Dr Anitha MA BHMS,MD(Hom)
Dr.Padiyar Homoeopathic Medical College.Kerala
Anatomical functional areas of the cerebellum:–
Anatomically, the cerebellum is divided into three lobes by two deep fissures.
1.The anterior lobe
2.The posterior lobe
3.The flocculo-nodular lobe.
Longitudinal functional divisions of the anterior and posterior lobes:-
- In the centre of the cerebellum, there is a narrow band separated from the remainder of the cerebellum by shallow grooves. This is called the vermis. In this area, most cerebellar control functions for the muscle movement of the axial body, neck, shoulder and hips are located.
- To each side of the vermis is a large, laterally protruding cerebellar hemispheres, and each of these hemispheres is divided into an intermediate zone and a lateral zone.
- The intermediate zone of the hemisphere is concerned with controlling muscle contractions in the distal portions of the upper and lower limbs, especially the hands and fingers and feet and toes.
- The lateral zone of the hemispheres operates at a much more remote level, because this area joins with the cerebral cortex in the overall planning of sequential motor movements
- Topographical representation of the body in the vermis and intermediate zone:-
- The axial portions of the body lie in the vermal part of the cerebellum, wheras the limbs and facial regions lie in the intermediate zones. These topographical representations receive afferent nerve signals from all the respective parts of the body as well as from corresponding topographical motor areas in the cortex and brainstem. In turn, they send motor signals into the same respective topographical areas of the motor cortex, as well as topographical areas of the red nucleus and reticular formation in the brain stem.
The large lateral portions of the cerebellar hemispheres do not have topographical representations of the body. These areas of the cerebellum receive their input signals almost exclusively from the cerebral cortex, especially from the motor and premotor areas of the frontal cortex, and from the somatosensory and sensory assosciation areas of the parietal cortex.
Neuronal circuit of the cerebellum:-
The human cerebellar cortx is actually a large folded sheet, about 17cms wide, by 120cms long, with the folds lying crosswise. Each fold is called a folium. Lying deep beneath the folded mass of cerebellar cortex are the deep cerebellar nuclei.
Input pathways to the cerebellum:-
1. Afferent pathways from other parts of the brain.
2. Afferent pathways from the periphery.
1. Afferent pathways from other parts of the brain :–
a. Corticopontocerebellar pathway- originates in the motor and premotor cortex and also in somatosensory cortex. It then passes by way of the pontine nuclei and pontocerebellar tracts mainly to the lateral divisions of the cerebellar hemispheres on the opposite sides.
b.Olivocerebellar pathway:-
c.Vestibulocerebellar pathway
d.Reticulocerebellar pathway.
2. Afferent pathways from the periphery:-
a. Dorsal spinocerebellar tract-2
b. Ventral spinocerebellar tract-2
The dorsal tract enters the cerebellum through the inferior cerebellar peduncle and terminates in the vermis and intermediate zones of the cerebellum on the same side as its origin. The ventral tract enters the cerebellum through superior cerebellar peduncle, but it terminates in both sides of the cerebellum.
The signals transmitted in the dorsal spinocerebellar tracts come mainly from the muscle spindles and to a lesser extent from other somatic receptors throughout the body such as golgi tendon organs, large tactile receptors of the skin and joint receptors. All these signals apprise the cerebellum of the momentary status of 1)muscle contraction 2)degree of tension of the muscle tendons 3)positions and rates of movements of the parts of the body and 4) forces acting on the surfaces of the body.
The ventral spinocerebellar tracts are mainly excited by the motor signals arising in the anterior horn of the spinal cord from 1)the brain through the corticospinal and rubrospinal tracts and 2) the internal motor pattern generators in the cord itself. Thus this ventral fibre pathway tells the cerebellum which motor signals have arrived at the anterior horn.
The spinocerebellar pathways can transmit impulses at velocities of upto 120m/sec, which is the most rapid conduction in any pathway in the central nervous system.
In addition, there are the spinoreticular pathway and spinoolivary pathway. Then these signals are relayed from both of these areas to the cerebellum.
Output signals from the cerebellum:-
Deep cerebellar nuclei and the efferent pathways:
Located deep in the cerebral mass on each side are the deep cerbellar nuclei- the dentate, interposed and fastigial.
All the deep cerebellar nuclei receive signals from two sources: 1) the cerebellar cortex and 2)the sensory afferent tracts to the cerebellum.
Each time an input signal arrives in the cerebellum, it divides and goes in two directions:1)directly to one of the cerebellar deep nuclei and 2)to a corresponding area of the cerebellar cortex overlying the deep nucleus. Then a fraction of a second later, the cerebellar cortex relays an inhibitory output signal back to the same deep nucleus. Thus, all input signals that enter the cerebellum eventually end in the deep nuclei in the form of initial excitatory signals followed a fraction of a second later by inhibitory signals.
Pathways:-
1.A pathway that originates in the midline structures of the cerebellum(the vermis) and then passes through the fastigial nuclei into the medullary and pontine regions of the brain stem.
2.A pathway that originates in the 1)intermediate zone of the cerebellar hemisphere and then passes through 2)the interposed nucleus to3) the ventrolateral and ventroanterior nuclei of the thalamus and then to 4) the cerebral cortex and then to 5) several midline structures of the thalamus and then to 6) the basal ganglia and 7) the red nucleus and reticular formation of the upper portion of the brain stem.
3. A pathway that begins in the cortex of the lateral zone of the cerebellar hemisphere and then passes to the dentate nucleus, next to the ventrolateral and ventroanterior nuclei of the thalamus and finally to the cerebral cortex.
Functional unit of the cerebellar cortex:–
- The purkinje cell and the deep nuclear cell.
- The three major layers of the the cerebellar cortex are the molecular layer, the purkinje cell layer, and granule cell layer.Beneath these cortical layers, in the centre of cerebellar mass, are the deep nuclei.
- The output from the functional unit is from a deep nuclear cell. This cell is continuously under both excitatory and inhibitory influences.The excitatory influences arise from direct connections with afferent fibres that enter the cerebellum from the brain or the periphery.The inhibitory influences arise entirely from the purkinje cells.
- The afferent inputs to the cerebellum are mainly of 2 types: one called the climbing fibre type and the other called mossy fibre.The climbing fibres all originate from the inferior olives of the medulla. There is one climbing fibre for about 5 to 10 purkinje cells. After sending branches to several deep nuclear cells, the climbing fibres projects all the way to the outer layer of the cerebellar cortex, where it makes about 300 synapses with the soma and dendrites and each purkinje cell. This climbing fibre is distinguished by the fact that a single impulse in it will always cause a single, prolonged(upto 1 second), peculiar type of action potential in each purkinje cell with which it connects, beginning with a strong spike and followed by a trail of weakening secondary spikes. This action potential is called the complex spike.
The mossy fibres are the other fibres that enter the cerebellum from multiple sources.The higher brain, brain stem and the spinal cord. These fibres also sent collaterals to excite the deep nuclear cells. Then they proceed to the granule cell layer of the cortex, where they synapse with hundreds to thousands of granule cells. In turn, the granule cells send very very small axons, less than 1micrometer in diameter, upto the outer surface of the cerbellar cortex to the molecular layer. Here the axons divide into two branches that extend 1 to 2 millimeters in each direction parallel to the folia. There are some 500 to 1000 granule cells for every 1 purkinje cell. It is into this molecular layer that the dendrites of purkinje cell project and 80,000 to 200,000 of these parallel fibres synapse with each purkinje cell.
The mossy fibre input to the purkinje cell is quite different from the climbing fibre input because their synaptic connections are weak, so that large number of mossy fibres must be stimulated simultaneously to excite a purkinje cell. Furthermore, activation usually takes the form of a much weaker-short duration action potential called a simple spike.Direct stimulation of the deep nuclear cells by both the climbing and the mossy fibres excites them. By contrast, signals arising from the purkinje cells inhibit them.
Other inhibitory cells in the cerebellum -Basket cells, stellate cells and golgi cells.Basket cells and stellate cells are located in the molecular layer of the cortex. Golgi cells , however lie beneath the parallel fibres.
Functions of the cerebellum in overall motor control:-
The nervous system uses the cerebellum to coordinate motor control functions at three levels, as follows:
1.The vestibulocerbellum:This consists principally of the small flocculonodular cerebellar lobes(that lie under the posterior cerebellum) and adjacent portions of the vermis. It provides the neural circuits for most of the body’s equilibrium movements.
2.The spinocerebellum:- This consists of most of the vermis of the posterior and anterior cerebellum plus the adjacent intermediate zones on both sides of the vermis. It provides the circuitry for coordinating mainly the movements of the distal portions of the limbs, especially of the hands and fingers.
3.The cerebrocerebellum:- This consists of the large lateral zones of the cerebellar hemispheres, lateral to the intermediate zones. It receives virtually all its input from the motor cortex and adjacent premotor and somatosensory cortices of the brain.
Summary:-
The main functions of the cerebellum is the control of movements:
Lesions of the cerebellum cause
1) Disturbance in posture and postural tone.
2) Disturbance in voluntary movement and fails to pick up any substance due to miscalculation of the distsnce and of speed of movement of the limb. flocculonodular lobe is concerned with standing and lesion in this lobe disturb the standing posture. Total cerebellectomy causes disturbance in tone of the muscle. The degree and extend of movement for performing a particular work is being determined by the feed back or error controlling activity of cerebellum.
Clinical abnormalities of the cerebellum:-
(a) Defects of posture
(b) Defects of voluntary movement
Defects of posture:-
1.Hypotonia—muscle tone is lost and becomes flabby.
2.Defective attitude—face turns towards the opposite side
3.Postponding and vertigo—defective arm deviates to the side of lesion when raised straight in front with eyes closed (Barany’s pointing test)
4.Static tremor develop when head or limb is held steadily
5.Nystagmus and deviation of eyes—jerky movements of eye balls—especially looking at the side of the lesion
6.Deep reflexes—weak and sluggish, and knee jerk becomes pendular.
Defects of voluntary movement:-
1.Asthenia—weakness of movement
2.Ataxia—inco-ordinated movement- 3 parts
a. decomposition of movement—a complex movement is carried out in parts
b. asynergia—incoordination between the protagonists and antagonists.
c. dysmetria—wrong judgement about the force and extend of movement.
3. Gait—reeling, legs wide apart,deviates to the same side
4. Speech—scanning—dysarthria
5. Intentional tremor—tremor during voluntary movement
6.Adidochokinesis—inability of the patients to execute pronation and supination of the forearm rapidly, or extension and flexion of fingers
References:-
1.Textbook of medical physiology-Guyton
2.Physiology- Ganong
3.Human physiology- C.C. Chatterjee
4.Understanding of medical physiology- R.L. Bijilani
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