SUMMARY1. The electrical activity of Purkinje cells was studied in guinea-pig cerebellar slices in vitro. Intracellular recordings from Purkinje cell somata were obtained under direct vision, and antidromic, synaptic and direct electroresponsiveness was demonstrated. Synaptic potentials produced by the activation of the climbing fibre afferent could be reversed by direct membrane depolarization.2. Input resistance of impaled neurones ranged from 10 to 19 MQ and demonstrated non-linearities in both hyperpolarizing and depolarizing directions.3. Direct activation of a Purkinje cell indicated that repetitive firing of fast somatic spikes (s.s.) occurs, after a threshold, with a minimum spike frequency of about 30 spikes/sec, resembling the '2-class' response of crab nerve (Hodgkin, 1948).4. As the amplitude of the stimulus was increased, a second form of electroresponsiveness characterized by depolarizing spike bursts (d.s.b.) was observed and was often accompanied by momentary inactivation of the s.s. potentials. Upon application of tetrodotoxin (TTX) or removal of Na+ ions from the superfusion fluid, the s.s. potentials were abolished while the burst responses remained intact. However, Ca conductance blockers such as Co, Cd, Mn and D600, or the replacement of Ca by Mg, completely abolish d.s.b.s.5. If Ca conductance was blocked, or Ca removed from the superfusion fluid without blockage of Na conductance, two types of Na-dependent electroresponsiveness were seen: (a) the s.s. potentials and (b) slow rising all-or-none responses which reached plateau at approximately -15 mV and could last for several seconds. These all-or-none Na-dependent plateau depolarizations outlasted the stimulus and were accompanied by a large increase in membrane conductance. Within certain limits the rate of rise and amplitude of the plateau were independent of stimulus strength. The latency, however, was shortened as stimulus amplitude was increased. These potentials were blocked by TTX or by Na-free solutions.6. Substitution of extracellular Ca by Ba or intracellular injection of tetraethylammonium generated prolonged action potentials lasting for several seconds and showing a plateau more positive than those obtained in normal circumstances by either non-inactivating Na or Ca currents. basic spontaneous activity and its frequency unaltered, indicating that Ca spiking and Ca-dependent K conductance changes are the main events underlying this oscillatory behaviour.8. It is concluded that Purkinje cells demonstrate three types of voltage-dependent electroresponsiveness at the soma: (a) a fast Na-dependent spike, (b) a noninactivating Na conductance which generates prolonged plateaus, where the amplitude of the plateau is produced by an equilibrium between the voltage-dependent Na and K conductances and (c) Ca-dependent spiking which is TTX-insensitive but responds to substances which block slow Ca conductances.9. The Ca spikes are most prominent in the dendrites (see accompanying paper) and are followed by prolonged K conductance changes. ...

SUMMARY1. Intradendritic recordings from Purkinje cells in vitro indicate that white matter stimulation produces large synaptic responses by the activation of the climbing fibre afferent, but antidromic potentials do not actively invade the dendritic tree.2. Climbing fibre responses may be reversed in a manner similar to that observed at the somatic level. However, the reversal does not show the biphasicity often seen at somatic level.3. Input resistance of these dendrites was found to range from 15 to 30 MQ. The non-linear properties seen at the somatic level for depolarizing currents are also encountered here. However, there seems to be less anomalous rectification.4. Detailed analysis of repetitive firing of Purkinje cells elicited by outward DC current shows that, as in the case of the antidromic invasion, the fast somatic potentials (s.s.) do not invade the dendrite actively. However, the dendritic spike bursts (d.s.b.s) interposed between the s.s. potentials are most prominent at dendritic level.5. Two types of voltage-dependent Ca responses were observed. At low stimulus level a plateau-like depolarization is accompanied by a prominent conductance change; further depolarization produces large dendritic action potentials. These two classes of response are TTX-resistant but are blocked by Cd, Co, Mn or D600, or by the removal of extracellular Ca.6. Following blockage of the Ca conductance, plateau potentials produced by a non-inactivating Na conductance are observed mainly near the soma indicating that this voltage-dependent conductance is probably associated with the somatic membrane.7. Spontaneous firing in Purkinje cell dendrites is very similar to that observed at the soma. However, the amplitude of these bursts is larger at dendritic level. It is further concluded that these TTX-insensitive spikes are generated at multiple sites along the dendritic tree.8. Six ionic conductances seem to be involved in Purkinje cell electroresponsiveness: (a) an inactivating and (b) a non-inactivating Na conductance at or near the soma, (c) a spike-and (d) a plateau-generating Ca conductance, and (e) voltagedependent and (f) Ca-dependent K currents.9. The possible role of these conductances in Purkinje cell integration is discussed.

Increases in intracellular calcium concentration are required for the release of neurotransmitter from presynaptic terminals in all neurons. However, the mechanism by which calcium exerts its effect is not known. A low-sensitivity calcium-dependent photoprotein (n-aequorin-J) was injected into the presynaptic terminal of the giant squid synapse to selectively detect high calcium concentration microdomains. During transmitter release, light emission occurred at specific points or quantum emission domains that remained in the same place during protracted stimulation. Intracellular calcium concentration microdomains on the order of 200 to 300 micromolar occur against the cytoplasmic surface of the plasmalemma during transmitter secretion, supporting the view that the synaptic vesicular fusion responsible for transmitter release is triggered by the activation of a low-affinity calcium-binding site at the active zone.