Cranial nerves

Cranial nerves are nerves that emerge from or enter the brain stem. Humans have 12 pairs of cranial nerves (nervi craniales). They are designated by Roman numerals according to the order in which they are located. Each nerve has its own name:

• I pair - olfactory nerves (nn. olfactorii)
• II para-optic nerve (n. opticus)
• III pair - oculomotor nerve (n. oculomotorius)
• IV paratrochlear nerve (n. trochlearis)
• V para- trigeminal nerve (n. trigiinus)
• VI pair - abducens nerve (n. abducens)
• VII pair - facial nerve (n. facialis)
• VIII pair - vestibulocochlear nerve (n. vestibulocochlearis)
• IX para- glossopharyngeal nerve (n. glossopharyngeus)
• X para - vagus nerve (n. vagus)
• XI pair - accessory nerve (nп. accessorius)
• XII para - hypoglossal nerve (n. hypoglossus)

The cranial nerves innervate all the organs of the head. In the neck, their innervation area extends to the esophagus and trachea. In addition, the vagus nerve innervates the viscera located in the chest and abdominal cavities (to the transverse colon).

All cranial nerves do not have a correct segmental arrangement. Unlike the spinal nerves, which are similar in origin and development, cranial nerves are divided into several groups, which differ in the features of development, structure and function.

The first group consists of nerves of the sense organs. This group includes the olfactory (I pair), optic (II pair) and vestibulocochlear (VIII pair) nerves. The olfactory and optic nerves develop as outgrowths of the anterior cerebral vesicle. These nerves do not have peripheral sensory nodes.

The second group is the motor cranial nerves: oculomotor (III pair), trochlear (IV pair), abducens (VI pair) and hypoglossal (XII). In origin and function, they correspond to the anterior roots of the spinal nerves. The posterior roots of these nerves do not develop. The hypoglossal nerve innervates the muscles of the tongue.

The third group of cranial nerves (nerves of the branchial arches) is represented by the trigeminal (V pair), facial (VII pair), glossopharyngeal (IX pair), vagus (X pair) and accessory (XI pair) nerves. Initially, each of these nerves innervates one of the branchial arches of the embryo, and subsequently - its derivatives. The nerves of the branchial arches are organized in accordance with the principles of metamerism, since the branchial arches are laid down in the form of metameric formations, making up a series of successive structures similar to each other. These nerves are devoid of anterior and posterior roots, all their roots exit the brainstem on its ventral surface. Cranial nerves, unlike spinal nerves, do not form plexuses, they have connections only along the periphery, on the paths to the innervated organs.

The sensory parts of the nerves of the branchial arches have nerve nodes (ganglia), in which the bodies of peripheral sensory neurons are located. The sensory ganglia of the trigeminal and facial nerves are located in the cranial cavity, the glossopharyngeal and vagus nerves are outside the skull.

Some cranial nerves (III, IV, VII, X pairs) contain vegetative parasympathetic fibers, which are processes of the vegetative nuclei of these nerves located in the brainstem. These fibers end in parasympathetic nodes, which are located on the periphery near the internal organs or in their thickness. The cranial nerves contain sympathetic fibers that join them as branches of the sympathetic trunk or from the perivascular sympathetic plexuses. Vegetative fibers can pass from one cranial nerve to another.

The cranial nerves are laid down in the fetus very early (from the 5th to the 6th week). Myelination of nerve fibers occurs earliest in the vestibular nerve (at 4 months), and in most other nerves - in the 7th month.