An investigation into mechanisms of reflex reinforcement by the Jendrassikmanoeuvre

Tendon jerk and H-reflexes are both potentiated by the Jendrassik manoeuvre , but the mechanism of potentiation remains uncertain. We investigated seve ral possibilities in human subjects. Evidence for fusimotor activation duri ng the Jendrassik manoeuvre was sought by recording the tendon jerk reflex as surface EMG in triceps surae after the muscles had been conditioned to l eave their spindles in a slack, insensitive state. Interposing a Jendrassik manoeuvre between conditioning and the test reflex should have increased r eflex amplitude by restoring spindle sensitivity, but this was not the case . In humans, a close synergist of the triceps surae is the quadriceps. A po ssible presynaptic disinhibitory mechanism was investigated by testing the effect of a Jendrassik manoeuvre on facilitation of the soleus H-reflex pro duced by a quadriceps afferent volley. The Jendrassik manoeuvre failed to i ncrease facilitation, contrary to what would be expected if it reduced the level of tonic presynaptic inhibition; the assumption being that the inhibi tion acts on both homonymous and synergist efferent terminals. The Jendrass ik manoeuvre did not increase the level of ongoing EMG in the soleus during a weak voluntary contraction, indicating that it does not operate by direc t facilitation of motoneurones. There was found to be less potentiation of soleus tendon jerk and H-reflexes by the Jendrassik manoeuvre under conditi ons when spindles in the soleus were likely to have a high resting discharg e rate. A remaining possibility is discussed: that the Jendrassik manoeuvre operates by modulation of oligosynaptic pathways that may contribute to th e largely monosynaptic reflex response. These experiments demonstrate, with new, more sensitive methods than previously used, that neither is the fusi motor system involved in reinforcement nor are direct excitatory or presyna ptic disinhibitory effects on motoneurones. While this con firms the previo usly prevailing view, none of the lingering uncertainties associated with t he methods used now remains.