Effect of Intra-Arterial Injections of Adrenaline on Spinal Extensor and Flexor Reflexes

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Abstract
Doses of 5–40 µg/kg of adrenaline, injected into the left renal artery, increase monosynaptic extensor reflexes evoked by single shocks in acute and chronic spinal cats and sustained extensor reflexes in the latter. This effect reaches a peak 1/2–1 minute after the start of injection and lasts several minutes. Low doses evoke only an increase but higher doses may cause more complicated effects: a) a brief early depression sometimes appears; b) following reflex enhancement a late, sometimes irreversible, depression may occur, evidently due to massive action of large doses of adrenaline. Monosynaptic and polysynaptic flexor reflexes are diversely affected by adrenaline. No evidence for a consistent reciprocal action of adrenaline on extensor and flexor reflexes has been obtained. It is suggested that the drug action on cells of the central nervous system may be fairly general, involving modification of some factor determining cellular excitability.