Structural organization of a cerebellum.

It is located above medulla oblongata and pons cerebri. It consists of two hemispheres, vermis and three pedunculi pairs (nervous fibers).

The hemispheres are divided into anterior and posterior lobes. They consist of cortex of a cerebellum and nuclei (nervous cells).

Upper peduncles connect it with midbrain, medium peduncles – with the pons, inferior peduncles – with medulla oblongata.

Neuronic structure of cerebellar cortex

The cortex consists of three layers:

• 1-st – molecular: dendrites of Purkinje cells, piriform cells (Purkinje's cells), bodies of basket cells, stellate cells.

• 2-nd – ganglionic: bodies of piriform cells;

• 3-rd – granular: bodies of association [intercalary, internuncial] neurons.

Two types of afferent fibers (climbing and mossy) come to cortex.

There are nuclei in white matter of hemisphere and vermis of cerebellum: fastigial (3), globosus (1), emboliformis (4), dentatus nuclei (2).

Pedunculi of cerebellum are conductive tracts.

Upper (1) are directed to corpora quadrigemina of midbrain;

medium (3) bind cerebellum to pons;

inferior (7) bind with medulla oblongata.

Somatotopic projection in a cortex of vermis and hemispheres of a cerebellum

Collector of afferent impulses in cerebellum is fastigial nucleus, which sends them to piriform neurones (Purkin’e cells) of cerebellar cortex according to a somatic projection. Upper extremities are represented in anterior departments of hemispheres, inferior ones are represented in back departments, head and neck are represented in anterior parts of vermis cortex and trunk is represented in posterior part. The proximal parts of extremities are projected more medially, distal parts – more laterally, hemispheres are responsible for coordination of locomotions of extremities, vermis – of trunk.

Functions of cerebellum

1. Center of movements coordination.

2. Control of posture and muscle tone.

3. Senso-motor coordination of posture and purposive movements.

4. Participation in arbitrary locomotions and coordination of fast (phasic) and slow (tonic) components of motor act.

5. Coordination of fast purposive movements.

Cerebellum and its connections

Cerebellum receives afferent impulses from all receptors stimulating during locomotion (from proprioreceptors, vestibular, visual, and acoustical). Cerebellum influences onto red nucleus and reticular formation of brainstem, which send impulses to spinal cord gamma-motoneurons regulating muscle tone.

Signs of a cerebellum lesion

Static ataxy – it is infringement of statics in rest (shaking).

Dynamic ataxy – it is locomotion coordination disorder at performance of targeted movements. A tremor of extremities – so-called intentional tremor – is observed. Walking is shaky, “drunk”.

Cause of cerebellar ataxy is the disturbance of сonjugative work of muscles – agonists and antagonists, disproportion of locomotion – dysmetria. Rapid change of one locomotion by others, inverse ones – adiadohokinesis (pronation and supination) is impeded.

Asynergia – it is disturbance of mutual conjugated movements.

Atony – it is loss of muscle tone (anterior part of cerebellar posterior lobe is responsible).

Astasia – it show up in ability of muscles to realize oscillating motions (tremor, shaking) and in disabilities to conjoint tetanic contraction.

Tremor – it is oscillating motions in various body segments.

The excision of cerebellum does not cause disappearance of reflectory responses; tonic reflexes of brainstem are non-changed.

A ssays (probes) for an examination of cerebellar functions

Examination of coordination and dynamic ataxy

Assay of Romberg – it is examination of equilibrium in rest. Ataxias arise at a lesion of spinal cord posterior funicles and vestibular apparatus.

In case of a cerebellar hemispheres lesion the patient is inclined to the side of the focus of a lesion.

In case of a lesion of big hemispheres cortex patient falls to the side opposite to the focus.

Phalanx walking – it is step-by-step movements sideways. In case of a lesion leg are excessively extended and are thrown out forward.

Test for an assessment of functions of a cerebellum

Examination of coordination and dynamic ataxy

Finger-nose test (digitonasal sign) – locomotor ataxia, dysmetrias, tremor of fingers are detected.

Genucalcaneal (heel-knee) test – presence of locomotor ataxia and dysmetrias of lower extremities are detected.

Diadochokinesia test – synchronism and uniformity of movements are detected. On the side of a cerebellum lesion the lag of extremities is determined.

Examination of an asynergia

Babynsky test. In reclining position with crossed arms it is necessary to get up. At a cerebellum lesion leg rise, and the body is remained to lie.

Оzhehovsky test. Support by palms of stretched arms should be done. At sudden disappearance of support the examinated person in norm is inclined back, the patient falls forward.

Stuart-Holms test. Forearm and hand are in position of pronation, hand is clenched in fist. The doctor tries to extend patient's forearm. Healthy person's muscles-antagonists contract quickly and stroke is prevented.

In case of cerebellum lesion

Speech is slowed, explosed (loss of smoothness), scanding (accents are not on necessary syllable) (dysarthria).

Nystagmus is more often horizontal;

Hypomyotonia (muscular hypotonia). Volume of movements is enlarged;

Handwriting disorder is observed. Handwriting is irregular, bold (macrography).

Extrapyramidal part of nervous system

Strio-pallidal part.

Extrapyramidal system includes structures of big hemispheres cortex, subcortical ganglions, cerebellum, reticular formation, descending and ascending tracts.

Functions: it provides involvement of all motor systems of brain, perfects movements making them economic and automatic.

Strio-pallidal system:

Striatum (more young formation) includes caudate nucleus and putamen.

Pallidum includes globus pallidus, black matter, red nucleus, subthalamic nucleus.

Myelination of striatal tracts comes to end of 5-th month age. Newborns possess pallidal features.

Cortical center of extrapyramidal part includes premotor area (field 6), temporal and occipital areas.

Connections of strio-pallidal system.

1. Tract connecting to ending motor tract and muscle.

2. Mutual connections with various parts of extrapyramidal system and big hemispheres cortex

3. Afferent pathways:

- rubrospinal tract of Monakov

- reticulospinal tracts

- tegmental-spinal tract from corpora quadrigemina

- tracts to motor nuclei of cranial nerves.

Afferent signals from thalamus, cerebellum, reticular formation, great brain cortex create continuous corrective flow.