The jaw jerk reflex is referred to as the master reflex.
This stretch reflex is used to test the status of a patient’s trigeminal nerve (cranial nerve V).
The reflex can help distinguish an upper cervical cord compression from lesions that are above the foramen magnum.
The mandible is tapped at a downward angle just below the lips at the chin while the mouth is held slightly open.
As a response the master muscles will jerk the mandible upwards.
Normally the jaw jerk reflex is absent or very slight.
With upper motor neuron lesion the jaw jerk reflex can be pronounced.
It is a dynamic stretch reflex with a monosynaptic response.
The sensory neurons of the trigeminal mesencephalic nucleus sending axons to the trigeminal motor nucleus, which in turn innervates the master muscle.
The jaw jerk reflex tests the integrity of the upper motor neurons projecting to the trigeminal motor nucleus.
Cranial nerve V mediates the sensory and motor aspects of this reflex.
The reflex is not part of a standard neurological examination, but performed when there are other signs of damage to the trigeminal nerve.
The presence of a hyperactive positive jaw jerk reflex suggests a neurological abnormality that exists above the foramen magnum.
A normal jaw jerk reflex, as seen with a cervical spondylotic myelopathy, distinguishes the clinical presentations from multiple sclerosis or amyotrophic lateral sclerosis which have a hyperactive reflex.
There is a significant effect of gender difference on the jaw jerk reflex as measured by electromyographic impulse within the muscle, with females showed a significantly higher amplitude than males.
There has been a slight increase in latency of this reflex with increasing age.
52% of the elderly exhibit an absence of jaw jerk reflex.