Spinal sympathetic reflex and random activity in the isolated amphibian spinal cord

Preganglionic sympathetic neurons in the fourth ramus communicantes discharged in response to electrical stimulation of somatic or visceral afferents entering the spinal cord at segments 2, 3, or 4. Intrasegmental reflexes consisted of a large peak followed by a second, smaller one, whereas only a single peak was observed in intrasegmental reflex. Reflex wave forms were similar to those in the cats except a monosynaptic component was not usually observed. In addition latencies and central delays were different in frogs and cats. Spontaneous activity was rarely recorded although poststimulus random activity was usually observed. Medium to high threshold afferents were responsible for eliciting spinal sympathetic reflexes from spinal nerve two. In frogs the interaction between spinal sympathetic reflexes elicited by the same or different afferents was qualitatively similar to that in cats. It appears therefore that the amphibian spinal cord is a suitable preparation for analyzing sympathetic preganglionic neurons.