ELECTRICAL INVESTIGATION OF THE MONOSYNAPTIC PATHWAY THROUGH THE SPINAL CORD

Abstract

The electrical events involved in transmission through a monosynaptic pathway of the spinal cord (quadriceps afferent fibers to quadriceps motoneurones) have been directly recorded by means of a fine needle electrode (insulated to the tip and inserted into the quadriceps nucleus) and an indifferent electrode. The focal potentials so obtained have been subjected to exptl. analysis and to interpretation in the" light of the theory of focal recording. It has thus been possible to identify the 3 main components of the complex electrical potential wave set up in the quadriceps nucleus of the spinal cord by a volley in the quadriceps afferent fibers: the diphasic spike set up by presynaptic impulses propagating to their terminals; the synaptic potential of the motoneurones; the spike response of the motoneurones when they initiate the discharge of an impulse. The focal potential is simplified by making the orthodromic volley small and by deep anaesthesia. Both these procedures eliminate not only the spike responses of the motoneurones, but also the action on them of impulses relayed through interneurones. The focal potential then is produced by a virtually synchronous volley of impulses in the presynaptic terminals and the resulting synaptic potential of the motoneurones, and it has a standard time course. The synaptic potential begins 0.3-0.45 msec. after the arrival of the presynaptic impulses, and it rises to a rounded summit in about 1 msec. thereafter decaying approx. exponentially with a half-time of about 1.5 msec. The synaptic potential recorded from the emerging ventral root has an identical synaptic delay, but a much slower course of rise and decay. It is shown that this divergence should occur if both potentials were produced by a soma depolarization having an intermediate time course. The excitability of the motoneurones before and during the synaptic potential has been tested by direct and by antidromic stimulation. No significant change is detectable during the early part of the synaptic delay, but there is an abrupt rise of excitability slightly preceding the rise of the synaptic potential. This excitability then decays more slowly than the focal synaptic potential, but possibly with much the same time course as the soma depolarization. It is shown that these excitability changes agree with those disclosed by synaptic excitation (the familiar facilitation curve), and that all 3 may be attributable to the synaptic potential. Thus even direct recording from the synaptic region reveals no post-synaptic excitatory process other than the synaptic potential and the soma spike potential initiated thereby. All the observations agree closely with the predictions of the electrical hypothesis of synaptic transmission.