An HRP study of the motoneurons supplying the rat hypobranchial muscles: central localization, peripheral axon course and soma size.

The locations of motoneurons (MNs) supplying the rat hypobranchial muscles (lingual, geniohyoid, and infrahyoid) and the peripheral courses of axons of these MNs were investigated by using a method of HRP injection into the hypoglossal nerve or these muscles in combination with severing of the hypoglossal and/or cervical components of the plexus hypoglossocervicalis. Moreover, some sizes of the MNs were investigated in both transverse and horizontal sections. The hypobranchial MNs formed a sequence of cell columns extending caudally from the hypoglossal nucleus, via the supraspinal nucleus, to the medial and then the ventrolateral subnuclei of the ventral horn of C1 to C3. The lingual and geniohyoid MNs were located in the hypoglossal nucleus. The majority of their axons passed solely through the hypoglossal nerve, whereas a small number of axons, whose somata lay in the caudal hypoglossal nucleus, passed through the first cervical nerve and then through the ansa cervicalis to reach the hypoglossal nerve. The infrahyoid MNs were located in the supraspinal nucleus and the ventral horn, and their axons passed through the first to third cervical nerves. The hypobranchial MNs were divided into three groups according to their size: the lingual, the geniohyoid and thyrohyoid, and lastly the other infrahyoid MNs, in order of smaller to larger size. The sizes of lingual, geniohyoid, and thyrohyoid MNs displayed a unimodal distribution both in transverse and horizontal sections, whereas the other infrahyoid MNs showed a bimodal distribution.