Generality
The cranial nerves are the nerves that originate from the brain and connect the latter to different regions of the head, neck and trunk.
Organized in pairs, the cranial nerves are altogether 12 pairs, for a total of 24 nerve structures.
The cranial nerves are known by a simple name, based on the use of the first 12 Roman numerals, and with a more articulated denomination, which refers to anatomy and / or function.
Short review of what Nerves are
Resulting from the combination of a group of axons, the nerves are the structures of the nervous system which have the important task of spreading the signals necessary for muscular movement, transporting sensitive information and controlling reflex responses.
The human nervous system includes 3 types of nerves:
- The efferent nerves (or motor nerves ), which carry information from the central nervous system (CNS) to the so-called periphery. These nerves control the movements of the muscles, so I am in charge of the motor sphere.
- Afferent nerves (or sensory nerves ), which carry information from the periphery to the central nervous system. These nerves have the task of picking up what happens on the skin surface, on the organs, etc. and communicate it to the SNC, waiting for a reply.
The afferent nerves are at the head of the sensitive sphere.
- Mixed nerves, which play the role of both efferent nerves and afferent nerves.
What is the axon?
The axon is the characteristic extension of neurons (ie the cells of the nervous system) which is used for long-distance transport of nerve signals.
Without the axon, neurons could not communicate with each other and could not control their muscles, skin sensitivity, etc.
What are cranial nerves?
The cranial nerves are the nerves that originate from the brain and connect the latter to different regions of the head, neck and trunk .
Comparable to electrical cables, cranial nerves are equal nerve structures - that is, organized in pairs (or pairs ) - which is fundamental for their more or less symmetrical distribution on both halves of the human body.
According to the most classic of anatomical visions, in adult humans, cranial nerves are organized into 12 pairs, for a total of 24 nerves; it is necessary, however, to point out from these first lines that, for several years, a debate has been underway on the existence and inclusion of a thirteenth nervous structure in the list of cranial nerves.
Among the cranial nerves, there are:
- Motor nerves, that is intended to control muscles;
- Sensitive nerves, that is to say, controlling the sensitivity of a particular part or organ of the human body;
- Mixed nerves, ie provided with axons with motor function and axons with sensory function.
Cranial nerves, spinal nerves and peripheral nervous system
Together with the spinal nerves - the nerves that originate from the spinal cord - the cranial nerves constitute the so-called peripheral nervous system ( SNP ).
The peripheral nervous system is the set of nerves appointed to:
- Check communication between organs of the central nervous system (these organs are the brain and spinal cord) e
- Transmit information from the central nervous system to the periphery and vice versa (NB: by periphery we mean muscles, skin surface, internal organs, glands etc.).
Name
For the cranial nerves, the neuro-anatomists have thought of a double denomination : a simpler name, based on the use of the first 12 Roman numerals (therefore I, II, III ... XII), and a more articulated denomination, which refers to the function and / or anatomy .
The result of this double denomination is the following:
- The I (first) pair of cranial nerves (or I cranial nerve) is, in the more articulated denomination, the so-called olfactory nerve ;
- The second pair of cranial nerves (or II cranial nerve) is the so-called optic nerve :
- The III pair of cranial nerves (or III cranial nerve) is the so-called oculomotor nerve ;
- The IV pair of cranial nerves (or IV cranial nerve) is the so-called trochlear nerve ;
- The V pair of cranial nerves (or V cranial nerve) is the so-called trigeminal nerve ;
- The VI pair of cranial nerves (or VI cranial nerve) is the so-called abducent nerve ;
- The VII pair of cranial nerves (or VII cranial nerve) is the so-called facial nerve or facial nerve ;
- The VIII pair of cranial nerves (or VIII cranial nerve) is the so-called vestibulocochlear nerve ;
- The IX pair of cranial nerves (or IX cranial nerve) is the so-called glossopharyngeal nerve ;
- The X pair of cranial nerves (or X cranial nerve) is the so-called vagus nerve ;
- The XI pair of cranial nerves (or XI cranial nerve) is the so-called accessory nerve ;
- The XII pair of cranial nerves (or XII cranial nerve) is the so-called hypoglossal nerve .
As readers will have noticed, when the name based on the use of Roman numerals is used, it is possible to refer to the cranial nerves as both a singular formula (eg: X cranial nerve) and a plural formula (eg: X pair of cranial nerves ); when instead we use the denomination that considers function and / or anatomy, we tend to refer to the cranial nerves exclusively with a singular formula (eg: vagus nerve), although we are talking about a pair of nervous structures.
The XIII pair of Cranial Nerves or Terminal Nerve
For those who admit the existence of 13 pairs of cranial nerves, the further nervous structure is the so-called XIII pair of cranial nerves or terminal nerve .
Did you know that ...
The terminal nerve is also known as zero nerve or N nerve .
Features
All cranial nerves arise from the brainstem, except the first pair (or olfactory nerve), which originates from the telencephalon, and the second pair (or optic nerve), which derives from the diencephalon .
Each pair of cranial nerves is a pair of axon bundles that follows a precise course (the course also includes the exact point of connection with the brain), which could be divided into branches (the branches are the branches of a nerve) and which covers a specific function.
To understand the next steps ...
- The encephalon can be divided into four main portions: the telencephalon (or brain proper), the diencephalon, the cerebellum and the brainstem.
- The brainstem is, in fact, the portion of the brain that precedes the spinal cord located inside the spinal column; this important nervous structure comprises three sections: an upper section, called the mesencephalon, an intermediate section, called the Varolio bridge, and a lower section, called elongated medulla .
I pair of Cranial Nerves or Olfactory Nerve
The olfactory nerve is the pair of cranial nerves with a sensory function responsible for transmitting olfactory information to the brain; the olfactory nerve, in fact, is connected, through the so-called olfactory bulb, to the olfactory epithelium of the nasal cavities, which has the important task of capturing the smells and turning them into signals understandable to the human nervous system.
The olfactory nerve is born in the telencephalon, it joins the olfactory bulbs that dominate the ethmoid bone (a skull bone), passes through the holes of the cribroso plate (a particular portion of the aforementioned ethmoid) and, finally, ends at the level of the receptors of olfactory epithelium.
The pair of Cranial Nerves or Optic Nerve
The optic nerve is the pair of cranial nerves with sensory function, having the task of transmitting visual information to the brain; the optic nerve, in fact, is connected to the retina of the eyes, which is the seat of the receptors responsible for capturing light and transforming it into signals understandable to the nervous system.
The nerve is born in the diencephalon and, before connecting to the ocular retina, passes through the so-called optical channel of the sphenoid bone (a skull bone) and is the protagonist of a crossing of nerve fibers known as the optic chiasm .
III pair of Cranial Nerves or Oculomotor Nerve
The oculomotor nerve is the pair of cranial nerves with motor function, designed to control most of the extrinsic musculature and the intrinsic musculature of the eyes ; the oculomotor nerve, in fact, is connected to:
- The elevator of the upper eyelid, that is the striated skeletal muscle designated to the elevation of the upper eyelid;
- The superior rectum, the medial rectum, the inferior rectum and the inferior oblique, or 4 of the 6 striated skeletal muscles that govern movement of the eyeball;
- The pupillary sphincter, that is the smooth muscle that allows the constriction of the pupil (miosis);
- The ciliary muscle, which is the smooth muscle that regulates the accommodation of sight in relation to objects placed at different distances.
The oculomotor nerve emerges from an anterior area of the brainstem, located between the midbrain and the Varolio bridge, and known as the ponto-mesencephalic junction; from here, it proceeds towards the aforementioned muscles, taking, to escape from the skull, the upper orbital fissure, a characteristic opening of the sphenoid bone.
IV pair of Cranial Nerves or Trochlear Nerve
The trochlear nerve is the pair of cranial nerves with motor function, in charge of innervating (therefore controlling) one of the 6 striated skeletal muscles involved in the movement of the eyeball: the so-called upper oblique muscle (NB: previously 4 others are indicated: the superior rectum, the medial rectum, the inferior rectum and the inferior oblique).
The trochlear nerve originates from the brainstem, to be precise from a posterior area of the midbrain; from here, it proceeds towards the eyes, taking, to get out of the skull, the already mentioned superior orbital fissure.
Did you know that ...
The upper oblique muscle allows lateral rotation of the eye.
V pair of Cranial Nerves or Trigeminal Nerve
The trigeminal nerve is the pair of cranial nerves with a mixed (ie, sensory and motor) function, which, at some point in its course, separates into 3 branches:
- The sensory branch known as ophthalmic nerve, whose job is to send to the brain the sensitive information coming from:
- The forehead and the front part of the scalp;
- Frontal sinuses and ethmoid sinuses;
- Upper eyelids and associated conjunctivae;
- The corneas;
- The dorsal region of the nose.
- The sensory branch known as the maxillary nerve, whose job it is to send to the brain the sensitive information coming from:
- The lower eyelids and associated conjunctivae;
- The dura mater encephalic (it is the most external meninge of the encephalon);
- Cheeks and maxillary sinuses;
- The mucosa of the nasal cavities and the lateral areas of the nose;
- The upper lip;
- The upper dental arch (incisors, canines and molars) and the associated gingiva;
- The hard palate and the soft palate.
- The mixed branch known as the mandibular nerve, which has two tasks:
- Send to the brain the sensitive information coming from the mucosa of the floor of the mouth, from the mucosa of the cheeks, from the external ear, from the 2/3 anterior of the tongue, from the lower lip, from the chin, from the lower dental arch (incisors, canines and molars) and from the gingiva associated with the latter, e
- Innervate the muscles of mastication (medial pterygoid, lateral pterygoid, masseter and temporal muscle), the tensor vein of the palatal veil, the tensor tympanic muscle, the digastric muscle at the level of the so-called anterior abdomen and the muscle mylohyoid.
The trigeminal nerve emerges from the sides of the Varolio bridge, in the brainstem; from here, it proceeds towards the so-called Meckel cable, crosses it and immediately afterwards splits into the three branches mentioned above.
Each of the three branches of the trigeminal nerve takes different roads to exit the skull: the ophthalmic nerve passes through the upper orbital fissure of the sphenoid, the maxillary nerve passes through the round hole of the sphenoid and, finally, the mandibular nerve passes through the oval hole of the sphenoid.
Did you know that ...
The trigeminal nerve can be the object of a neurological condition known as trigeminal neuralgia .
An example of neuropathy, trigeminal neuralgia is a chronic syndrome, which mainly causes severe pain in areas of the face innervated by the trigeminal nerve.
VI pair of Cranial Nerves or Abducent Nerve
The abducent nerve is the pair of cranial nerves with motor function, responsible for innervating one (the last, for this article) of the 6 striated skeletal muscles involved in the movement of the eyeball: the so-called lateral rectus muscle .
The abducent nerve emerges from an area of the brainstem located between the Varolio bridge and the medulla oblongata, and known as the bulbo-pontine (or medulla-pontine ) junction ; from here, it proceeds towards the aforementioned muscle, taking the upper orbital fissure to escape from the skull.
Did you know that ...
The lateral rectus muscle is the striated skeletal muscle of the human body on which the possibility of moving the eyes sideways depends.
VII pair of Cranial Nerves or Facial Nerve
The facial nerve is the pair of cranial nerves with mixed function (therefore both sensory and motor), which, through a series of branches, provides:
- Innervating the muscles on which facial expression (or facial expression) depends;
- Innervating the digastric muscle at the level of the posterior belly and the stylohyoid muscle, two muscles involved in the swallowing process;
- Check the sensitivity of the external ear;
- Check the lacrimal glands and the sublingual and submandibular salivary glands;
- Check the perception of taste on the front 2/3 of the tongue and on the hard and soft palate;
- Innervate the muscle stapedius of the ear, whose task is to regulate the movement of the three bones of the middle ear.
The facial nerve emerges from the brainstem, at the bulbo-pontine junction (lateral to the VI pair of cranial nerves); from here, after a short while, it already makes a first branch, which leads it to escape from the skull through two distinct openings, both however located on the temporal bone : the so-called internal acoustic meatus and the stylomastoid hole ; outside the skull, therefore, it separates further, so as to be able to fulfill all the functions indicated above.
VIII pair of Cranial Nerves or Vestibulocochlear Nerve
The vestibulocochlear nerve is the pair of cranial nerves with sensory function, charged with controlling balance and transmitting to the brain the acoustic information captured by the ear; the vestibulocochlear nerve, in fact, is the nervous structure from which it originates:
- The nerve connected to the vestibular apparatus, ie the component of the inner ear on which the ability to balance depend. This branch of the vestibulocochlear nerve is the so-called vestibular nerve ;
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- The nerve connected to the cochlea, ie the component of the inner ear on which the ability to pick up vibrations and turn them into signals understandable to the nervous system depends. This branch of the vestibulocochlear nerve is the so-called cochlear nerve .
The vestibulocochlear nerve emerges from the brainstem, at the level of the often mentioned bulbo-pontine junction (lateral to the VII pair of cranial nerves); from here, to escape from the skull, he heads towards the internal acoustic meatus of the temporal bone and crosses it; after leaving the skull, it is divided into the aforementioned branches, buccal and cochlear.
IX pair of Cranial Nerves or Glossopharyngeal Nerve
The glossopharyngeal nerve is the pair of cranial nerves with mixed function (therefore both sensory and motor), which, through a series of branches, provides:
- Innervate the stilopharyngeal muscle, one of the longitudinal muscles of the pharynx;
- Check the parotid, the most voluminous salivary gland present in the human body;
- Check the sensitivity of the posterior third of the tongue;
- Check the sensitivity of the carotid sinuses, the carotid body, the pharynx (oropharynx in particular), the tonsils, the middle ear, the Eustachian tube.
The glossopharyngeal nerve emerges from the section of the brainstem known as medulla oblongata (to be exact from a posterior area of the latter); from here, it proceeds towards the jugular hole - an opening located at the base of the skull, between the temporal bone and the occipital bone - and take it, in order to escape from the skull; once out of the skull, it separates into the branches through which it provides the aforementioned functions.
X pair of cranial nerves or vagus nerve
Also known as the pneumogastric nerve, the vagus nerve is the pair of cranial nerves with a mixed function (therefore both sensory and motor), which, through a series of branches, is distributed at the level of the ear, neck, thorax and abdomen, with the purpose of:
- Send sensory information from the external auditory canal and the tympanic membrane to the brain;
- Innervating the mucosal glands of the pharynx and larynx;
- Innervate the striated skeletal muscles of the pharynx (except the stilopharyngeal muscle) and of the larynx, the muscles destined for phonation and the extrinsic muscle of the tongue, necessary for swallowing, called palatoglossus ;
- Send sensory information from the root of the tongue from the epiglottis to the brain;
- Innervate the smooth muscles of the trachea, bronchi, lungs and heart and of the organs of the digestive tract used for peristalsis (esophagus, stomach and intestine);
- Send to the brain the sensitive information coming from the thoracic organs trachea, esophagus, bronchi, lungs and heart, and from the abdominal organs pancreas, spleen, stomach, kidneys, adrenals and intestine;
- Send to the brain the sensitive information coming from the large blood vessels of the neck (eg: carotids) and the chest (eg: aortic arch);
- When needed, stimulate gastric, pancreatic, biliary and / or enteric secretions, reduce heart rate, promote peristalsis and produce vasodilation.
The vagus nerve emerges from a posterior portion of the medulla oblongata; from here, to escape from the skull, he heads towards the already mentioned jugular hole and crosses it; once outside the skull, it branches off into numerous branches, which allow it to reach the ear, neck, chest and abdomen.
The vagus nerve is the longest cranial nerve pair in the human body.
Did you know that ...
The vagus nerve is the main representative of the nervous structures that make up the parasympathetic nervous system .
XI pair of Cranial Nerves or Accessory Nerve
The accessory nerve is the pair of cranial nerves with motor function, responsible for controlling the musculature that serves to rotate, flex and extend the neck (and the head), and the musculature that allows the lifting and adduction of the shoulders ; the accessory nerve, in fact, is connected to the sternocleidomastoid muscle (for the movement of the neck and head) and to the trapezius muscle (for the movement of the shoulders).
The accessory nerve emerges from a posterior area of the medulla oblongata; from here, he heads towards the jugular hole and takes it, to get out of the skull; once outside the skull, it moves towards the aforementioned muscles.
XII pair of Cranial Nerves or Hypoglossal Nerve
The hypoglossal nerve is the pair of cranial nerves with motor function, responsible for controlling the musculature that governs the movement of the tongue and the control of a pair of neck muscles involved in the swallowing process; the hypoglossal nerve, in fact, is connected to:
- The extrinsic muscles of the tongue known as genioglosso, ioglosso and stiloglosso ;
- The so-called intrinsic language muscles ;
- The muscles of the genioid and thyroid neck.
The hypoglossal nerve emerges from an anterior area of the medulla oblongata; from here, to escape from the skull and then move towards the aforementioned muscles, he starts towards the hypoglossal canal - an opening of the occipital bone - and passes through it.
XIII pair of Cranial Nerves or Terminal Nerve
The XIII pair of cranial nerves is a nervous structure whose precise function is still unclear; according to the most reliable hypotheses, however, it would seem to be involved in the mechanisms of the reception of pheromones .
From the purely anatomical point of view, the XIII pair of cranial nerves emerges near the first pair of cranial nerves (olfactory nerve) and imitates the path, except not to stop at the olfactory bulb.
