PTN Webbed Reprint Collection
William H. Calvin
University of Washington
Box 351800
Seattle WA 98195-1800 USA
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Journal of Neurophysiology 35:311-325 (1972).

copyright ©1972 by authors and American Physiological Society.

Membrane-potential trajectories between spikes underlying motoneuron firing rates.

PETER C. SCHWINDT and WILLIAM H. CALVIN
Departments of Neurological Surgery and of Physiology and Biophysics,
University of Washington School of Medicine,
Seattle, Washington 98195


SUMMARY

1. Repetitive firing was studied in cat spinal motoneurons by injecting constant currents through the recording microelectrode. The shape of the membrane potential between spikes (the trajectory) was studied to examine how its changes corresponded to the linear relation between injected current and firing frequency (the f-I curve).

2. In all motoneurons firing in the primary range of the f-I curve, the trajectory can be subdivided descriptively into an early descending and bottoming portion (the scoop) and a subsequent rise toward firing level for the next spike (the ramp).

3. In all cells, injected current increased the firing rate over the greater part of the primary range by making the scoop shallower and shorter; ramps from different firing rates were superimposable.

4. In most cells, injected currents increased the firing rate near the threshold of rhythmic firing by making the ramp slope steeper.

5. In the steeper secondary range of the f-I curve, a characteristic trajectory was seen where a deep notch developed after a spike followed by an upward-convex scoop and a ramp. Some motoneurons which lacked a secondary range also developed this same special trajectory at higher currents.

6. In cells that had a secondary range, injected currents increased the firing rates by making the upward-convex scoop steeper and shorter; ramps from different firing rates remained superimposable with primary-range ramps.

7. When interspike intervals lengthen during the adaptation which follows a stepwise increment of current, the ramp portion of the trajectories do not change. The interspike interval is lengthened by changes in the duration of the scoop, even near rhythmic-firing threshold where ramp slope changed under different driving currents.

8. Adaptation in the secondary range was often characterized by a decline in firing rate after the first spike, followed by a rise to the steady-state rate. The notch in the early trajectory develops as the firing rate recovers to the steady-state value.

9. The results are discussed in terms of a separate "early trajectory process" which controls firing rates, and a stereotyped "late trajectory process." It is concluded that the processes underlying cat spinal motoneuron rhythmic firing are significantly different from those in commonly studied invertebrate preparations.

10. Discussion of the early trajectory process in terms of spatially distributed membrane conductance changes emphasize the need to account for the spatial inhomogeneity of the cat spinal motoneuron in studying repetitive-firing behavior.


This is the second of a pair of papers. The first is:

Calvin, W. H., and Schwindt, P. C. (1972). Steps in production of motoneuron spikes during rhythmic firing. Journal of Neurophysiology 35:297-310.


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