Mauthner Cell - Morphology and Connections - Axon Cap

The Mauthner cell axon hillock is surrounded by a dense formation of neuropil, called the Axon cap. The high resistance of this axon cap contributes to the typical shape of the Mauthner cell field potential (see below). In its most advanced form the axon cap consists of a core, immediately adjacent to the Mauthner cell axon, and containing a network of very thin unmyelinated fibers, and a peripheral part. This peripheral part contains the large unmyelinated fibers of the PHP neurons (see below) that mediate the inhibitory feedback to the Mauthner cell; the Mauthner cell itself also sends small dendrites from its axon hill to the peripheral part of the axon cap. Finally, the surface of the axon cap is covered with a cap wall composed of several layers of astrocyte-like glial cells. Both glial cells and the unmyelinated fibers are coupled with each other by means of gap junctions.

Evolutionarily, the axon cap is a more recent development than the Mauthner cell itself, so some animals, such as lampreys and eels, while having functional Mauthner cells, don't have axon cap at all, while some other animals, such as amphibia and lungfish, do have a very simplified version of it.