Cranial nerves – types, structure, functions, damage. How many cranial nerves are there?

Proven content

Cranial nerves are the most important element among all transmitters of sensory stimuli: sight, hearing, smell, taste, touch. Information received by the sense organs goes to the brain, where it is decoded and processed. The cranial nerves act as the main transmitter and decoder of all the operations that take place in the area of ​​our senses.

Cranial nerves – what are they?

Cranial nerves are nerves that come directly from the brain (including the brainstem). In contrast, spinal nerves emerge from sections of the spinal cord. The cranial nerves transmit information between the brain and parts of the body, mainly to and from the head and neck areas. There are 12 cranial nerves, each named for its function or structure.

Each cranial nerve is paired and occurs on both sides of the brain. The numbering of cranial nerves is based on the order in which they exit the brain from front to back (brainstem). Terminal nerves, olfactory nerves (I), and optic nerves (II) emerge from the brain or forebrain, and the remaining ten pairs emerge from the brainstem, which is the lower part of the brain. The cranial nerves are considered parts of the peripheral nervous system.

Their functions are usually classified as sensory or motor. Sensory nerves are involved in the senses such as smell, hearing, and touch. Motor nerves control the movement and function of muscles or glands.

Spinal nerves emerge sequentially from the spinal cord, with the spinal nerve closest to the head (C1) emerging in the space above the first cervical vertebra. The cranial nerves come out of the central nervous system above this level.

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Cranial nerves – types

The following types of cranial nerves are distinguished:

1. olfactory nerve (I) – it is a nerve, instrumental for the sense of smell, it is one of the few nerves capable of regeneration,
2. optic nerve (II) – this nerve carries visual information from the retina of the eye to the brain,
3. oculomotor nerve (III) Controls most eye movements, constriction of the pupil and keeps the eyelid open
4. block nerve (IV) – the motor nerve that innervates the upper oblique muscle of the eye, which controls rotation
5. nerw trójdzielny (V) – is responsible for the feeling and motor functions of the face and mouth,
6. nerve abductor (VI) – the motor nerve that innervates the lateral rectus muscle of the eye that controls lateral movements
7. facial nerve (VII) – controls the muscles of facial expression and works in transmitting the taste sensations from the front two-thirds of the tongue and mouth,
8. vestibulocochlear nerve (VIII) – this is the nerve responsible for transmitting sound information and balance from the inner ear to the brain
9. glossopharyngeal nerve (IX) – this nerve receives sensory information from the tonsils, throat, middle ear and the rest of the tongue
10. vagus nerve (X) – it is the nerve responsible for many tasks, including heart rate, gastrointestinal motility, sweating and muscle movement in the mouth, including speech and keeping the larynx open while breathing.
11. accessory nerve (XI) – this nerve controls specific muscles in the shoulder and neck
12. sublingual nerve (XII) – this nerve controls tongue movements during speech, food manipulation and swallowing.

The olfactory nerve (I) – structure and functions

The olfactory nerve, or I cranial nerve, is the first of the 12 cranial nerves and is responsible for the sense of smell. The olfactory nerve is the shortest of the 12 cranial nerves and only one of the two cranial nerves (the other is the optic nerve) that do not connect to the brainstem. The cranial nerve I is a special somatic afferent nerve that innervates the olfactory mucosa of the nasal cavity.

The sense of smell arises when the olfactory (or olfactory) receptors are stimulated by small molecules with different spatial, chemical and electrical properties that pass through the nasal epithelium in the nasal cavity during inhalation. These interactions are converted into electrical activity in the olfactory bulb, which then transmits the electrical activity to other parts of the olfactory system and the rest of the central nervous system via the olfactory tract.

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Optic nerve (II) – structure and functions

The optic nerve, or cranial nerve II, is a special somatic afferent nerve that innervates the retina of the eye and provides visual information to the brain.

The optic nerve is considered by physiologists to be part of the central nervous systembecause it stems from a bulge in the diencephalon during embryonic development. The optic nerve conveys all visual information, including brightness perception, color perception, and contrast. It also conducts visual impulses that are responsible for two important neurological reflexes: the light reflex and the accommodative reflex.

The light reflex refers to the constriction of both pupils that occurs when light falls on one eye; the accommodative reflex refers to the swelling of the lens of the eye that occurs when looking at a close object, such as when reading

Oculomotor nerve (III) – structure and functions

The oculomotor nerve (III cranial nerve) controls eye movement. The oculomotor nerve is the third paired cranial nerve. It enters through the upper orbital slit and controls most eye movements, including constriction of the pupil and keeping the eyelid open by innervation of the levator eye muscle of the upper eyelid.

The oculomotor nerve is derived from the basal plate of the embryonic midbrain. The IV and VI cranial nerves are also involved in controlling eye movement.

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Block nerve (IV) – structure and functions

The block nerve (IV cranial nerve) is the motor nerve that innervates a single muscle: upper oblique muscle of the eye. The block nerve is in many ways unique among cranial nerves in that it is the smallest nerve in the number of axons it contains and has the longest intracranial length. It is the only cranial nerve that leaves the dorsal part of the brainstem.

There are two main clinical syndromes that can manifest as block nerve injury:

1. vertical double vision – damage to the block nerve causes reduced downward movement of the eyeballs and, consequently, vertical double vision,
2. torsion double vision – inversion weakness causes torsional double vision, in which two different fields of vision, inclined to each other, are visible at the same time. To compensate for this, patients with block nerve palsy tilt their heads to the opposite side to fuse the two images into a single field of view.

Trigeminal nerve (V) – structure and functions

The trigeminal nerve (V cranial nerve) is the fifth cranial nerve and is responsible for sensation and motor functions of the face and mouth. It is the largest cranial nerve and has sensory and motor functions. The trigeminal nerve is divided into three groups: the optic nerve (CN V1), the maxillary nerve (CN V2) and the mandibular nerve (CN V3).

The sensory function of the trigeminal nerve is to provide tactile sensations, movement, position, pain in the face and mouth, and its motor function activates the muscles of the jaw, mouth and inner ear.

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The abduction nerve (VI) – structure and functions

The abduction nerve (VI cranial nerve) is the somatic drainage nerve that controls the movement of a single muscle in humans: the lateral rectus muscle of the eye that moves the eye horizontally.

The abduction nerve begins in the pons of the brainstem, enters an area known as the Dorello canal, passes through the cavernous sinus, and ends in the lateral rectus muscle in the bone sockets.

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Facial nerve (VII) – structure and functions

The facial nerve (VII cranial nerve) determines the facial expressions and the taste sensations of the tongue. It also functions in transmitting taste sensations from the front two-thirds of the tongue and mouth, and delivers pre-ganglion parasympathetic fibers to several ganglia of the head and neck.

Voluntary facial movements, such as frowning, showing teeth, closing eyes, pursing lips, and puffing out cheeks, are all accounted for by the facial nerve.

Vestibulocochlear nerve (VIII) – structure and functions

The vestibulocochlear nerve (also known as the auditory vestibular nerve and the VIII cranial nerve) contains axons that carry the auditory and balance modalities.

It consists of two parts, the cochlear part and the vestibular part:

1. worm part – specialized cells in the ear detect sound vibrations based on the volume and pitch of the sound. This generates nerve impulses that are transmitted to the cochlear nerve.
2. vestibular part – koil cell set in this part can track both linear and rotational head movements. This information is passed to the vestibular nerve and used to adjust the balance.

Injury to the vestibulocochlear nerve can cause hearing loss, dizziness, a false sense of movement, loss of balance in dark places, nystagmus, motion sickness, and tinnitus.

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Glossopharyngeal nerve (IX) – structure and functions

The glossopharyngeal nerve is the ninth of the 12 pairs of cranial nerves. It exits the brainstem from the sides of the upper spine, only rostrally (closer to the nose) to the vagus nerve.

A benign primary intracranial tumor of the vestibulocochlear nerve is called a vestibular neuroma (also called an acoustic neuroma). The glossopharyngeal nerve has both motor and sensory functions.

The sensory function picks up information from the throat, tonsils, middle ear, and the back of the tongue. It is also involved in tasting the back of the tongue.

Vagus nerve (X) – structure and functions

The vagus nerve, also known as the X cranial nerve, is the tenth of the twelve paired cranial nerves. After leaving the core between the spinal pyramid and the lower pedicel of the cerebellum, it passes through the jugular opening, then into the carotid sheath between the internal carotid artery and the internal jugular vein below the head, to the neck, chest and abdomen, where it contributes to the innervation of the viscera.

In addition to outputting to various organs in the body, the vagus nerve transmits sensory information about the state of the organs of the body to the central nervous system. Eighty to 90% of the nerve fibers in the vagus nerve are afferent (sensory) nerves that tell the brain the state of its bowels.

The vagus nerve is responsible for tasks as diverse as:

1. heart rate
2. gastrointestinal peristalsis,
3. sweating,
4. oral muscle movements including speech (via the retrograde laryngeal nerve)
5. swallowing and keeping the larynx open for breathing (through the action of the posterior cricoid muscle, the only abductor of the vocal folds).

The accessory nerve (XI) – structure and functions

The accessory nerve (XI cranial nerve) controls the sternocleidomastoid and trapezius muscles of the shoulder and neck. It begins in the central nervous system (CNS) and exits the skull through an opening. The accessory nerve provides motor innervation from the CNS to the sternocleidomastoid and trapezius muscles of the neck. The sternocleidomastoid muscle allows the head to tilt and rotate, while the trapezius has several actions for the scapula, including elevation of the shoulder and adduction of the scapula.

Unlike the remaining 11 cranial nerves, the accessory nerve begins outside the skull.

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The sublingual nerve (XII) – structure and functions

The sublingual nerve is the twelfth cranial nerve (XII) and innervates all the external and internal muscles of the tongue, except the palatine tongue muscle. The sublingual nerve controls tongue movements during speech, food manipulation, and is responsible for swallowing.

Proper function of the sublingual nerve is important for the performance of the lingual movements associated with speech. Many languages ​​require the specific, and sometimes atypical, use of the nerve to create unique speech sounds, which can contribute to the difficulties some adults have in learning a new language.

Cranial nerves – damage

Damage to the area of ​​the cranial nerves are among the most serious in terms of consequences for the functioning of the human body. They occur as a result of mechanical damage, such as injuries, accidents, blows, dents, but also as a result of various injuries resulting from diseases.

This includes circumstances such as stroke, heart attack, and long-term illnesses such as multiple sclerosis or Lyme disease. Damage can also occur as a result of poisoning the body with poisonous substances (drugs, drugs, alcohol associated with Korsakoff’s disease).

Damage to the cranial nerves they can be reversible and irreversible, it all depends on the degree of damage and the speed of the medical response. Some damage, especially in the area of ​​the sensory nerves, is very difficult to reverse. This means that it is easier to regain movement abilities, e.g. the ability to adopt appropriate facial expressions, than sight or smell. However, the forecasts are very optimistic. Currently, the use of artificial intelligence allows to repair the so far considered irreversible consequences cranial nerve injuries.