Aspects of the Organization of Central Nervous Pathways in Aplysia Depilans

Abstract

1. The organization of the central nervous system of Aplysia depilans has been investigated in whole animal and isolated ganglion preparations using mechanical and electrical stimulation. 2. Intracellular micro-electrodes have been used to record activity in nerve cells of the abdominal ganglia in situ. Some cells are spontaneously active and quite unaffected by mechanical stimulation, whereas others show varying degrees of responsiveness. Those which are unaffected may exhibit regular rhythmic activity or intermittent bursts which are intrinsic to the cells themselves but in other cases are due to synaptic input from other central neurones. 3. In isolated central nervous system preparations a special study of the pleural ganglion has revealed many types of cell with electrical activity similar to that shown in isolated abdominal ganglion preparations. A notable feature of the pleural ganglion cells was the large size of the excitatory post-synaptic potentials recorded in response to stimulation of pre-synaptic fibres. 4. Different types of branching of cells of the pleural ganglia were investigated. By observing the somatic potential it was possible to decide in which nerve a particular cell sent collateral branches and which nerves contained fibres affecting the cell synaptically. By this means it was clear that a large number of pathways connect the cerebral and pleural ganglia on each side. 5. A large number of direct pathways were found of nerve fibres passing through ganglia without any synapse. 6. Synaptic pathways varied in the number and intrinsic properties of the individual synapses along their route. Synapses between fibres in the nerves innervating the foot and parapodial lobes of the two sides were not as common as has been described for Ariolimax. 7. In general the results have shown a great variety in the extent to which afferent stimulation may affect the whole or part of the central nervous system. They have also revealed the great multiplicity in the pathways whereby this is achieved.