Involvement of the basal nucleus of Meynert on regional cerebral cortical vasodilation associated with masticatory muscle activity

Recently, attention has focused on the oral health conditions, especially mastication in the context of cognitive deficits. Studies suggested a possible causal relationship between mastication and cognitive function. Loss of masticatory function is also associated with increased disability and mortality. Consequently, being able to chew properly is of utmost important for elderly to maintain a healthy diet and preserve cognitive function. It has been demonstrated that mastication is of great importance for peripheral sensory input to the neocortex and hippocampus for preserving and promoting the cognitive function. Accumulating evidence indicates that impaired masticatory function causes morphological and functional alterations of the neocortex and hippocampus. Indeed mastication or chewing may actually help to maintain the cerebral function. With various methods like ultrasound Doppler and fMRI, it has been shown that the regional cerebral blood flow (rCBF) in widespread cortices is increased in humans during chewing. The laser Doppler method has also shown that there is an increase in the rCBF of the frontal lobe in conscious cats during mastication. However, the neural mechanisms responsible for these increases associated with mastication are yet to be determined.

In this study, Japanese scientists: Dr. Harumi Hotta and Dr. Harue Suzuki from Tokyo Metropolitan Institute of Gerontology together with Dr. Tomio Inoue from Showa University, and Dr. Mark Stewart from the State University of New York Downstate Medical Center in the USA, identified the neural mechanisms involved in increasing rCBF in the neocortex associated with mastication. They found that the activation of the nucleus basalis of Meynert (NBM) is partly responsible for rCBF increase related to masticatory muscle activity. The work is published in the Journal of Cerebral Blood Flow & Metabolism.

The authors observed that changes in rCBF and NBM activity were associated with spontaneous jaw muscle activity and cortical desynchronization. In addition, changes in rCBF were found to be induced by electrical stimulation of cortical masticatory areas (e.g., A-area). During spontaneous jaw movement, there was an increase in NBM neurons prior to an increase in rCBF. Furthermore,  A-area stimulation resulted in a bilateral increase in rCBF in large areas of the parietal, frontal and occipital lobes.

Following the administration of muscimol to inhibit NBM activation on rCBF response induced by A-area stimulation, they observed that A-area stimulation caused a selective reduction of rCBF responses on the side ipsilateral to the muscimol administration.

The research team compared A-area stimulation responses before and after immobilization using vecuronium (a muscle relaxant) under artificial respiration. Without the administration of vecuronium, the control responses of rCBF, local field potential, systemic mean arterial pressure and jaw muscle electromyographic activity were similar under artificial and spontaneous respiration. They found that changes in rCBF and local field potential in the cerebral cortex were similar both before and after muscle relaxation, whereas, the jaw muscle electromyographic activity observed during A-area stimulation disappeared after the administration of vecuronium.

Moreover, stimulation of the pyramidal tract to induce jaw muscle activity was conducted and it was found to have no effect on rCBF, compared to the rCBF changes observed when jaw muscle activity was stimulated by the cortical masticatory areas.

The authors have been able to demonstrate that the activation of NBM vasodilator neurons partly contributes to the increase in rCBF associated with masticatory muscle activity, and that the NBM activation is induced by central commands from the motor cortex and is not affected by feedback from contracting muscles or brainstem central pattern generator. Future studies will be required to examine in more detail the association between mastication and the cognition and the influence of aging on cognition.