The organization of the emotion-related somatic motor behavior, including jaw movements, is governed not only by the cortical limbic system but also by the subcortical limbic system including the amygdala and hypothalamus. GABAergic neurons in the central amygdaloid nucleus (CeA) and glutamatergic neurons in the posterior lateral hypothalamus (PLH) exert inhibitory and excitatory influences, respectively, upon premotor neurons for the motor trigeminal nucleus (Vm) in the parvicellular reticular format ion (RFp) of the medulla oblongata. The CeA also has an inhibitory influence on non-dopaminergic (probably GABAergic) neurons in the retrorubral field of the midbrain that send their axons to the RFp. Furthermore, the CeA and lateral hypothalamus including the PLH may also modulate Vm neurons via projections to the mesencephalic trigeminal nucleus which contains the cell bodies of primary afferent neurons conveying inputs from the masticatory muscle spindles or the periodontal ligament receptors to jaw closing motoneurons within the Vm. These pathways from the subcortical limbic structures to the trigeminal motor system in the lower brainstem underlie the regulation of emotional jaw movements.
Jaw movements are carried out mainly by muscles attached to the mandibular bone, such as the masticatory and suprahyoid muscles. The masticatory muscles as well as the mylohyoid muscle and anterior belly of the digastric muscle are innervated by motoneurons in the motor trigeminal nucleus (Vm). Within the Vm, the neurons innervating each muscle are assembled and constitute subgroups. The motoneurons innervating the jaw-closing muscles, such as the masseteric and tempolaris muscles, are assembled in the dorsolateral part of the Vm (Vm-dl), whereas those innervating the jaw-opening muscles, such as the mylohyoid muscle and anterior belly of the digastric muscle, are assembled in the ventromedial part of the Vm (Vm-vm).
The organization of the emotion-related somatic motor behavior, including jaw movements, is governed by neural circuits in the limbic system, which include the amygdala, hypothalamus, and several regions of the cerebral cortex. The amygdala has been known to play a crucial role in the development and expression of emotional behavior, such as the fear reaction and defense response (for review, see [
In this communication, I summarize recent neuroanatomical and neurochemical studies on the control pathways to the Vm from the subcortical limbic structures, such as the amygdala and hypothalamus.
Jaw movements are carried out by Vm motoneurons activated by inputs from the periphery and/or from the upper motor centers. Most of these inputs are transmitted to the Vm via interneurons called premotor neurons. The Vm-premotor neurons send their axons directly to the Vm and many of them are distributed in the lower brainstem, such as the pons and medulla oblongata [
to Vm motoneurons for controlling jaw movements [
As for the possible pathways relaying output signals of the amygdala to the Vm, the direct pathway from the central amygdaloid nucleus (CeA) to the Vm has been suggested by Mascaro et al. [
According to Gonzales and Chesselet [
Gonzales and Chesselet [
Furthermore, the CeA sends projection fibers to the Vmes and Vmes neurons are closely apposed by CeA axonal varicosities [
With respect to the neurotransmitter of the CeA neurons, recent in situ hybridization studies demonstrated that
almost all the CeA neurons are positive for glutamic acid decarboxylase (GAD) 65 mRNA [
The SN contains many dopaminergic neurons, which are distributed mainly in the pars compacta, referred to as the A9 dopamine cell group. The RRF also contains numerous dopaminergic neurons that are referred to as the A8 dopamine cell group. However, SN neurons projecting to the brainstem regions, such as the inferior colliculus [
The RFp is heterogeneous with respect to neurotransmitter phenotypes; glutamatergic, GABAergic, cholinergic, and nitrergic neurons are all found in the RFp [
reactive for GAD or glycine are also distributed in the RFp [
Vmes neurons have been indicated to be glutaminergic [
The hypothalamic projection to the Vm has been examined by using retrograde tracers [
Shammah-Lagnado et al. [
Interestingly, anterograde tracing studies with Phaseolus vulgaris-leucoagglutinin [
Neumerous hypothalamic neurons labeled with CTb injected into the Vm region have been revealed to display glutamate-like immunoreactivity [
As for the neuropeptides, by using retrograde tracing in combination with immunohistochemical methods, recent studies in the cat [
This article summarizes neuroanatomical and neurochemical organization of the control pathways to the Vm from the subcortical limbic structures, such as the amygdala and hypothalamus. The summary diagram is shown in
The author is grateful for the support of Drs. R. Fujimoto, K. Fujita, T. Goeku, M. Kuzushima, T. Mukai, Y. Munenaga, T. Sakurai, Y. Sakurai, H. Taguchi, K. Tanaka, K. Urashima, H. Yamaoka, S. Yamazaki and K. Yoshida. This study was partly supported by Grant-in-Aid for Scientific Research (C) (19500294, 21500325 and 24500412) from the Ministry of Education, Science, Sports and Culture of Japan.