Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Nervous System (General Outline of the Structure and Development of the Nervous System)
Peripheral Nervous System
Cranial Nerves

There are 12 pairs of Cranial Nerves that emerge from the Brainstem. Each cranial nerve has a specific name and a sequential number designated by a Roman numeral: I — olfactory; II — optic; III — oculomotor; IV — trochlear; V — trigeminal; VI — abducens; VII — facial; VIII — vestibulocochlear; IX — glossopharyngeal; X — vagus; XI — accessory; XII — hypoglossal.

Based on their structural features and predominant fiber composition, cranial nerves are divided into three groups: sensory, motor, and mixed.

The sensory nerves include the olfactory (cranial nerve I), optic (cranial nerve II), and vestibulocochlear (cranial nerve VIII) nerves.

The olfactory nerves (cranial nerve I) consist of the central processes of sensory (receptor) Cells located in the mucous membrane of the olfactory region of the Nasal cavity. Comprising 15–20 filaments (nerves), the olfactory nerves pass into the cranial cavity through the openings of the cribriform plate located on the roof of the nasal cavity. Within the cranial cavity, the fibers of the olfactory nerves enter the olfactory bulbs and synapse with the second-order Neurons. From the olfactory bulbs, Olfactory nerve impulses travel via the fibers of the olfactory tracts and other structures of the rhinencephalon to the cerebral hemispheres.

The optic nerves (cranial nerve II) are formed by the processes of the ganglion Cells of the retinal layer of the eye. Upon entering the cranial cavity, the right and left optic nerves partially decussate anterior to the sella turcica and continue as the optic tracts. These tracts lead to the lateral geniculate bodies, the superior colliculi of the tectum of the Midbrain, and the pulvinar (posterior part) of the thalamus, which serve as subcortical visual centers. From these subcortical centers, nerve impulses are relayed to the visual cortex in the occipital lobes of the cerebral hemispheres.

The Vestibulocochlear nerve (cranial nerve VIII) is formed by the central processes of bipolar neurons located in the vestibular and cochlear ganglia of the Inner ear. The peripheral processes of these ganglionic cells form bundles that terminate in receptors within the vestibular division of the membranous labyrinth of the inner ear and the Cytology/practical/79.html">Spiral organ of the cochlear duct, respectively. The central processes extend into the Brain to nuclei situated in the tegmentum of the Pons near the border with the Medulla Oblongata. From the superior (dorsal) and inferior (ventral) cochlear nuclei, impulses are transmitted to the superior temporal gyrus of both the right and left cerebral hemispheres.

The motor cranial nerves are the Oculomotor nerve (cranial nerve III), Trochlear nerve (cranial nerve IV), Abducens nerve (cranial nerve VI), Accessory nerve (cranial nerve XI), and Hypoglossal nerve (cranial nerve XII).

The oculomotor nerve (cranial nerve III) contains both motor and parasympathetic fibers that emerge from the motor and autonomic (accessory) nuclei located in the midbrain. The nerve enters the Orbit via the superior orbital fissure. Its motor fibers supply the extraocular Muscles—the superior, inferior, and medial rectus muscles, the inferior oblique Muscle, and the levator palpebrae superioris. The parasympathetic fibers run to the ciliary ganglion, from which postganglionic fibers extend to the sphincter pupillae and ciliary muscles.

The trochlear nerve (cranial nerve IV) originates from a motor Nucleus also situated in the midbrain at the level of the inferior colliculi. The nerve enters the orbit through the superior orbital fissure and innervates the superior oblique muscle of the eye.

The abducens nerve (cranial nerve VI) is formed by the processes of motor neurons located in the tegmentum of the pons. Passing through the superior orbital fissure into the orbit, it innervates the lateral rectus (abducting) muscle of the eye.

The accessory nerve (cranial nerve XI) arises from several motor rootlets emerging from nuclei located in the medulla oblongata and the upper segments of the Spinal Cord. The nerve exits the cranium through the jugular foramen (along with the glossopharyngeal and vagus nerves) and innervates the sternocleidomastoid and trapezius muscles.

The hypoglossal nerve (cranial nerve XII) has its motor nucleus in the medulla oblongata. The processes of these neurons form the nerve, which leaves the cranial cavity via the hypoglossal canal and innervates the Muscles of the Tongue. Nerve fibers from the first cervical spinal nerve join the hypoglossal nerve; these fibers contribute to The formation of the ansa cervicalis, which innervates the Infrahyoid muscles.

The mixed cranial nerves are the trigeminal, facial, glossopharyngeal, and vagus nerves.

The Trigeminal nerve (cranial nerve V) has two roots: sensory and motor. The sensory ROOT is formed by the central processes of cells from the trigeminal ganglion, which is located near the apex of the petrous part of the Temporal bone. The motor root comprises processes originating from the motor Nucleus of the trigeminal nerve.

The trigeminal nerve divides into three major branches: the first, second, and third. The First and Second branches are sensory, containing the peripheral processes of the trigeminal ganglion cells. The third branch consists of both sensory and motor fibers.

The first branch of the trigeminal nerve is the ophthalmic nerve, which enters the orbit through the superior orbital fissure and divides into three branches: the lacrimal, frontal, and nasociliary nerves. These branches innervate the Eyeball, upper eyelid, the mucosa of the anterior nasal cavity and Paranasal Sinuses (frontal, sphenoid, and ethmoid air cells), as well as the Skin of the forehead.

The second branch is the maxillary nerve, which passes through the foramen rotundum into the pterygopalatine fossa, where it gives rise to the infraorbital and zygomatic nerves, as well as ganglionic branches. Its branches supply the mucous membrane of the nasal cavity, Hard and Soft palate, paranasal sinuses (maxillary and ethmoid cells), the skin of the zygomatic region, lower eyelid, Nose, and upper lip, the maxillary Teeth, and the dura mater in the middle cranial fossa. The ganglionic Branches of the maxillary nerve run to the parasympathetic pterygopalatine ganglion. Processes from this ganglion travel within the branches of the maxillary nerve to the Glands of the nasal and oral mucosa, as well as the lacrimal gland.

The third branch, the mandibular nerve, is a mixed nerve that exits the cranial cavity through the foramen ovale and divides into the auriculotemporal, buccal, lingual, and inferior alveolar nerves. It supplies the dura mater in the middle cranial fossa and emits motor branches to the Muscles of Mastication (masseter, temporalis, medial and lateral pterygoids), as well as to the mylohyoid muscle, the anterior belly of the digastric muscle, the tensor veli palatini, and the tensor tympani. The auriculotemporal nerve innervates the skin of the temporal region, auricle, and external acoustic meatus, and conveys parasympathetic fibers (derived from the Glossopharyngeal nerve) destined for the Parotid salivary gland. The buccal nerve innervates the buccal mucosa, while the lingual nerve supplies the mucosa of the anterior two-thirds of the tongue. The lingual nerve also contains taste fibers terminating in the taste buds of the tongue, alongside parasympathetic fibers (from the Facial Nerve). These parasympathetic fibers proceed to the submandibular and sublingual ganglia, which in turn innervate the corresponding Salivary Glands. The inferior alveolar nerve enters the mandibular canal, supplies the lower teeth and Gums, and finally emerges through the mental fossa to innervate the skin of the chin.

The facial nerve (cranial nerve VII) is a mixed nerve containing motor, sensory, and autonomic (parasympathetic) fibers. The motor fibers are processes of cells belonging to the motor nucleus of the facial nerve, situated in the tegmentum of the pons. The Cell bodies of the sensory neurons are located in the geniculate ganglion, deep within the facial canal inside the petrous part of the temporal bone. The central processes of these cells terminate in the sensory nucleus, also located in the tegmentum of the pons. The parasympathetic fibers are axons of neurons from the superior salivatory nucleus, which lies medial to the sensory nucleus. Upon emerging from the brain, the facial nerve enters the internal acoustic meatus, where the facial canal begins. Initially, the greater petrosal nerve branches off, exiting through the hiatus foramen on the apex of the petrosal pyramid to deliver parasympathetic fibers to the pterygopalatine ganglion. The stapedius nerve supplies the stapedius muscle in the tympanic cavity. The chorda tympani, carrying taste fibers for the tongue and parasympathetic fibers for the submandibular and sublingual salivary glands, passes straight through the tympanic cavity without giving off branches. Upon exiting the tympanic cavity, the chorda tympani joins the lingual nerve. After leaving its canal via the stylomastoid foramen, the facial nerve gives off branches to the posterior belly of the occipitofrontalis muscle and the auricular muscles, then pierces the parotid gland and terminates in branches supplying the Muscles of facial expression, including the platysma.

The glossopharyngeal nerve (cranial nerve IX) is a mixed nerve containing motor, sensory, and parasympathetic fibers. It exits the cranial cavity through the jugular foramen. Its motor fibers originate from The Nucleus ambiguus (shared with the Vagus nerve) and innervate the pharyngeal muscles. The sensory fibers are processes of cells forming the superior and inferior ganglia of this nerve; they extend to the mucous membrane of the posterior pharyngeal wall and the posterior third of the tongue, providing general and gustatory sensation. Parasympathetic fibers, originating from the inferior salivatory nucleus, are directed to the parotid salivary gland.

The vagus nerve (cranial nerve X), which also contains motor, sensory, and parasympathetic fibers, exits the cranial cavity through the jugular foramen alongside the glossopharyngeal and accessory nerves and the Internal jugular vein. Its motor fibers, emerging from the nucleus ambiguus (shared with the glossopharyngeal nerve), innervate the muscles of the Pharynx and Larynx. The sensory fibers are processes of cells forming the superior and inferior ganglia of the vagus nerve, transmitting sensory impulses from Internal Organs, the External ear, and the dura mater of the posterior cranial fossa. The parasympathetic fibers originate from the dorsal motor nucleus of the vagus. The vagus nerve innervates The Heart, respiratory organs, most of the digestive tract (down to the sigmoid colon), and the Kidneys. In the neck, running close to the common carotid artery and internal jugular vein, the vagus nerve gives off branches to the larynx, pharynx, and Esophagus. Further down, the left vagus nerve descends along the anterior surface of the esophagus and continues onto the anterior surface of The Stomach, whereas the right vagus nerve courses along the posterior surface of the esophagus and stomach. Within the walls of internal organs, including the heart, the parasympathetic fibers of the vagus nerves synapse with intrinsic enteric (intramural) ganglia, which regulate the smooth musculature and glandular secretions of these organs.

Review and Self-Assessment Questions:

1. Describe The Structure of a nerve.

2. Explain the basis for the Classification of nerves in The Human Body. What groups comprise the Peripheral Nervous system?

3. List the sensory, motor, and mixed cranial nerves and describe their characteristics.

4. Explain which cranial nerves contain autonomic (parasympathetic) fibers. Which organs do these fibers innervate?

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Fig. 106. Peripheral nervous system:

1 — brain; 2 — spinal cord; 3 — Cervical plexus; 4 — Brachial Plexus; 5 — intercostal nerves; 6 — radial nerve; 7 — Lumbar plexus; 8 — Sacral Plexus; 9 — sciatic nerve; 10 — tibial nerve; 11 — common fibular nerve; 12 — superficial fibular nerve; 13 — deep fibular nerve; 14 — saphenous nerve; 15 — obturator nerve; 16 — femoral nerve; 17 — ulnar nerve; 18 — median nerve; 19 — Sympathetic trunk