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Adrienne Mueller, Ric Robinson; Sources of tonic firing properties of saccade-related cerebellar neurons. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1928. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Clear vision requires accurate eye movements, for which the cerebellum is vital. Two areas of medial cerebellum contribute accurate saccades: the oculomotor vermis (OMV) in the cerebellar cortex, and the caudal part of the cerebellar fastigial nucleus (CFN). Purkinje cells in the OMV fire action potentials tonically and increase (burst) or decrease (pause) their firing rate for saccades. Neurons in the CFN also fire tonically and burst for saccades. The saccade-related responses of these cells are well-documented, but our study will investigate how the cerebellum produces these signals from the input it receives.
We recorded saccade-related Purkinje cells (P-cells) in the OMV while monkeys make eye movements. Multi-barreled iontophoretic pipettes allowed us to inject the γ-aminobutyric acid (GABA) antagonist gabazine close to the neurons from which we recorded. We compared the normal responses of P-cells with responses recorded during application of gabazine. In a related experiment we investigated if the tonic firing of cerebellar nuclear neurons results from pacemaker activity intrinsic to these neurons or is influenced by excitatory input, e.g., from climbing fiber collaterals. We iontophoresed the non-specific glutamate-receptor antagonist Kynurenic acid onto cerebellar nuclear neurons while we record their activity.
Our results show that P-cells that pause for saccades are normally under persistent GABA-mediated inhibition that suppresses both phasic and tonic P-cell activity. Also, we can completely suppress the tonic firing of cerebellar nuclear neurons by removing glutamatergic input. This indicates that excitatory glutamatergic input contributes significantly to the tonic activity of neurons in the cerebellar nuclei.
Together these results show that both Purkinje cells and cerebellar nuclear neurons receive constant signals from other cells that determine their high tonic firing rates. They also receive specific signals that modulate their activity during behavior. The activity of these cells is therefore a combination of two distinct types of input, which together will generate the necessary signals for accurate saccades. Future work will determine how excitatory and inhibitory input contribute to CFN activity.
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