Active contraction of nerve bundle and identification of a nerve-contractor motoneuron in Aplysia

1. The active contraction or shortening of the nerves of Aplysia kurodai and Aplysia juliana was examined. 2. A particular neuron in the abdominal ganglion controlled the contractions of the nerves, and the neuron was identified as a gill- and siphon-contractor motoneuron previously identified as L7. This was based on the following observations, which agreed closely with those reported for L7: the position, shape, and color of the cell body of the neuron; the synaptic and antidromic activation of the neuron by stimulation of various nerves; and the configurations of the major axons as revealed by dye injection into the soma of the neuron. Finally, the activation of the neuron contracted the gill, and the characteristics of the contraction were those reported for L7. 3. L7 was the only neuron we could find in the abdominal ganglion that directly controlled the contractions of the nerves. This was supported by the following observations: 1) direct or synaptic activation of L7 made the nerves shorter in a firing frequency-dependent manner. 2) Even single action potentials of L7 contracted the nerves. 3) When L7 was destroyed, no contractions of the nerves were observed however strongly a stimulation was applied. 4) When every synaptic connection in the abdominal ganglion was blocked by bathing the ganglion in Cafree artificial seawater, direct or spontaneous activation of L7 still made the nerves shorter. 4. A sensitive marker, cobaltic-lysine complex solution, was injected into the cell body of L7, and many fine axonal branches were newly found in the epineurium of the nerves. With the use of spike-triggered averaging, small potentials were recorded from the contracting nerves. This suggested that the action potentials were conducted along the newly found fine branches of the axon to innervate the epineurial muscles. 5. We found that L7 is a multifunctional motoneuron that contracts nerves.