The Journal of physiology
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The Journal of physiology · May 1979
Effects of nerve impulses on threshold of frog sciatic nerve fibres.
1. The firing thresholds of single myelinated fibres of frog sciatic nerves were monitored as a function of impulse activity in the fibre. The threshold was given by the number of coulombs in current pulses that excited a particular fibre half the time when delivered to the whole nerve. ⋯ The duration of the superexcitable phase interpreted as a relative change in excitability was roughly the same regardless of the level of depression. 8. The magnitude of the oscillation in threshold was give to ten times larger than the grey region (the range of stimuli for which response is probabilistic). It is concluded that at regions of low conduction safety such as axonal branches, where weak forces can influence whether an impulse will pass, such pronounced and long-lasting after-effects of firing can be expected to modulate conduction of nerve impulses.
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The Journal of physiology · May 1979
The initiation and maintenance of bradycardia in a diving mammal, the muskrat, Ondatra zibethica.
1. The cardiac and respiratory responses shown by muskrats in both unrestrained and restrained dives have been compared with responses elicited by stimulation of a number of cardio-depressant receptor inputs, in an attempt to determine which are most important in initiating and maintaining diving bradycardia. 2. In unrestrained voluntary dives heart rate fell from 310 +/- 3 to 54 +/- 3 beats min-1 in 1 to 2 sec, which was significantly below that seen in dives by restrained unanaesthetized or anaesthetized animals. 3. ⋯ Artificial ventilation of paralysed muskrats with 5% CO2 in N2 caused bradycardia when Pa, O2 reached 8.4 +/- 0.8 kPa and heart rate declined to 76 +/- 7 beats min-1 at 4 kPa. Bilateral section of the sinus nerve delayed bradycardia until Pa, O2 reached 4.5 +/- 0.5 kPa. 8. These results suggest that the cardiac response to submergence could be the expression of input from three groups of receptors, nasal, lung and carotid chemoreceptors, although it is not clear how they interact with one another to generate the cardiac responses displayed by unrestrained animals during submergence.