THE NEW IN THE WORLD OF MASTICATORY MUSCLES PHYSIOLOGY AND FUNCTIONAL ANATOMY
Masticatory function is defined by the complex interaction of masticatory musculature, temporal-mandibular joint, teeth and nervous system during biting, jewing, swallowing and speech. Maticatory muscles comprise temporal one, masseter, medial pterygoid and lateral pterygoid paired muscles.
Masseter and medial pterygoid muscle serve first of all as an origin of powerful tension while the temporal and lateral pterygoid muscle are responsible for mandible stabilization. Masticatory musculature functions in a complex with epihyoideal and subhyoideal muscles as well as with the ones of tongue, lips and cheeks. Neck muscles also influence indirectly on the stabilization, participate into the head position changing during the mastication.
Muscular mass is maintained due to physical activity as well as due to natural steroid hormones and growth hormone. Insufficient activity leads to muscular hypotrophy while the training results in hypertrophy; muscular fibers change their length at this. Muscular tissue enzymes react to the releasing energy linked with aerobic and anaerobic loading. Capillaries get adapted to the activity level as well. Characteristics and functional peculiarities of masticatory muscles differ from the ones of the extremities muscles because masticatory musculature has better capillary support and other organization of fibers. The I-st-typed fibers responsible for durable contractions (slow) and resilient to the fatigue are dominant in masticatory muscles. Durable activity and muscles powerful contractions, without resting periods, result in intramuscular tension increasing and then in local ishemy, increased membranous permeability, edema and even in the cellular damage. Besides, there re easy edema and hyperemy after the loading to the healthy muscles even at chewing gum usage. They consider that masticatory muscles insufficient power can be the predisposition factor for their “wearing”. Muscular tissue decomposition can lead both to fibrosis and to muscular fibers regeneration from the cells-satellites (inactive myoblasts) which are also participate into muscles growth. The biggest masticatory muscles are the ones rising mandible, especially the masseter and the temporal one. Masticatory force depends on their activity level; the muscle width, the fibers size and location are also valuable. Male masticatory muscles are stronger than the female ones but the force also depends on the age and occlusive contacts. There is a link between masticatory force and face morphology: the more powerful the muscles are, the less is face vertical height and mandibular angle. Thus, weak masticatory muscles are the prolonged faces characteristics while the stronger ones – for the square ones. If masticatory muscles activity is lowered because of the diminished necessity in mastication, teeth loss or constant pain, their hypotrophy can develop. And, on the contrary, masticatory muscles excessive activity for instant at night bruxism results in their hypertrophy.
Mouth closage id est mandible lifting appears due to two-sided symmetric activity of masseters, temporal and medial pterygoid muscles though muscles work non-symmetrically, with their bigger activity at the working side during the mastication. The gravitation action to mandible is equaled with the temporal muscles positive tone. They say that the latest ones play important role in the mandible positioning in the space. Also masseters and medial pteryoid muscles are activated at the teeth closage. Mouth opening or mandible lowering is performed due to suprahyoid muscles (digastric muscle anterior belly, omy-hyoid and mylohyoid one) with the lateral pterygoid muscles participance. Suprahyoideal muscles are attached to the mandible and hyoid bone. When hyoid bone is fixed due to subhyoid muscles than suprahyoid muscles can participate in mandible lowering. Mandible symmetric protrusion (movement forward) is achieved by lateral pterygoid muscles two-sided action. Mandible retrusion (movement back) is performed by temporal muscles posterior part, suprahyoid muscles and masseter deep fibers. Laterotrusion (mandible shift on the right or on the left) takes place at contralateral pterygoid muscle contraction as well as ipsylateral lateral pterygoid muscle contraction. Though laterotrusion is usually combined to the protrusion on opposite side while antero-lateral movement forming.
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