FIG. 3.

Motor neuron postsynaptic potentials elicited by stimulation of Mauthner axons. A: diagram of the circuit that underlies escape behavior. Mauthner neurons, M, in the hindbrain extend their axons the length of the spinal cord. Axons make excitatory synapses (▴) on primary motor neurons, P. Mauthner axons are also electrically coupled (=) to inhibitory interneurons () the axons of which cross the spinal cord to inhibit Ps on the opposite side. Axons of the Ps exit the spinal cord to innervate peripheral fast muscle (hatched parallelograms). S_Ipsi: stimulation of the ipsilateral Mauthner axon triggers an action potential. S_Contra: stimulation of the contralateral Mauthner axon triggers an inhibitory postsynaptic potential (IPSP) followed by a small depolarization. B: the relative amplitudes of the individual excitatory and inhibitory components of the synaptic potentials are dependent on membrane potential of the motor neuron. Synaptic potentials were triggered by stimulating the contralateral Mauthner axon in 50–58 hpf wild-type fish. Representative individual sweeps taken between −80 and 10 mV are shown. Near −40 mV the IPSP is negative-going and the excitatory postsynaptic potential (EPSP) is sufficiently positive to generate an action potential. Near −80 mV, both components are positive-going.