Medullary reticular neurons in the Japanese toad: morphologies and excitatory inputs from the optic tectum

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Abstract
To elucidate the neural mechanisms that mediate visual responses of optic tectum (OT) to medullary and spinal motor systems, we analyzed medullary reticular neurons in paralyzed Japanese toads (Bufo japonicus). We examined their responses to electrical stimulation of OT, and stained some neurons intracellularly. Responses to stimulation of the glossopharyngeal nerve (IX) were also analyzed. Extracellular single unit recording revealed excitatory responses of medullary neurons to OT and IX stimulation. Among 92 units encountered, 79 responded to OT stimuli, 10 to IX stimuli, and 3 to both. Some units responded to successive stimuli of short intervals with relatively stable lags. Intracellular recording and staining experiments revealed morphologies of reticular neurons that received excitatory inputs from OT. Thirteen units were identified after complete reconstruction of somata and dendrites. Neurons in the nucleus reticularis medius received excitatory inputs from bilateral OT. They had wide dendrites in ventral, ventrolateral and lateral funiculi, and single axons descending in the ipsilateral ventral funiculus as far caudally as the cervical spinal cord. Some collaterals of these axons projected directly to the hypoglossal and spinal motor nuclei. Some neurons in other medullary nuclei (nuc. reticularis superior, pretrigeminal nucleus, nuc. reticularis inferior, and nuc. tractus spinalis nervi trigemini) also responded to the OT stimulation. Activities in bilateral OT converge onto medullary reticular neurons, which may directly control medullary and spinal motor systems.