Spinal and supraspinal inhibition of somato-sympathetic reflexes by conditioning afferent volleys

Summary

• 1. In chloralose anaesthetized cats sympathetic mass reflex discharges were recorded from preganglionic fibres in the lumbar white rami after somatic afferent stimulation. An inhibitory process of reflex excitability was studied for various reflex discharge components by application of conditioning and test shocks to afferent nerves.

• 2. The early (spinal) reflex evoked by a test shock was inhibited by a prior conditioning shock. Three main inhibitory mechanisms were noted: (1) common post-excitatory depression, (2) afferent inhibitory process at the spinal level, and (3) powerful descending inhibitory influence from the supraspinal level. The descending inhibition was considered to be generalized inhibition at various segmental levels.

• 3. The test early (spinal) reflex was affected neither by the prior conditioning very late (suprapontine) reflex nor by the medullary component of the conditioning Group IV afferent-reflex.

• 4. The late (medullary) reflex discharge was completely inhibited by the prior conditioning late reflex response, but less inhibited by the very late reflex or by the Group IV afferent-reflex.

• 5. The test Group IV afferent-reflex was never completely inhibited by the prior conditioning Group IV afferent-reflex. The reflex responses of conditioning and test shocks could fuse together if the interval of both shocks was short.