Cranial nerves

The cranial nerves or cranial nerves are a set of 12 pairs of nerves that emerge directly from the brain. The first two nerves, the olfactory and optic, arise from the brain , while the remaining ten arise from the brain stem.  The names of the cranial nerves are related to their function and are also identified numerically in Roman numerals ranging from I to XII.

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, which can be anterior to posterior.

What are the cranial nerves

The cranial nerves are as follows:

• Olfactory nerve or cranial nerve I: transmits nerve information detected through smell, and therefore is an afferent fiber. It is the shortest of all pairs.
• Optic nerve or cranial nerve II: it is an afferent fiber and transmits information to the brain from the eye.
• Oculomotor nerve or cranial nerve III: it is called the common ocular motor nerve, and it sends orders to the muscles that participate in the movement of the eyes, causing the pupil to dilate or contract.
• Trochlear nerve, pathetic nerve or IV cranial nerve: participates in the movement of the eyes. It arises in the midbrain .
• Trigeminal nerve or cranial nerve V: it is a mixed cranial nerve because they have motor and sensory functions. It sends commands to muscles for chewing and collects tactile, proprioceptive, and pain information in the face and mouth.
• Abducent nerve or cranial nerve VI: make the eye move.
• Facial nerve or cranial nerve VII: sends commands to muscles of the face to develop facial expressions and collects taste data from the tongue.
• Vestibulocochlear nerve or cranial nerve VIII: collects information from the auditory area. It receives data that is heard and allows balance.
• Glossopharyngeal nerve or cranial nerve IX: it is sensitive and motor and influences the tongue and pharynx . It receives information from the taste buds.
• Vagus nerve or cranial nerve X: carries orders to the muscles of the pharynx and larynx . Receive taste information from the epiglottis.
• Accessory nerve or cranial nerve XI: It is known as the spinal nerve. Activates the trapezius and sternocleidomastoid muscles to move the head and shoulders.
• Hypoglossal nerve or cranial nerve XII: activates muscles of the tongue to be able to swallow.

Function of the cranial nerves

The function of the cranial nerves is according to each cranial pair. In this way we can say that the functions are:

• Cranial nerves I and II: they  are the olfactory and optic nerves . They include the common ocular motor nerve, the external ocular motor nerve, and the trochlear nerve . They work by controlling the sense of smell and carrying the information that enters through the eyes to the brain.
• Cranial nerves III, IV and VI:  control movement . Among them are the trochlear nerve and the external ocular motor nerve . Cranial nerve III also regulates the ocular motor nerve, the movement of the eye and the pupil. They are responsible for controlling the pupil when there is much or little light.
• Cranial nerve V: it  is the trigeminal nerve and is responsible for controlling touch , pain sensations in the face and head. It also controls the muscles used for chewing, closing the jaws, and facial sensitivity.
• Cranial nerves VII and VIII: they are the facial nerve and the cochlear vestibule . Control taste, receive information from the ear and control facial expression.
• Cranial Nerves IX and X: The glossopharyngeal nerve controls swallowing function , taste on the tongue, tonsils, and pharynx. Control pain and digestion, heart rate.
• Cranial nerve XI: it is the hypoglossal nerve and controls the different muscles in the tongue.

Real and apparent origin

Each skull pair has a real and an apparent origin. The apparent origin is the place of emergence that the nerves have through the encephalic mass and the real origin is the place where the nerve fibers that make up the nerve originate .

Classification of the cranial nerves

The cranial nerves can be classified into:

• Sensitive nerves: olfactory, optic, acoustic state.
• Motor nerves: common ocular motor, external ocular motor, pathetic, spinal, hypoglossal.
• Mixed nerves: trigeminal, facial, glossopharyngeal, vagus.

Nuclei

Sensory nuclei are somatic and visceral afferents . They include all the sensitive parts that a cranial pair has in the nerve cells which form extensions, thus creating first degree neurons, second degree and third order neurons in which the axons terminate in the cerebral cortex .

Pathologies

The disorders that occur in them can affect taste , sight , facial sensitivity , facial expressions, hearing , speech and balance , some of them can even affect the neck muscles . Some of the pathologies that can occur in the cranial nerves are the following:

• Hyposmia : affects olfactory stimuli and the olfactory nerve. It damages the mucosa and the olfactory bulb.
• Hyperosmia: increased perception of smell.
• Dysosmia : perception of aberrant odor.
• Agnosia : you cannot recognize particular odors.
• Visual acuity and field defects such as Argyll Robertson’s pupil and Horner’s Syndrome.
• Detachment of retina , toxoplasmosis and retinitis
• Optic neuritis that damages the otic nerve.

Examination of the cranial nerves

The scan is done to determine the presence of a disease that affects the cranial nerves. Some of the methods are:

• Offer the patient smells to assess their olfactory ability.
• Visual acuity, campimetry and fundus exams.
• Eyelid inspection.
• Examination of fixation and sustained gaze.
• Assessment of pupillary size and symmetry.
• Photomotor and consensual examination.