Neuronal Activity in Cerebellar Cortex and Nuclei
Functions of cerebellum
are executed mainly by the impulses discharged from cerebellar nuclei. However,
cerebellar cortex controls the discharge from nuclei constantly via the fibers of Purkinje cells. It is done in
accordance with the signals received by cerebellar cortex from different parts
of the brain and body via climbing and mossy fibers. Entire
process involves a series of neuronal
activity:
1. Climbing fibers excite the Purkinje cells directly and cerebellar nuclei via collaterals, by releasing aspartate. Excitatory effect of climbing fiber on Purkinje cell is very strong because each climbing fiber ends on a single Purkinje cell.
2. Mossy fibers excite the Purkinje cells indirectly. In the glomeruli, mossy fibers release glutamate and excite the granule cells and Golgi cells. Collaterals of mossy fibers activate the cerebellar nuclei also by glutamate.
3. Granule cells, which are activated by mossy fibers in turn, excite the Purkinje cells, stellate cells and the basket cells through the parallel fibers. Neurotransmitter utilized by granule cells is glutamate or aspartate. Granule cells are the only excitatory cells in cerebellar cortex, while all other cells are inhibitory in function. Each mossy fiber innervates many Purkinje cells indirectly via granule cells. So, the excitatory effect of mossy fibers on Purkinje cells is weak.
4. Stellate cells and basket cells, which are activated by granule cells, inhibit the Purkinje cells by releasing GABA. This type of inhibition is called feed forward inhibition.
5. 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
6. Cerebellar nuclei are excited by collaterals from climbing and mossy fibers. In turn, the nuclei send excitatory impulses to thalamus and different nuclei in brainstem.
7. However, signals
discharged from Purkinje cells inhibit cerebellar nuclei via GABA. Purkinje
cells.
Thus, it is clear that the
cerebellar cortex plays an important role in modulating the excitatory signals
of following pathways:
i. From cerebellar nuclei
to cerebral cortex via thalamus
ii. From
final common motor pathway via brainstem and spinal cord.
Because of this activity
of cerebellar cortex, movements of body are well organized and
coordinated.
CEREBELLAR NUCLEI
Cerebellar nuclei are the
masses of gray matter scattered in the white matter of cerebellum. There are
four nuclei on either side.
1. Fastigial
Nucleus
Fastigial nucleus is also
known as nucleus fastigi. Phylogenetically, it is the
oldest cerebellar nucleus. It
is placed near the midline
on the roof of IV ventricle.
2. Globosus
Nucleus
Globosus nucleus is situated
lateral to nucleus fastigi. This is also known as nucleus
globosus.
3. Emboliform
Nucleus
Emboliform nucleus is also
called nucleus emboliformis. This nucleus is below the nucleus
fastigi and
nucleus globosus.
4. Dentate Nucleus
Dentate nucleus is also
called nucleus dentatus. It is the largest cerebellar
nucleus. As it is crenated, it is
called dentate nucleus. It
is situated lateral to all the other nuclei.
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