Effects of mechanical and chemical stimulation of fine muscle afferents upon primate spinothalamic tract cells.

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1. Injections of algesic chemicals were made into the arterial circulation of the triceps surae muscles in anaesthetized monkeys. 2. The responses of a sample of primary muscle afferents suggest that what is known about the activation of muscle afferents in the cat by algesic agents applies also to the monkey. One exception to this is the activation of many group I afferents by KCl in the monkey, but not in the cat. 3. Many spinothalamic tract cells were powerfully excited by the intra-arterial injection of algesic chemicals (bradykinin, 5-hydroxytryptamine (5-HT), KCl) in preparations in which the hind limb was denervated except for the nerves to the triceps surae muscles. The excitatory action of bradykinin had a slower time course than did that of 5-HT or KCl. 4. A number of the spinothalamic tract cells which failed to respond to chemical activation of muscle afferents were located in lamina I of the spinal cord. 5. Repeated injections of bradykinin produced similar responses, whereas the effects of 5-HT injections showed marked tachyphylaxis. 6. No evidence was obtained that activation of muscle spindle afferents by succinylcholine injections resulted in the excitation of spinothalamic tract neurones in the population sampled. 7. Injections of hypertonic NaCl into muscle or tendon produced a prolonged excitation of many spinothalamic tract cells. 8. It is concluded that a substantial proportion of primate spinothalamic tract cells receive a convergent input from cutaneous and muscle receptors. The muscle receptors involved appear to include primary afferents of group III and IV calibre. The possibility is suggested that such cells could play a role in the production of poorly localized pain.