SURAL NERVE EFFECTS ON MEDIAL GASTROCNEMIUS MOTONEURONS IN THE CAT

  • 1 January 1979
    • journal article
    • research article
    • Vol. 288  (MAR) , 401-410
Abstract
Excitability cycles for medial gastrocnemius motoneurons were recorded following sural nerve stimuli with amplitudes of 1.5, 3, 5, 10 and 40 times the nerve threshold. The monosynaptic reflex was time-integrated to quantify the motoneuronal output and some implications of this technique are discussed. It is calculated that this method specifically examines the 15-25% most excitable motoneurons of the medial gastrocnemius pool. In most cases high amplitude stimulation of the sural nerve caused a triphasic (excitatory-inhibitory-excitatory) change in excitability. Arguments that this represents a corresponding postsynaptic alteration in medial gastrocnemius motoneuron potential is supported. This pattern illustrated the nature of the sural nerve projections to this portion of the medial gastrocnemius pool. The 1st period of excitation began after a latency, corrected to be compatible with intracellular recordings of 2.8 ms and had a mean duration of 4.0 ms. The minimum stimulus level necessary for this effect lay between 1.5 and 3 times the nerve threshold. The maximum amplitude of this facilitation occurred with stimuli of 3-5 times threshold and was 60-90% of the amplitude of the monosynaptic facilitation which followed stimulation of the lateral gastrocnemius-soleus nerve. A period of inhibition followed immediately after the period of excitation and had a mean duration of 16 ms. The minimum stimulus level necessary for this effect lay between 1.5 and 3 times the nerve threshold and the degree of inhibition increased with stimuli up to 40 times threshold. The 2nd period of excitation lasted a mean period of 50 ms and was due to activity in high threshold fibers. On the average its amplitude was 50% that of the initial excitation. Exceptions to this triphasic variation of excitability were found. The presence of projections with opposing actions which were simultaneously activated by sural stimuli is indicated.