Horseradish peroxidase (hrp) is used to trace axonal connections from the motor end-plate to the driving neuron. This technique has confirmed that the neurons activating the sternothyroid muscle are located in the cervical spinal cord, while those controlling the posterior cricoarytenoid (pca) are found in the nucleus ambiguus ipsilaterally. Eight rabbits underwent a sternothyroid ansa pedicle implantation to the pca at the time of sectioning the recurrent laryngeal nerve ipsilaterally. After two months, four of these animals received hrp injections into the previously implanted pca. Brainstem staining hrp did not reveal any retrograde transport to the motor neurons that were known to control the sternothyroid. Possible reasons for the failure of retrograde transport are discussed.
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