Analysis of muscle receptor connections by spike-triggered averaging. 2. Spindle group II afferents

Abstract

1. The spike-triggered averaging (STA) method has been used to study synaptic connections of nine spindle group II afferents from medial gastrocnemius to 151 motoneurons of leg muscles in the cat. 2. EPSPs were found in 40 cells, predominantly of triceps surae with latency from cord entry ranging from 0.3 to 4.2 ms. Those with latency less than or equal to 1.4 ms were deduced to be monosynaptic in confirmation of Kirkwood and Sears (13). Mean amplitude for MG-LGS cells was 30.1 muV and mean rise time 1.0 ms (compared with 65.4 muV and 1.0 ms for monosynaptic Ia EPSPs from the preceding report (26)). It is argued that monosynaptic latency for spindle group II afferents could be as large as 1.65 ms. 3. The occurrence of a presynaptic spike permitted the division of EPSP latencies into central conduction time and synaptic delay components. Sindle group II central conduction times were significantly longer than those of Ia afferents, while there were no differences in the synaptic delays associated with the two afferent types. 4. EPSPs of longer latency were judged to be di- or trisynaptic. They were smaller and had longer rise times than the monosynaptic effects. Evidence is presented to show that short rise time of an individual PSP does not guarantee that it is monosynaptic. Rise times are different only on a population basis. 5. Inhibitory responses were found with latencies and mean rise times appropriate for di- and trisynaptic connections. Their mean amplitude was 4.6 muV. 6. The distribution of EPSPs and IPSPs was generally consistent with their exerting stretch reflex effects similar to that of Ia afferents and inconsistent with the inclusion of them in the grouping known as "flexor reflex afferents."

The measurement of single motor‐axon recurrent inhibitory post‐synaptic potentials in the cat.

“…These extracellular averages had peak-to-peak noise levels generally in the range of 3-6 ,uV (cf. Stauffer et al 1976;Watt et al 1976), thereby providing additional support for the present judgement of positive responses. Similarly, the noise levels of the intracellular averages with no i.p.s.p.s were generally < 7-5 ,uV.…”

Field potentials generated by group II muscle afferents in the middle lumbar segments of the cat spinal cord.

“…The monosynaptic excitation of motoneurones by group afferents also requires a direct projection of these afferents to the ventral parts of the grey matter. The segmental delays of the motoneuronal e.p.s.p.s evoked by secondary muscle spindle afferents (up to 1-2 ms; Kirkwood & Sears, 1975;Stauffer et al 1976;Liischer et al 1979;Munson et al 1980) were in good agreement with the intraspinal conduction times of these afferents. In the present series of experiments we have also demonstrated a direct projection of afferents with the correct activation thresholds to the region of the ventral field potentials.…”

“…However, these field potentials were small, and were superimposed on much larger field potentials evoked by group I afferents, whether they are evoked in the ventral horn or in the dorsal horn. This suggests that group II afferents either have relatively weak actions on neurones in the caudal lumbar segments, as has been demonstrated for motoneurones (Stauffer et al 1976;Liischer et al 1979;Munson et al 1980), or that the neurones they influence are distributed over a wide area and are mixed with other neurones.…”

Synaptic Control of Motoneuronal Excitability

“…Monosynaptic input to spinal motoneurons from sources outside the neuraxis originate exclusively from muscle spindle Ia and group II afferents (147, 502,543,830,1184). The Ia projection likely uses glutamate as a transmitter (939).…”