Summary
Recording was made in the C3-C4 segments from cell bodies of propriospinal neurones identified by their antidromic activation from more caudal segments. Monosynaptic excitatory effects from descending motor pathways and primary afferents were investigated by electrical stimulation of higher motor centres and peripheral nerves in the forelimb and neck.
The cell bodies were located mainly laterally in Rexed's layer VII. Threshold mapping for single axons showed that they descend in the lateroventral part of the lateral funicle. Antidromic stimulation at different spinal cord levels showed that some neurones terminated in the forelimb segments, others in the thoracic cord or in the lumbar segments. Terminal slowing of the conduction velocity suggested axonal branching over some segments.
Monosynaptic EPSPs were evoked in the neurones by stimulation of the contralateral pyramid, red nucleus and dorsal tegmentum-superior colliculus. It is concluded that corticospinal, rubrospinal and tectospinal fibres project directly to both short and long propriospinal neurones. There was marked frequency potentiation in tectospinal synapses. Convergence from two descending tracts was common and in half of the tested cells all three tracts contributed monosynaptic excitation. Experiments with collision of descending volleys and antidromic volleys from the brachial segments demonstrated that the corticospinal and rubrospinal monosynaptic projection to the propriospinal neurones is by collaterals from fibres continuing to the forelimb segments.
Stimulation of cervical primary afferents in the dorsal column gave monosynaptic EPSPs in somewhat less than half of the tested propriospinal neurones. The further analysis with stimulation of forelimb nerves and C2-C3 dorsal rami showed that monosynaptic EPSPs may be evoked from low threshold cutaneous and group I muscle afferents in the forelimb and from C2-C3 neck afferents entering close to the spinal ganglia, possibly from joint receptors. Convergence from cervical afferents and at least two of the above descending tracts was common.
It is postulated that the propriospinal neurones previously indirectly defined by their action on motoneurones as relaying disynaptic excitation from higher motor centres to forelimb motoneurones (Illert et al., 1977) belong to those neurones of the C3-C4 propriospinal systems which terminate in the cervical enlargement. The function of the neurones projecting beyond the upper thoracic segments is discussed.
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Illert, M., Lundberg, A., Padel, Y. et al. Integration in descending motor pathways controlling the forelimb in the cat. Exp Brain Res 33, 101–130 (1978). https://doi.org/10.1007/BF00238798
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DOI: https://doi.org/10.1007/BF00238798