Sural nerve effects on medial gastrocnemius motoneurones in the cat

1. Excitability cycles for medial gastrocnemius motoneurones 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. In particular it is calculated that this method specifically examines the 15-25% most excitable motoneurones of the medial gastrocnemius pool.2. In most cases high amplitude stimulation of the sural nerve caused a triphasic (excitatory-inhibitory-excitatory) change in excitability. Arguments are given to support the conclusion that this represents a corresponding post-synaptic alteration in medial gastrocnemius motoneurone potential. It is concluded that this pattern illustrates the nature of the sural nerve projections to this portion of the medial gastrocnemius pool.3. The first 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.4. 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.5. The second period of excitation lasted a mean period of 50 ms and was due to activity in high threshold fibres. On average its amplitude was 50% that of the initial excitation.6. Exceptions to this triphasic variation of excitability were found. This result is interpreted as indicating the presence of projections with opposing actions which were simultaneously activated by sural stimuli.