Electrophysiological and ultrastructural changes in severed motor axons of the crayfish.

In many invertebrates, distal stumps of severed axons degenerate slowly and survive for long periods of time; this lengthy process allows the study of the physiological and structural changes involved in axonal degeneration processes. The following experiments demonstrate a reduction in EPSP amplitude, an increase in the distance between neighboring release sites, an extended duration of transmitter release, and a doubling in the average number of quanta released per stimulus at each release site. Ultrastructural examination of those stumps revealed various degrees of glial cell invasion. In the same distal stump, some axons were partially filled with glial cells, but adjacent axons could be completely filled by them. Glial cell invasion was greater at regions closer to the site of axotomy and increased as time progressed. The glia engulfing the stumps exhibited hypertrophy and changes in nuclear morphology. The nuclei of some of those glia cells were unusually close to the axonal membrane in the distal stumps. In spite of these severe morphological changes, the stumps were still capable of conducted action potentials and releasing transmitter at their synapses.