Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Functional Anatomy of the Peripheral Nervous System
Cranial Nerves

Cranial Nerves are the nerves connected anatomically and functionally with the Brain. There are 12 pairs of cranial nerves.

Pair I consists of the olfactory nerves, nn. olfactorii, which are represented by several bundles (15–20) originating from the olfactory region of the nasal mucosa. Composed entirely of sensory nerve fibers, they pass into the cranial cavity through the cribriform plate of the Ethmoid bone and terminate in the olfactory bulb. Their function is to conduct impulses to the rhinencephalon.

Pair II is the Optic nerve, n. opticus, a large trunk (up to 4–5 mm in diameter) originating from the Eyeball (in the region of the blind spot). It enters the cranial cavity through the optic canal. This nerve terminates in the optic chiasm, which unites the trunks of the right and left sides. This pair of cranial nerves consists exclusively of sensory fibers. They transmit impulses from the retina to the brain (see Fig. 14.2).

Pair III is the Oculomotor nerve, n. oculomotorius, which is mixed in composition, being formed by motor and autonomic fibers. It enters the Orbit via the superior orbital fissure. Its nuclei are located in the Midbrain. The motor fibers innervate the superior, inferior, and medial rectus Muscles, the inferior oblique Muscle of the eyeball, and the levator palpebrae superioris muscle. Autonomic preganglionic parasympathetic fibers pass (Fig. 15.2) to the ciliary ganglion. Postganglionic fibers innervate the ciliary muscle and the sphincter pupillae muscle.

Pair IV is the Trochlear nerve, n. trochlearis, which is a motor nerve. Its Nucleus is located in the midbrain. It enters the orbit through the superior orbital fissure and innervates the superior oblique muscle of the eyeball.

Pair V is the Trigeminal nerve, n. trigeminus, which is mixed in composition, possessing both motor and sensory nuclei. The motor nucleus is located in the Pons, while the sensory nuclei are situated in the Spinal Cord, pons, and midbrain.

The trunk of the trigeminal nerve emerges from the pons. After passing through the sensory (gasserian) ganglion, it divides into three branches: the ophthalmic, maxillary, and mandibular nerves (see Fig. 15.2), each of which sends fibers to the Meninges.

The ophthalmic nerve enters the orbit through the superior orbital fissure and gives off exclusively sensory branches: the lacrimal nerve, providing sensory innervation to the lacrimal gland; the nasociliary nerve, innervating the mucosa of the Nasal cavity and Paranasal Sinuses, as well as the conjunctiva of the eyeball; and the frontal nerve, innervating the Skin of the face above the palpebral fissure—the forehead, ROOT of the Nose, and upper eyelid.

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Fig. 15.2. Oculomotor, trigeminal, facial, and glossopharyngeal nerves (schematic diagram):

1 — lacrimal gland; 2 — frontal nerve; 3 — lacrimal nerve; 4 — nasociliary nerve; 5 — ciliary ganglion; 6 — ophthalmic nerve; 7 — oculomotor nerve; 8 — trigeminal ganglion; 9 — trigeminal nerve; 10 — midbrain; 11 — pons; 12 — Medulla Oblongata; 13 — Facial Nerve; 14 — Glossopharyngeal nerve; 15 — parotid plexus of the facial nerve; 16 — greater petrosal nerve; 17 — chorda tympani; 18 — tympanic nerve; 19 — lingual Branches of the glossopharyngeal nerve; 20 — otic ganglion; 21 — maxillary nerve; 22 — mandibular nerve; 23 — lingual nerve; 24 — submandibular ganglion; 25 — pterygopalatine ganglion; 26 — infraorbital nerve; 27 — Tongue; 28 — Cytology/practical/98.html">Submandibular salivary gland; 29 — sublingual salivary gland

The maxillary nerve enters the pterygopalatine fossa through the foramen rotundum. It branches into a series of sensory nerves: the zygomatic nerve innervates the corresponding region; the infraorbital nerve innervates the skin of the nose, lower eyelid, upper lip, cheek, and temporal region, as well as the maxillary Teeth; ganglionic branches pass to the mucosa of the nasal cavity and palate. Thus, the maxillary nerve innervates the middle part of the face between the palpebral fissure and the angle of the Mouth.

The mandibular nerve exits the cranial cavity through the foramen ovale. It contains both sensory and motor fibers. The main branches of the mandibular nerve perform the following Functions:

1) the lingual nerve provides general sensation to the anterior ²/₃ of the organ; it carries fibers of the chorda tympani (a branch of the facial nerve) that provide taste sensation to this part of the tongue;

2) the buccal nerve provides general sensation to the cheek;

3) the auriculotemporal nerve innervates the Parotid salivary gland, the capsule of the temporomandibular joint, and part of the auricle; it carries postganglionic parasympathetic fibers from the otic ganglion (from the IX pair);

4) the mylohyoid nerve branches off from the inferior alveolar nerve, innervating the mylohyoid muscle and the anterior belly of the digastric muscle; the nerve then courses through the mandibular canal, supplying branches to the teeth and Gums, and terminates as the mental nerve, which exits through the mental foramen to supply the skin of the chin and lower lip;

5) muscular (motor) branches innervate the Muscles of Mastication.

Thus, the fibers of the mandibular nerve innervate the lower part of the face (below the angle of the mouth) and the muscles of mastication.

Pair VI is the Abducens nerve, n. abducens, consisting exclusively of motor fibers. It emerges from the substance of the pons and enters the orbit via the superior orbital fissure. It innervates the lateral rectus muscle of the eyeball.

Pair VII is the facial nerve, n. facialis, which is mixed in composition. It emerges from the substance of the pons at the cerebellopontine angle and travels with the VIII pair toward the internal acoustic meatus. There it enters the facial canal. In the region of the first bend of the canal lies the sensory ganglion of the nerve, known as the geniculate ganglion. Within the canal, the greater petrosal nerve and the chorda tympani branch off from the main trunk. The greater petrosal nerve enters the pterygopalatine fossa, where it terminates on the Neurons of the pterygopalatine ganglion, providing parasympathetic innervation to the lacrimal gland and the mucosal Glands of the nasal and oral cavities. The chorda tympani, having separated from the main trunk of the facial nerve, joins the lingual nerve (a branch of the mandibular nerve) and travels to the tongue. Its fiber composition is mixed: it provides taste sensation to the anterior ²/₃ of the tongue and also carries parasympathetic fibers to the submandibular and sublingual Salivary Glands.

After exiting the canal, the facial nerve consists exclusively of motor fibers that form the parotid plexus within the parenchyma of the parotid salivary gland. From this plexus arise the temporal, zygomatic, buccal, cervical, and marginal mandibular branches (Fig. 15.3), which innervate all the Muscles of facial expression, the platysma, the stylohyoid muscle, and the posterior belly of the digastric muscle.

Pair VIII is the Vestibulocochlear nerve, n. vestibulocochlearis, consisting solely of sensory fibers. It comprises two parts: the cochlear (auditory) and vestibular parts. Each of these has its own nuclei in the Central Nervous system. The nerve emerges from the cerebellopontine angle, adjacent to the facial nerve. It does not exit the Skull base, as it transmits impulses from the Organs of Hearing and Equilibrium located within the petrous part of the Temporal bone. Its spiral (cochlear) ganglion is located in the spiral canal of the cochlea and receives impulses from The Organ of Corti. The vestibular ganglion is located at the Fundus of the internal acoustic meatus. Fibers conducting impulses from the Organ of Equilibrium run from this ganglion to the brain.

Pair IX is the glossopharyngeal nerve, n. glossopharyngeus, which has a mixed fiber composition. It emerges from the medulla oblongata and leaves the skull through the jugular foramen.

Its largest branch is the tympanic nerve (see Fig. 15.2). It provides sensory innervation to the tympanic cavity and continues as the lesser petrosal nerve, which exits the petrous part of the temporal bone through the lesser petrosal canaliculus. It consists of parasympathetic fibers that terminate on the neurons of the otic ganglion, providing parasympathetic innervation to the parotid salivary gland. It also gives off pharyngeal branches that innervate the mucosa and longitudinal Muscles of the Pharynx. The nerve then penetrates the root of the tongue, where it divides into the terminal lingual branches. These provide general and taste sensation to the posterior third of the tongue.

Thus, the glossopharyngeal nerve is responsible for general and gustatory sensation in the posterior third of the tongue; sensory Innervation of the mucous membrane of the pharynx, palatine Tonsils, tympanic cavity, auditory tube, and carotid sinus; motor innervation of the stylopharyngeus muscle; and parasympathetic innervation of the parotid salivary gland.

Fig. 15.3. Branches of the facial nerve and Cervical plexus:

1 — auriculotemporal nerve; 2 — temporal branches; 3 — frontal nerve; 4 — zygomatic branches; 5 — buccal branch; 6 — marginal mandibular branch; 7 — parotid plexus; 8 — superficial cervical loop; 9 — transverse cervical nerve; 10 — supraclavicular nerves; 11 — great auricular nerve; 12 — lesser occipital nerve; 13 — greater occipital nerve

Cranial nerve X is the Vagus nerve, n. vagus, which is mixed in its fiber composition. It emerges from the medulla oblongata and exits the cranial cavity via the jugular foramen. Its Structure is divided into cranial, cervical, thoracic, and abdominal segments.

In the cranial segment, its sensory branches supply the External ear and the dura mater.

In the neck, the nerve runs as part of the neurovascular bundle alongside the carotid artery and Internal jugular vein. The cervical segment gives rise to the superior cardiac and pharyngeal branches (Fig. 15.4), as well as the superior laryngeal nerve, which innervates the laryngeal mucosa above the vocal folds and the cricothyroid muscle. The recurrent laryngeal nerve originates from the vagus nerve as the latter enters the thoracic cavity. The left recurrent laryngeal nerve loops around the aortic arch, while the right loops around the Subclavian Artery. The recurrent laryngeal nerve branches into tracheal, esophageal, pericardial, and inferior cardiac branches, which contain sensory and parasympathetic fibers. Afterwards, the recurrent laryngeal nerve continues as the inferior laryngeal nerve, which innervates the laryngeal mucosa below the vocal folds and most of the laryngeal muscles, thereby playing a key role in phonation.

Fig. 15.4. NERVES OF THE HEAD, neck, and thoracic cavity:

1 — trigeminal nerve; 2 — glossopharyngeal nerve; 3 — Accessory nerve; 4 — vagus nerve; 5 — branches of the cervical plexus; 6 — phrenic nerve; 7 — middle cervical ganglion of the Sympathetic trunk; 8 — Brachial Plexus; 9 — thoracic ganglion of the sympathetic trunk; 10 — recurrent laryngeal nerve; 11 — pulmonary plexus of the vagus nerve; 12 — cardiac branches; 13 — inferior laryngeal nerve; 14 — pharyngeal branches of the vagus nerve; 15 — Hypoglossal nerve; 16 — superior laryngeal nerve; 17 — lingual nerve

In the thoracic segment, the tracheal, bronchial, esophageal, pericardial, and thoracic cardiac branches of the vagus nerve supply the thoracic organs. Here, the right and left vagus nerves form plexuses along the course of the Esophagus as well as the pulmonary plexus.

Upon entering the Abdominal cavity (abdominal segment), the fibers of the vagus nerve extend to all abdominal viscera, dividing into corresponding branches containing parasympathetic and sensory fibers: gastric, hepatic, splenic, intestinal, renal branches, and others.

Thus, the vagus nerve features a remarkably extensive innervation zone: sensory and parasympathetic innervation of the Organs of the neck, Thorax, and abdomen (excluding the pelvic organs); and motor innervation of the muscles of the Larynx, pharynx, and esophagus.

Cranial nerve XI is the accessory nerve, n. accessorius, which is motor in its fiber composition. It emerges via several rootlets from the medulla oblongata and spinal segments I–VI. It exits the cranial cavity through the jugular foramen and innervates the trapezius and sternocleidomastoid muscles.

Cranial nerve XII is the hypoglossal nerve, n. hypoglossus, which is motor in its fiber composition. It emerges from the medulla oblongata and exits the cranial cavity through the hypoglossal canal, innervating the muscles of the tongue.