Jaw jerk reflex

The jaw jerk reflex or the masseter reflex is a stretch reflex used to test the status of a patient's trigeminal nerve (CN V) and to help distinguish an upper cervical cord compression from lesions that are above the foramen magnum. The mandible—or lower jaw—is tapped at a downward angle just below the lips at the chin while the mouth is held slightly open. In response, the masseter muscles will jerk the mandible upwards. Normally this reflex is absent or very slight. However in individuals with upper motor neuron lesions the jaw jerk reflex can be quite pronounced.

The jaw jerk reflex can be classified as a dynamic stretch reflex. As with most other reflexes, the response to the stimulus is monosynaptic, with sensory neurons of the trigeminal mesencephalic nucleus sending axons to the trigeminal motor nucleus, which in turn innervates the masseter. This reflex is used to judge the integrity of the upper motor neurons projecting to the trigeminal motor nucleus. Both the sensory and motor aspects of this reflex are through CN V.

It is not part of a standard neurological examination. It is performed when there are other signs of damage to the trigeminal nerve.

The clinical presentation of cervical spondylotic myelopathy can be similar to multiple sclerosis (MS) or amyotrophic lateral sclerosis (ALS), however, a hyperactive jaw reflex suggests the pathology is above the foramen magnum. In other words, a normal jaw jerk reflex points the diagnosis toward cervical spondylotic myelopathy and away from MS or ALS.

Studies have shown that there is a significant effect of gender on the jaw jerk reflex. Electromyographs are used to measure the impulse within the muscle, allowing the amplitude of the impulse to be known and shown on a graph. The ECMs were focussed on the masseter muscle and temporalis muscle. Females showed a significantly higher amplitude – meaning that the impulse was larger – than males. This should be taken into account when interpreting ECM results, as a female’s graphs will normally show a higher peak to peak amplitude than a male. The mean latency of the impulse was also found to be shorter in females than in males. This variation in women appears to be constant, and is not affected by the menstrual cycle.

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