zones are rarely seen clinically.
• It is extremely unlikely that damage to one zone could happen without damaging other parts of the cerebellum and possibly parts of the brainstem.
• As a result, what is typically seen clinically are problems referable to the flocculonodular lobe or to one or both sides of the corpus cerebelli as a whole.

• Tumours called medulloblastomas occasionally arise in the roof of the 4th ventricle.
• This is the most common cause of damage to the flocculonodular lobe.

• Affected individuals have:

1. A general loss of equilibrium;
2. They sway from side to side when standing;
3. Walk with a staggering, wide-based gait;
4. And tend to fall over.
5. When the trunk is supported movements of the arms and legs are normal (as the basic mechanisms used in moving the limbs are unaffected);
6. There is no tremor;
7. Both reflexes and muscle tone remain normal.

• A variety of eye movement difficulties may also be seen:

1. Problems with pursuit eye movements;
2. With maintaining eccentric gaze;
3. Or with making accurate voluntary eye movements.

• As a further consequence of these tumours, the lateral and median apertures of the fourth ventricle may be squeezed shut.
• This may cause non-communicating hydrocephalus.

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• The malnutrition often accompanying chronic alcoholism causes a degeneration of the cerebellar cortex.
• This tends to start at the anterior end of the anterior lobe and spread backwards.
• A great deal of the anterior lobe is occupied by vermis and paravermis, and the legs are represented most anteriorly.

• The result is a syndrome (called the anterior lobe syndrome) in which the legs are primarily affected.
• Symptoms of this include:

1. A broad-based, staggering gait;
2. A general ataxia of leg movements, even when the trunk is supported.

• Most of the cerebellum is made up of the lateral hemisphere.
• This is often the region most heavily damaged in lesions of the corpus cerebelli.
• The result is called the neocerebellar syndrome.
• This is characterised by:

1. Widespread decreases in muscle tone (hypotonia) may follow small lesions, so that the limbs offer little resistance to passive movement and muscles feel abnormally soft and flaccid;
2. The reduction of stretch reflexes (hyporeflexia);
3. As a result of the hypotonia a limb may swing back and forth after a reflex contraction (pendular reflexes);
4. The lack of co-ordination of voluntary movements caused by a fundamental deficit in the timing of movements and the regulation of their rates;
5. Voluntary movements take longer than usual to initiate;
6. There are problems in stopping or changing direction;
7. Patients are likely to overshoot or undershoot targets (dysmetria);
8. Corrective movements when the patient nears a target have the appearance of a tremor (intention tremor);
9. And rapid alternating movements may be especially difficult (adiadochokinesia).

• When complex movements involving more than one joint are performed, the timing of different parts may be defective.
• This leads to a decomposition of movement.

• The complex movements used in speaking may be affected in this way.
• The normal flow and rhythm of speech is disrupted.
• Successive syllables may emerge slowly and separated from each other (scanning speech).

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