39. Jaw Jerk Reflex

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What is Being Tested?

Facial sensation can be impaired by lesions of the trigeminal nerve (CN V), the trigeminal sensory nuclei in the brainstem, or ascending sensory pathways to the thalamus and somatosensory cortex in the postcentral gyrus (see Neuroanatomy Through Clinical Cases 3e, Figures 7.9A,B and 12.8). Extinction in the presence of intact primary sensation is usually caused by right parietal lesions.

The corneal blink reflex is mediated by polysynaptic connections in the brainstem between the trigeminal (CN V) and facial (CN VII) nerves and can be impaired by lesions anywhere in this circuit (see Neuroanatomy Through Clinical Cases 3e, KCC 12.4).

Weakness of the muscles of mastication can be due to lesions in the upper motor neuron pathways synapsing onto the trigeminal (CN V) motor nucleus, in the lower motor neurons of the trigeminal motor nucleus in the pons, in the peripheral nerve as it exits the brainstem to reach the muscles of mastication, in the neuromuscular junction, or in the muscles themselves.

Presence of a jaw jerk reflex is abnormal, especially if it is prominent. It is a sign of hyperreflexia associated with lesions of upper motor neuron pathways projecting to the trigeminal motor nucleus. Both the afferent and the efferent limbs of the jaw jerk reflex are mediated by CN V (see Neuroanatomy Through Clinical Cases 3e, KCC 12.4).