Briefly outline the functional significance of the internal cerebellar circuitry



The deep cerebellar nuclei maintain a continuous excitatory output due to the excitation by climbing and mossy fibers. This tonic excitatory output decreases when there is discharge of the inhibitory Purkinje cells of the cerebellar cortex.

There are three minor circuits inside the cerebellum that are meant for ensuring that the discharge of the granule cells and Purkinje cells are extremely precise and short-lasting (neural sharpening). This helps in accurate timing of action potential. These circuits are as follows.

(1) When the granule cell discharges, it excites the Purkinje cell. However, the excitation is extremely short lasting. This is because the granule cell inhibits the Purkinje cell (through the basket cell) shortly after stimulating it (feed-forward inhibition).

(2) The mossy fiber stimulates the granule cell. However, the stimulation is extremely short-lived. This is because the mossy fiber inhibits the granule cell (through the Golgi cell) shortly after stimulating it.

(3)The mossy fiber causes a brief excitation of the granule cell. The duration of excitation of the granule cell is further reduced through feedback inhibition, i.e. the discharge of the granule cells brings about its own inhibition through the Golgi cells.

There are two major circuits in the cerebellum whose functions are identifiable. These are the motor correction circuit and the reverberation circuit. The motor correction circuit helps the cerebral cortex in achieving precision in motor control. When the cortex issues a motor command to the muscles, the cerebellum receives a copy (the efference copy) of the command.

The cerebellum also receives peripheral feedback from the muscles indicating to what extent the cortical motor commands have been carried out. The cerebellum compares the efference copy and the peripheral feedback to calculate the error in motor effect and accordingly issues a motor correction signal to compensate for the error.

The main function of the reverberating circuit is to form Hebbian synapses. New synapses are formed (rather, non-functional synapses are revived and strengthened) when two neurons repeatedly discharge synchronously. This principle was first enunciated by Hebb and hence, synapses strengthened through repeated use are called Hebbian synapses.

The reverberating circuit can explain why the cerebellum is required for alternately supinating and pronating the hand rhythmically (diadochokinesia). This capability is impaired in cerebellar disorders and is called adiadochokinesia.