1. A study has been made of the increase in fluorescence of the desheathed cervical vagus nerve that occurs after electrical stimulation (usually 5 sec at 30/sec) of its non‐myelinated fibres. 2. At room temperature this increase in fluorescence is normally masked by the decrease in fluorescence caused by mitochondrial oxidative phosphorylation. However, at higher temperatures (30–35° C) the increasing fluorescence phase predominates and the net change on stimulation is an increase. 3. At room temperature the increase in fluorescence is seen clearly only when ATP splitting has been prevented by ouabain, by bathing the nerve in lithium‐Locke solution, or when oxidative phosphorylation has been prevented by metabolic inhibitors. 4. The increasing fluorescence response is absent when calcium is removed from the external medium; it increases with increasing calcium concentration. 5. It is argued that the increasing fluorescence response is due to an increase in glycogenolysis (leading to an increase in the reduced pyridine nucleotide concentration) brought about by the increased calcium entry during the action potential. This calcium presumably increases the activity of phosphorylates a or phosphofructokinase. 6. Calcium entry also speeds mitochondrial oxidative phosphorylation.
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