Cranial Nerves: What are they? Origin, Modalities and Classification

They are a set of 12 pairs of nerves that arise directly from the brain.

The first two nerves (olfactory and optical) arise from the brain, while the remaining ten emerge from the brainstem .

The names of the cranial nerves are related to their function and also identified numerically in Roman numerals (I-XII).

Origin of the cranial nerves

There are twelve cranial nerves in total. The olfactory nerve (CN I) and the optic nerve (CN II) originate in the brain.

The cranial nerves III – XII arise from the brainstem. They may arise from a specific part of the brainstem (mesencephalon, protuberance or marrow) or from a union between two parts:

  • Mesenteric brain: the trochlear (IV) nerve comes from the posterior side of the mesencephalon. It has the longest intracranial length of all the cranial nerves.
  • Mesencephalic-pontine junction – oculomotor (III).
  • Pons – trighemino (V).
  • Ponto-medullary junction – abducens, facial, vestibulocochlear (VI-VIII).
  • Medulla oblongata posterior to the olive: glossopharyngeal, vagus, accessory (IX-XI).

The cranial nerves are numbered by their location in the brainstem (superior to inferior, then medial to lateral) and the order of their exit from the skull (anterior to posterior).

Modalities

Simplified, each cranial nerve can be described as sensory, motor or both. They can transmit more specifically seven types of information; three are exclusive of the cranial nerves (SSS, SVS and SVM).

Sensory modalities (afferent):

  • General somatic sensorial (GSS): general feeling of the skin.
  • General visceral sensory (GVS): general feeling of the viscera.
  • Special somatic sensorial (SSS): senses derived from the ectoderm (sight, sound, balance).
  • Sensory visceral special (SVS): senses derived from the endoderm (taste).

Motor modalities (efferent):

  • General somatic motor (GSM): skeletal muscles.
  • General visceral motor (GVM): smooth muscles of the intestine and the autonomous motor.
  • Special visceral motor (SVM): muscles derived from the pharyngeal arches.

Classification of the 12 pairs of cranial nerves

Each cranial nerve is paired and is present on both sides. There are twelve pairs of cranial nerves, which are assigned Roman numerals I-XII, sometimes also includes the cranial nerve zero. There are XII cranial nerves in the left hemisphere of the brain and exactly the same in the right hemisphere.

The numbering of the cranial nerves is based on the order in which they arise from the brain and the function they perform.

Before beginning, it is important to point out that the order that this explanation will have will be according to the corresponding Roman number assigned to the cranial nerve.

I- Olfactory nerve:

It is the first of the 12 pairs of cranial nerves. It is a sensory nerve, in charge of transmitting olfactory stimuli from the nose to the brain. Its real origin is given by the cells of the olfactory bulb. It’s the shortest cranial pair of all.

II- Optic nerve:

This cranial pair is the second of the 12 pairs of cranial nerves and is responsible for performing visual stimuli from the eye to the brain. It is made of axons of the ganglion cells of the retina, which carry information from the photoreceptors to the brain, where it will later be integrated and interpreted.

III- Oculomotor:

This cranial nerve is also known as the common ocular motor nerve. It is the third of the 12 pairs of cranial nerves. It controls eye movement and is also responsible for the size of the pupil. It originates in the mesencephalon.

IV- Trochlear:

This nerve has a motor and somatic functions that are connected to the superior oblique muscle of the eye, being able to cause the eyeballs to move and rotate. Its nucleus also originates in the mesencephalon and ocular motor nerve. It is the fourth of the 12 pairs of cranial nerves.

V- Trigeminal:

It is a mixed cranial nerve (sensitive, sensory and motor), being the largest of all cranial nerves, it is the fifth of the 12 pairs of cranial nerves.

Its function is to bring sensitive information to the face, to transmit information for the chewing process. The sensory fibers transmit sensations of touch, pain and temperature from the front of the head, including the mouth and also from the meninges.

VI nerves;

It is also known as the cranial nerve of the external ocular motor and is the sixth of the 12 pairs of cranial nerves. It is a cranial motor torque, responsible for transmitting motor stimuli to the external rectus muscle of the eye and, therefore, allows the eye to move towards the opposite side to the one we have in the nose.

VII-Facial o Intermedio:

This is another mixed cranial pair since it consists of several nerve fibers that perform different functions, such as ordering the muscles of the face to create facial expressions and also send signals to the salivary and lacrimal glands. On the other hand, it collects flavor information through the language.

It is the seventh of the 12 pairs of cranial nerves.

VIII- Vestibulo-Cochlear:

It is a sensory cranial nerve. It is also known as the auditory and vestibular nerve, thus forming vestibulocochlear. He is responsible for balance and orientation in space and auditory function. It is the eighth of the 12 pairs of cranial nerves.

IX-Glossopharyngeal:

It is a nerve whose influence lies in the tongue and pharynx. Collect information from the taste buds (tongue) and sensory information from the pharynx. Take orders to the salivary gland and several neck muscles that help swallow. It also monitors blood pressure. It is the ninth of the 12 pairs of cranial nerves.

X-Vagus:

This nerve is also known as pneumogastric. It emerges from the medulla and supplies nerves to the pharynx, esophagus, larynx, trachea, bronchi, heart, stomach and liver.

Like the anterior nerve, it influences the action of swallowing, but also in the sending and transmission of signals to our autonomous system, to help regulate the activation and control of stress levels or send signals directly to our sympathetic system . It is the tenth of the 12 pairs of cranial nerves.

XI-Accessory:

This cranial nerve is called the spinal nerve. It is a motor nerve and could be understood as one of the most “pure”. It governs the movements of the head and shoulders by supplying the sternocleidomastoid and trapezius muscles in the regions (anterior and posterior) of the neck.

The spinal nerve also allows us to throw our heads back. Therefore, we would say that it intervenes in the movements of the head and shoulders. It is the eleventh of the 12 pairs of cranial nerves.

XII-hypoglossal:

It is a motor nerve that, like the vagus and the glossopharyngeal, is involved in the muscles of the tongue, swallowing and speech. It is the twelfth of the 12 pairs of cranial nerves.