Identification of motoneurons and interneurons in the spinal network for escapes initiated by the Mauthner cell in goldfish

Abstract

We used intracellular recording and staining techniques to study the spinal circuitry of the escape behavior (C-start) initiated by the Mauthner axon (M-axon) in goldfish. Simultaneous intracellular recordings from one or both M-axons and a spinal neuron, followed by HRP labeling of the spinal cell, show that each M-axon makes monosynaptic, chemical excitatory synapses onto 2 populations of ipsilateral spinal neurons. The first consists of the large primary motoneurons that, based on earlier work (Fetcho, 1986), innervate exclusively the faster, white muscle fiber types in the myomeres. The second group of cells is formed by previously undescribed descending interneurons with ipsilateral axonal branches that have contacts with primary and secondary motoneurons spread over 2 or more body segments. Indirect evidence suggests that these descending interneurons are excitatory, and they may explain the polysynaptic activation of motoneurons observed in earlier studies of the spinal circuitry (Diamond, 1971). Both classes of neurons excited by the ipsilateral M-axon are disynaptically inhibited by the contralateral one. The morphology and physiology indicate that this inhibition is mediated by interneurons that are electrotonically coupled to one M-axon and have processes that cross the cord to inhibit contralateral neurons in the region where these postsynaptic cells receive excitatory input from the other M-axon. We have identified interneurons with the physiological and morphological features of these predicted crossed inhibitory interneurons. These cells are electrotonically coupled to the ipsilateral M-axon and receive a chloride-dependent disynaptic inhibitory input from the contralateral M-axon. Their very simple somata give rise to a process that crosses the spinal cord between the 2 M-axons. Once on the opposite side of the cord, the crossing process sends myelinated branches that run rostrally and caudally, roughly parallel to the contralateral M-axon. Processes that arise from these longitudinal branches terminate in a striking association with collaterals of the M-axon; nearly every M-axon collateral along the longitudinal course of an interneuron is met by a branch or branches of the interneuron whose terminals are apposed to neurons postsynaptic to the collateral. The identified cells can account for some of the major features of the escape behavior initiated by the M-cell, including the massive excitation of muscle via mono- and polysynaptic activation of motoneurons on the side of the active M-axon and the inhibition of motoneurons and interneurons on the opposite side. The description of the spinal network that emerges from our work confirms several conclusions from Diamond and Yasargil's early studies of the circuitry (Yasargil and Diamond, 1968; Diamond and Yasargil, 1969; Diamond, 1971), refutes others, and introduces some new neurons in the network. The M-cell circuitry has several similarities with the spinal circuits for swimming in lampreys and anuran embryos. These parallels suggest that many vertebrates might have common arrangements of spinal circuitry, portions of which are involved in such different motor behaviors as escape (C-starts) and swimming.