Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Brain
Cranial nerves

General Information. Mammals, including humans, have 12 pairs of Cranial Nerves, while fish and amphibians have 10, as their XI and XII nerve pairs arise from the Spinal Cord.

Cranial nerves contain afferent (sensory) and efferent (motor) fibers of the Peripheral Nervous System. Sensory nerve fibers begin with terminal receptor endings that detect changes occurring in the external or internal environment of the body. These receptor endings can be part of the Sensory Organs (Hearing, balance, Vision, taste, smell) or, like Skin Receptors, form encapsulated and unencapsulated endings sensitive to tactile, thermal, and other stimuli. Sensory fibers transmit impulses to the CNS. Similar to Spinal Nerves, the sensory Neurons of cranial nerves lie outside the CNS in ganglia. The dendrites of these neurons extend to the periphery, while their axons travel to the Brain, primarily the Brainstem, and reach their respective nuclei.

Motor fibers innervate skeletal Muscles. They form neuromuscular synapses on Muscle fibers. Depending on which fibers predominate in a nerve, it is classified as sensory or motor. If a nerve contains both types of fibers, it is called a mixed nerve. In addition to these two fiber types, some cranial nerves contain fibers of the Autonomic nervous system, specifically its parasympathetic division.

I pair — olfactory nerves (n. olfactorii) and II pair — Optic nerve (n. opticus) occupy a unique position: they are classified as part of the conducting pathway of Sensory systems and are described alongside their respective sensory organs. They develop as outgrowths of the Forebrain vesicle and represent pathways (tracts) rather than typical nerves.

III—XII cranial nerves differ from spinal nerves because the developmental conditions of the HEAD and brain differ from those of the trunk and spinal cord. Due to the reduction of myotomes in the head region, few neurotomes remain. Consequently, the cranial nerves innervating myotomes are homologous to an incomplete spinal nerve, which consists of ventral (motor) and dorsal (sensory) roots. Each somatic cranial nerve contains fibers homologous to one of these two roots. Since derivatives of the branchial apparatus participate in head formation, cranial nerves also include fibers,

that innervate structures developing from the musculature of the visceral arches.

III, IV, VI and XII pairs of cranial nerves — oculomotor, trochlear, abducens, and hypoglossal — are motor nerves and correspond to the ventral, or anterior, roots of spinal nerves. However, in addition to motor fibers, they contain afferent fibers that carry proprioceptive impulses from the musculoskeletal apparatus. Nerves III, IV, and VI branch within the extraocular muscles, which originate from the three anterior (preotic) myotomes, while nerve XII branches in the Muscles of the Tongue, which develop from occipital myotomes.

VIII pair — Vestibulocochlear nerve consists only of sensory fibers and corresponds to the dorsal ROOT of spinal nerves.

V, VII, IX and X pairs — trigeminal, facial, glossopharyngeal, and vagus nerves contain sensory fibers and are homologous to the dorsal roots of spinal nerves. Like the latter, they consist of the axons of sensory ganglion Cells of the respective nerve. These cranial nerves also contain motor fibers belonging to the visceral apparatus. Fibers running within the Trigeminal nerve innervate the musculature derived from the muscles of the I visceral (mandibular) arch; those in the Facial Nerve innervate derivatives of the musculature of the II visceral (hyoid) arch; those in the Glossopharyngeal nerve innervate derivatives of the I branchial arch; and those in the Vagus nerve innervate derivatives of the mesoderm of the II and all subsequent branchial arches.

XI pair — Accessory nerve consists only of motor fibers of the branchial apparatus and acquires The Significance of a cranial nerve only in higher vertebrates. The accessory nerve innervates the trapezius muscle, which develops from the musculature of the last branchial arches, and the sternocleidomastoid muscle, which separated from the trapezius in mammals.

III, VII, IX, X cranial nerves also contain unmyelinated parasympathetic fibers of the autonomic nervous system. In III, VII, and IX nerves, these fibers innervate the smooth muscles of THE EYE AND the Glands of the head: salivary, lacrimal, and mucous. X nerve carries parasympathetic fibers to the glands and smooth muscles of the Internal Organs of the neck, thoracic, and abdominal cavities. This extensive distribution of the vagus nerve (hence its name) is explained by the fact that in the Cytology/cytology/16.html">Early stages of phylogenesis, the organs it innervates lay near the head and in the region of the branchial apparatus, and then, during evolution, gradually shifted backward, pulling the nerve fibers along with them.

Branches of cranial nerves. All cranial nerves, except for IV, emerge from the Base of the brain (Fig. 3.15).

III pair — Oculomotor nerve (n. oculomotorius) is formed by the axons of cells of the oculomotor Nucleus, which lies anterior to the periaqueductal Gray matter of the cerebral aqueduct (Atl. Fig. 150). In addition, this nerve has an accessory (parasympathetic) nucleus. It is a mixed nerve, emerging on the brain surface near the anterior border of the Pons between the cerebral peduncles and entering the Orbit through the superior orbital fissure. Here, the oculomotor nerve innervates almost all muscles of the Eyeball and the upper eyelid (Atl. Fig. 114). After entering the orbit, the parasympathetic fibers leave the nerve and head to the ciliary ganglion (Atl. Fig. 149). Sympathetic fibers from the internal carotid plexus also run within the nerve.

IV pair — Trochlear nerve (n. trochlearis) consists of fibers from the trochlear nucleus, located anterior to the aqueduct. The axons of this nucleus's neurons decussate to the opposite side, form the nerve, and emerge on the brain surface from the superior medullary velum (Fig. 3.27). The nerve winds around the cerebral peduncle and enters the orbit through the superior orbital fissure, where it innervates the superior oblique muscle of the eye (Atl. Fig. 114).

V pair — trigeminal nerve (n. trigeminus) emerges on the brain surface between the pons and the middle cerebellar peduncles as two roots: a large sensory root and a small motor root (Atl. Fig. 115).

The sensory root consists of the axons of sensory neurons of the trigeminal ganglion, which is located on the anterior surface of the petrous part of the Temporal bone, near its apex. Upon entering the brain, these fibers terminate in three Relay nuclei located in the pontine tegmentum, along the Medulla Oblongata and the cervical spinal cord, and on the sides of the aqueduct. The dendrites of the trigeminal ganglion cells form the three main Branches of the trigeminal nerve (hence its name): the ophthalmic, maxillary, and mandibular nerves, which innervate the skin of the forehead and face, the Teeth, and the mucous membranes of the tongue, Oral Cavity, and Nasal cavity (Atl. Fig. 115; Fig. 3.28). Thus, the sensory root of the V pair of nerves corresponds to the dorsal sensory root of a spinal nerve.

The motor root contains the processes of cells of the motor nucleus, which lies in the pontine tegmentum, medial to the principal sensory nucleus. Upon reaching the trigeminal ganglion, the motor root bypasses it, joins the mandibular nerve, exits the Skull through the foramen ovale, and supplies all masticatory and other muscles developing from the mandibular arch. Thus, the motor fibers of this root are of visceral origin.

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Fig. 3.28. Trigeminal nerve (sensory root):

1 — mesencephalic nucleus; 2 — principal sensory nucleus; 3 — IV ventricle; 4 — spinal nucleus; 5 — mandibular nerve; 6 — maxillary nerve; 7 — ophthalmic nerve; 8 — sensory root; 9 — trigeminal ganglion

VI pair — Abducens nerve (n. abducens) consists of fibers from cells of the homonymous nucleus located in the Rhomboid fossa. The nerve emerges on the brain surface between the pyramid and the pons, passes through the superior orbital fissure into the orbit, where it innervates the lateral rectus muscle of the eye (Atl. Fig. 114).

VII pair — facial nerve (n. facialis) consists of fibers from the motor nucleus located in the pontine tegmentum. The intermediate nerve, whose fibers join it, is considered alongside the facial nerve. Both nerves emerge on the brain surface between the pons and the medulla oblongata, lateral to the abducens nerve. Through the internal acoustic meatus, the facial nerve, together with the intermediate nerve, enters the facial canal, which traverses the petrous part of the temporal bone. The geniculate ganglion—the sensory ganglion of the intermediate nerve—lies within the facial canal. It is named for the sharp bend (genu) that the nerve forms within the canal. After passing through the canal, the facial nerve separates from the intermediate nerve, exits through the stylomastoid foramen into the substance of the Parotid salivary gland, where it divides into terminal branches forming the "pes anserinus major" (Atl. Fig. 158). These branches innervate all Muscles of facial expression, the platysma, and other muscles derived from the mesoderm of the hyoid arch. Thus, the nerve belongs to the visceral apparatus.

The intermediate nerve consists of a small number of fibers that originate from the geniculate ganglion, located in the initial part of the facial canal. Upon entering the brain, these fibers terminate in the pontine tegmentum (on cells of the solitary tract nucleus). The dendrites of the geniculate ganglion cells run within the chorda tympani—a branch of the intermediate nerve—and then join the lingual nerve (a branch of the V pair) to innervate the gustatory (fungiform and foliate) papillae of the tongue. These fibers, carrying impulses from the taste organs, are homologous to the dorsal roots of the spinal cord. The remaining fibers of the intermediate nerve are parasympathetic, originating from the superior salivatory nucleus. These fibers reach the pterygopalatine ganglion.

VIII pair — vestibulocochlear nerve (n. vestibulocochlearis) consists of the sensory fibers of the cochlear nerve and the vestibular nerve.

The cochlear nerve conducts impulses from the Organ of Hearing and is represented by the axons of cells of the spiral ganglion, which lies within the bony cochlea.

The vestibular nerve carries impulses from the vestibular apparatus, signaling THE POSITION OF the head and body in space. The nerve is composed of the axons of cells from the vestibular ganglion, located at the bottom of the internal acoustic meatus.

The axons of the vestibular and cochlear nerves unite within the internal acoustic meatus to form the common vestibulocochlear nerve, which enters the brainstem adjacent to the intermediate and facial nerves, lateral to the olive of the medulla oblongata.

The fibers of the cochlear nerve terminate in the dorsal and ventral cochlear nuclei of the pontine tegmentum, while the fibers of the vestibular nerve end in the vestibular nuclei of the rhomboid fossa (Atl. Figs. 154, 155).

Cranial nerve IX — the glossopharyngeal nerve (n. glossopharyngeus), emerges On the surface of the medulla oblongata, lateral to the olive, via several rootlets (4 to 6); it exits the cranial cavity as a single trunk through the jugular foramen. The nerve consists primarily of sensory fibers that innervate the vallate papillae and the mucous membrane of the posterior third of the tongue, as well as the mucosa of the Pharynx and Middle ear (Atl. Fig. 116). These fibers are the dendrites of cells from the sensory ganglia of the glossopharyngeal nerve, located near the jugular foramen. The axons of these ganglion cells terminate in the relay nucleus (Nucleus of the solitary tract) beneath the floor of the Fourth ventricle. Some fibers project to the dorsal nucleus of the vagus nerve. This described portion of the glossopharyngeal nerve is homologous to the dorsal roots of spinal nerves.

The nerve is mixed. It also contains motor fibers of branchial origin. They originate from the motor (nucleus ambiguus) nucleus of the tegmentum of the medulla oblongata and innervate the pharyngeal muscles. These fibers represent the nerve of the I branchial arch.

The parasympathetic fibers carried by the nerve originate from the inferior salivatory nucleus.

Cranial nerve X — the vagus nerve (n. vagus), the longest of the cranial nerves, emerges from the medulla oblongata posterior to the glossopharyngeal nerve via several rootlets and exits the skull through the jugular foramen along with cranial nerves IX and XI. Near the foramen lie the ganglia of the vagus nerve, which give rise to its sensory fibers (Atl. Figs. 116, 117). Descending the neck within its neurovascular bundle, the nerve enters the thoracic cavity and runs along the Esophagus (Atl. Fig. 119); the left nerve gradually shifts to its anterior surface, and the right to its posterior surface, due to the Rotation of the Stomach during Embryogenesis. Passing through the Diaphragm into the Abdominal cavity alongside the esophagus, the left nerve branches on the anterior surface of The Stomach, while the right nerve contributes to the celiac plexus.

The sensory fibers of the vagus nerve innervate the mucous membrane of the pharynx, Larynx, and root of the tongue, as well as the dura mater of the brain, and represent the peripheral processes (dendrites) of its sensory ganglion cells. The central processes of these cells terminate in The Nucleus of the solitary tract. This nucleus, like the nucleus ambiguus, is shared by cranial nerves IX and X.

The motor fibers of the vagus nerve arise from the cells of the nucleus ambiguus in the tegmentum of the medulla oblongata. These fibers belong to the nerve of the II branchial arch; they innervate its mesodermal derivatives: the muscles of the larynx, palatine arches, soft palate, and pharynx.

The bulk of the vagus nerve consists of parasympathetic fibers that originate from the cells of the dorsal nucleus of the vagus nerve and innervate the viscera.

Cranial nerve XI — the accessory nerve (n. accessorius), consists of fibers from the cells of the nucleus ambiguus (shared with nerves IX and X) located in the medulla oblongata lateral to the central canal, and fibers from its spinal nucleus, which is situated in the anterior horns of the spinal cord across the upper 5–6 cervical segments. The rootlets of the spinal nucleus unite to form a common trunk, entering the skull through the foramen magnum, where they join the rootlets of the cranial nucleus. The latter, numbering 3 to 6, emerge posterior to the olive, immediately behind the rootlets of cranial nerve X.

The accessory nerve exits the skull along with the glossopharyngeal and vagus nerves through the jugular foramen. Here, the fibers of its internal branch join the vagus nerve (Atl. Fig. 118).

The external branch of cranial nerve XI, composed of motor fibers, enters the Cervical plexus and innervates the trapezius and sternocleidomastoid muscles, which are derivatives of the branchial apparatus.

Fig. 3.29. Hypoglossal nerve:

1 — intrinsic and 2 — extrinsic muscles of the tongue; 3 — Mandible; 4 — Hyoid bone; 5 — hypoglossal canal; 6 — pyramid; 7 — inferior olive; 8 — rootlets of the hypoglossal nerve

Cranial nerve XII — the hypoglossal nerve (n. hypoglossus), a motor nerve, consists of fibers from the cells of the corresponding nucleus in the rhomboid fossa; it emerges from the brainstem via numerous rootlets between the pyramid and the olive, which unite into a single trunk. Branches arise from it to the dura mater. The nerve exits the skull through the hypoglossal canal and innervates the intrinsic and extrinsic muscles of the tongue (Fig. 3.29).

Review Questions

1. What are the Divisions of the brain?

2. Which divisions of the brain make up the brainstem?

3. The medulla oblongata: its Location, Structure, pathways, and nuclei.

4. The pons: its Location and Structure. Pathways and nuclei of the pons.

5. The rhomboid fossa. Which cranial nerve nuclei are located within it?

6. The Cerebellum: its location and structure.

7. Pathways, nuclei, and connections of the cerebellum.

8. The Midbrain, its location, structure, pathways, and nuclei.

9. The reticular Formation of the brainstem and its connections with the reticular formation of the spinal cord.

10. Ventricles of the brainstem and their interconnections.

11. List the cranial nerves. Which of them are classified as sensory, motor, and mixed?

12. Which cranial nerves innervate the structures of the orbit?

13. Which structures of the head are innervated by the trigeminal and facial nerves?

14. Which nerves innervate the structures of the oral cavity?

15. List the CHARACTERISTICS OF THE vagus nerve.