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Cortico-spinal connections in the rat. I. Monosynaptic and polysynaptic responses of cervical motoneurons to epicortical stimulation

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Summary

In rats the fast cortico-spinal pathway and its linkage to motoneurons of the distal portions of the forelimbs were examined by electrophysiological techniques:

  1. 1.

    Unilateral epicortical stimulation with a single pulse led to “flick”-like movements of corresponding muscle groups in the ipsi- and contralateral limbs. The effective stimulation points of the forelimbs were localized in fronto-central areas, those of the hindlimbs in centroparietal regions of the cortex.

  2. 2.

    After a single epicortical stimulus a cord dorsum potential appeared in all cervical, thoracal and lumbar segments, the early part of which could be shown to reflect the activity of fast cortico-spinal fibres conducting with a velocity of 60 m/s. The sites of origin of the cortical-spinal fibres mapped by antidromic stimulation exceeded the cortical regions determined as motor areas by orthodromic stimulation in parietal and occipital direction.

  3. 3.

    Single stimuli applied to the ipsi- and contralateral cortical surface caused a sequence of EPSP in all studied cervical motoneurons of the distal forearm. In nearly half of the impaled neurons the first EPSP was monosynaptic. Postsynaptic reactions following ipsi- and contralateral epicortical stimulation did not show significant differences both in flexor and extensor motoneurons. A minimum increase of the stimulus intensity beyond threshold level of the first EPSP led to action potentials in the early part of the synaptic response.

The results demonstrate that in non-primates, showing skilled movements of the hand and digits, a powerful direct cortico-motoneuronal connection exists which is of predominant functional significance.

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Elger, C.E., Speckmann, E.J., Caspers, H. et al. Cortico-spinal connections in the rat. I. Monosynaptic and polysynaptic responses of cervical motoneurons to epicortical stimulation. Exp Brain Res 28, 385–404 (1977). https://doi.org/10.1007/BF00235718

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