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The Lowest Four Cranial Nerves

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The Lowest Four Cranial Nerves The lowest four cranial 15 nerves HYPOGLOSSAL NERVE of the spinal cord. The nucleus of origin is a column of a and g motorneurons in the basolateral anterior gray The hypoglossal nerve (cranial nerve XII) contains somatic horn. efferent fibers for the supply of the extrinsic and intrinsic The nerve runs upward in the subarachnoid space, behind muscles of the tongue. Its nucleus lies close to the midline in the denticulate ligament. It enters the cranial cavity through the floor of the fourth ventricle and extends almost the full the foramen magnum and leaves it again through the jugular length of the medulla (Figure 15.1). The nerve emerges as a foramen. While in the jugular foramen, it shares a dural series of rootlets in the interval between the pyramid and the sheath with the cranial accessory nerve, but there is no olive. It crosses the subarachnoid space and leaves the skull exchange of fibers (Figure 15.3). Upon leaving the cranium, through the hypoglossal canal. Just below the skull, it lies it crosses the transverse process of the atlas and enters the close to the vagus and spinal accessory nerves (Figure 15.2). It sternomastoid muscle, in company with twigs from roots C2 descends on the carotid sheath to the level of the angle of the and C3 of the cervical plexus. It emerges from the posterior mandible, then passes forward on the surface of the hyo- border of the sternomastoid and crosses the posterior trian- glossus muscle where it gives off its terminal branches. gle of the neck to reach the trapezius. It pierces the tra- In the neck, proprioceptive fibers enter the nerve from the pezius in company with twigs from roots C3 and C4 of the cervical plexus, to accept afferents from about 100 muscle cervical plexus. In the posterior triangle, the nerve is vulner- spindles in the same half of the tongue. able, being embedded in prevertebral fascia and covered only by investing cervical fascia and skin. Phylogenetic note The spinal accessory nerve provides the extrafusal and In reptiles, the lingual muscles, the geniohyoid muscle, and intrafusal motor supply to the sternomastoid and trapezius. the infrahyoid muscles develop together from the uppermost The branches from the cervical plexus are proprioceptive in mesodermal somites. The somatic efferent neurons supply- function to the sternomastoid and to the craniocervical part ing this hypobranchial muscle sheet form a continuous ribbon of the trapezius. The thoracic part of the trapezius, which of cells extending from lower medulla to the third cervical arises from the spines of all the thoracic vertebrae, receives spinal segment. In mammals, the hypoglossal nucleus is its proprioceptive innervation from the posterior rami of located more rostrally and its rootlets emerge separately the thoracic spinal nerves. Some of the afferents supplying from the cervical rootlets. However, the caudal limit of the muscle spindles in the thoracic trapezius do not meet up hypoglossal nucleus remains linked to the cervical motor cell with the fusimotor supply before reaching the spindles. This column by the supraspinal nucleus, from which the thyrohy- is the only instance, in any muscle known, where the fusimo- oid muscle is supplied via the first cervical ventral root. In tor and afferent fibers to some spindles travel by completely rodents, some of the intrinsic muscle fibers of the tongue independent routes. receive their motor supply indirectly, from axons which leave the most caudal cells of the hypoglossal nucleus and emerge in the first cervical nerve to join the hypoglossal nerve trunk GLOSSOPHARYNGEAL, VAGUS, AND in the neck. Whether this arrangement holds for primates is CRANIAL ACCESSORY NERVES not yet known. Especially relevant to nerves IX, X, and cranial XI are the soli- Supranuclear supply to the tary nucleus and the nucleus ambiguus. The solitary nucleus hypoglossal nucleus extends from the lower border of the pons to the level of the gracile nucleus. Its lower end merges with its opposite The hypoglossal nucleus receives inputs from the reticular number in the midline; hence the term commissural formation, whereby it is recruited for stereotyped motor rou- nucleus for the lower part of the solitary nucleus. tines in eating and swallowing. For delicate functions includ- Anatomically, the nucleus is divisible into eight ing articulation, most of the fibers from the motor cortex parts. Functionally, four regions have been clarified (Figure cross over in the upper part of the pyramidal decussation; 15.4): some remain uncrossed and supply the ipsilateral hypoglos- sal nucleus. 1 The uppermost region is the gustatory nucleus, which Supranuclear, nuclear, and infranuclear lesions of the receives primary afferents supplying taste buds in the hypoglossal nerve are described together with lesions of the tongue and palate. accessory nerve (see Clinical Panels 15.1–15.3). 2 The lateral midregion is the dorsal respiratory nucleus SPINAL ACCESSORY NERVE (see Ch. 19). 3 The medial midregion is the baroreceptor nucleus, The spinal accessory nerve (cranial nerve XI) is a purely which receives the primary afferents supplying blood motor nerve attached to the uppermost five segments pressure detectors in the carotid sinus and aortic arch. 155 15 THE LOWEST FOUR CRANIAL NERVES Dorsal nucleus of vagus Nucleus ambiguus Inferior salivatory nucleus Hypoglossal nucleus IX IX IX X X Cranial XI Spinal XI Spinal nucleus of V Accessory nerve XII Solitary nucleus Lateral cell column of anterior horn of spinal cord A B CD Figure 15.1 (A) Sensory nuclei (left) and motor nuclei (right) serving cranial nerves IX–XII. (B) shows the special visceral efferent cell column giving a contribution to the glossopharyngeal nerve and forming the cranial accessory nerve. (C) shows the general visceral efferent cell column contributing to the glossopharyngeal and vagus nerves. (D) shows the somatic efferent cell column giving rise to the hypoglossal nerve. Internal carotid nerve and artery Spinal XI IX Superior cervical sympathetic Taste buds ganglion Sternomastoid X Trapezius XII Middle pharyngeal constrictor Figure 15.2 Semischematic illustration of the lowest four cranial nerves and the internal carotid branch of the superior cervical ganglion. ICA, internal carotid artery. 4 The most caudal region, including the commissural From the nucleus ambiguus, special visceral efferent nucleus, is the major visceral afferent nucleus of the fibers supply the constrictor muscles of the pharynx, brainstem. It receives primary afferents supplying the stylopharyngeus, levator palati, intrinsic muscles of 156 alimentary tract and respiratory tract. the larynx, and (via the recurrent laryngeal nerve) the GLOSSOPHARYNGEAL, VAGUS, AND CRANIAL ACCESSORY NERVES 15 striated muscle of the upper one-third of the eling to five separate afferent nuclei in the brainstem. The esophagus. largest of its peripheral territories is the oropharynx, which is bounded in front by the back of the tongue; hence the Glossopharyngeal nerve name for the nerve. The glossopharyngeal rootlets are attached behind the The glossopharyngeal nerve is almost exclusively sensory. It upper part of the olive. The nerve accompanies the vagus carries no less than five different kinds of afferent fibers trav- through the anterior compartment of the jugular foramen (the posterior compartment contains the bulb of the inter- nal jugular vein). Within the foramen, the nerve shows small superior and inferior ganglia; these contain unipolar sensory neurons. Immediately below the skull, the glossopharyngeal is in the company of three other nerves (Figure 15.2): the vagus, the spinal accessory, and the internal carotid (sympathetic) branch of the superior cervical ganglion. Together with the Nucleus ambiguus stylopharyngeus, it slips between the superior and middle constrictor muscles to reach the mucous membrane of the oropharynx. Functional divisions and branches • Before emerging from the jugular foramen, the IX nerve Cranial XI gives off a tympanic branch which ramifies on the Spinal XI tympanic membrane and is a potential source of referred pain (see later). The central processes of the XI tympanic branch synapse in the spinal nucleus of the trigeminal nerve (Figure 15.1). Wall of jugular • Some fibers of the glossopharyngeal tympanic branch foramen are parasympathetic. They pierce the roof (tegmen tympani) of the middle ear as the lesser petrosal nerve, IX leave the skull through the foramen ovale, and synapse in the otic ganglion. Postganglionic fibers supply secretomotor fibers to the parotid gland (Figure 10.3). X • The branchial efferent supply to the stylopharyngeus Pharyngeal branch comes from the nucleus ambiguus. Superior laryngeal • Branches serving ‘common sensation’ (touch) supply the branch Lateralmost anterior mucous membranes bounding the oropharynx (throat), Recurrent laryngeal gray horn including the posterior one-third of the tongue. The branch To sternomastoid neurons synapse centrally in the commissural nucleus. To trapezius The glossopharyngeal branches provide the afferent limb of the gag reflex – contraction of the pharyngeal Figure 15.3 Course and distribution (in red) of special visceral constrictors in response to stroking the wall of the efferent fibers derived from the nucleus ambiguus. oropharynx. (The gag reflex is unpleasant because of Geniculate Gustatory nucleus ganglion (VII) Dorsal respiratory nucleus Gustatory IX Baroreceptor nucleus Respiratory Baroreceptor X Visceral Commissural nucleus Respiratory Figure 15.4 Functional Baroreceptor composition of
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