The jaw jerk reflex or the masseter reflex is a stretch reflex used to test the status of a patient's

trigeminal nerve In neuroanatomy, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and chew ...

( cranial nerve V) and to help distinguish an upper cervical cord compression from lesions that are above the foramen magnum. The

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

—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 muscle In human anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it ...

s will jerk the mandible upwards. Normally this reflex is absent or very slight. However, in individuals with

upper motor neuron Upper motor neurons (UMNs) is a term introduced by William Gowers in 1886. They are found in the cerebral cortex and brainstem and carry information down to activate interneurons and lower motor neurons, which in turn directly signal muscles t ...

lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals. Types There is no designated classif ...

s 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

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...

s to the trigeminal motor nucleus, which in turn innervates the

masseter In human anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it ...

. 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 A neurological examination is the assessment of sensory neuron and motor responses, especially reflexes, to determine whether the nervous system is impaired. This typically includes a physical examination and a review of the patient's medical hist ...

. 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.

Factors affecting the reflex

Gender

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

and

temporalis In anatomy, the temporalis muscle, also known as the temporal muscle, is one of the muscles of mastication (chewing). It is a broad, fan-shaped convergent muscle on each side of the head that fills the temporal fossa, superior to the zygomatic a ...

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.

Age

Studies have shown that there has been a slight general increase in latency of this reflex with increasing age. Latency is defined here as the time taken between the chin tap to the first obvious deflection as seen on the subject. The most prominent decline in masseteric activity was recorded in patients aged 75 years and older, which might be due to the reduction in both

tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...

and

superficial Superficial may refer to: *Superficial anatomy, is the study of the external features of the body *Superficiality, the discourses in philosophy regarding social relation *Superficial charm, the tendency to be smooth, engaging, charming, slick and ...

reflexes. A study also reports that 52% of the elderly exhibit an absence of jaw jerk reflex, in an average age of 81.8 years. Jaw muscles do not display very prominent changes in muscular tissue with age in healthy elderly, as their oral cavities are in constant motor movement (i.e.: performing tasks such as talking and chewing etc.). This motion delays the decrease in lean body mass and aids protein retention that comes with age, preventing the muscular tissues from wearing and tearing.

History

American neurologist Morris Lewis (1852–1928) first described the jaw jerk. But several textbooks of neurology and clinical neurophysiology attribute discovery of the jaw jerk reflex to Armand de Watteville (1846–1925) as he correctly predicted that the jaw jerk would be valuable to detect disease affecting bulbar nuclei.

References

* Nolte, J. ''The Human Brain'', 5th ed. Mosby: Missouri; 2002, p. 307. * Blumenfeld, H. ''Neuroanatomy Through Clinical Cases''. Sinauer Associates: Massachusetts; 2002, p. 484.

Footnotes

{{Reflex Reflexes de:Eigenreflex#Masseterreflex