Two Pathways from Brain Stem to Gamma Ventral Horn Cells.

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Abstract
Summary.: Spikes in single muscle spindle afferents from the dorsal roots (cat, rabbit) or individual gamma efferents from the ventral roots can be driven by shocks to the mesencephalic tegmentum or to afferent nerves (crossed as well as ipsilateral). The latent periods from the shock to the spike have been measured.The experiments revealed the presence of a fast path from the mesencephalic tegmentum to the gamma ventral horn fibres with a latent period of only 8—9 msec (about 30 cm conduction distance). This path is crossed and, at the level of L3 to L6, can be destroyed by deep incisions into the lateral column of the spinal cord (see Fig. 8).Another pathway is activated by repetitive stimulation of the mesencephalic tegmentum. Spikes cannot be driven over this route which elicits a recruiting protracted response. This path is widely distributed over the spinal cord and stands extensive crisscross sectioning of the latter. Often the whole cord has to be divided for complete removal of the recruiting type of response.It is suggested that the fast path serves spindle and motor cooperation in fast action and that the other path acts as a general ‘volume control’ of slow tonic effects.In addition measurements are reported on gamma reflex times and the time required for the gamma effect to traverse the loop formed by gamma root efferent — muscle spindle — spindle root afferent.