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1. 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 L7 or S1.

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.

5. When the cell is stimulated by injected current to maintained repetitive firing, the `silent period' in contraction begins with the phase of hyperpolarization and ends with the hump of depolarization as described above.

6. Later during relaxation, delayed inhibition, may or may not follow often accompanied by hyperpolarizing activation noise and sometimes also visible as an extension of the silent period of a firing cell. There is, however, no marked hyperpolarization of the motoneurone in delayed inhibition.

7. In the Discussion the events described above are related to previous studies employing monosynaptic testing or electromyography for the analysis of the variations of excitability caused in extensor motoneurones by autogenetic contractions as well as to known properties of stretch receptors.