Abstract: In the Crucion Carp,the axon of the Mauthner cell and,at the same time,the spinal afferent pathway were activated by stimulation of the spinal cord with single rectangular pulse and the evoked responses of the Mauthner cell in the medulla oblongata were intracellularly recorded. The results showed that although spinal stimulation always induced antidromic action potential with a notch on its descending limb, there was no inevitable relation between the notch and the action potential itself. Each of them could be independently induced by suitable stimulation. The spike like notch potential and the successive small depolarizing response were inseparable and made up a compound EPSPwhich was named by us the spinal pre-EPSP(simply pre-EPSP).The pre-EPSPwas also induced by stimulation of the spinal afferents as the spinal EPSP,but with smaller amplitude,shorter latency and lower threshold, so that it was often activated togather with Maxon.

Key words: Mauthner cell, the spinal EPSP, the spinal pre-EPSP