Human Anatomy - A Course of Lectures - Kostylenko Yu.P. 2015

Peripheral Nervous System. Cranial Nerves

Lecture Outline:

9.1. General characteristics of the Peripheral Nervous System.

9.2. Structure of Spinal Nerves and Plexuses.

9.3. Cranial Nerves.

9.1 General CHARACTERISTICS OF THE Peripheral Nervous System

The peripheral part of The Nervous System is represented by a set of spinal and cranial nerves. It includes: nerve trunks, ganglia, nerve endings, and nerve plexuses. Thus, the peripheral nervous system integrates all neural structures located outside the Spinal Cord and Brain.

Peripheral nerves consist of a certain number of nerve fibers with varying structures and functional properties, surrounded by Connective Tissue sheaths. Depending on the presence or absence of a myelin sheath, fibers are classified as myelinated or unmyelinated.

As mentioned above, nerves possess their own system of sheaths. The outermost sheath, the epineurium, covers the nerve trunk externally, isolating it from surrounding Tissues, and consists of loose irregular connective tissue containing Blood Vessels that supply the nerve. The loose connective tissue of the epineurium filling the spaces between individual bundles of nerve fibers is called the internal epineurium.

The perineurium surrounds individual bundles of nerve fibers, serving as a structural framework for them. The perineurial sheaths of one nerve connect with those of adjacent nerves, allowing fibers to transition from one nerve to another through these connections (similar to anastomoses between blood vessels).

The innermost sheath, the endoneurium, surrounds individual nerve fibers with a delicate connective tissue layer. The endoneurial space of a nerve bundle plays an intermediary role in metabolic processes between microvessels and nerve fibers (Fig. 9.1).

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Fig. 9.1 Diagram of the nerve trunk structure:

1 - bundles of nerve fibers;

2 - endoneurium;

3 - perineurium;

4 - internal epineurium;

5 - external epineurium.

9.2 Structure of Spinal Nerves and Plexuses

The human nervous system contains 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

Each spinal nerve is formed by the union of the anterior and posterior spinal roots. The anterior roots consist of efferent fibers, which are processes of neurocytes located in the Gray matter of the anterior horns of the spinal cord. These fibers transmit signals to skeletal Muscles. Therefore, the anterior roots are also referred to as motor roots; their damage primarily results in impaired motor function.

The anterior roots of the thoracic and lumbar nerves also contain preganglionic autonomic (sympathetic) fibers originating in the lateral horns of the gray matter of the spinal cord, which are likewise efferent.

The posterior roots contain afferent fibers, which are axons of pseudounipolar Cells of the spinal ganglia. These fibers transmit sensory impulses from receptors in the Skin, Musculoskeletal System, Internal Organs, and blood vessels to the spinal cord. Thus, the posterior roots are sensory, and their damage primarily leads to sensory deficits.

Each posterior ROOT forms a dilation—the spinal ganglion—which houses sensory pseudounipolar Neurons whose axons form the posterior root, while their dendrites serve as the afferent fibers of the spinal nerves. Within the intervertebral foramina, the anterior and posterior roots join to form the trunk of the spinal nerve, or the spinal nerve proper.

A spinal nerve contains both afferent and efferent fibers; hence, all spinal nerves are mixed in composition. The trunk of a spinal nerve is relatively short (0.5–1 cm) and does not completely fill the intervertebral foramen, allowing the nerve to lie freely without being compressed during movements of THE Vertebral Column. With age or pathology, the reserve spaces around the spinal nerves diminish, which can lead to nerve trunk compression and pain.

Upon exiting the vertebral canal, the spinal nerves divide into four branches:

1. Meningeal branches: these return to the vertebral canal and innervate the Meninges of the spinal cord and its nourishing blood vessels.

2. White communicating branches: these run to the autonomic ganglia of the Sympathetic trunk and contain preganglionic sympathetic fibers.

3. Posterior (dorsal) branches: these HEAD toward the back. With the exception of the posterior branch of the first cervical (I), the fourth and fifth sacral (IV, V), and the coccygeal spinal nerves, all posterior branches divide into medial and lateral branches. The posterior Branches of the spinal nerves are segmentally arranged, innervating the deep back muscles and the skin from the occiput down to the gluteal region.

4. Anterior (ventral) branches: these innervate the anterolateral walls of the trunk and the limbs. These are the largest branches of the spinal nerves. Only the Anterior branches of the Thoracic nerves retain a segmental arrangement, known as the intercostal nerves; the remaining nerves interconnect via loops to form nerve plexuses. From these plexuses, in turn, arise the nerves categorized by fiber type into muscular (motor), cutaneous (sensory), and mixed.

The spinal nerve plexuses include the cervical, brachial, lumbar, sacral, and coccygeal plexuses. The lumbar, sacral, and coccygeal plexuses together form the Lumbosacral plexus.

Cervical plexus: Formed by the anterior branches of the upper four cervical spinal nerves. It lies at the level of the upper four cervical vertebrae on the anterolateral surface of the deep Neck Muscles, covered anteriorly and laterally by the sternocleidomastoid Muscle. Its branches include muscular, cutaneous, and mixed nerves.

Brachial Plexus: Formed by the anterior branches of the lower four cervical spinal nerves, part of the anterior branch of the fourth cervical (IV) nerve, and the first thoracic (I) spinal nerve. It is located in the interscalene space (between the anterior and middle scalene muscles), superior and posterior to the Subclavian Artery. The branches of the brachial plexus are divided into short and long.

Short branches innervate the Muscles of the neck, the shoulder girdle (except the trapezius muscle), and the muscles of the shoulder joint region. Long branches supply the muscles and skin of the upper limb.

Lumbar plexus: Formed by the anterior branches of the upper three lumbar spinal nerves, part of the anterior branch of the twelfth thoracic (XII) nerve, and the fourth lumbar (IV) spinal nerve. It lies anterior to the transverse processes of the lumbar vertebrae, beneath the psoas major muscle and on the anterior surface of the quadratus lumborum muscle. Its branches innervate the muscles of the lumbar region, the muscles and skin of the anterior abdominal wall, and the muscles and skin of the anterior thigh.

Sacral Plexus: Formed by the anterior branches of the fifth lumbar (V), upper four sacral, and part of the anterior branch of the fourth lumbar (IV) spinal nerves. It is situated on the pelvic surface of the sacrum. The anterior branch of the fifth lumbar nerve and part of the anterior branch of the fourth lumbar nerve form the lumbosacral trunk.

The branches of the sacral plexus are divided into short and long. The short branches terminate in the Pelvic Girdle region, whereas the long branches extend to the muscles and skin of the free lower limb.

Coccygeal plexus: Formed by the anterior branches of the fifth sacral (V) and coccygeal spinal nerves. This plexus is located within the lesser pelvis on the coccygeus muscle and the sacrotuberous (sacrospinous) ligament. The anococcygeal nerves emerging from the plexus innervate the skin in the region of the coccyx and the anus.

GENERAL PATTERNS OF nerve course and branching: In their course and ramification, nerves share many similarities with blood vessels. Within the body walls, nerves, much like blood vessels, are segmentally arranged (intercostal NERVES AND Arteries). Major nerve trunks are predominantly located on the flexor surfaces of joints. Notable exceptions include the ulnar nerve in the upper limb and the sciatic nerve in the lower limb.

Running alongside arteries and Veins within neurovascular bundles, nerves pass through protected pathways, specifically muscular and osteofascial grooves and canals.

9.3 Cranial Nerves

The twelve pairs of cranial nerves lack a regular segmental arrangement and cannot be considered homologues of the spinal nerves. Unlike spinal nerves, cranial nerves lack anterior and posterior roots, do not form plexuses, provide cutaneous innervation in a strictly pathway-based manner, and show no segmental arrangement at their branching sites.

Unlike spinal nerves, which share similar developmental patterns and a uniform structural plan, cranial nerves are divided into several groups that differ in origin, structure, and functional significance.

The first group comprises the sensory nerves: the olfactory nerves (nn. olfactorii) (pair I), the Optic nerve (n. opticus) (pair II), and the Vestibulocochlear nerve (n. vestibulocochlearis) (pair VIII). These nerves will be examined in detail in The Study of Sensory Organs.

The second group (motor nerves) combines the oculomotor (n. oculomotorius) (pair III), trochlear (n. trochlearis) (pair IV), abducens (n. abducens) (pair VI), and hypoglossal (n. hypoglossus) (pair XII) nerves. They consist predominantly of somatic motor fibers and embryologically correspond to the anterior roots of spinal nerves. Pairs III, IV, and VI innervate the muscles of the Eyeball, while the Hypoglossal nerve supplies the muscles of the Tongue.

The third group is represented by mixed nerves. It includes the so-called branchial arch nerves: the trigeminal (n. trigeminus) (pair V), facial (n. facialis) (pair VII), glossopharyngeal (n. glossopharyngeus) (pair IX), vagus (n. vagus) (pair X), and accessory (n. accessorius) (pair XI) nerves. Each of these nerves originally innervates one of the embryonic branchial arches and subsequently supplies the organs derived from that corresponding arch.

Thus, the Trigeminal nerve is associated with the first (I) branchial arch, the Facial Nerve with the second (II) arch, the Glossopharyngeal nerve with the third (III) arch, and the Vagus nerve with the fourth and fifth (IV, V) arches. The Accessory nerve is a derivative that split off from the vagus nerve during development; structurally, it consists entirely of motor fibers.

Outline for describing cranial nerves in Practical Sessions:

3. Name of the nerve (Ukrainian, Latin).

4. General characteristics of the nerve.

5. Name, Location, and characteristics of the nuclei.

6. The site of exit of the nerve onto the Base of the brain.

7. The site of exit of the nerve from the cranial cavity.

8. Characteristics of Individual branches of the nerve and their areas of innervation.

Motor nerves. The Oculomotor nerve, n. oculomotorius, is a mixed nerve containing motor and autonomic (parasympathetic) fibers. It innervates most of the extraocular muscles: the inferior rectus, inferior oblique, medial rectus, superior rectus, and the levator palpebrae superioris muscle.

The nuclei of this nerve are located in the tegmentum of the Midbrain at the level of its superior colliculi. There is a paired motor Nucleus, nucleus motorius nervi oculomotorii, and two nuclei belonging to the Autonomic nervous system: the central unpaired nucleus, nucleus centralis impar, and the accessory nucleus, nucleus accessorius nervi oculomotorii (Yakubovich's nucleus, Edinger-Westphal nucleus). Two smooth muscles receive innervation from these nuclei: the sphincter pupillae and the ciliary muscle.

The oculomotor nerve emerges from the sulcus on the Medial surface of the cerebral crus and enters the Orbit through the superior orbital fissure. Within the orbit, it divides into superior and inferior branches. The first innervates the superior rectus and the levator palpebrae superioris muscle, while the second innervates the inferior and medial rectus muscles as well as the inferior oblique muscle. The oculomotor root, radix oculomotoria, containing parasympathetic fibers, arises from the inferior branch and proceeds to the ciliary ganglion, where these fibers synapse. Damage to the oculomotor nerve results in ptosis (drooping of the upper eyelid), divergent strabismus, mydriasis (pupillary dilation), and accommodation disorders.

The Trochlear nerve, n. trochlearis, is a motor nerve that innervates only a single extraocular muscle, the superior oblique. The Nucleus of this nerve is located in the tegmentum of the midbrain at the level of the inferior colliculi. The trochlear nerve is the only cranial nerve that emerges from the dorsal surface of the Brainstem. The nerve wraps around the cerebral crus, passes through the cavernous sinus, and enters the orbit via the superior orbital fissure.

Damage to the trochlear nerve leads to paralysis of the superior oblique muscle. As a result, the eyeball is turned upward and inward, the patient has difficulty seeing objects beneath their feet, and experiences double Vision (diplopia).

The Abducens nerve is a motor nerve, n. abducens, that innervates the lateral rectus muscle of the eyeball. The Nucleus of the nerve is located in the pontine tegmentum and projects in the upper part of the Rhomboid fossa corresponding to the facial colliculus. The nerve fibers emerge from the brain near the midline at the border between the Pons and the pyramid of the Medulla Oblongata. Further on, the nerve trunk passes through the cavernous sinus, lying adjacent to the Internal Carotid Artery, and enters the orbit through the superior orbital fissure.

Damage to the abducens nerve results in the loss of mobility of the eyeball, causing it to deviate medially (convergent strabismus).

The hypoglossal nerve, n. hypoglossus, is the motor nerve of the tongue. Its nucleus lies in the medulla oblongata and projects in the inferomedial part of the rhomboid fossa, corresponding to the hypoglossal triangle. The rootlets of the nerve emerge from the medulla oblongata between the pyramid and the olive. The hypoglossal nerve gives off branches to the meninges, exits the Skull through the hypoglossal canal, and reaches the tongue, where it divides into branches that innervate individual muscles.

Injury to the hypoglossal nerve causes paralysis of the tongue muscles. This is typically unilateral, in which case the tongue deviates toward the side opposite the lesion. Bilateral paralysis of the tongue muscles impairs chewing, swallowing, and speech.

NERVES OF THE branchial arches.

The trigeminal nerve, n. trigeminus, is mixed in terms of its innervation pattern and serves as the principal sensory nerve of the head.

The trigeminal nerve has 4 nuclei. The motor nucleus of the trigeminal nerve, nucleus motorius nervi trigemini, is located in the pontine tegmentum and projects in the upper part of the rhomboid fossa. The remaining nuclei are sensory: the mesencephalic nucleus (responsible for tactile sensitivity), the chief (pontine) sensory nucleus (proprioceptive sensitivity), and the spinal trigeminal nucleus (pain and Temperature sensitivity).

The rootlets of the trigeminal nerve emerge from the pons at the line where it joins the middle cerebellar peduncles. The sensory root is thicker than the motor root. Near the apex of the petrous part of the Temporal bone, the sensory root of the trigeminal nerve forms the trigeminal ganglion, ganglion trigeminale (s. semilunare Gasseri, Gasserian ganglion). It contains pseudounipolar cells similar to those of spinal ganglia.

Three branches arise from the trigeminal ganglion: the ophthalmic nerve, n. ophthalmicus, the maxillary nerve, n. maxillaris, and the mandibular nerve, n. mandibularis. The ophthalmic and maxillary nerves contain only sensory fibers. The mandibular nerve also contains motor fibers, making it a mixed nerve. The mandibular nerve contains the largest number of nerve fibers, followed by the maxillary nerve, while the ophthalmic nerve is the thinnest.

Each of these nerves gives off a meningeal branch before exiting the cranial cavity. The ophthalmic nerve enters the orbit via the superior orbital fissure, the maxillary nerve passes through the foramen rotundum into the pterygopalatine fossa, and the mandibular nerve reaches the external base of the skull through the foramen ovale. All three branches innervate the skin of the facial region (Fig. 9.2).

Fig. 9.2 Zones of Innervation of the trigeminal nerve branches:

1 - ophthalmic nerve;

2 - maxillary nerve;

3 - mandibular nerve;

The facial nerve, n. facialis. It comprises two independent nerves: the facial nerve proper, which is motor, and the intermediate nerve, which contains autonomic and sensory fibers; therefore, it would be more accurate to call this nerve the nervus intermediofacialis.

The facial nerve (proper) emerges from the brain at the cerebellopontine angle, at the border between the pons, medulla oblongata, and Cerebellum. The nerve enters the internal acoustic meatus and then runs within the petrous part of the temporal bone inside the facial canal, which opens at the stylomastoid foramen on the external base of the skull.

From here, the facial nerve enters the parotid gland, passes through it, and divides into branches that interconnect to form the parotid plexus, plexus parotideus. The temporal, zygomatic, buccal, marginal mandibular, and cervical branches emerge from the plexus. Most of these branches innervate the Muscles of facial expression, a topography that must be considered during surgical interventions in the facial region.

The intermediate nerve, n. intermedius (Wrisberg's nerve), contains sensory (gustatory) and autonomic (parasympathetic) fibers. The nerve emerges from the brain between the rootlets of the facial and vestibulocochlear nerves, but subsequently merges with the facial nerve. The taste fibers are processes of neurons of the geniculate ganglion, ganglion geniculi. They pass through the chorda tympani into the lingual nerve and innervate the taste buds of the anterior two-thirds of the tongue. Upon entering the brainstem, the central branches of these fibers run descendently as part of the solitary tract, tractus solitarius (Gierke's tract), and terminate in the solitary nucleus, nucleus solitarius, which lies in the medulla oblongata and projects to the lower part of the rhomboid fossa.

The parasympathetic fibers of the facial nerve originate from the superior salivatory nucleus, nucleus salivatorius superior. Via the branches of the chorda tympani and greater petrosal nerve, they reach the pterygopalatine and submandibular ganglia, which provide innervation to the lacrimal glands, Nasal cavity glands, minor Salivary Glands, as well as the submandibular and sublingual salivary glands.

Damage to the facial nerve results in facial muscle paralysis, impaired taste sensitivity, altered lacrimation and salivation, and disturbed sound perception (hyperacusis). The latter is caused by the fact that the facial nerve innervates the stapedius muscle.

The glossopharyngeal nerve, n. glossopharyngeus, is a mixed nerve containing sensory, motor, and parasympathetic fibers. The motor fibers originate in the ambiguus nucleus, nucleus ambiguus, located in the medulla oblongata and projecting onto the inferior division of the rhomboid fossa within the vagal trigone. The sensory taste fibers converge on the solitary nucleus, which is shared with the facial and vagus nerves. The glossopharyngeal nerve also possesses a parasympathetic inferior salivatory nucleus, nucleus salivatorius inferior, which provides innervation to the Cytology/practical/97.html">Parotid salivary gland.

The rootlets of the glossopharyngeal nerve emerge from the brain dorsal to the olive, while the nerve trunk exits the cranium through the jugular foramen. Here it forms the superior ganglion (Müller's ganglion) and inferior ganglion (Andersch's ganglion), where its peripheral afferent neurons are located.

The innervation zone of the glossopharyngeal nerve encompasses the derivatives of the third pharyngeal arch. Its sensory branches innervate the tympanic cavity, auditory tube, palatine arches and Tonsils, the mucous membrane of the posterior third of the tongue, and a small skin area anterior to the auricle. The motor branches innervate the superior pharyngeal constrictor, the stylopharyngeus muscle, and the levator veli palatini.

The glossopharyngeal nerve has connections with the vagus nerve. Both participate in The formation of the pharyngeal plexus and the innervation of the palate. Injury to the glossopharyngeal and vagus nerves impairs the swallowing act and causes paralysis of the soft palate.

The vagus nerve (n. vagus) represents the nerve of the fourth and fifth pharyngeal arches. It possesses three nuclei located in the dorsal part of the medulla oblongata: the ambiguous nucleus (motor), shared with the glossopharyngeal nerve; the posterior nucleus (parasympathetic); and the nucleus of the solitary tract (sensory).

The vagus nerve is the longest of the cranial nerves. Its innervation field extends from the dura mater of the brain down to the sigmoid colon. The nerve rootlets emerge from the brain substance posterior to the olive of the medulla oblongata and, together with the glossopharyngeal and accessory nerves, pass through the jugular foramen. The nerve forms two ganglia (homologs of spinal ganglia): the superior ganglion, lying within the jugular foramen, and the inferior ganglion, located 1-2 cm lower.

Topographically, the vagus nerve is divided into cranial, cervical, thoracic, and abdominal segments.

In the cranial segment, the vagus nerve gives off the following branches: the meningeal branch to the dura mater of the brain; the auricular branch, consisting of sensory fibers supplying the skin of the posterior wall of the external acoustic meatus; and communicating branches to the glossopharyngeal and accessory nerves, as well as to the superior cervical sympathetic ganglion.

In the cervical segment, numerous sensory, motor, and parasympathetic branches arise from the vagus nerve trunk. Together with corresponding branches of the glossopharyngeal and cervical sympathetic nerves, they form the pharyngeal plexus, superior and inferior cervical cardiac branches, and the superior laryngeal nerve.

In the thoracic cavity, the vagus nerve trunk gives off the thoracic cardiac branches; bronchial branches, which form the pulmonary plexus; branches to the Pericardium, mediastinal Pleura, and Thoracic duct; branches to the Esophagus, which contribute to the esophageal plexus; and the recurrent laryngeal nerve.

In the abdominal segment, the vagus nerve branches to The Stomach, Liver, and Kidneys, and also gives rise to celiac branches (rr. coeliaci) that participate in forming the celiac plexus. Via the branches of the celiac plexus, the vagus nerve reaches most of the internal Organs of the Abdominal cavity.

The accessory nerve, n. accessorius (Willis's nerve), is a motor nerve consisting of fibers originating from two nuclei: the nucleus ambiguus in the medulla oblongata and the spinal nucleus located in the upper 4 cervical segments. Accordingly, the nerve has cranial and spinal rootlets that unite within the cranial cavity to form the nerve trunk. It exits the cranium together with the glossopharyngeal and vagus nerves through the jugular foramen and divides into internal and external branches. The internal branch, r. internus, joins the vagus nerve; it contains predominantly fibers from the cranial rootlets of the accessory nerve, which contribute to the innervation of the Pharynx and Larynx.

The external branch, r. externus, supplies motor fibers to two large muscles: the trapezius and the sternocleidomastoid.