ISSN:
1432-2013
Keywords:
Motoneuron
;
Excitation
;
Inhibition
;
Spinal Cord
;
Cat
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary 1. In extensor alpha motoneurones, the effects of increasing the number of afferent fibres stimulated electrically, and the doubling of the threshold stimulus frequency for optimal repetitive firing was analysed quantitatively. 2. Three types of motoneurones could be distinguished according to their relative sizes, and to their discharge patterns during orthodromic stimulation of various strength and frequency: small (tonic), medium size (tonic) and large (phasic) motoneurones. 3. The excitability of the motoneurones was inversely related to their cell sizes. The order of inhibitibility depended on the experimental conditions. When the number of afferent fibres was increased by increasing the threshold stimulus strength by 2–4 X, at constant stimulus frequency, the mean firing rates of the small motoneurones were decreased very markedly. This procedure had practically no effect on the repetitive activity of the medium size motoneurones, whereas the large motoneurones could be driven better. 4. When the stimulus frequency was increased at constant stimulus strength, the large motoneurones were inhibited markedly, the medium size motoneurones less and the small motoneurones least,i. e., the inhibitibility of motoneurones was directly related to their cell sizes. 5. It was concluded that the high input resistance values in connection with postsynaptic inhibitory mechanisms could be responsible for more susceptibility of small motoneurones to inhibition, caused by electrically increasing the number of afferent fibres tetanized at constant stimulus frequency. To explain the greater susceptibility of large motoneurones to inhibition, caused by increasing the stimulus frequency from 100–200 Hz, a presynaptic mechanism was suggested. 6. The results were discussed according to several aspects of the size principle.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00592686
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