jaw

jaw The ‘jaw’, in everyday language and in descriptions of facial characteristics, commonly refers to the lower jaw that gives the chin its shape. The word is derived from the Anglo-Saxon for ‘chew’ and is properly applied to both participants in this function: the upper as well as the lower jaw. Jaw movements are fundamental to the processes of both eating and speaking.

In mammals, the upper jaw or maxilla is one of the bones forming the facial part of the skull; the lower jaw or mandible forms a joint with the skull (temporo-mandibular joint) and is suspended by a set of jaw-closing muscles, attached to the temple and to the cheek bone as well as to the maxilla. Forceful jaw closure is a function of these muscles; the masseter that runs from the cheek bone to the angle of the jaw can easily be felt bulging and hardening when the teeth are clenched.

Below the mandible on each side there is a Y-shaped set of muscles. The two upper arms of the ‘Y’ are attached respectively to the mandible in front and to the base of the skull above and behind. The stalk of the Y is attached to the sternum (breastbone) below. The small U-shaped hyoid bone, which lies below the mandible at the base of the tongue, is at the junction of the three arms of the ‘Y’ on each side. In effect both mandible and hyoid are suspended within a chain of muscles. Lowering of the jaw is produced by contraction of the muscles forming the front upper limb of the ‘Y’ only if the other muscles of the ‘Y’ are contracting and thus fixing the hyoid; if the other muscles of the ‘Y’ are relaxed, the hyoid will be moved forwards.

The posture of the jaw at rest depends on the length of the jaw elevator muscles, and the factors determining this are similar to those controlling posture in the body generally. Muscle length depends upon the operation of muscle-length detecting receptors (muscle spindles) that reflexly cause a proportionate muscle contraction if the muscle is stretched. Jaw-closing muscle tone is thus maintained so that, while awake, gravity does not pull the mouth open. The muscles passing from the mandible to the hyoid and to the tongue are of importance in maintaining tongue posture and airway patency, so jaw position is a factor in keeping the airway open. The converse is that restriction of the nasal airway results in a mouth-open posture.

Conscious or voluntary jaw movement is produced by activity in the primary motor cortex that projects directly to the motor neurons of the jaw muscles in the brain stem. The automatic, rhythmic jaw movements of feeding are, however, produced by a central pattern generator (CPG) in the brain stem that can be activated by pathways descending from the motor cortex and/or by sensory input to the brain stem from the mouth. The activity of the CPG then produces a sequential activation of the motor neurons of the jaw opening and closing muscles as well as activation of other muscles involved in feeding.

In normal function, the movements of the jaw vary directly in magnitude with the hardness of the ingested food while the extent of tongue movement varies inversely. Consequently, as hard food is converted into a soft bolus in the mouth, the amplitude of jaw movements reduces while the amplitude of tongue/hyoid movement increases. There is, consequently, evidence for sensory feedback from the mouth influencing the expression of the activity of the CPG.

The teeth are supported in their sockets by the periodontal ligament that transmits the jaw-closing force to the teeth. Sensory nerve endings in this ligament detect the loading and produce neural signals proportionate to the load. These signals cause reflex inhibition of jaw-closing motoneurons and have been considered to represent a device for preventing excessive forces being applied to any individual tooth.

Allanz Thexton

See also movement, control of; pharynx; skull; teeth; tongue.alimentary system.