SUMMARY1. The electroresponsive properties of guinea-pig thalamic neurones were studied using an in vitro slice preparation.2. A total of 650 cells were recorded intracellularly comprising all regions of the thalamus; of these 229 fulfilled our criterion for recording stability and were used as the data base for this report. The resting membrane potential for thirty-four representative neurones which were analysed in detail was -64 + 5 mV (mean + S.D.), input resistance 42 + 18 MK2, and action potential amplitude 80 + 7 mV.3. Intracellular staining with horseradish peroxidase and Lucifer Yellow revealed that the recorded cells had different morphology. In some their axonal trajectory characterized them as thalamo-cortical relay cells.4. Two main types of neuronal firing were observed. From a membrane potential negative to -60 met, anti-or orthodromic and direct activation generated a single burst of spikes, consisting of a low-threshold spike (l.t.s.) of low amplitude and a set of fast superimposed spikes. Tonic repetitive firing was observed if the neurones were activated from a more positive membrane potential; this was a constant finding in all but two of the cells which fulfilled the stability criteria.5. The l.t.s. response was totally inactivated at membrane potentials positive to -55 mV. As the membrane was hyperpolarized from this level the amplitude of the l.t.s. increased and became fully developed at potentials negative to -70 mV. This increase is due to a de-inactivation of the ionic conductance generating this response. After activation the l.t.s. showed refractoriness for approximately 170 ms. Deinactivation of l.t.s. is a voltage-and time-dependent process; full de-inactivation after a step hyperpolarization to maximal l.t.s. amplitude (-75 to -80 mV) requires 150-180 ms.6. Membrane depolarization positive to -55 mV generated sudden sustained depolarizing 'plateau potentials', capable of supporting repetitive firing (each action potential being followed by a marked after-hyperpolarization, a.h.p.). The a.h.p. and the plateau potential controlled the voltage trajectory during the interspike interval and, with the fast spike, constitute a functional state where the thalamic neurone displayed oscillatory properties.

SUMMARY1. The ionic requirements for electroresponsiveness in thalamic neurones were studied using in vitro slice preparations of the guinea-pig diencephalon.2. Analysis of the current-voltage relationship in these neurones revealed delayed and anomalous rectification.3. Substitution of Na+ with choline in the bath or addition of tetrodotoxin (TTX) abolished the fast spikes and the plateau potentials, described in the accompanying paper. K+ conductance was blocked with 4-aminopyridine. 8. It is proposed that the intrinsic biophysical properties of thalamic neurones allow them to serve as relay systems and as single cell oscillators at two distinct frequencies, 9-10 and 5-6 Hz. These frequencies coincide with the a and 0 rhythms of the e.e.g. and, in the latter case, with the frequency of Parkinson's tremor.

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SUMMARY1. The properties of neurones of the guinea-pig deep cerebellar nuclei in a slice preparation were investigated by intracellular recording.2. The recorded population of cells did not differ morphologically from nuclear cells in vivo as judged from neurones stained with Lucifer Yellow.3. Fifty-two out of sixty cells were spontaneously active with a regular firing pattern and a mean frequency of 26 + 14 (mean + S.D.) impulses/s. The action potentials lasted 0-41 + 0 07 ms (n = 60) with an amplitude of 58 + 8 mV. Input resistance was 44+10 MCI and the time constant of the membrane 13 + 3 ms.4. When stimulated with intracellularly injected depolarizing current pulses the cells responded with trains of action potentials. Near the threshold for the spike the stimulation produced firing of constant frequency and from more hyperpolarized levels an initial acceleration sometimes followed by a deceleration was seen.5. At levels less than 15 mV from the spike threshold there was a rebound train of spikes as a response to a hyperpolarizing current injection. At more hyperpolarized levels there was only a small depolarizing potential after the hyperpolarizing stimulation.6. Three types of subthreshold potentials were recorded. Spikelets rose from base line as 3-10 mV depolarizing wavelets with a duration between 5 and 10 ms. They served as trigger potentials for the action potential. Plateau potentials were slow depolarizing potentials often reaching the spike threshold and thus generating long trains of action potentials. After-hyperpolarizations followed each spike with a time course dependent on the previous activity of the cell.7. Plots of the firing frequency versus injected current were linear at the first and second interspike interval, after 50 ms of activity and at steady state. Plots of the voltage versus injected current were upward concave demonstrating anomalous rectification of the cell membrane.8. It is concluded that neurones in the deep cerebellar nuclei in vitro are spontaneously active because of the electroresponsive properties of their membranes. The physiological importance may be that the cerebellar output from these cells can be rapidly and efficiently modulated by synaptic potentials generated by Purkinje cells and mossy and climbing fibres. 5PHY 372