Cranial Nerves: The 12 Nerves Leaving the Brain

The cranial nerves are a set of nerves that leave directly from the brain, unlike the rest of the nerves of the nervous system.

In this article we will see what they are and what characterizes them, what is their location, and what functions they have exactly in the human body.

What are the cranial nerves?

In general, it can be said that the human brain communicates with almost all the nerves of the brain through the spinal cord.

Thus, for example, the information that reaches us about what we touch with our hands is collected by nerves that run through the arm until reaching the spinal cord, and from there to the brain, from where the order will be issued to continue examining the object. This order efferent it will also leave the brain through the spinal cord, and will reach the corresponding arm through the nerve fibers that come out of it.

However, this is not a rule that is always followed, since there are also some nerves that leave directly from the brain, without being born in the spinal cord.

These are the cranial nerves, or cranial nerves, which arise from the lower part of the brain and reach their destination areas through small holes scattered around the base of the skull. From these orifices, the cranial nerves communicate with peripheral areas.

Also, although it may seem strange, not all of these cranial nerves have the function of reaching areas and organs that are in the head. Some extend to the neck and even the abdomen area.

How are the cranial nerves classified and distributed?

The cranial nerves They are so called because they are counted in pairs, as there is one on both the right and the left side of the brain. Thus, there are twelve cranial nerves pointing towards the right hemisphere and another twelve pointing towards the left, symmetrically.

Each pair is numbered with a Roman numeral according to whether the position from which they emerge from the brain more or less close to the frontal area. In fact, cranial nerves can be grouped and categorized according to two criteria: the place from which they depart and their function.

Cranial nerves classified according to their position

• Starting from areas above the brainstem are the pairs I and II.
• Starting from the midbrain (the upper part of the brainstem), are the cranial nerves III and IV.
• Starting from the Varolio bridge (or brainstem bridge), there are the cranial nerves V, VI, VII and VIII.
• Starting from the medulla oblongata (in the lowest part of the brainstem) are the nerves IX, X, XI and XII.

Cranial nerves classified according to their function

• Sensitive: pairs I, II and VIII.
• Related to eye movements (and its parts) and the eyelids: cranial nerves III, IV and VI.
• Related to the activation of muscles of the neck and tongue: cranial nerves XI and XII.
• Mixed cranial nerves: pairs V, VII, IX and X.
• Parasympathetic fibers: nerves III, VII, IX and X.

What are the cranial nerves?

We are going to know below which are the cranial nerves one by one, and their main functions.

1. Olfactory nerve (cranial nerve I)

As its name suggests, This cranial nerve is dedicated to specifically transmitting nervous information about what is detected through the sense of smell, and therefore it is an afferent fiber. It is the shortest of the cranial nerves, since its destination is very close to the area of ​​the brain from which it arises.

2. Optic nerve (cranial nerve II)

It is also part of the afferent fibers, and is responsible for transmitting to the brain the visual information that is collected from the eye. It arises from the diencephalon.

3. Oculomotor nerve (cranial nerve III)

Also know as common ocular motor nerve, this cranial nerve commands most of the muscles involved in eye movement, and causes the pupil to dilate or contract.

4. Trochlear, or pathetic nerve (IV cranial nerve)

Like the oculomotor nerve, this cranial nerve deals with eye movement. Specifically, it signals the superior oblique muscle of the eye. The place from which this pair of nerves arises is the midbrain.

5. Trigeminal nerve (cranial nerve V)

It is one of the mixed cranial nerves, because has both motor and sensory functions. As a motor nerve, it commands the muscles responsible for chewing movements, while that as a sensory cranial nerve collects tactile, proprioceptive and pain information from various areas of the face and mouth.

6. Abducent nerve (6th cranial nerve)

This is another of the cranial nerves in charge of making the eye move. Specifically, it is responsible for producing abduction, that is, the eye moves to the opposite side to where the nose is.

7. Facial nerve (cranial nerve VII)

It is one of the mixed cranial nerves. It is in charge of sending orders to muscles of the face dedicated to creating facial expressions (thus allowing to socialize and communicate correctly) as well as the lacrimal and salivary glands. It also collects taste data from the tongue.

8. Vestibulocochlear nerve (cranial nerve VIII)

It is one of the sensory cranial nerves, and collects information from the auditory area. Specifically, it receives data related to what is heard and the position in which we are with respect to the center of gravity, which allows us to maintain our balance.

9. Glossopharyngeal nerve (cranial nerve IV)

It is both a sensitive and a motor nerve And, as its name suggests, it has an influence on both the tongue and the pharynx (the tube that connects the mouth with the stomach). It receives information from the taste buds of the tongue, but it also sends commands to both the parotid (salivary) gland and neck muscles that facilitate swallowing.

10. Vagus nerve (cranial nerve X)

This cranial pair carries commands to most pharyngeal and laryngeal muscles, sends nerve fibers from the sympathetic system to the viscera that are in the area of ​​our abdomen and receives taste information that comes from the epiglottis. Like the glossopharyngeal nerve, it is involved in the action of swallowing, so it is very relevant given the importance of this vital function.

11. Accessory nerve (cranial nerve XI)

To this cranial pair also it is known as the spinal nerve.

It is one of the pure cranial nerves, and activates the trapezius and sternocleidomastoid muscles, which are involved in the movement of the head and shoulders, so that their signals are noticed in part of the upper chest area. Specifically, it allows the head to be tilted to one side and to be tilted back.

12. Hypoglossal nerve (cranial nerve XII)

Like the vagus and glossopharyngeal nerves, activates muscles of the tongue and participates in the action of swallowing. Thus, it works together with cranial nerves IX and X to allow swallowing to be carried out correctly, something essential for the good state of the organism.

Bibliographic references:

• Cardinali, D.P. (2000). Neurophysiology Manual. Madrid: Díaz de Santos Editions.
• Chrisman, C., Morales, M. (2003). Handbook of practical neurology. Multimedia.
• Davis, M. C., Griessenauer, C. J., Bosmia, A. N.; Tubbs, R. S., Shoja, M. M. "The naming of the cranial nerves: A historical review". Clinical Anatomy. 27 (1): pp. 14 - 19.
• Müller, F and O'Rahilly R (2004). "Olfactory structures in staged human embryos". Cells Tissues Organs (Print) 178 (2): pp. 93 - 116.
• Purves, D. (2011). Neuroscience. Sunderland: Sinauer.
• Snell, R.S. (2003). Clinical neuroanatomy. Mexico D.F.: Panamericana.