Neuroscience/Objectives/Lectures 41-42

From PhysioWiki

Cerebellum and cerebellar diseases

Introduce the contribution of the cerebellum to sensorimotor processing.

The cerebellum processes sensory and motor information to maintain:

• equilibrium,
• posture,
• muscle tone,
• coordination, and
• accuracy

It also provides signals for the initiation and termination of movements. Its motor cortex and spinal cord afferents allow the cerebellum to compare motor plans with their outcomes, putting the cerebellum in a position to correct and fine-tune movements and motor programs.

Define the regional anatomical organization of the cerebellum.

Anterior and posterior lobes

The anterior and posterior lobes are separated from one another by the primary fissure.

Flocculonodular lobe

The flocculonodular lobe is separated from the posterior lobe by the posterolateral fissure. It contains the medial nodulus and the flocculus, a pair of lateral lobes.

Lobules

In lecture, we were told that we would not be responsible for this.

Folia

Convolutions of the cerebellar cortex, separated by fissures, give the cerebellum a foliate appearance.

Describe the anatomical organization of the deep cerebellar nuclei.

Dentate nucleus

The paired dentate nuclei sit deep within the lateral cerebellar hemispheres. They are the most lateral of the deep cerebellar nuclei.

Interposed nucleus

Medial to the dentate nuclei are two pairs of interposed nuclei. They are located deep to the paravermal (intermediate) zone.

Fastigial nucleus

Paired fastigial nuclei sit deep to the vermis and medial to the interposed nuclei.

Understand the functional and anatomical subdivisions of the cerebellum.

Vestibulocerebellum (balance)

The vestibulocerebellum corresponds to the flocculonodular lobe, which is phylogenetically the oldest part of the cerebellum (hence another synonym is archicerebellum). This region of the cerebellum sends fibers to and receives fibers from the vestibular nuclei, giving the vestibulocerebellum control over balance: controlling eye movements and body equilibrium.

Spinocerebellum (motor execution)

Composed of the vermis and paravermis, the spinocerebellum receives a large number of spinocerebellar afferents. Its cortex overlies the fastigial, globose, and emboliform deep cerebellar nuclei, which send efferent projections to modulate alpha motor neurons of the spinal cord. The spinocerebellum plays an important role in motor execution: exerting control over equilibrium, posture, and the ongoing execution of limb movements.

Cerebrocerebellum (motor planning)

Corresponding to the paired lateral hemispheres, the cerebrocerebellum receives most of its input from the motor cortex via the pons (hence corticopontocerebellar pathway). It sends recurrent projections to the motor cortex, giving the cerebrocerebellum control over motor planning.

Illustrate the biochemical and cellular organization of the cerebellar cortex.

List the major neurotransmitters in the cerebellar circuit.

• Excitatory: glutamate
• Inhibitory: GABA

Describe the input-output organization of the cerebellum.

Inputs to the cerebellum come in two flavors, which were arbitrarily designated A and B in lecture. A-type fibers project only to the cerebellar cortex, while B-type fibers project both to the cortex and to the deep cerebellar nuclei. Neurons of the cerebellar cortex in turn project to the deep cerebellar nuclei, which send fibers that constitute the major output of the cerebellum.

(Note that the vestibulocerebellum does not send its outputs via the deep cerebellar nuclei. Instead, the vestibulocerebellar cortex projects directly onto vestibular nuclei. Thus, functionally the vestibular nuclei behave as the deep nuclei of the vestibulocerebellum.)

List and draw cerebellar pathways.

Define hypotonia, ataxia, intention tremor, asynergia, dysmetria.

See glossary.

Discuss the clinical aspects of cerebellar lesions.

Ipsilateral deficits

Because cerebellar pathways often decussate twice before reaching their targets, cerebellar lesions often result in ipsilateral motor deficits.

Posterior lobe syndrome

Lesions to the posterior lobe result in ataxia (including dysmetria and dysdiadochokinesia), hypotonia, and intention tremor.

Anterior lobe syndrome

Often resulting from chronic alcoholism, anterior lobe patients suffer a loss of coordination mainly in the lower limbs.

Flocculonodular lobe syndrome

Lesions of the vestibulocerebellum often result in balance disturbances, chiefly presented as a lack of axial muscle coordination (truncal ataxia). Consequently, the patient walks with a wide-spaced gait with the trunk constantly swaying even when at rest.