Experimental Brain Research

, Volume 42, Issue 3–4, pp 282–298 | Cite as

Integration in descending motor pathways controlling the forelimb in the cat

8. Ascending projection to the lateral reticular nucleus from C3-C4 propriospinal neurones also projecting to forelimb motoneurones
  • B. Alstermark
  • S. Lindström
  • A. Lundberg
  • E. Sybirska
Article

Summary

An analysis has been made of the ascending projection to the lateral reticular nucleus (LRN) from the previously described C3-C4 “propriospinal” neurones (PNs) which are monosynaptically activated from several higher motor centres and project caudally, some of them directly to forelimb motoneurones (Illert et al. 1977, 1978).

Extra- and intracellular recording was made from cells in the C3-C4 segments which could be antidromically activated both from the lateral funicle in C7 and from the ipsilateral LRN. The ascending projection to LRN was found in 84% of the PNs terminating rostral to Th9 but at the most in 11% of the PNs projecting beyond Th9. Threshold mapping in and around the LRN showed that the stem axons of the ascending collaterals enter the nucleus from a position dorsomedial to its caudal part and terminate at different levels, along the entire rostrocaudal extent of the nucleus. Termination was not restricted to the forelimb region (the A-zone, Clendenin et al. 1974a) but was found also in the ventral part of the LRN. The conduction velocity was generally slower in the ascending than in the descending branch (mean values 26 and 44 m/s). The conduction velocity was higher in the PNs projecting beyond Th9 (mean value 101 m/s).

Stimulation in the LRN evoked large monosynaptic EPSPs in forelimb motoneurones as would be expected from the double projection of C3-C4 PNs. These EPSPs are elicited from the regions where the collaterals from C3-C4 PNs ascend and terminate. Their latency and time course are those expected for EPSPs mediated by the bifurcating axons of C3-C4 PNs. It is concluded that they are produced by antidromic activation of ascending neurones also projecting to forelimb motoneurones. The monosynaptic EPSPs from the LRN were found in all motor nuclei tested but were larger in motoneurones to elbow flexors than to elbow extensors (mean values 4.5 and 2.9 mV). Motoneurones classified as fast or slow from the duration of the afterhyperpolarization received EPSPs from the LRN even if pyramidal volleys evoked excitation in the former and inhibition in the latter. Double stimuli in the LRN revealed considerable frequency potentiation of the EPSPs.

Stimulation in the LRN gives marked facilitation of transmission in the reciprocal Ia inhibitory pathway to motoneurones. The effective LRN region, threshold strength and time course is the same as for the monosynaptic EPSPs in motoneurones. The Ia inhibitory interneurones receive a direct projection from C3-C4 PNs (Illert and Tanaka 1978) and it is postulated that these PNs also have an ascending collateral to the LRN which antidromically mediate monosynaptic excitation to the Ia inhibitory interneurones.

It is suggested that the ascending collaterals are a link in an intrinsic feed-back by which the brain controls how the C3-C4 PNs govern forelimb movements. This mode of ascending information — a mirror of the activity reaching forelimb motoneurones and Ia inhibitory interneurones — is discussed in relation to the more complex information in other ascending systems signalling intrinsic spinal activity.

Key words

C3-C4 propriospinal neurones Bifurcating descending, ascending projection Motoneurones LRN 

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Copyright information

© Springer-Verlag 1981

Authors and Affiliations

  • B. Alstermark
    • 2
  • S. Lindström
    • 2
  • A. Lundberg
    • 2
  • E. Sybirska
    • 2
  1. 1.Department of NeurophysiologyNencki Institute of Experimental BiologyWarsawPoland
  2. 2.Department of PhysiologyUniversity of GöteborgGöteborgSweden

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