Evidence that the masticatory muscles receive a direct innervation from cell group k in the rabbit

These experiments have shown that a group of neurons lateral to the trigeminal motor nucleus innervates the muscles of mastication. The work began to describe the location of digastric last-order interneurons, using the technique of transneuronal labeling with wheatgerm agglutinin-conjugated horseradish peroxide injected into the left digastric muscle of rabbits under general anaesthesia. Four to eight days later, the animals were killed with an overdose of anaesthetic and perfused. Coronal sections of the frozen brainstem were cut at 20 microns thickness and processed for peroxidase activity. Motoneurons in the ventral and caudal divisions of the trigeminal motor nucleus were labeled in all animals as expected. An additional population of neurons located ventrolaterally to the motor nucleus in cell group k were also found to be labeled if the survival time was five days or more. In an attempt to determine whether cell group k neurons were labeled transynaptically, two series of control experiments were carried out. In the first, crystals of fluorescein- and rhodamine-conjugated dextran amines and horseradish peroxidase were applied directly to central ends of cut digastric nerves. In the second, central ends of cut digastric nerves were enclosed in cuffs containing 40-60% horseradish peroxidase solutions. Again, neurons in both the trigeminal motor nucleus and cell group k were labeled suggesting that neurons within cell group k project to the digastric muscle. Similar experiments using dextran amines and wheatgerm peroxidase were carried out on the masseter muscle. Motoneurons in the dorsomedial and rostral half of the trigeminal motor nucleus, as well as primary afferent cell bodies in the mesencephalic nucleus of the trigeminal nerve, were labeled in all experiments. In addition, a population of neurons in cell group k, dorsal to those associated with the digastric muscle, were found to contain each one of the reaction products. Since it is thought that only the wheatgerm agglutinin-conjugated horseradish peroxidase transferred from one neuron to another, we conclude that cell group k neurons provide an additional innervation to the digastric and masseter muscles.