Spinal network of the Mauthner cell.

Most swimming vertebrates, particularly fishes and amphibians, avoid predators by producing an escape behavior initiated by a single action potential in one of a pair of cells, the Mauthner cells, located in the hindbrain. The most prominent feature of this behavior is a rapid, forceful bend of body and tail which leads to a characteristic C bend (stage 1) early in the escape. The spinal output of the Mauthner cell is largely responsible for this bend. Each Mauthner cell sends an axon down the length of the spinal cord on the side opposite the soma. When one Mauthner axon fires, it massively excites the ipsilateral musculature by (1) monosynaptic excitation of the large primary motoneurons that innervate the fast white muscle fibers and (2) polysynaptic excitation of motoneurons which is most likely mediated through an identified class of descending interneurons. While motoneurons on the side of the C bend are excited, excitation of those on the opposite side is blocked by inhibition of primary motoneurons and descending interneurons. This inhibition is mediated by commissural interneurons that are electrotonically coupled to the Mauthner axon and cross the spinal cord to monosynaptically inhibit cells on the opposite side. They inhibit not only primary motoneurons and descending interneurons, but also the commissural inhibitory interneurons on the opposite side. The inhibition of contralateral primary motoneurons and descending interneurons prevents motor activity on the side opposite the C bend from opposing that bend, while the inhibition of commissural interneurons prevents them from interfering, via their inhibitory connections, with excitation of motoneurons on the side of the bend. The spinal network responsible for the bend has several similarities with the spinal network for swimming in other anamniotic vertebrates, including lampreys and embryonic frogs. These similarities reveal important, primitive features of axial motor networks among vertebrates.