Assessing changes in presynaptic inhibition of I a fibres: a study in man and the cat.

Abstract

1. A method to assess changes in presynaptic inhibition of Ia afferent terminals in man is proposed. The soleus H reflex was facilitated by a heteronymous Ia volley from quadriceps and the amount of reflex facilitation was used to estimate the size of the conditioning Ia excitatory post-synaptic potential (e.p.s.p.). It is argued that the size of this e.p.s.p. as measured by the resulting amount of reflex facilitation reflects the amount of presynaptic inhibition on the corresponding Ia fibres. A decrease in the reflex facilitation may then be ascribed to an increase in presynaptic inhibition of the Ia fibres mediating the conditioning volley. 2. That the heteronymous Ia facilitation from quadriceps to soleus is caused by a purely monosynaptic e.p.s.p. is a prerequisite for the validity of the method. Experimental evidence is therefore given in an Appendix that in man the earliest part (first 0.5 ms) of this heteronymous Ia facilitation is mediated through a monosynaptic pathway. Evidence is also given that this earliest facilitation is not yet contaminated by any polysynaptic effects from Ia or Ib afferents. 3. The validity of the method was established in animal experiments in which presynaptic inhibition of Ia fibres and post-synaptic events in motoneurones could be assessed by direct tests. It was found that the amount of test reflex facilitation produced by a conditioning Ia volley was decreased when Ia fibres were subjected to presynaptic inhibition but remained unchanged when the motoneurone pool in which the test reflex was elicited received pure post-synaptic inhibition. 4. In man, presynaptic inhibition of Ia fibres was evoked by a short-lasting (three shocks at 200 Hz) vibration applied to the tibialis anterior tendon. Such a vibratory burst reduced the efficiency of the heteronymous Ia volley in facilitating the soleus H reflex. By contrast, during a pure post-synaptic inhibition of soleus motoneurones the efficiency of the conditioning volley in facilitating the test reflex remained unchanged. It is therefore argued that the amount of heteronymous Ia facilitation can indeed be used to assess the amount of ongoing presynaptic inhibition exerted onto heteronymous Ia fibres from the quadriceps muscle to soleus motoneurones. 5. The short-lasting tibialis anterior vibration used here evoked a long-lasting (300-500 ms) depression of soleus and quadriceps H reflexes. Even though several mechanisms contribute to this depression, it is argued that presynaptic inhibition of Ia fibres mediating the afferent volley of the reflex is the only mechanism responsible for the reflex depression when the test reflex is evoked 40-60 ms after the onset of vibration. Within this time interval, therefore, the measurement of the vibratory inhibition of the H reflex assesses the excitability of the interneurones mediating presynaptic inhibition of homonymous Ia extensor fibres.