Widespread mechanosensory activation of the serotonergic system of the medicinal leech

The serotonergic system of the medicinal leech comprises a small number of iterated, identified neurons, of which the Retzius (Rz) neurons are major components. Activity in pressure mechanosensory (P) cells sufficient to elicit locomotory and defensive behaviors also excites Rz neurons. We characterized the interactions between P and Rz neurons within the ganglion and at different distances along the nerve cord. 2. Within a ganglion 1) P cells excited both Rz neurons, electrically close to the site of electrical coupling between the Rz neurons; 2) each of the four P cells had similar effects on the Rz neurons; and 3) homologous contralateral P cells shared interneuronal pathways. These data show that P cells provide nearly identical bilateral information onto Rz neurons. 3. Along the nerve cord 1) every P cell excited Rz neurons in ganglia anterior and posterior to the site of stimulation; 2) the signal was carried the entire length of the nerve cord along interneuronal pathways with similar overall (but regionally different) conduction velocities in the two directions; 3) the amplitude of the Rz responses was smaller as the distance to the activated P cell increased; 4) the rate of change of the amplitude along the cord was larger when the signal traveled from front-to-back than in the opposite direction. 4. These data shows that mechanosensory input from any segment could excite Rz neurons along the cord, in proportion to the intensity of the stimulus.