Brain Mechanisms Involved in Mastication

Abstract

The last frontier of science is to understand the biological basis of consciousness and the mental processes by which we perceive, act, learn, and remember. The brain science has recently been promoted to remarkable development by virtue of a rapid advance in the methodology in such scientific fields as the science of information, molecular biology, health sciences, and so on. The purpose of the present review is to explain roughly“What the brain science is, ”giving a couple of examples from synaptic plasticity, motor learning in the cerebellum, with special reference to our recent experiments on masticatory motor learning in young rats. Finally, working memory in the prefrontal cortex will briefly be introduced as an up-to-date topic.
1. Synaptic plasticity
Synapses denote the functional juxtaposition of one neuron to another. Three kinds of synapse were recognized-chemical, electrical, and mixed. In the general sense, synaptic transmission exhibits synaptic plasticity. In the brain, synapses occur not so much on the dendritic shafts as on spines that project from the dendrites. According to F. Crick's hypothesis (1982), spines could increase in girth when their synaptic contacts are activated. This would lead to a decrease in electrical resistance between spines and its parent dendrite with a consequent potentiation of subsequent synaptic bombardment. On the other hand, Dr. N. Tsukahara (1976) succeeded in an experiment of synaptic spontaneous turnover that occurred in the absence of tissue damage between efferent fibers from the cerebral cortex pyramidal cells and the red nucleus cells in cats.
2. Motor learning in the cerebellum-with special reference to masticatory motor learning in young rats
David Marr (1965) suggested that the cerebellum is capable of storing and executing specific sequences of actions, by means of a gradual process in which the sequences are first generated consciously while the subject is attempting to master the task, but as the result of repetition are gradually taken over by the cerebellum itself. The Purkinje cells are powerfully and specifically excited by their climbing fibers, and one important source of these fibers, via the inferior olive, is the cerebral cortex. During the initial learning phase, the Purkinje cells are driven by these climbing fibers -activated by some kind of volitional process. Every time we carry out a motor act, it necessarily results in a kind of echo that comes back to us through our sense. Each motor act generates a particular pattern of sensory feedback that will be quite specific to that particular action and to no other.This pattern will be reflected in the pattern of activity of the parallel fibers, which convey information of the most diverse kinds to the dendrites of the Purkinje cells. If we suppose that the condition for the synapses between parallel fibers and Purkinje cell (these synapses are plastic) getting stronger is that the parallel fiber should often fire at the same time as the Purkinje cell, then we have a system that will learn to recognize the context associated with a particular action, and eventually respond to it automatically by generating the action itself.
Mastication can be considered to be a learned skill (Welford, 1979) . If the Purkinje cells are interrupted their volitional inputs via the climbing fibers, what kinds of disorder would be caused in chewing movements of young rats during motor learning of mastication. In our experiments using young rats, specific chemical lesion of the inferior olive by i. p. administration of 3-acetylpyridine resulted in an increased number of chewing strokes required for breaking down a certain quantity of chow until swallowing.
3. Working memory in the prefrontal cortex
The article“Working memory and the mind, ”written by P. S. Goldman-Rakic, appeared in the Scientific American, Vol. 267, September 1992, was summarized along with several slides.