IDENTIFICATION OF INTERNEURONS WITH CONTRALATERAL, CAUDAL AXONS IN THE LAMPREY SPINAL-CORD - SYNAPTIC-INTERACTIONS AND MORPHOLOGY

Propriospinal interneurons with axons projecting contralaterally and caudally (CC interneurons) were studied with intracellular recordings in the lamprey. CC interneurons were identified by recording their axon spikes extracellularly in the spinal cord during intracellular stimulation of the cell body. The axon projections of CC interneurons were confirmed after intracellular injection and development of horseradish peroxidase. Intracellular stimulation of CC interneurons produced synaptic potentials in myotomal motoneurons, lateral interneurons and other CC interneurons that lay caudally on the opposite side of the spinal cord. Most CC interneurons were inhibitory, but some were excitatory. CC interneurons were divided into 3 classes (CC1, CC2 and CC3) on the basis of reticulospinal Mueller cell inputs. CC1 interneurons were excited by the ipsilateral Mueller cell B1 and the contralateral Mauthner cell. CC1 interneurons were inhibitory. CC2 interneurons were also inhibitory, but they were distinguished from CC1 interneurons by their excitation from the ipsilateral Mueller cells B2-4. CC3 interneurons were excitatory and were inhibited by the ipsilateral Mueller cell I1. Inhibitory CC interneurons may be important for motor coordination in the lamprey. Movements of the lamprey body during reflexes and swimming consist of contraction and relaxation of myotomal muscles on opposite sides of the body. By being coactive with ipsilateral myotomal motoneurons, inhibtory CC interneurons could contribute to the inhibition of contralateral motoneurons during these movements.