STUDIES ON EFFECTS OF PYRAMID STIMULATION UPON FLEXOR AND EXTENSOR MOTONEURONES AND GAMMA MOTONEURONES

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In order to analyse the function of the pyramidal tract, studies were made on the effects of pyramid stimulation upon flexor and extensor motoneurones and also upon gamma motoneurones innervating the hindlimb of cats.
Special care was taken to eliminate all the possible indirect effects of pyramid stimulation upon the above noted neurones due to current spread to the structure in the medulla, through collaterals of pyramidal fibers to the structure of the brain. stem and also through the gamma-loop at the spinal segmental level. For this purpose, medullar and pontine structures except the pyramidal tract was entirely removed by means of electrocoagulation and suction, and the pyramidal tract was transected at the midpontine level. All ventral roots from L4 downwards were cut bilaterally or unilaterally.
1. Effective points of stimulation upon spinal motoneurones were found to be distributed in the contralateral, vertical plane in the pyramid 1.0 mm lateral from the midline. However, those upon gamma motoneurones were dispersed rather diffusely in the pyramid.
2. With repetitive stimulation of the pyramid monosynaptic and polysynaptic reflex discharges from flexor motoneurones increased in amplitude in all 16 tested cases, whereas those from extensor ones showed appreciable decrease in amplitude in 12 out of 16 tested cases.
3. Following single pulse conditioning stimulation of the pyramid, increase in amplitude of the flexor monosynaptic reflex discharge was observed for 30-40 msec in all 10 tested cases. Whereas, monophasic decrease in amplitude for about 30 msec and inhibitory effect for about 30 msec with initial slight facilitatory one were recognized respectively in 14 and 3 of 17 tested cases of extensor motoneurones.
4. As to the effect of pyramid stimulation upon individual motoneurones, facilitatory effect was obtained in the case of flexor motoneurones, whereas inhibitory one was observed frequently in the case of extensor ones judging from the firing index method.
On the other hand, EPSPs were evoked in 12 flexor and 6 extensor motoneurones, IPSPs were evoked in 8 extensor ones and mixed form of EPSP and IPSP were observed in 2 extensor motoneurones.
5. These results provide basis for the suggestion that the pyramidal tract as a whole has a stronger facilitatory control over flexor motoneurones than over extensor ones and that these facilitatory pyramidal fibers to flexor motoneurones may exert a certain inhibitory effect upon extensor motoneurones.
6. Facilitatory effect upon gamma neurones: increase in frequency of spontaneous discharges or initiation of discharges, as well as inhibitory effect: decrease in frequency of spontaneous discharges, were observed respectively in 4 and 10 out of 30 tested cases with pyramid stimulation.