Abstract
Intra-arterial injections of capsaicin and benzonatate (Tessalon) have been used to demonstrate the differential sensitivity of primary and secondary spindle endings to depressant drugs. Capsaicin causes initial stimulation in both types of endings followed by a stage of depression. The primary endings are depressed first and recover later than the secondary endings. At least the depressant phase of capsaicin response appears to be due to direct action on the sensory endings. Tessalon causes immediate depression of the receptors with the primary endings again being more sensitive. These effects are contrary to the local anaesthetic action of Tessalon on nerve fibres. Desensitization to subsequent doses is prominent with capsaicin but not with Tessalon. Greater sensitivity of primary endings to excitatory drugs has already been well established. The present results are discussed along with the findings of earlier workers to emphasise the greater safety factor of the secondary endings to external disturbances than the primary endings.
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