Medical Neurosciences 731 Afferent Home
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NUCLEUS AMBIGUUS

This nucleus lies dorsal and lateral to the inferior olive. Cells in nucleus ambiguus contain motor neurons associated with three cranial nerves (rostral pole =C.N. IX=glossopharyngeal; middle part =C.N. X=vagus; caudal pole =C.N. XI=spinoaccessory). Axons arising from nucleus ambiguus pass laterally and slightly ventrally to exit the medulla just dorsal to the inferior olive. These axons then course with the three cranial nervesIX (glossopharyngeal), X (vagus) and XI (spinoaccessory)to innervate the striated muscles of the soft palate, pharynx, larynx, and upper part of the esophagus. Since these muscles have developed embryologically from branchial arches 3, 4 and 5, the cells that innervate them are called branchiomotor. Remember, NUCLEUS AMBIGUUS is “SHARED” by three cranial nerves (IX, X, XI). The general pattern of motor innervation below does not have to be memorized.

IX stylopharyngeus muscle

X palatal muscles; levator veli palatini (with assistance from V for the tensor veli palatini; of little clinical significance), most of the pharyngeal muscles (with assistance from IX), laryngeal muscles and striated muscles of the esophagus-palatoglossus too!

XI laryngeal muscles (cranial portion)

THINK SOFT PALATE, PHARYNX, LARYNX



A unilateral lesion of nucleus ambiguus will result in atrophy and paralysis of all palatine muscles ipsilateral to the lesion, except the tensor veli palatini (C.N. V). Because of the palate paralysis, the patient’s speech may be nasal. This is because air is allowed to escape into the nose during speaking. Normally, the soft palate elevates in order to reduce the nasopharyngeal aperture during speaking. This elevation of the soft palate detours the air through the mouth, the path of least resistance. Due to the hemiplegic palate the patient may complain of nasal regurgitation of liquids since he/she is unable to shut off completely the nasopharynx from the buccal cavity. Moreover, during phonation (say ahhh!) the soft palate is elevated on the normal side and the UVULA DEVIATES TOWARDS THE NORMAL SIDE (contralateral to the lesion; contrast this with lesions of the hypoglossal nucleus). Remember from Gross Anatomy that the levator veli palatini raises the soft palate and, in doing so, also pulls it backward. Also, some awkwardness of swallowing, called dysphagia, may occur due to the unilateral paralysis of the constrictors of the pharynx. Due to paralysis of the laryngeal muscles, the patient exhibits dysphonia, his/her voice being husky or hoarse (speech requires phonation by the vocal cords; phono=voice, sound).

Bilateral lesions of nucleus ambiguus increase the difficulties I have just described following ipsilateral lesions. Nasal regurgitation is more distressing and permanent. Dysphagia is more pronounced and speech and respiratory disorders may be profound. Respiratory disorders, induced by the paralysis of the abductor muscles (of the larynx) bilaterally may lead to suffocation unless treated by intubation.





Corticobulbar fibers (you can voluntarily swallow!) to nucleus ambiguus are BILATERAL (both crossed and uncrossed). Therefore, muscles supplied by the nucleus ambiguus are NOT noticeably weakened in the event of unilateral lesions of the corticobulbar system (i.e., in the motor cortex). This means that there is NO deviation of the uvula following cortical lesions. Don’t confuse the results of lesions of the corticobulbar projection to nucleus ambiguus with lesions of nucleus ambiguus! Also don’t confuse the bilateral corticobulbar input to nucleus ambiguus with the primarily CROSSED corticobulbar input to the HYPOGLOSSAL nucleus. What it boils down to is that BILATERAL corticobulbar input is GREAT for you as students, since you don’t have to remember which way something deviates following its interruption. It is only those corticobulbar projections that are not equally bilateral (so far only that to the HYPOGLOSSAL, but more to come) that you need to worry about.

Motor fibers of C.N. XI that arise from the nucleus ambiguus join the vagus outside of the skull and innervate muscles of the larynx (recurrent [inferior] vagus). These fibers comprise the CRANIAL branch of C.N. XI. REMEMBER: CRANIAL XI=AMBIGUUS. In contrast, the SPINAL portion of C.N. XI consists of motor axons whose cell bodies lie in the lateral part of the ventral horn of the first five or six cervical SPINAL CORD segments. The axons of these cells pass dorsal and laterally (that is they do not exit via the ventral root), leave the spinal cord between the dorsal and ventral roots and unite to ascend in the spinal canal to enter the skull via the foramen magnum. They then exit the skull via the jugular foramen along with cranial nerves IX and X and eventually innervate the sternocleidomastoid and the upper fibers of the trapezius. REMEMBER: CAUDAL XI=SPINAL CORD.



Lesions involving C.N. XI fibers to these two muscles result in atrophy of the muscles. Since the RIGHT sternocleidomastoid rotates the head to the LEFT (opposite), a lesion of the RIGHT C.N. XI will result in the chin being turned slightly to the RIGHT (paralyzed) side, especially when the head is flexed. The same RIGHT side lesion will result in paralysis of the RIGHT upper trapezius and slight sagging of the RIGHT shoulder.



As for cortical input to the cells of origin of the spinal part of XI, you are lucky since it is bilateral.

CORTICOBULBAR TO SPINAL PART XI=BILATERAL=GOOD=TAKE A BREAK!!!!



The normal gag reflex is a mass contraction of both sides of the posterior oral and pharyngeal musculature and an indication by the patient of an unpleasant experience. Sensory information (painful) comes into the brain stem over C.N.s IX and X (cell bodies in superior ganglia), enters the spinal tract V and terminates in caudal spinal nucleus V. Cells in spinal nucleus V then project bilaterally to nucleus ambiguus (we cannot identify these axons in our sections, but they travel over the TTT). The contractions of the pharyngeal musculature ipsilateral to the stimulus is called the DIRECT response, while the contractions of the musculature contralateral to the stimulus is called the CONSENSUAL response (consensus=agreement). Don’t forget that with a lesion of nucleus ambiguus the efferent or motor part of the GAG REFLEX is lost IPSILATERAL to the lesion. Sensory stimulation from the soft palate and pharynx can reach spinal nucleus V (via C.N.s IX and X; superior ganglia), and, via the TTT, both nuclei ambiguui. However, there is contraction of only the muscles innervated by cells in the “alive” nucleus ambiguus. Look at the diagram and contrast the effects of lesions involving (1) C.N.s IX and X, (2) caudal spinal nucleus V and (3) nucleus ambiguus. Also, do the practice questions on the next page. Good luck!





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