CEREBELLUM AND ATAXIA

Human motility is characterized by amazing accuracy of purposeful movements, which is provided by commensurate work of many muscle groups managed not only arbitrary, but also automatically. This multifunctional system is realized by multineuronal coordinating apparatus which controls the body's equilibrium, stabilizes the centre of gravity, regulates the tone and coordinated various muscles activities. To perform the coordination of movements it is required to have clear and continuous return afferentation with information about the relative position of the joints, muscles state, pressure on them, control for the trajectory of the movement.

Centre for coordination is the cerebellum. In functional terms it consists of the body of the cerebellum, in which there are two hemispheres, vermis cerebelli and three pairs of legs.

Fastigial nucleus (nucl. fastigii)is the collector of afferent impulses coming to the cerebellum by different ways. Having scattered information from different sources this nucleus sends it for processing of the pear-shaped neurons (Purkyne cells) of the cerebellar cortex in accordance with the somatic projection (in anterior parts of the cerebellum hemispheres are the upper limbs, in posterior regions - the lower limbs, in the anterior cortex of the vermis cerebelli are head and neck, and in the posterior parts is trunk). Proximal parts of limbs are projected medially, distal-laterally; hemispheres are responsible for areas (precentral and frontal gyrus) (fig. 50). Therefore, muscle-cerebellar-cortical tract can be attributed with conductors of joint-muscle sense to the motor (kinesthetic) analyzer (fig. 51).

The main function of the cerebellum is carried out on subconscious level. Efferent impulses from the cerebellum nucleus regulate the proprioceptive reflexes on tension. When muscle contracts there is excitement of proprioceptor (muscle spindle) as muscles synergist and antagonist muscles. However normally the transformation of voluntary movement in a complex reflex does not occur due to inhibitory effect of cerebellar impulses. Therefore, in cerebellar affection the disinhibition of segmental proprioceptive reflexes is shown by limbs’ movements in type of ataxia. The cerebellum has many afferent and efferent connections.

Flechsig's tract. The first neuron is in the spinal ganglia, his dendrites are connected with muscles’ propriceptors, tendons, ligaments and periosteum; axon in the dorsal root passes to the Clark cells (at the base of the dorsal horn) through the dorsicornu. The fibres of these second neurons are sent to the outer layers of the rear lateral side of its cord, rise along the spinal cord and at the level of medulla oblongata in the inferior cerebellar peduncle enters into the cerebellar vermis. This tract is referred to as the tractus spinocerebellaris dorsalis (posterior) or the Flechsig’s tract. In the root of cerebellar vermis there is a third neuron which is in the contact with the pear-shaped neurons in the root of the cerebellar hemisphere. Axons of the latest pass to the dentate nucleus (nucl. dentatus). Fibres of this fifth neuron consist of the upper cerebral peduncle. The right and left upper cerebral peduncles intersect (decussation of superior cerebellar peduncles) and end at the cells of red nucleus on opposite side. The axons of red nucleus (nucl. ruber) cells are immediately transferred to the opposite side of the mid-brain and form a ventral cross in the midbrain operculum (Forel’s cross), they pass as a part of the lateral spinal cord (in front of the pyramidal tract), reach the anterior horn cells (α- and γ- motoneurons). The whole set of axons of the red nucleus cells are called tractus rubrospinalis (Monakow's tract). It is underdeveloped among humans. The main downward effect of the cerebellum is transmitted by the reticulospinal tract.

Gowers' tract (tractus spinocerebellaris anterior). The first neuron is located in the spinal ganglia, the second neuron is in the posterior horn’s cell, but its axons pass to the opposite side and they are sent up to the spinal cord in the front part of the lateral cord, pass through the medulla oblongata (the bridge of the brain), at the level of the superior medullary velum they pass to the opposite side and in the superior cerebellar peduncle reach the cells of the nuclei of the cerebellum. The further course of the efferent impulses is the same as by the posterior spinocerebellar tract.

The cerebellum gets afferent proprioceptive impulses not only by Flechsig's and Gower’s tracts, but also by axons of nuclei’ cell of tract of Goll and Burdach's tract, some of which pass not directly to the optic thalamus but pass to its vermis cerebelli through inferior cerebellar peduncle.

Axons of cells’ vestibular nucleus with inferior cerebellar peduncle pass to the cerebellum - generally from the vestibular lateral nucleus (Deiters’ nucleus), they end up in the nucleus of declive cerebellum. The cells’ fibres of this nucleus with superior cerebellar peduncle and probably inferior cerebellar peduncle fit to the cells of reticular formation of the brain stem and to vestibular lateral nucleus from which the conductors form a descending tracts – vestibular-spinal and reticular-spinal tracts, ending in the anterior horn cells of the spinal cord. The regulation of body balance is carried out by this cerebellar-vestibular-muscular tract.

Connections with the nuclei of the oculomotor nerve (with medial longitudinal fasciculus) establish from cerebellum through the vestibular lateral nucleus.

The function of the cerebellum is corrected by the various divisions of the cerebral cortex. It is obvious from numerous connections of almost every lobes of the brain with the cerebellum. The most massive of them are 2 fascicles – fronto-pontine-cerebellar and occipito-temporal-cerebellar.

Fronto-pontine-cerebellar (tractus fronto-ponto-cerebellaris) is a set of axons of cells of predominantly anterior parts of upper and middle frontal convolutions. In the depths they compact in a fascicle and form the anterior peduncle of the internal capsule. Then they pass to the basement of foot cerebral peduncle and on their own side ends up by the synapse at the cerebellar peduncle cells. The axons of these secondary neurons pass to the opposite side of the cerebellar peduncles and with the middle cerebellar peduncles enters its hemisphere and contact with the cells of the cerebellar cortex. Appendixes of these neurons in the cerebellar cortex pass to the gear core. The fibres of dentate nucleus cells with superior cerebellar peduncle reach the opposite side of the red nucleus and by reticular-spinal tract carry impulses controlling humans’ postures in vertical position particularly walking and standing.

Occipito-temporal-cerebellar tract (tractus occipito-temporo-ponto-cerebellaris). Its first neurons are located in the cortex of the occipital and temporal lobes (in part parietal), their axons pass to the subcortical white substance and then with the posterior part of the inner thigh capsules pass at the base of the midbrain nuclei to pontine. The axons of the cells of the pontine pass to the opposite side and by the middle cerebral peduncle reach the cerebellar cortex. Fibres of these cells pass to the dentate nucleus which has a connection with the brainstem. With the help of these tracts there is the coordination of the cerebellum with the organs of vision and hearing.

The existing crosses of the cerebellar afferent and efferent systems lead, ultimately, to the homolateral connection of one hemisphere of the cerebellum and the limbs. With the lesion of the cerebellar here appear hemispheres dysfunctions on the corresponding side of the body. Nidus in lateral funiculus of spinal cord also cause cerebellar disorders in their half of the body. Hemispheres of the brain are connected to the opposite hemisphere in the cerebellum. Therefore, with the lesion of the hemispheres of great brain or the red nucleus cerebellar disorders will be identified on the opposite side of the body.

Many of the symptoms of the cerebellar dysfunction are connected with lesion of reciprocal innervation of antagonist muscles. There is the essence of this phenomenon. With any movement motor neurons of muscle agonists and antagonists (e.g. flexor and extensor muscles) are in the opposite excited state. If, for example, neurons of muscles-flexors are excited, the neurons of the extensor muscles are inhibited. The mechanism of this dual (reciprocal) inhibition of spinal motor centres is that the axons of receptor cells (their bodies are located in the spinal ganglia) in the spinal cord are divided into branches, some of them excite motor neurons of the flexor muscles, and others contact with the intercalary cells that have an inhibitory effect on the cells of the extensor muscles. Thus the mechanism of reciprocal innervation is carried out by segmental spinal unit. However cerebellar impulses are involved in its complex integrative functions.

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