Anatomy-Lexicon

Cranial nerves

Synonyms in the broadest sense

Cranial nerve, cranial nerves, optic nerve, olfactory nerve, oculomotor nerve, trochlear nerve, trigeminal nerve, facial nerve, abducens nerve, vestibulocochlear nerve, glossopharyngeal nerve, vagus nerve

definition

The generic term cranial nerves (Nervi craniales) denotes 12 specific nerves of outstanding importance on each half of the body. For practical reasons, these are usually abbreviated with Roman numerals, i.e. from I (1) to XII (12). All but one (the 11th cranial nerve, accessory nerve) have the common feature that they either arise from the brain and leave it or flow directly into it.

By definition, cranial nerves are actually so-called "peripheral nerves", e.g. comparable to annoythat from the Spinal cord step out and take care of our arms (body nerves). Ultimately, however, there are also deviations from this (e.g. with the 2nd cranial nerve, optic nerve), which mainly are based on the fact that the meninges are grouped differently around the actual nerve fibers of the nerve. Incidentally, behind the medical term of a normal peripheral nerve (e.g. a cranial nerve) there is always a large number of nerve fibers including special connective tissue and not, for example, the extension (axon) of a single nerve cell (Neuron). The finished nerves and their nerve branches then pass partially in the skull or outside of the skull, in certain connective tissue spaces, or also on the neck a very characteristic course, which is described in more detail for the individual nerves. The attending physician must of course not only observe this process during surgical interventions, the cranial nerves are systematically checked as part of a typical neurological examination.

Note: cranial nerve designation

In the following, the cranial nerves are referred to in the text with the Roman numerals I-XII.

Core areas

The order 1-12 of the cranial nerves reflects the height of the position of their core areas (nuclei) for most of the cranial nerves Brain stem again (due to certain peculiarities this assignment does not apply to the 1st, 2nd and 11th cranial nerves, see there). That's how they lie Neuronswhich in their entirety make up the core area of the 4th cranial nerve, for example further up (cranial) in the brain stem than the nerve cells that bundle together to form the 12th cranial nerve.

Each cranial nerve is formally assigned to 1-4 core areas in the brain, smaller nerves such as the 4th cranial nerve originate / flow out of / into only one core area, larger nerves such as the 5th cranial nerve from / into 4 nuclei!
It is also possible that a single core shares fibers with several cranial nerves: The core responsible for our sensitivity in the head area (nucleus spinalis trigeminalis) contains inflows from three cranial nerves, namely V, IX and X.

Function of the cranial nerves

What exactly do the cranial nerves do, what do we need them for?

In short: they guide the sensations of our sensory organs, i.e. what we do see (II), hear (VIII), taste (VII, IX, X), smell (I), in the area of the Head feel (V), information of ours Sense of balance (VIII), information about the Blood pressure in large cervical arteries (IX and X from the carotid body) etc. directly into our brain for further processing. In addition, they transmit commands from the brain to a variety of muscles, such as the Eye muscles (III, IV, VI), the chewing muscles (V), the mimic muscles of the face (VII), the muscles of the throat (IX, X), the muscles of the larynx (X), some neck muscles (XI) and the tongue muscles ( IX, XII). In addition to muscles, glands are also controlled, e.g. Lacrimal gland and Salivary glands as well as numerous reflexes such as the swallowing reflex or the eyelid closing reflex (corneal reflex). Only the Xth cranial nerve, Vagus nerve, supplies areas to us that far exceed the head- / neck area, such as all organs of the chest and abdomen. This shortened list already shows how important our 12 cranial nerves actually are.

Cranial nerve I: Olfactory nerve (also olfactory nerve)

Main function: Conveys olfactory sensations to the brain (see nose)
Common illnesses / failure: The inability to smell (anosmia) or a weakening (hyposmia). Often with anterior skull base fractures.

Cranial nerve II: Optic nerve (also optic nerve)

Main function: conveys the images recorded by the eye to the brain
Common illnesses / failure: Defects in the field of vision, dark spots, hemianopsia, quadrant anopia, blinders phenomenon, etc. Even with high pressure inside the skull: "congestion papilla" (see vision; visual pathway)

Cranial nerve III: Oculomotor nerve

Main function: Controls eye movements as well as all inner eye muscles (e.g. ciliary muscles) and the size of the pupil
Common illnesses / failure: Unequal pupils (anisocoria), narrow pupils (miosis), wide pupils (mydriasis). Gaze paralyzes, the eye looks outwards and downwards, therefore double images possible. Problems with “focusing” on what one sees (accommodation disorders). Drooping upper eyelid (ptosis) (see also course vision, presbyopia)

Cranial nerve IV: Trochlear nerve

Main function: Controls a specific eye muscle
Common illnesses / failure: Eye looks towards the nose and upwards, also double vision

Cranial nerve V: trigeminal nerve (also triple nerve)

Main function: Consists of 3 strong nerve branches, hence the name triple nerve. It conducts sensitive information from the head area to the brain and innervates, among other things. the masticatory muscles
Common illnesses / failure: Failure of the entire nerve is rare, individual branches tend to fall out. Then sensory disturbances in the head (peripheral or central type). When the mouth is opened, the lower jaw moves towards the "diseased side". Painful hypersensitivity of the nerve (trigeminal neuralgia) without an ascertainable cause or also with sinus infections. Failure of the corneal reflex, rarely hearing difficulties (hypacusis)

Cranial nerve VI: Abducens nerve

Main function: Innervates a specific eye muscle
Common illnesses / failure: Eye looks towards the nose, double vision. Often in pathological processes on the sinus cavernosus first visible on VI

Cranial nerve VII: Facial nerve

Main function: Controls i.a. the muscles of the facial expressions convey the taste perception in the front parts of the tongue and innervate a multitude of glands in the head area
Common illnesses / failure: Paralysis in the muscles of the face (difference between central and peripheral facial paralysis). Noises are heard too loud (hyperacusis), loss of taste sensation on the front tongue, burning eyes due to drying out of the conjunctiva / cornea (see conjunctivitis)

Cranial Nerve VIII: Vestibulocochlear nerve (also auditory and equilibrium nerves)

Main function: Conveys the sense of hearing and balance from the inner ear to the brain (see ear)
Common illnesses / failure: Hearing impairment, deafness, disruption in spatial orientation, gaze stabilization and posture. Also dizziness, nausea, tendency to fall and twitching of the eyes (nystagmus)

Cranial nerve IX: Glossopharyngeal nerve

Main function: Directs the taste sensation of the back of the tongue to the brain and innervates many muscles of the throat as well as parts of its sensitivity, also important for the movement of the tongue. Indispensable for the swallowing reflex. Innervated a.o. also the largest salivary gland on the head
Common illnesses / failure: Failure of the gag reflex, problems swallowing and drinking, uvula deviates to the healthy side. Loss of taste in the back of the tongue (especially for the "bitter" sensation). Nasal sounding speaking

Cranial nerve X: Vagus nerve

Main function: Directs taste sensations from the pharynx to the brain, completely supplies the larynx (sensory and motor). Is an important part of the autonomic nervous system and embodies the "parasympathetic nervous system" well into the abdominal area. It also regulates the heart rate, gastric acid secretion, intestinal movement and numerous other functions of internal organs
Common illnesses / failure: Hoarseness when a certain branch (recurrent laryngeal nerve) becomes trapped due to lung cancer (bronchial carcinoma) or bulging of the main artery (aorta, aortic aneurysm) or thyroid surgery. With regard to swallowing and the uvula, as in IX, everything is conceivable. Severe shortness of breath if a certain muscle of the larynx fails (posticus paralysis), change in heart rate (faster and slower possible), less stomach acid and bowel movements etc. possible

Cranial nerve XI: Accessory nerve

Main function: Proportionally supplies 2 important muscles of the neck. The effective fibers of XI actually come from the cervical spinal cord, the name cranial nerve comes from other peculiarities
Common illnesses / failure: Often damage after ENT surgery or lymph node removal on the neck. Then tilt the head to the opposite side of the damage and turn the head to the same side of the damage. Weakness in lifting arm

Cranial nerve XII: Hypoglossal nerve

Main function: Supplies a multitude of muscles in the tongue
Common illnesses / failure: Tongue deviates to the diseased side when sticking out. Speech disorders and difficulty swallowing

Common illnesses

In view of the diverse functions of our cranial nerves, there are theoretically typical failure symptoms or diseases for each of them (see table). However, certain combinations of failures often occur, e.g. Damage to IX, X and XI because they are close to each other at the base of the skull and run through a common hole (jugular foramen).

So-called double vision occurs with disorders (lesions) of III, IV and VI whenever the visual axes of both eyes no longer coincide. This is the case when the eyeballs (bulbi) are aligned differently in the eye socket (orbit).