Pyramidal excitation in long propriospinal neurones in the cervical segments of the cat

Summary

1. The effect of stimulating the contralateral pyramid has been investigated with intracellular recording from 128 long propriospinal neurones (long PNs) in the C3-Th1 segments of the cat. Long PNs were identified by the antidromic activation from the Th13 segment. They were located in laminae VII–VIII of Rexed. Single pyramidal stimulation evoked monosynaptic EPSPs in 15/40 of the long PNs in cats with intact pyramid. In 15 other long PNs, a train of three to four pyramidal stimuli evoked EPSPs with latencies indicating a minimal disynaptic linkage. The remaining 25% of the long PNs lacked mono- or disynaptic pyramidal EPSPs. In a few cases longer latency excitation was observed. 2. The location of the intercalated neurones which mediate the disynaptic pyramidal EPSPs was investigated by making four different lesions of the corticofugal fibres: 1) at the border of the C5 and C6 segments, 2) at the border of the C2 and C3 segments, 3) at the caudal part of the pyramid; three mm rostral to the decussation and 4) at the level of the trapezoid body. Stimulation of the corticofugal fibres was made either rostral to lesion 3 (rPyr) in order to activate neurones in a cortico-bulbospinal pathway or caudal to lesion 3 (cPyr) to activate neurones in a corticospinal pathway. In the former case, in one experiment, stimulation was made in the pyramid between lesions 3 and 4 (double pyramidal lesion). In case of cPyr stimulation, lesions 1 and 2 were added sequentially in order to investigate if the corticospinal excitation was mediated via C3–C4 PNs. All lesions were made mechanically, except lesion 2 which in some of the experiments was performed by reversible cooling. 3. Stimulation in the pyramid rostral to lesion 3 and in between lesions 3 and 4 evoked disynaptic EPSPs in the long PNs, which shows that they were mediated via reticulospinal neurones. Stimulation in cPyr after lesion 3 elicited disynaptic EPSPs, which remained after lesion 1 but were abolished after adding lesion 2. It is concluded that the disynaptic cPyr EPSPs were mediated via intercalated neurones in the C3–C4 segments. 4. When the disynaptic cPyr EPSP was conditioned with a single volley in nucleus ruber and/or in tectum, it was markedly facilitated, especially when the conditioned volley was applied simultaneously with the effective cPyr volley. The results show that the intercalated neurones in the C3–C4 segments receive monosynaptic convergence from cortico-, rubro- and tectospinal] fibres. Stimulation in the lateral reticular nucleus (LRN) evoked monosynaptic EPSPs. These EPSPs had similar latencies and shapes as those previously recorded in forelimb motoneurones and which have been shown to be due to activation of ascending branches of the C3–C4 PNs. This finding in addition to the striking similarity of the descending input pattern of long PNs as compared to the forelimb motoneurones strongly suggest that short C3–C4 PNs project both to long PNs as well as to forelimb motoneurones. 5. Spatial facilitation of disynaptic EPSPs in long PNs was also observed between rPyr volleys and tectal volleys. The results suggest that common reticulospinal neurones which project to the long PNs receive monosynaptic convergence from corticofugal and tectofugal fibres but in some of the reticulospinal neurones the main input is cortical and in others tectal. Monosynaptic EPSPs were evoked from the medial part of the reticular formation, from 2 mm caudal to 6 mm rostral of the obex level. These EPSPs were presumably due to direct activation of reticulospinal neurones. 6. Convergence of disynaptic excitation mediated by cortico-propriospinal and cortico-reticulospinal routes was observed in about 12% of the long PNs. Convergence of monosynaptic corticospinal and disynaptic corticoreticulospinal and/or cortico-propriospinal input was observed in about 15% of the long PNs. 7. The role of the monosynaptic pyramidal input and disynaptic corticoreticulospinal and cortico-propriospinal (mediated by short C3–C4 PNs) inputs to long PNs is discussed in relation to postural control during movements of head and forelimb.