The identification of serotonergic neurons in the nucleus raphe magnus by conduction velocity

Histochemical studies have described serotonergic neurons in the nucleus raphe magnus, and since their axons are small, they would be expected to have low conduction velocities. However, previous studies have reported few slow-conducting units in the nucleus. In these studies, raphe-spinal neurons, detected by antidromic activation, were found to exhibit a wide range of conduction velocities, including numbers of slow-conducting units. In a second set of experiments, the number of raphe-spinal units found in control rats and those treated with 5,7-dihydroxytryptamine, a serotonin neurotoxin, were compared. Two groups of slow-conducting units were reduced in treated animals and those units, conducting between 0.7-1.0 m/sec and 3.1-6.0 m/sec, were presumed to be serotonergic. These neurons comprised about 40% of the total found in the nucleus.