Bilateral innervation of the anterior digastric muscle by trigeminal motor neurons.

The locations and numbers of trigeminal motor neurons supplying the anterior digastric muscle in the rat were observed following retrograde transport of horseradish peroxidase. Unilateral injection of horseradish peroxidase into the muscle resulted in bilateral labelling of cells in the ventromedial region of the caudal trigeminal nucleus. The numbers of cells on the contralateral side were consistently lower than those in the ipsilateral nucleus (30 cells compared to 171). Bilateral injection of horseradish peroxidase resulted in higher numbers of cells on both sides than was seen ipsilaterally after unilateral injection: these numbers corresponded roughly to the sum of ipsilateral and contralateral labelled cells after a single injection. The contralateral reaction product survived removal of the opposite muscle and nerve immediately before injection. Ablation of one trigeminal motor nucleus caused degeneration in the opposite nucleus, and some degenerating fibres crossed the mid-line at the level of the motor nucleus, in the bundle of fibres decussating in the dorsal pons.