Synaptic organization of monosynaptic connections from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat

Abstract

Synaptological characteristics of synapses between axonal boutons of the trigeminal mesencephalic nucleus (Vme) neurons and the hypoglossal nucleus (XII) motoneurons (MNs) were studied using biotinylated dextran amine (BDA) anterograde labeling combined with horseradish peroxidase (HRP) retrograde transport in the rat. BDA was initially iontophoresed into Vme unilaterally and 7 days later HRP was injected into the anterior two-thirds of the ipsilateral tongue. After histochemical reactions, BDA anterogradely labeled boutons were seen to appose closely to somata and dendrites of HRP retrogradely labeled MNs in XII by light microscopy. A total of 212 BDA-labeled Vme boutons were examined ultrastructurally, which had an average diameter of 1.3 +/- 0.4 microm and contain small clear spherical vesicles. Eighty-eight percent of Vme boutons (187/212) synapsed on dendrites of HRP-labeled XII MNs. Twenty-five Vme boutons (25/212, 12%) made synapses with somata of HRP-labeled XII MNs. Thirty-five percent (74/212) of BDA-labeled Vme boutons were also contacted by unlabeled P-type terminals. Presynaptic P-type terminals contained spherical (47%, 35/74), pleomorphic (43%, 32/74), and flattened (10%, 7/74) synaptic vesicles. Thus, P-type terminals (as a presynaptic element), BDA-labeled Vme boutons, and XII MNs constitute axoaxodendritic and axoaxosomatic synaptic triads. There are four types of synaptic microcircuits in XII neuropil: synaptic convergence, synaptic divergence, presynaptic inhibition synaptic circuits, and feedforward regulation circuits. This detailed ultrastructure examination of the synaptic organization between Vme neurons and XII MNs provides insights into the synaptic mechanisms of the trigeminal proprioceptive afferents involved in the jaw-tongue reflex and coordination during oral motor behaviors.

Ultrastructural and biochemical changes of the medial pterygoid muscle induced by unilateral exodontia

“…Maeda et al (1990) suggested a crossing pathway of sensorial stimuli, from the extracted molar to the contralateral motor neuron to explain the alteration phenomena in neuromuscular spindles. The afferent pathways from jaw-muscle spindles have projection areas to trigeminal motor nucleus (Luo and Dessem, 1999;Luo et al, 2001) and to hypoglossal motoneurons (Zhang et al, 2003) coordinating masticatory muscle stretch reflexes (Luo et al, 1995), and oral motor behaviors (Zhang et al, 2003). Therefore, the neuromuscular spindle morphologic abnormalities caused by occlusal alteration in medial pterygoid muscle may play significant alteration on masticatory function, since this muscle activity was associated with early masticatory closing cycle.…”

Role of the trigeminal mesencephalic nucleus in rat whisker pad proprioception

“…To date, it has been thought that masticatory muscles possess only a proprioceptive reflex that is mediated by trigeminal motor neurons. However, it has been shown that axons of the TMne also target the ipsilateral hypoglossal nucleus, and proprioceptive afferents from the masseter muscle directly project to the hypoglossal nucleus [54-57]. Hypoglossal efferences have been also demonstrated in the extrinsic muscles of the mystacial pad and in masseter neuromuscular spindles [2,3].…”

Distribution of Na/K‐ATPase alpha 3 isoform, a sodium‐potassium P‐type pump associated with rapid‐onset of dystonia parkinsonism (RDP) in the adult mouse brain

“…The Journal of Comparative Neurology | Research in Systems Neuroscience the complex control of mastication, respiration, and speech, suggesting that failure of the pump function in these areas may contribute to the observed symptoms (Luo et al, 2001;Zhang et al, 2003).…”