The Nervous System, The Cerebellum

The cerebellum is the control centre for the coordination of:

1.  voluntary muscle activity, 
2. equilibrium, and 
3. muscle tonus. 

It does not initiate movement and therefore a person who has cerebellar activity does not become paralysed. Rather his movements are:

1. slow, clumsy, temulous and uncoordinated
2. The muscles may be hypotonic and the person unable to walk steadily and tends to sway, stagger and fall.

To carry out its three functions, the cerebellum needs to receive information concerning:

1.  what "orders" are being sent from the cerebral motor cortex
2. the "equilibrium state of the body
3. the position and state of muscles and joints and the amount of tonus present.

Inputs are then integrated and feedback for regulation  and control motor activity, equilibrium and muscle tonus.

Cerabellar Organisation: Lobes and Regions 
The cerebellum has 3 major Lobes, each associated with distinct syndromes if damaged:

1. Anterior Lobe
2. Posterior Lobe
3. Flocculonodular lobe

Functional organisation:

1. Vermis - midline
1. Fastigial deep nuclei
2. Paravermis
1. globulose and emboliform deep nuclei
3. Lateral Hemisphere
1. dentate deep nuclei

The deep nuclei provide the "coarse adjustments" upon which the superimposed "fine adjustments" from the cerebellar cortex.

It regulates the 3 tracts:

1. Reticulospinal tract and Vestibulospinal tract
2. rubrospinal tract and 
3. corticospinal tract

The Spinocerebellar pathways

1. Information concerning the condition of the muscles, the amount of tonus, and the position of the body is supplied by unconscious proprioceptive fibers, whose receptors are found in joints, tendons and muscles.  
2. The cell bodies are in the dorsal root ganglion; and the axons pass into the cord, from which they reach the cerebellum by either one of two tracts.
3. The lower part of the body enter the dorsal root ganglion - synapse
1. ascend same side - ventral spinocerebellar tract of the lateral columns, and enter the cerebellum from the superior peduncle.
2. cross over and ascend contralateral side - then enter ventral spinocerebellar tract of the lateral columns, and enter the cerebellum from the superior peduncle. However before passing into the superior cerebellar peduncle they cross back to the same side from which they started.
4. Proprioceptive receptors from the upper part of the body use the dorsal spinocerebellar tract:
1. Here primary neurons synapse with secondary ones in CLARKE'S Nucleus which is only found in the upper part of the spinal cord - C8 to L2
2. Secondary columns pass into the lateral columns on the same side from the spinocerebellar tracts - and enter the cerebellum from the inferior cerebellar peduncle. 

VESTIBULOCEREBELLAR TRACT:

1. From the superior and the lateral vestibular nucleus arise the fibers that supply information concerning the equilibrium state of the body.
2. Enter through the ipsilateral (homolateral) inferior peduncle and pass to the cerebellar cortex, especially that of the focculus.

CORTICOPONTOCEREBELLAR TRACTS

1. Information regarding cerebral motor cortex discharges to lower motor neurons - its nature, what muscle groups, strength, etc - through the Corticopontocerebellar tracts
2. Cerebral cortex - descend through the internal capsule, and at the level of the pons, synapse with second order neurons in the pontine nuclei. 
3. The second order neurons now cross over to the other side and enter the cerebellum through the middle peduncle.

FEEDBACK PATHWAYS

1. After integration feedback is given to Broadmann area's 4, 4s and 6, and
2. pathways to the Lower Motor Neurons

From the cerebellar cortex, short neurons pass to the cerebellar nuclei:

1. the emboliform, fastigeal, globulose and dentate nuclei.
2. The dentate nuclei sends fibers through the superior peduncle that decussate and enter the RED NUCLEUS of the midbrain - the dentorubro fibers - or the dentorubrothalamic tract because some of them bypass the red nucleus and go up to the thalamus. 
3. The red nucleus can discharge to the up to the thalamus which relays information to the cerebral motor cortex. Completing the feedback circuit. 
4. The Red Nucleus can also discharge down to the lower motor neurons by means of the rubrospinal tract  - influence the corticospinal impulses at the spinal level

The cerebellum also discharges back via the fastigial nucleus, to the vestibular nuclei. These in turn relay stimuli to the lower motor neurons by means of the vestibulospinal tract.

Finally, the cerebellum can influence the lower motor neurons by discharging to the reticular area and the nuclei of the pons, midbrain and the medulla, which relay the discharge by the lateral and medial reticulospinal tracts.

Pathology:

Lesions to the cerebellum and its afferent or efferent pathways produce signs on the same side of the body.

1. Asynergia -is the loss of coordination in performing motor acts. One sees decomposition of movement - movement is done in jerky stages rather than smoothly 
2. Dysmetria - inability to judge distance and to stop movement at a chosen spot. Thus, in reaching for an object, a patients hand will overreach or under reach it. Pass-pointing when a patient is asked to touch her nose. 
3. Adiadochokinesia - inability to perform rapid alternating movements, such as pronation and supination of the hands.
4. Intention tremor -  occurs in voluntary movement and not at rest. This is opposite to what is seen in Parkinson's disease - here there is a resting tremor.
5. Abnormal Gait (ataxia) - to compensate the patient walks with feet spread apart
6. Falling - may occur on the injured side
7. Hypotonia  - the muscles may be floppy and weak or hypertonic and rigid.
8. Dysphonia - slurred explosive speech
9. Nystagmus - may be present

Medulloblastoma is the most common CNS tumour in children of 4 to 8 years. It is situated on the vermis of the roof of the 4th ventricle, and the first characteristic signs are atexia (ie stumbling gait) and frequently unexplained falls. The tumour is sensitive to radiation with a 5-year survival of 60%.