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Injection of depolarizing and hyperpolarizing currents into the nonimpulsive S and T sensory fibres has allowed a quantitative analysis of the input-output relationships of this sensory-reflex system.
Graded depolarization of the T fibre typically results in sigmoid voltage-frequency relationships for motoneurones Pm1–3, the maximum ‘slopes’ in the most sensitive preparation being 31, 47.5 and 33 Hz/mV respectively, without taking into account further afferent attenuation within the thoracic ganglion.
Graded depolarizations of the S fibre recruit four motoneurones Pmi-4 common to the T fibre reflex pathway, although activation thresholds are much higher and the maximum slope or ‘gain’ of the voltage frequency relationships is much reduced compared to that of the T fibre reflex pathways. Thus, although the S fibre synapses either directly or indirectly with promotor motoneurones, typical 5–10 mV stretch-induced receptor potentials remain sub-threshold for a contribution to the reflex output, albeit under experimental conditions and at all but the most extended receptor muscle lengths.
Differentiating between motor impulse amplitudes, and using simultaneously recorded promotor muscle junction potentials as a further aid, establishes at least nine motoneurones as being reflexly recruited by the S and T fibres: Pm1–8 by the T fibre and Pm1–4 and Pm9 by the S fibre.
Hyperpolarization of the T and S fibres confirms that ongoing tonic motor activity is peripherally determined by receptor length prescribed T fibre ‘resting’ potentials, rather than by the more linearly related S fibre ‘resting’ potentials at different receptor muscle lengths. Moreover, suppression of the reflex response by sensory fibre hyperpolarizations coincident with stretch stimuli leaves little doubt that it is the T rather than the S fibre that provides the sensory drive for the stretch reflex in vitro.
By using long duration, constant value T fibre depolarizing potentials, the postsynaptic component of adaptation for Pm1–3 has been found to be slight, in contrast to the more rapid adaptation of the higher threshold Pm 5–8 motoneurones. Moreover, adaptation rates for Pm 1–3 discharges are lower the smaller the afferent drive. It is suggested therefore that reflexly activated promotor MNs can be differentiated into ‘tonic’ and ‘phasic’ categories.
Motoneurone Pm2 sometimes discharges in the form of ‘rigid’ (7–8 ms intervals) impulse couplets. At low mean reflex frequencies all Pm2 impulses are locked into couplets (‘complete’ patterning). While both ‘rigid’ and ‘complete’ patterning break down abruptly as mean frequencies increase, re-establishment is more gradual (hysteretic) with decreasing mean frequencies. Patterning, albeit a particularly labile phenomenon, is almost certainly an intrinsic, if unknown, property of the motoneurone itself.