Ultrastructure and synaptic organization of the spinal accessory nucleus of the rat

Abstract

The accessory nucleus is composed of neurons in the medial column that innervate the sternocleidomastoid muscle, and neurons in the lateral column that innervate the trapezius muscle. We retrogradely labeled these neurons by injection of cholera toxin conjugated horseradish peroxidase into the sternomastoid (SM) or the clavotrapezius (CT) muscles, and investigated fine structure and synaptology of these neurons. Almost all SM and CT motoneurons had the appearance of alpha-motoneurons, i.e., large, oval or polygonal cells containing well-developed organelles, Nissl bodies, and a prominent spherical nucleus. More than 60% of the somatic membrane was covered with terminals. The SM motoneurons (34.4 x 52.2 microm, 1,363.1 microm(2) in a section) were slightly larger than the CT motoneurons (32.8 x 54.2 microm, 1,180.8 microm(2)). The average number of axosomatic terminals in a section was 52.2 for the SM, and 54.2 for the CT motoneurons. More than half of them (58.0%) contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II) with the SM motoneurons, while 57.9% of them contained round vesicles and made asymmetric synaptic contacts (Gray's type I) with the CT motoneurons. A few C-terminals were present on the SM (3.5) and the CT (3.7) motoneurons. About 60% of the axodendritic terminals were Gray's type I in both the SM and the CT motoneurons. A few labeled small motoneurons were also found among the SM and the CT motoneurons. They were small (19.2 x 26.2 microm, 367.0 microm(2)), round cells containing poorly developed organelles with a few axosomatic terminals (9.3). Only 20% of the somatic membrane was covered with the terminals. Thus, these neurons were presumed to be gamma-motoneurons. These results indicate that the motoneurons in the medial and the lateral column of the accessory nucleus have different ultrastructural characteristics.

Sources of the motor and somatic sensory innervation of the trapezius muscle in the rat

“…Neuronal tracing studies performed on the rat showed tracer labelled neurons in the ventral horn of the spinal cord. Neurons traced with cholera toxin-conjugated horseradish peroxidase were found in the C 2 to C 4 spinal segments (Hayakawa et al, 2002). In other studies traced neurons were found in the C 2 to C 6 spinal segments (Ullah et al, 2007).…”

Early emergence of axons and formation of accessory nerve (XI) in avian embryos

“…Recently, Ullah et al, 2007 reported that there are three longitudinal columns that innervate the sternocleidomastoid muscle in addition to one trapezius column in the adult rat (Ullah et al, 2007), whereas Flieger, 1964 andClavenzani et al, 1994 found two groups of longitudinal nuclei for the sheep SAN. Hayakawa et al, 2002 reported that there are two columns, one each for the sternocleidomastoid and trapezius muscles. Finally, a single longitudinal column was identified in the rat by retrograde labeling of the SAN branches with DiI (Yan et al, 2007), and similarly in the baboon (Augustine and White, 1986) only one column of somata in the spinal cord has been reported.…”

“…The somata of trapezius-innervating neurons whose axons grow through the spinal accessory nerve are larger than those of neurons that pass through the cervical nerves. The medial column, which innervates the sternomastoid and cleidomastoid muscles, is located at the dorsomedial edge of the ventral horn at the level of C1 to C3, and the lateral column, which innervates the trapezius and cleido-occipital muscles, is located in the dorsolateral part of the ventral horn at the level of C2 to C6 (Matesz and Szekely 1983;Krammer et al, 1987;Hayakawa et al, 2002). In avians, on the other hand, the spinal motor axons that innervate the cucullaris sprout from dorsal roots, and these neurons exhibit branchiomotor properties by expression of Phox2b and Isl1 (Kobayashi et al, 2013;Hirsch et al, 2007).…”

Localization of motoneurons that extend axons through the ventral rami of cervical nerves to innervate the trapezius muscle: A study using fluorescent dyes and 3D reconstruction method

“…The motor neurons supplying the SCM have been identified using HRP tracing and electron microscopy in rat. The motoneurons include two types-a-motoneurons and gmotoneurons (Hayakawa et al, 2002). In our study, the motoneurons passing through the cervical nerves are generally smaller in diameter than those passing directly through the SAN.…”