1. The origin of primary afferent depolarization (PAD) of the central terminals of group II afferent fibres of tibialis anterior and extensor digitorum longus muscles has been investigated in the cat. Changes in the excitability of the terminals to intraspinal stimuli, upon application of conditioning stimuli to muscle nerves (quadriceps, sartorius, gracilis, posterior biceps-semitendinosus, anterior biceps-semimembranosus, gastrocnemius-soleus, deep peroneal), cutaneous nerves (sural, superficial peroneal) and the posterior nerve to the knee joint, were used as a measure of PAD. 2. PAD was most readily evoked by conditioning stimuli which were maximal for group II muscle afferents. However, some PAD was also evoked from group I afferents and evidence is presented that group Ia afferents contributed. Afferents of posterior biceps-semitendinosus and sartorius muscles appeared to be most effective. PAD was also evoked by stimulation of cutaneous and joint nerves, often in the same fibres which were affected by group Ia afferents. 3. It is concluded that there are several common sources of PAD of group II and group Ia afferent terminals on the one hand, and group Ib afferent terminals on the other. 4. The properties of PAD of group II afferents are discussed in relation to the problem of how PAD affects transmission from fibres with long terminal branches of small diameter.
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