A 49-year-old woman comes to the office due to muscle weakness. For the past 3 months, the patient has had worsening weakness in her right hand. She has also been tripping. The patient experiences twitching and cramping in her arms and legs. She has no sensory deficits. On examination, there is increased tone and fasciculations in the lower extremity muscles. Atrophy of the interosseus muscles of the right hand and the bilateral intrinsic muscles of the feet is present. When the physician taps on the chin while the mouth is slightly open, the jaw jerks upward briskly. Which of the following cranial nerves mediate this reflex?

Question

Quizplus · Accepted Answer

11ec160b_35ce_0c82_9249_47f3ac9061b4_MD0001_00 This patient with findings of both upper (spasticity, increased tone) and lower (atrophy, fasciculations) motor neuron lesions most likely has amyotrophic lateral sclerosis (ALS). Tapping on the chin with the mouth slightly open causes the jaw to briskly close, which represents a positive (ie, pathologic) jaw jerk reflex. This reflex is normally absent or very weak but is present in patients with bilateral upper motor neuron lesions such as ALS. The jaw jerk reflex begins with afferent signals from muscle spindles of the masseter that are carried along the mandibular division of the trigeminal nerve (CN V3) to the trigeminal mesencephalic nucleus, which is involved with reflex proprioception of the muscles of mastication. This center then coordinates with the trigeminal motor nucleus to send motor (efferent) signals back along CN V3 to the muscles of mastication, causing the jaw to close. The jaw jerk reflex is the only tendon reflex mediated above the plane of the foramen magnum. Therefore, the pathologic presence of this reflex suggests bilateral lesions of the upper motor neuron pathways above the level of the pons (eg, ALS, multiple sclerosis, pseudobulbar palsy). (Choices A and B) Sensory fibers of the glossopharyngeal nerve (CN IX) are involved in both the gag and the carotid sinus reflexes. The gag reflex is mediated via afferent information from the glossopharyngeal nerve (CN IX), and efferent information from the vagus nerve (CN X). The carotid sinus reflex is mediated by afferent signals from arterial baroreceptors which are carried to the brainstem by CN IX and CN X; efferent signals are then carried by parasympathetic fibers to the sinoatrial and atrioventricular nodes. (Choice C) The corneal reflex is mediated by afferent fibers from the ophthalmic division of the trigeminal nerve (CN V1) and efferent fibers of the facial nerve (CN VII) projecting to the orbicularis oculi muscle, causing a blink. (Choice E) Both the afferent and the efferent limbs of the cough reflex are mediated by the vagus nerve (CN X). Educational objective: The jaw jerk reflex is a pathologic cranial nerve reflex present in patients with bilateral upper motor neuron lesions, such as amyotrophic lateral sclerosis. Both the afferent and efferent pathways are mediated by the mandibular division of the trigeminal nerve (CN V3).

A 49-Year-Old Woman Comes to the Office Due to Muscle