Interneuronal activity in cerebellum
|Climbing fibers||Purkinje cells and Cerebellar nuclei||Excitation||Aspartate|
|Mossy fibers||Granule cells Golgi cells and Cerebellar nuclei||Excitation||Glutamate|
|Granule cells||Purkinje cells Stellate cells Basket cells||Excitation||Glutamate/ Aspartate|
|Stellate cells||Purkinje cells||Inhibition||GABA|
|Basket cells||Purkinje cells||Inhibition||GABA|
|Golgi cells||Granule cells||Inhibition||GABA|
|Purkinje cells||Cerebellar nuclei Vestibular nuclei||Inhibition||GABA|
GABA—Gamma aminobutyric acid
The climbing fibers excite the Purkinje cells directly and the cerebellar nuclei via the collaterals by releasing aspartate. Each climbing fiber ends on a single Purkinje cell. Because of this, the excitatory effect of climbing fiber on Purkinje cell is very strong. Mossy fibers excite the Purkinje cells indirectly. In the glomeruli, the mossy fibers release glutamate and excite the granule cells and Golgi cells. The collaterals of mossy fibers activate the cerebellar nuclei also by glutamate. The granule cells, which are activated by mossy fibers in turn, excite the Purkinje cells, stellate cells and the basket cells through the parallel fibers. The neurotransmitter utilized by granule cells is glutamate or aspartate. The granule cells are the only excitatory cells in cerebellar cortex while all the other cells are inhibitory. Each mossy fiber innervates many Purkinje cells indirectly via granule cells. So, the excitatory effect of mossy fibers on Purkinje cells is weak. The stellate cells and basket cells, which are activated by the granule cells, inhibit the Purkinje cells by releasing gamma aminobutyric acid (GABA).
The Golgi cell that is activated by mossy fibers in turn provides feedback inhibition to granule cells by releasing GABA, i.e. it inhibits the transmission of impulse from mossy fiber to granule cell.
The cerebellar nuclei are excited by collaterals from climbing and mossy fibers. In turn, the nuclei send excitatory impulses to thalamus and different nuclei n brainstem. However, the signals discharged from Purkinje cells inhibit the cerebellar nuclei via GABA. The Purkinje cells inhibit the activities of vestibular nuclei also. Thus, it is clear that the cerebellar cortex plays an important role in modulating the excitatory signals of following pathways:
1. From cerebellar nuclei to cerebral cortex via thalamus.
2. From final common motor pathway via brainstem and spinal cord.
Because of this, the movements of body become well organized and coordinated.
Cerebellar nuclei are the masses of gray matter scattered in the white matter of cerebellum. There are four nuclei on either side (Fig.25).
1. Fastigial Nucleus
This is also known as nucleus fastigi. Phylogenetically, this is the oldest cerebellar nucleus. It is placed near the midline on the roof of IV ventricle.
2. Globosal Nucleus
This is situated lateral to nucleus fastigi. This is also known as nucleus globosus.
3. Emboliform Nucleus
It is also called nucleus emboliformis. This nucleus is slow the nucleus fastigi and nucleus globosus.
4. Dentate Nucleus
Is is also called nucleus dentatus. This is the largest cerebellar nucleus. As it is crenated, it is called dentate nucleus. It is situated lateral to all the other nuclei.
Fig.25. Cerebellar nuclei.
Pile matter of cerebellum is formed by afferent and perent nerve fibers. These nerve fibers are classified into three groups.
Projection fibers: Projection fibers leave or enter the paleocerebellum and connect cerebellum with other parts of central nervous system.
Association fibers: Association fibers connect different tons of same cerebellar hemisphere.
Commissural fibers: The commissural fibers connect fhe areas of both halves of cerebellar cortex.
A. Inferior cerebellar peduncles between cerebellum and medulla oblongata.
B. Middle cerebellar peduncles between cerebellum and pons.
C. Superior cerebellar peduncles between cerebellum and midbrain.
A. Inferior Peduncles
The inferior cerebellar peduncles are otherwise called restiform bodies and contain predominantly afferent fibers. These nerve fibers transmit the impulses from tactile receptors, proprioceptors and receptors in vestibular apparatus to cerebellum.
B. Middle Peduncles
The middle cerebellar peduncles are otherwise called brachia pontis. These penduncles contain predominantly the afferent fibers. Most of the fibers of the middle cerebellar peduncles are commissural fibers, which connect the areas of both the halves of cerebellar cortex.
C. Superior Peduncles
The superior cerebellar peduncles are otherwise called brachia conjunctivae and contain predominantly efferent fibers.
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