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SYNOPSISThe contribution of the fusimotor system to reflex reinforcement such as the Jendrassik manoeuvre was investigated by recording single unit activity with tungsten electrodes from muscle spindle afferent nerves in unanaesthetized normal human subjects. Muscle spindle afferent activity was recorded before, during, and after the reinforcement test. When the leg muscles remained relaxed during the Jendrassik manoeuvre, spindle activity recorded in the tibial nerve was accelerated. Also in the median nerve, activity from muscle spindle afferent fibres was increased during a remote contraction of the ipsilateral quadriceps muscle. Comparing the time course of the phasic reflex reinforcement and the muscle spindle facilitation during the remote contraction, a marked after-effect was recorded in both responses. Present results show an increased spontaneous muscle spindle activity in relaxed muscles during a remote muscle contraction, and provide evidence for the contribution of the fusimotor system to the enhancement of phasic reflexes by reinforcement manoeuvres.

SUMMARY1. Intracellular records have been taken from cat motoneurones belonging to gastrocnemius and soleus or to popliteal synergists during contractions of gastrocnemius and soleus, acting separately or jointly. Such contractions were elicited by brief tetani or single shocks to the peripheral end of the cut ventral roots L 7 or S 1.2. Hyperpolarization of the motoneurone accompanies rise of tension in contraction. The amount of it increases when at constant extension the contraction of the muscle is increased by increasing stimulus strength, as well as when it is increased by augmenting extension at constant stimulus strength. It is therefore tension-sensitive.3. The duration of the hyperpolarization induced in this manner reflects the duration of the contraction itself, being considerably longer in the slow soleus than in the faster gastrocnemius. It is often preceded by a brief wavelet of depolarization ascribed to the so-called back-response.4. Early in relaxation there occurs a transient 'hump' of membrane depolarization. This corresponds to the moment characterized by phasic bursts from the spindle primaries. The 'hump' terminates hyperpolarization.

The effects of picrotoxin on post‐synaptic inhibitions were intracellularly investigated in cat popliteal, common peroneal and hamstring motoneurones which were activated by stretch of the triceps surae, tibialis anterior or semitendinosus muscles. In some experiments a single electrical shock to the nerve was also used as a stimulus. Inhibitions from muscle stretch were studied by the following criteria: i) hyperpolarization of the post‐synaptic membrane, ii) decrease in size of the monosynaptic excitatory post‐synaptic potential, iii) reduction of firing rate of a motoneurone stimulated by injected depolarizing current, iv) ‘synaptic activation noise’ in a hyperpolarizing direction, and v) sensitivity of the post‐synaptic inhibition to an increased intracellular concentration of chloride ions. Post‐synaptic inhibitions remaining after picrotoxin (picrotoxin‐resistant) and those abolished by picrotoxin (picrotoxin‐sensitive) were identified. Both types of inhibition were shown definitely to act on the post‐synaptic membrane. As with strychnine‐resistant post‐synaptic inhibition (Kellerth and Szumski 1966), picrotoxin‐sensitive post‐synaptic inhibition was found to have characteristics similar to those described for presynaptic inhibition. In no case was a post‐synaptic inhibition found to be resistant to both strychnine and picrotoxin. and in strychninized cats, the remaining post‐synaptic inhibitions were always abolished by picrotoxin.