Fibre composition of the hypoglossal nerve in the rat.

The fibre composition of the hypoglossal nerve of the rat has been investigated by means of (a) retrograde neuronal labelling following application of fluorescent dyes to the XII nerve and/or to the ansa cervicalis, lingual nerve and facial nerve; and (b) anterograde fibre degeneration in silver-impregnated sections of the tongue following interruption of one or more of these nerves. A search for neuromuscular spindles was carried out in coronal paraffin sections of eight control tongues. The ansa cervicalis was found to make three significant contributions to the distal XII trunk: one set of axons originated in the medial subnucleus of the ventral tier of the XII nucleus and contributed to the motor innervation of intrinsic lingual muscles: a second set originated in the lateral subnucleus of the ventral tier and contributed to the motor innervation of the posterior part of the geniohyoid muscle. A third set, having cell bodies in the two uppermost cervical ganglia, provided proprioceptive afferents to the tongue and geniohyoid. A small number of somata in the caudal end of the facial nucleus contributed axons to the XII nerve prior to its emergence from the medulla oblongata. The salivatory nucleus (pons) also contributed pre-emergence fibres, which supplied autonomic ganglia destined for the supply of small arteries in the tongue. The fibre composition of the XII trunk was completed by an afferent contribution from the jugulo-nodose ganglion complex of the vagus nerve, and by sympathetic fibres from the superior cervical ganglion. In control material, eight neuromuscular spindles were detected in five out of eight tongues. Six occupied the longitudinalis inferior, one the hyoglossus, and one the genioglossus. All eight were close to the surface of the tongue base.