IX,X,XI,XII CRANIAL NERVES

 

Glossopharyngeal nerve (IX cranial nerve)

Special visceral efferent (SVE), general visceral efferent (GVE), general visceral afferent (GVA) and special visceral afferent (SVA)

 

The glossopharyngeal nerve leaves the brain stem at the level of the medulla oblongata, caudally to the trapezoid body.

 

The parasympathetic preganglionic fibers of the glossopharyngeal nerve form the tympanic nerve (GVE). This nerve enters the middle ear to form the tympanic plexus. Fibers from this plexus form the minor petrosal nerve that leaves the skull through a small foramen (located dorsocaudally to the oval foramen) to reach the otic ganglion. The parasympathetic postganglionic fibers join the auriculotemporal and buccal nerves (both are branches of the mandibular nerve) to innervate the parotid and zygomatic salivary glands respectively.

 

The remaining fibers of the glossopharyngeal nerve exit the cranial cavity through the jugular foramen and, then, through the tympano-occipital fissure along with the vagus and accessory nerves. The glossopharyngeal nerve sends a sensory branch (GVA) to the carotid sinus and carotid body. Subsequently, it divides into lingual and pharyngeal branches.

 

The lingual branch (GVE, GVA and SVA) innervates the tonsils and the root of the tongue. The pharyngeal branch and the vagus nerve form the pharyngeal plexus, which innervates the muscles and mucosa of the pharynx. This plexus is composed of motor fibers (SVE and GVE) and sensory fibers (GVA), and is joined by sympathetic postganglionic fibers from the cranial cervical ganglion.

 

In the dog, the soma of the sensory neurons is located in a ganglion at the level of the jugular foramen.

 

Vagus nerve (X cranial nerve)

Special visceral efferent (SVE), general visceral efferent (GVE), general visceral afferent (GVA), special visceral afferent (SVA) and general somatic afferent (GSA)

 

The vagus nerve leaves the brain stem at the level of the medulla oblongata, caudal to the glossopharyngeal nerve.

 

Most of the vagal fibers are visceral sensory (GVA). The rest are made up of visceral motor (GVE), special visceral motor (SVE), sensory to the taste buds of the epiglottis (SVA) and somatic sensory (GSA) to the ear canal.

 

The vagus nerve leaves the cranial cavity through the jugular foramen and, then, through the tympano-occipital fissure. At the level of the jugular foramen it sends a sensory auricular branch (GSA) to join the facial nerve in order to reach the skin of the ear canal through the lateral internal auricular nerve.

 

The proximal vagal ganglion (also named jugular ganglion) is located at the level of the jugular foramen. It is formed by the soma of the somatic afferent neurons (GSA). The soma of the GVA and SVA neurons is located at the distal vagal ganglion (also named nodose ganglion). This is situated outside the tympano-occipital fissure and topographically related with the glossopharyngeal, accessory and hypoglossal nerves, and the cranial cervical ganglion.

 

The pharyngeal ramus (SVE, GVE and GVA) leaves the vagus nerve at the level of the distal vagal ganglion. It is joined by fibers from the glossopharyngeal ramus and sympathetic postganglionic fibers (from the cranial cervical ganglion) to form the pharyngeal plexus. This innervates the pharyngeal muscles and the cranial part of the esophagus.

 

The cranial laryngeal nerve branches off from the vagus nerve also at the level of the distal vagal ganglion. It divides into an external ramus and an internal ramus. The external ramus (SVE) innervates the cricothyroideus muscle. The internal ramus (GVE and GVA) innervates the mucosa of the larynx, rostral to the vocal folds, and the taste buds of the epiglottis (SVA).

 

The vagus nerve descends into the thoracic cavity together with ascending sympathetic fibers, forming the vagosympathetic trunk. At the level of the middle cervical ganglion, the vagus nerve leaves the sympathetic fibers and sends out cardiac branches formed by parasympathetic preganglionic fibers (GVE) and sensory fibers (GVA). On the left side, the nerve forms the left recurrent laryngeal nerve that passes around the aortic arch. On the right side, the right recurrent laryngeal nerve turns around the right subclavian artery. Both recurrent laryngeal nerves run cranially becoming the left and the right caudal laryngeal nerves. They innervate the trachea (GVE and GVA), the esophagus (SVE, GVE and GVA), and the laryngeal muscles (SVE), except for the cricothyroid muscle that is innervated by the external ramus of the cranial laryngeal nerve. The laryngeal nerves also innervate the mucosa of the larynx caudal to the vocal folds (GVE and GVA). In its course the recurrent laryngeal nerve runs parallel to an smaller dorsally located nerve, the pararecurrent laryngeal nerve. This nerve sends branches to the trachea and esophagus.

At the middle mediastinum, the vagus nerve sends branches to the bronchi, esophagus and heart. Just caudal to the bronchi, each left and right vagus nerves form a dorsal and a ventral branch. The dorsal branches (left and right) unite, as do the ventral ones, to form the dorsal and ventral vagal trunks respectively. In the abdominal cavity, the dorsal vagal trunk joins the aortico-abdominalis plexus so the fibers may follow the vessels to reach the viscera. The ventral vagal trunk innervates the lesser curvature of the stomach and the liver. These fibers are made of preganglionic parasympathetic (GVE) and visceral sensory (GVA) neurons.

 

Although the pharyngeal and laryngeal muscles were considered lacking of spindles, spindle-like receptors to stretch have been found indicating the possibility of GSA fibers for proprioception in the glossopharyngeal and vagus nerves.

 

Accessory nerve (XI cranial nerve)

General somatic efferent (GSE) and special visceral efferent (SVE)

 

The accessory nerve consists of cranial and spinal roots. The neurons that form the cranial roots have their bodies in the nucleus ambiguus in the medulla oblongata. The neurons of the spinal roots have their bodies in the motor nucleus of the accessory nerve, located in the dorsolateral portion of the ventral horn of the cervical spinal cord segments (from C1 to C7).

 

The spinal roots leave the spinal cord between the dorsal and ventral roots of the cervical spinal nerves, and lie dorsal to the denticulate ligament (under the dorsal spinal roots). They run rostrally to enter the cranial cavity, through the foramen magnum, and join the cranial roots. These leave the medulla oblongata vetrolaterally. Cranial and spinal accessory nerve roots unite to become the external branch of the accessory nerve (GSE). Some fibers from the cranial roots join the vagus nerve to form the internal branch of the accessory nerve. This branch becomes part of the recurrent laryngeal nerve (SVE).

 

Upon exiting the cranial cavity through the jugular foramen and, then, through the tympano-occipital fissure, the external branch of the accessory nerve descends to innervate the trapezius, omotransversarius, sternocephalicus and cleidocephalicus muscles. The latter two muscles are also innervated by ventral branches of the cervical spinal nerves.

 

As there are communications between the accessory nerve and the cervical spinal nerves, it has been proposed that the soma of sensory cells (GSA) for proprioception may be located in the dorsal root ganglia.

 

Hypoglossal nerve (XII cranial nerve)

General Somatic efferent (GSE)

 

The hypoglossal nerve is motor to muscles of the tongue (intrinsic and extrinsic) and to the genihyoideus and thyrohyoideus muscles. It leaves the medulla oblongata laterally to the pyramids by means of several rootlets. The nerve exits the skull through the hypoglossal canal and goes ventrorostrally, lateral to the external carotid artery, to run parallel to the lingual artery. In its course, the hypoglossal nerve communicates with the ventral branch of the first cervical spinal nerve forming, the cervical loop (ansa cervicalis).

 

Although the hypoglossal nerve is considered a motor nerve, there are afferents that respond to stretch. As this nerve has no sensory ganglion, it has been proposed that the soma of the afferent neurons may be located at the dorsal root ganglion of C1 (the connection will be through the ansa cervicalis).