There are 12 cranial nerves, a series of sensory and motor nerves that, unlike the common ones that leave the spinal cord, are born directly from the brain.
With more than 86 billion neurons, our brain is the organ par excellence next to the heart, as it centralizes the activity of the nervous system. This small mass of tissue of only about 1,500 grams allows us to think, move and, really, perform any function in the physiological and emotional field.
It is shocking to think that a structure that accounts for less than 2% of the weight of an adult individual has such essential and diverse functionality. However, for information to reach the nerve center, there must be nerve transport “highways” of equal importance, right?
Here come into play the pairs or cranial nerves, twelve pairs of nerves that arise directly from the brainto be distributed by different areas of our body. If you want to know more about the exciting world of neuroscience and specifically about these essential nerves, we encourage you to keep reading.
- We recommend you read: “Human brain: its main parts and functions”
Cranial Nerve Overview
As we have already said in the introductory paragraphs, we are facing 12 pairs of peripheral nerves that emerge from the brainstem, whichare related to motor and sensory activities of the head, neck and trunk. Some of these nerves have sensory functions (hearing, taste, and sight), while others control regions of facial musculature or secretion from certain glands. Cranial nerves can be divided according to their functionality.
- Pairs with sensory functions: olfactory (I), optic (II) and vestibulocochlear or statoacoustic (VIII).
- Pairs with motor functions: the common ocular motor or oculomotor (III), the pathetic or trochlear (IV), the abductor (VI), the spinal accessory (XI) and the hypoglossal (XII).
- Mixed pairs, that is, with sensory and motor functions: the trigeminal (V), the facial (VII), the glossopharyngeal (IX) and the vagus (X).
As we can see, the pairs of cranial nerves are designated by specific names and Roman numerals according to their order of location, from the frontal area of the brain to the back of it. Based on this location, we also classify them into different groups:
- Midbrain: pairs I and II.
- Mesencephalon: pairs III and IV.
- Protrusion: pairs V, VI, VII and VIII.
- Bulb: pairs IX, X, XI, XII.
So, we have a total of 12 pairs of cranial nerves. Although, as we will see in later lines, the first two pairs cannot be considered in a strict sense as authentic nerves (a fact that causes a slight nominal confusion in various medical areas).
The 12 types of cranial nerves
Once we have unraveled the location and general classification of the cranial nerves, it is time to dive briefly into each of them. Let’s go.
Olfactory nerve (pair I)
It collects the olfactory sensitivity of receptors in the nasal mucosa andconducts this information to the olfactory bulb. The extensions of the olfactory cells of the nasal mucosa are sieved through the lamina cribosa of the ethmoid. It should be noted that it is not a peripheral nerve as such, as it is actually peripheral extensions of the neural fiber tracts. In any case, we are not going to dwell on this debate on nomenclature.
Optic nerve (pair II)
It conducts visual impulses from the retina to the occipital cortex, that is, it connects the back of each eye to the brain. It passes through the optic foramen along with the ophthalmic artery and it should be noted that, like olfactory nerves, they are not considered a pair of peripheral nerves to use.
Oculomotor (par III), trochlear (par IV) and abductor (par VI)
We include them in a single group, since all of them are responsible for innervating muscles involved in the movement andmotor control of the eyes. These nerves reach the eye orbit through the superior orbital fissure.
Trigeminal nerve (pair V)
El nervio trigémino se describe como mixto, pues cuenta con una función tanto sensitiva como motora. Es el par craneal de mayor grosor y tiene mucha importancia, pues recoge la sensibilidad facial y es el nervio motor encargado de la musculatura implicada en la masticación. A su vez, el trigémino se divide en un nervio oftálmico (V1), nervio maxilar (V2) y nervio mandibular (V3).
Nervio facial (par VII)
Estamos ante otro nervio craneal mixto, pues presenta un 70 % de neuronas motoras y un 30 % de fibras sensitivas. Se trata del par craneal más complejo de todos, ya que presenta una infinidad de funciones. Entre ellas, podemos listar las siguientes: moviliza los músculos de la cara, participa en la sensibilidad gustativa de la lengua y en la sensibilidad general de algunas partes del oído externo y regula la secreción de saliva y lágrimas.
Nervio vestibulococlear (par VIII)
Nervio aferente de función sensorial que tiene como funciones la audición y el mantenimiento del equilibrio en el individuo. Está compuesto por dos raíces: el nervio coclear (auditivo) y el nervio vestibular (transporta información sobre el equilibrio).
Nervio glosofaríngeo (par IX) y nervio vago (par X)
De nuevo, englobamos a estos dos pares craneales en un mismo grupo por su funcionalidad compartida: gracias a ellos se produce la elevación del paladar y el reflejo nauseoso, es decir, un mecanismo que evita que materiales sólidos o líquidos entren en nuestras vías respiratorias.
Tanto el par glosofaríngeo como el vago son nervios mixtos, pues presentan funciones sensitivas en la lengua, la faringe y la amígdala, así como actividad motora a nivel muscular. Cabe destacar que el par glosofaríngeo presenta actividad moduladora de la glándula parótida, cuya función es la producción de saliva para ayudar a masticar y digerir los alimentos.
Nervio espinal (XI)
Un nervio exclusivamente motor. Este par, ante todo, nos permite realizar el movimiento de “echar la cabeza hacia atrás”. Es el responsable de inervar el músculo esternocleidomastoideo y las tres porciones del músculo trapecio (trapecio superior, medio e inferior).
Nervio hipogloso (XII)
El último de los pares craneales, pero no por ello el menos importante. Se trata de un nervio motor, el cual es el encargado de la coordinación de los movimientos de la lengua (de ahí su nombre).
Enfermedades
Como con todas las estructuras importantes, los fallos en cualquiera de ellas pueden comprometer el bienestar individual de forma drástica. Por ejemplo, pueden suceder trastornos que dañan directamente a los nervios craneales, como son los traumatismos directos, aparición de tumores, infecciones (como la producida por el virus herpes zóster), un aporte inadecuado de sangre o el consumo de ciertos fármacos y toxinas.
The pathology will depend on the affected pair, as the patient may experience total or partial loss of vision in one of the two eyes, its movement, loss of facial sensitivity, hearing difficulty, swallowing or speaking problems and many other pathologies related to the neck and head. Depending on whether the affected pairs are motor or sensory, muscle paralysis or decreased one of the senses will be the dominant symptoms in these disorders.
For all these reasons, there are multiple guides for physiotherapists and doctors that explore possible disorders in the cranial nerves: from difficulty swallowing to loss of balance, the symptoms are very varied.
Summary
As we have seen in these lines, cranial nerves are 12 pairs of nerves that arise directly from the brain and branch directly to the eyes, ear, nose and muscles, in order to receive sensory signals, maintain balance and modulate the motor activity of the individual.
After this brief summary of each of the cranial nerves, it is clear that each of these two nerves in each pair play a specific function in the head or its surroundings: from taste to tongue movement, through smell, hearing, balance and multiple motor activities, we can affirm thatcranial nerves are essential for our senses and proper physiological functioning.
All this information shows that yes, the brain is very important, but there are another series of nerves and “highways” without which human beings could not live. In the physiology of our species, every cell and tissue is essential to our overall functioning and well-being.
References
- Chapter 1: cranial nerves. Collected on October 30 in http://www.paidotribo.com/pdfs/769/769.0.pdf
- Exploration of cranial nerves, reminder of semiology, medfam.com. Collected on October 30 in https://medfamcom.files.wordpress.com/2012/08/pares-craneanos.pdf
- Gutiérrez, J. C., Pérez, P. C., & García, M. S. (2011). Pathology of cranial nerves. Medicine-Accredited Continuing Medical Education Program, 10(71), 4793-4805.
- Gutiérrez, J. C., Pérez, P. C., & García, M. S. (2011). Pathology of cranial nerves. Medicine-Accredited Continuing Medical Education Program, 10(71), 4793-4805.
- Introduction to cranial nerves, MSDmanuals.com. Collected at 30 October in https://www.msdmanuals.com/es-es/hogar/enfermedades-cerebrales,-medulares-y-nerviosas/trastornos-de-los-pares-craneales/introducci%C3%B3n-a-los-pares-craneales
- NERVOUS SYSTEM: clinical examination of cranial nerves, clinical propaedeutics and medical semiology (Volume I). Collected at October 30 in http://www.sld.cu/galerias/pdf/sitios/pdguanabo/cap15.pdf
To the classic question “what do you do?” I always answer “basically I am a psychologist”. In fact, my academic training has revolved around the psychology of development, education and community, a field of study influenced my volunteer activities, as well as my first work experiences in personal services.