The paper defines the discovery made by I. M. Sechenov in 1863, of inhibition of spinal reflexes in a frog produced by stimulation of the thalami optici with a crystal of NaCl -- first finding revealing inhibition in the central nervous system. The question is discussed, what parts of the nervous system participate in this inhibition and, in particular, along what pathways the excitation is transmitted from thalami optici to the spinal cord. According to I. M. Sechenov this pathway is the sympathetic nervous system, which is confirmed by numerous studies. P. G. Kostyuk (1969) describes this inhibition as presynaptic. However there are reasons to assume the participation of the sympathetic nervous system in the presynaptic inhibition as well.