Changes in the excitability of Renshaw cells due to orthodromic tetanic stimuli

Distribution of recurrent inhibition within a motor nucleus. II. Amount of recurrent inhibition in motoneurones to fast and slow units

The maximal recurrent inhibition was studied by intracellular recording from 43 triceps surae motoneurones (tentatively type-identified by the biophysical properties of the neurones) and from 67 medial gastrocnemius motoneurones (type-identified by the muscle unit properties; fast fatiguing, FF; fatigue resistant, FR and slow, S). Maximal homonymous recurrent IPSP (RIPSP) and input resistance (RN) were measured at 'resting' membrane potential and close to firing threshold. The 'synaptic current' at the peak of the RIPSP was estimated (RIPSP/RN). At 'resting' membrane potential the RIPSPs increased in the order FF less than FR less than S (0.9, 1.4, and 2.5 mV, respectively). This order was preserved when the 'current' (RIPSP/RN) rather than voltage was considered, although the overall range was much reduced. When investigated close to firing threshold there were no significant differences in synaptic 'current'. This apparent paradox may be explained by systematic differences in firing threshold between motor unit types; it increased in the order S less than FR less than FF (4.6, 8.9 and 13.5 mV, respectively).

Dynamic properties of Renshaw cells: equivalence of responses to step changes in recruitment and discharge frequency of motor axons

In decerebrate cats, the dynamic responses of Renshaw cells to step changes in input were determined separately both for changes in the number of alpha-axons excited and for changes in the frequency at which they were stimulated. Together, these two input variables to the Renshaw cells describe the level of activity in the motor output from the spinal cord. In either case, the dynamic responses of the interneurons depend only on their static activity before and after an input step occurs, but are otherwise indistinguishable. This favors the interpretation that the two input variables are equivalent under dynamic conditions, i.e., Renshaw cells respond to total motor output.

Static input-output relations in the spinal recurrent inhibitory pathway

The static discharge rate of Renshaw cells (studied in deafferented, intercollicularly decerebrate cats) has a nonlinear dependence on the frequency of trains of stimulus impulses to alpha-motor axons in the ventral root. This dependence is well described by a rectangular hyperbola that approaches saturation with increasing stimulus frequency. The tendency to saturate is independent of the number of motor axons exciting a Renshaw cell. On average, the stimulus frequency at which the discharge rate reaches half its saturation value lies between 10 and 15 Hz. The effect of Renshaw cell activity -- measured as the antidromic inhibition of individual alpha-motoneurons -- reflects the forms of the static frequency characteristics. An electric circuit analog of the Renshaw cell membrane is presented which serves to explain the qualitative features of the static input-output relations; the nonlinearity is the result of synapses with linear properties acting together at the cell membrane.

The relative dependence of the activity of Renshaw cells on recurrent pathways during contraction of the triceps muscle

Renshaw cell activity was recorded simultaneously with motoneuronal unit discharge during vibration and tetanic stimulation of triceps muscles in decerebrated cats. The experiments confirm that, in this preparation, the motoneurones are the main source of Renshaw cell firing during muscle stretch and vibration and when motoneuronal discharge was induced through the gamma loop. However they also show that a discharge of Renshaw cells, monosynaptically coupled with triceps motoneurones through their recurrent collaterals, could be elicited during contraction of the muscle at the time when the discharge of these motoneurones had been silenced. The recording of the stretch receptors and motoneuronal unit discharge during stretch, vibration, and ventral root stimulation gave evidence of the contribution of the withdrawal of excitation by primary endings to the occurrence of the silent period during tetanic contraction of the muscle. The measurements of the critical firing level in motoneuronal units responding reflexly to held stretch and vibration of the muscles, and silencing their discharge during muscle shortening, showed that these cells are amongst the lowest ranking in the pool For these reasons, these data suggest that Renshaw cell firing during vibration and tetanic contraction of the muscle cannot be attributed only to the alpha motoneurone excitation by the Ia fibres.

Neurochemical changes in the cat's spinal cord due to orthodromic tetanic stimuli. I: phospholipids

Posttetanic potentiation of monosynaptic reflexes has been used as a paradigm for neuronal plasticity. The explanation for this phenomenon is an increased responsiveness of the synaptic junctions. This would basically require chemical changes of the nervous structures involved. The ventral horn area of the spinal cord was therefore analyzed neurochemically. The determination of the phospholipids revealed an alteration of their composition. Sphingomyelin, phosphatidylcholine and phosphatidylinositide/serine behaved differently, whereas phosphatidylethanolamine and the total phospholipid content remained unchanged.