1. A study has been made of the changes in the fluorescence of desheathed rabbit cervical vagus nerves that occur during and after electrical stimulation of its non‐myelinated fibres. 2. Stimulation for 5 sec at 30 shocks/sec produces a maximal decrease, of about 1% of the resting fluorescence. Stimulation for less than 0·5 sec fails to produce responses visible above the inherent noise in the recording system. 3. A pharmacological dissection (with ouabain, metabolic inhibitors, and calcium) has revealed four phases of fluorescence change: (a) under conditions where the sodium pump is functioning, there is a prolonged decrease in the fluorescence following electrical activity; (b) even in the absence of pumping the mere entry of sodium into the nerve causes an initial decrease in fluorescence; (c) the entry of calcium ions with electrical activity also causes an initial rapid decrease in fluorescence; (d) following these phases of decreased fluorescence there is a phase of increased fluorescence. 4. These changes in fluorescence are related to changes in the NADH concentration in the nerve resulting from: (a) the splitting of ATP during sodium extrusion; (b) the initial binding of sodium to the sodium‐ and potassium‐dependent ATPase, which is the sodium pump; (c) the stimulation of mitochondrial respiration by calcium that has entered during the spike; and (d) an increased glycogenolysis as a result of the calcium entry during activity.
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