The cranial nerves are a set of nerves that come 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 their location is, and what functions they have in the human body .
What are the cranial pairs?
Generally speaking, it can be said that the human brain communicates with almost all the nerves in the brain through the spinal cord.
Thus, for example, the information that comes to us about what we touch with our hands is collected by nerves that run through the arm to the spinal cord, and from there to the brain, from where the order to further examine the object will be issued. This efferent order will leave the brain also 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 come directly from the brain, without being born in the spinal cord. These are the cranial pairs, or cranial nerves , which emerge from the lower part of the brain and reach their destination areas through small holes distributed at the base of the skull. From these holes, the cranial pairs 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 abdominal area.
How are the cranial pairs classified and distributed?
Cranial pairs are so called because they are counted in pairs, as there is one on both the right and left sides 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 depending on whether the position from which they exit the brain is closer to the front. In fact, the cranial nerves can be grouped and classified into categories according to two criteria : the place from which they start and their function.
Cranial pairs classified according to their position
- Starting from areas above the brain stem are pairs I and II .
- Starting from the midbrain (the upper part of the brain stem), there are the cranial pairs III and IV .
- Starting from the pons of Varolo (or trunk-brain bridge), there are the cranial nerves V, VI, VII and VIII .
- Starting from the spinal cord (in the lowest part of the brain stem) are the nerves IX, X, XI and XII .
Cranial pairs classified by function
- Sensitive : pairs I, II and VIII.
- Related to the movements of the eyes (and their parts) and the eyelids: the cranial pairs III, IV and VI.
- Related to activation of neck and tongue muscles : cranial pairs XI and XII.
- Mixed cranial nerves : pairs V, VII, IX and X.
- Parasympathetic fibres : nerves III, VII, IX and X.
What are the cranial pairs?
We will now know which are the cranial pairs 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 nerve information about what is detected through the sense of smell , and is therefore an afferent fibre. It is the shortest of the cranial pairs, as its destination is very close to the area of the brain from which it arises.
2. Optic nerve (cranial pair II)
It is also part of the afferent fibres, and is responsible for transmitting the visual information collected from the eye to the brain . It arises from the diencephalon.
3. Oculomotor nerve (cranial nerve III)
Also known as the common ocular motor nerve , this cranial nerve sends commands to most of the muscles involved in eye movement , and causes the pupil to dilate or contract.
4. Trochlear nerve, or pathetic (IV cranial nerve)
Like the oculomotor nerve, this cranial pair deals with eye movement . Specifically, it sends signals to the upper oblique muscle of the eye. The site from which this pair of nerves arises is the midbrain.
5. Trigeminal nerve (cranial nerve V)
It is one of the mixed cranial pairs, because has both motor and sensory functions . In its motor nerve facet, it sends commands to the muscles responsible for performing the movements of mastication, while as a sensory cranial nerve it collects tactile, proprioceptive and pain information from various areas of the face and mouth.
6. Abducent nerve (cranial nerve VI)
This is another of the cranial pairs responsible for making the eye move . Specifically, it is in charge of producing the abduction, that is, that the eye moves to the side opposite to where the nose is.
7. Facial nerve (cranial nerve VII)
It’s one of the mixed cranial pairs. It is in charge of sending commands to both the face muscles dedicated to creating facial expressions (thus allowing correct socialization and communication) and the lacrimal and salivary glands. It also collects taste data from the language.
8. Vestibulocochlear nerve (cranial nerve VIII)
It is one of the cranial sensory pairs, and collects information from the auditory zone . Specifically, it receives data relating to what is heard and the position in which we are with respect to the centre of gravity, which allows us to maintain our balance.
9. Glossopharyngeal nerve (cranial nerve IV)
It is both a sensory and a motor nerve and, as its name suggests, it influences both the tongue and the pharynx (the tube that connects the mouth to the stomach). It receives information from the taste buds of the tongue, but also sends commands to both the parotid gland (salivary) and the muscles of the neck that facilitate the action of swallowing.
10. Vagus nerve (cranial nerve X)
This cranial pair carries commands to most of the pharyngeal and laryngeal muscles , sends nerve fibres from the sympathetic system to viscera in the area of our abdomen and receives gustatory information from the epiglottis. Like the glossopharyngeal nerve, it is involved in the action of swallowing, so it is very important given the importance of this vital function.
11. Accessory nerve (cranial pair XI)
This cranial pair is also known as the spinal nerve .
It is one of the pure cranial pairs, and activates the trapezium and sternocleidomastoid muscles , which are involved in the movement of the head and shoulders, so that their signals are felt in part of the upper chest. In particular, it allows the head to be tilted to one side and to lean back.
12. Hypoglossal nerve (cranial nerve XII)
Like the vagus nerve and the glossopharyngeal, a activates muscles of the tongue and participates in the action of swallowing . Thus, it works together with the cranial pairs IX and X to allow the swallowing to be carried out correctly, something fundamental for the good state of the organism.
- Cardinali, D.P. (2000). Manual of Neurophysiology. Madrid: Ediciones Díaz de Santos.
- Chrisman, C., Morales, M. (2003). Manual of practical neurology. Multimedical.
- 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 City: Panamericana.