Control Of Ma Ti Cation 1





1- The idea which treat mastication as a conscious act , even as a ` learnt ` activity . The normal pattern of jaw movement is due to the activity of the higher centres , in particular the motor cortex supported by experimental works reported that stimulation of fairly localized regions in the motor cortex produces movements of the jaw , face & limbs & also lesions affecting the same areas in the human brain produce corresponding defects in voluntary action , BUT , animals were able to feed after removal of the entire cerebral cortices SO the centres subserving rhythmic jaw activity can not be in the cortex although cortex may normally exercise some control over them .



Electrical stimulation of structures such as internal capsule , limbic system & hypothalamus has been shown to elicit rhythmic jaw movements BUT even those structures are not essential as similar movements can also be elicited reflexly in the decerebrate animals ( decerebrate rabbits chewed & the human equivalent , anencephalic infant , suckling ) so one left with inescapable Conclusion that rhythmic oral activity does not necessarily arise in the higher centres although it may be influenced by them .

2- The idea which treat mastication as a series of reflexes 

Movement originating in interacting chains or sequences of reflexes which become self perpetuating . It relies upon the existence of a jaw opening reflex , elicited by mechanical stimulation of oral structures when food is taken into the mouth , which is then said to be followed by a central rebound phenomenon producing closure . Closure upon food in the mouth resulted in mechanical stimulation of the teeth or mucosa so that reflex jaw opening followed & the cycle is repeated This apparently explain the rhythmic jaw movement seen in decerebrate animal .



Jaw jerk reflex = reflex opening → stretching the muscles which close the jaw → their muscle spindles will be activated → reflex closure via monosynaptic reflex arc → results in force being exerted on food so that oral structures will be stimulated → initiate reflex opening .



One of the objection to the reflex chain concept is that reflex activation of muscle tends to be brief & of abrupt onset , whereas the regular bursts of muscle activity in mastication are of much longer duration & of gradual onset .

3- The idea which treat mastication as a rhythmic activity originates in the brain stem under the commands of high centres & from stimulation of oral receptors



The basic idea is that there is a centre for generating rhythmic activity in the brain stem ( Rhythmic generator = Pattern generator ) as it must generate the various rhythmic activities in the sequence in which the masticatory muscles have to be activated . The centre is considered to receive sensory inputs from the mouth & impulses from higher centres , either of which are capable of driving the generator into rhythmic activity . Such area or centre is close to the motor nucleus of the trigeminal nerve . It explains why rhythmic activity can be obtained on the one hand in the decerebrate animal by suitable intraoral stimulation & on the other hand by stimulation of cerebral cortex .





The afferent volleys in sensory pathways are seen as producing two quite separate central effects to drive a neuronal circuit into rhythmic ( cyclical ) activity , so that reciprocating movement of an anatomical structures are obtained by motorneurons innervating the agonist & antagonist muscles which must alternate rhythmically in their activity . If periodically active group of nerve cells has the power to inhibit another similar group of cells , rhythmic alternating firing will take place so when one group is firing , the other is having its activity depressed , is tiring or is recovering ( it has been suggested that the periodic failure to respond to excitation is due to the development of a prolonged refractory period . The group of neurons may thus be looked upon as firing , tiring & reviving ) .