11Feb19+Na+Ca+Na-Ca+spikes

11Feb19+Na+Ca+Na-Ca+spikes - 11Feb19 Na, Na ­Ca,...

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Unformatted text preview: 11Feb19 Na, Na ­Ca, and Ca spikes QUESTIONS: Does hyperpolarization of the pacemaker channels open Na channels in both the SA and AV nodes in the same manner (even though not simultaneously)? Or are there no Na channels at the AV node? ... Ach acts to decrease conduction velocity at the AV node by decreasing the calcium current and activating the K channels; whereas, I would have intuitively thought that to decrease conduction velocity Ach should decrease influx of Na (hence decrease the sodium current) since that causes a much faster spike than Ca. That is, I thought decreasing Na current would delay successive spikes more efficiently than Ca? ANSWERS: 1) The influx of Na into nodal cells is through the hyperpolarization ­activated ‘pacemaker channels’. These are normally not referred to as Na channels – to avoid confusing them with the depolarization ­ activated Na channels in spiking neurons and also in cardiac muscle (of the atria and ventricles). In this sense, yes, there are no “Na channels” in the nodes, but there are pacemaker channels that allow Na to enter the nodal cells. 2) Yes, Ach slows conduction by decreasing Ca current. Remember that these are the channels are activated when the nodal cells reach threshold. This is why the spikes in nodal cells are “calcium spikes” not “Na spikes” of spiking neurons and not “Na ­Ca” mixed spikes of atrial and ventricular muscle. Looking back, remember the following: 1) spiking neurons: the spikes in the axons we discussed (not including the axon terminal): triggering event: EPSP, disinhibition, or sensory stimulus at threshold: voltage ­gated Na channels open regeneratively repolarization: due to opening of voltage ­gated K channels (& closure of Na channels) so the spike is a Na spike 2) nodal cells: triggering event: pacemaker potential (the slow depolarization from –70 to –50 mV) due to Na influx thru pacemaker channels at threshold: voltage ­gated Ca channels open regeneratively (& pacemaker channels close) repolarization: due to opening of voltage ­gated K channels (& closure of the Ca channels) so the spike is a Ca spike 3) atrial & ventricular muscle cells: triggering event: spikes or depolarization reaching them via gap junctions or specialized conduction fibers at threshold: voltage ­gated Na channels open regeneratively, then voltage ­gated Ca channels open regeneratively repolarization: due to opening of voltage ­gated K channels (& closure of the Na & Ca channels) so the spike is a Na ­Ca spike This all allows Ach to affects Ca channels, not Na channels, in nodal cells — as written on the slide about conduction velocity at the AV node. The effect of Ach on Na influx is by effects on the pacemaker channels – as written on the slide about parasympathetic slowing of the heart rate. ...
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