Further Studies on the Electrophysiological Anatomy of the Left and Right Giant Cells in Aplysia

Abstract

1. Electrophysiological investigations have shown that the giant cell in the right upper quadrant of the abdominal ganglia (RGC) and the giant cell of the left pleural ganglion (LGC) have axonal branches in the main parapodial and pedal nerves on both sides of the body. There is also a branch of the RGC in the branchial nerve. 2. The A spike recorded in the LGC following antidromic stimulation of nerves on the left side is usually larger than that resulting from stimulation of nerves on the right side. Collision experiments suggest that transmission can always occur at the main branching from left to right, but that it is not always possible in the opposite direction. 3. The effectiveness of synaptic inputs to the LGC and RGC from the main nerves has been compared. The synaptic potentials recorded simultaneously in the two cells were very similar in their general form following stimulation of a given nerve, but there were differences in latency. Inputs from the left side tend to be more effective on the LGC and from the right side on the RGC, but only slightly and not in all preparations. Inputs from nerves which enter the abdominal ganglia were generally more effective on the RGC than the LGC, but again not in all cases. The threshold for spike initiation is usually lower for the LGC than the RGC. 4. The presence of a number of interneurones common to these two cells was shown by simultaneous recording. The precise location of such interneurones was not established, but the order of appearance of unitary EPSPs, and the interval between them, showed that some interneurones were in the abdominal ganglia and others in ganglia of the circumoesophageal ring. It is suggested that the presence of these interneurones accounts for the general similarity in the compound EPSPs recorded in the two cells in response to synaptic inputs. 5. There is a direct synaptic pathway between the cells from right to left, but evidence for a connexion in the reverse direction was only found in a single specimen of Aplysia fasciata. The unitary postsynaptic potential recorded for these direct connexions is biphasic in form. This direct connexion does not seem to be present in all species of Aplysia. 6. Comparison of some of the membrane properties of the LGC and RGC is made. Both cells are similar with respect to their resistance and capacitance and both show anomalous rectification in their current voltage curves. They are both H cells. The excitability of the LGC is usually greater than that of the RGC as indicated by the responses to intrasomatic stimulation. 7. It is concluded that the two giant cells are homologous cells which innervate many parts of the foot and parapodia on both sides and receive convergent input from all over the body. Possible ways in which they might integrate such inputs are discussed.