With ” cranial nerves “, also called ” brain nerves “, we indicate a group of nerves that originate directly from the brain, more precisely from thebrain stem, and which are part of the peripheral nervous system. In the human body there are 12 pairs of cranial nerves (24 in total, there is a right and left nerve), numbered with the Roman numeration. Considering their origin, they are numbered in the cranio-caudal sense, that is from top to bottom, and they are:
- I cranial nerve (first): olfactory nerve ;
- II cranial nerve (second): optic nerve ;
- III cranial nerve (third): oculomotor nerve ;
- IV cranial nerve (fourth): trochlear nerve ;
- V cranial nerve (fifth): trigeminal nerve ;
- VI cranial nerve (sixth): abducent nerve ;
- VII cranial nerve (seventh): facial nerve ;
- VIII cranial nerve (eighth): vestibulocochlear nerve ;
- IX cranial nerve (ninth): glossopharyngeal nerve ;
- Cranial nerve (tenth): vagus nerve ;
- XI cranial nerve (eleventh): accessory nerve ;
- XII cranial nerve (twelfth): hypoglossal nerve .
A “0” cranial nerve has also been identified, also called a “terminal nerve” or “N nerve”.
The cranial nerves are numbered according to their rostral-caudal (anterior-posterior) position, in the observation of the brain. If the brain is carefully removed from the skull, the nerves are typically visible in their numerical order, with the exception of the latter, the XII, which appears to emerge rostrally (above) in the eleventh.
The cranial nerves run both inside and outside the skull. Paths inside the skull are called “intracranial” while those outside “extracranial”. In the skull there are many holes called ” foramina ” through which the nerves can exit from it. All cranial nerves are paired, which means that they are located on both sides, right and left, of the body. If a nerve innervates a muscle, the skin or performs a function on the same side of the body from which it originates, its function is called “ipsilateral”, if instead it is on the opposite side this is known as “contralateral” function.
Cranial and spinal nerves
The cranial nerves are distinguished from the spinal nerves which instead originate from the spinal cord. Similarly to the spinal nerves they are lined with meninges (pious mother, arachnoid, dura mater) and emerge from holes delimited by bones and connective structures (intervertebral holes in the spinal nerves; holes with heterogeneous terminology in the case of the skull).
The intracranial course of the cranial nerves is important for the diagnosis of various intracranial lesions such as brain tumors and brain aneurysms. The dysfunction of one or more cranial nerves indicates compression or stimulation by some lesions. For example, an acoustic schwannoma may initially cause hearing impairment, but with further tumor growth it may involve other cranial nerves and the patient may thus also present pain, similar to trigeminal neuralgia when the tumor involves the trigeminal nerve. . or diplopia due to involvement of the abducent nerve. A patient presenting with ptosis may have a posterior communicating artery aneurysm that compresses the oculomotor nerve during its intracranial course.
The cell bodies of many of the neurons of most cranial nerves are contained in one or more nuclei in the brain stem. These nuclei are important in relation to cranial nerve dysfunction because damage to them, as in the case of a stroke or trauma, can simulate damage to one or more branches of a cranial nerve. In terms of specific nuclei of the cranial nerve, the mesencephalic portion of the brainstem possesses the nuclei of the oculomotor nerve (III) and the trochlear nerve (IV); the bridge has the nuclei of the trigeminal nerve (V), the abducent nerve (VI), the facial nerve (VII) and the vestibulocochlear nerve (VIII); and the medulla has the nuclei of the glossopharyngeal nerve (IX), the vagus nerve (X), the accessory nerve (XI) and the hypoglossal nerve (XII). The fibers of these cranial nerves exit the brainstem from these nuclei.
Some cranial nerves have sensory or parasympathetic ganglia (collections of cell bodies) of neurons, which are located outside the brain (but can also be located inside or outside the skull). The sensory ganglia are directly corresponding to the dorsal ganglia of the spinal nerves and are known as the cranial sensory ganglia. Sensory ganglia exist for nerves that have a sensory function, namely: V, VII, VIII, IX, X. There are also parasympathetic ganglia, which are part of the autonomic nervous system of the cranial nerves III, VII, IX and X.
- The trigeminal ganglia of the trigeminal nerve (V) occupy a space in the dura mater called the trigeminal cavity. This ganglion contains the cell bodies of the sensory fibers of the three branches of the trigeminal nerve.
- The geniculate ganglion of the facial nerve (VII) is located immediately after the nerve enters the facial canal; it contains the cell bodies of the sensory fibers of the facial nerve.
- The upper and lower ganglia of the glossopharyngeal nerve (IX) are located immediately after the nerve passes through the jugular foramen and contains the cell bodies of the sensory fibers of this nerve.
- The inferior ganglion of the vagus nerve (gnarled ganglion) is located under the jugular foramen and contains the cell bodies of the sensory fibers of the vagus nerve (X).
|Lamina cribrosa||Olfactory nerve (I)|
|Optical foramen||Optic nerve (II)|
|Upper orbital fissure||Oculomotor (III)
|Round foramen||Trigeminal V2
|Oval foramen||Trigeminal V3
|Stylomastoid foramen||Facial nerve (VII)|
|Internal auditory canal||Vestibulococlear (VIII)|
|Jugular fossa||Glossopharyngeal (IX) Vague (
|Hypoglossal canal||Hypoglossal (XII)|
After emerging from the brain, the cranial nerves run into the skull and some of them must emerge from this bone structure to reach their destinations. Nerves often pass through holes in the skull called “foramina”. Other nerves pass through the bone channels, longer paths enclosed by bones. These foramina and canals can contain more than one cranial nerve and can also contain blood vessels.
- The olfactory nerve (I), actually composed of many small separate nerve fibers, passes through perforations in the part of the cribral lamina of the ethmoid bone. These fibers end in the upper part of the nasal cavity and are used to transmit impulses containing information about odors to the brain.
- The optic nerve (II) passes through the optic foramen into the sphenoid bone as it runs towards the eye. It transmits visual information to the brain.
- The oculomotor nerve (III), the trochlear nerve (IV), the abducent nerve (VI) and the ophthalmic branch of the trigeminal nerve (V1) travel through the cavernous sinus into the upper orbital fissure, passing from the skull to the orbit. These nerves control the small muscles that move the eye and also provide sensory innervation to the eye and orbit.
- The maxillary division of the trigeminal nerve (V2) passes through the round foramen into the sphenoid bone in order to innervate the skin of the center of the face.
- The mandibular division of the trigeminal nerve (V3) passes through the oval foramen of the sphenoid bone to provide sensory innervation to the lower face. This nerve also sends branches to almost all the muscles that control chewing.
- The facial nerve (VII) and the vestibulocochlear nerve (VIII) both enter the internal auditory canal in the temporal bone. The facial nerve then reaches the side of the face using the stylomastoid foramen, also in the temporal bone. Its fibers expand to reach and control all the muscles of facial expression. The vestibulocochlear nerve reaches the organs that control balance and hearing in the temporal bone, and therefore does not reach the external surface of the skull.
- The glossopharynx nerve (IX), the vagus nerve (X) and the accessory nerve (XI) exit the skull through the jugular fossa to enter the neck. The glossopharyngeal nerve provides innervation to the upper portion of the throat and the back of the tongue, the vagus nerve provides innervation to the muscles in the voice and continues downward to provide parasympathetic innervation to the chest and abdomen. The accessory nerve controls the trapezius and sternocleidomastoid muscles in the neck and shoulder.
- The hypoglossal nerve (XII) exits the skull through the hypoglossal canal in the occipital bone and reaches the tongue to control almost all the muscles involved in the movements of this organ.
Functioning and malfunction of individual nerves
The cranial nerves provide motor and sensory innervation mainly to structures within the skull and neck. Sensory innervation includes both “general” sensation, such as temperature and touch, and “special” innervation such as taste, sight, smell, balance and hearing. The vagus nerve (X) provides sensory and autonomic (parasympathetic) innervation of movements to the structures of the neck and most of the thoracic and abdominal organs.
The olfactory nerve (I) transmits the sense of smell (i.e. allows the perception of odors).
An injury to the olfactory nerve can cause various pathologies of the sense of smell, which can be of two types: quantitative and qualitative.
The pathologies of quantitative alteration of the sense of smell are represented by hyposmia , or partial reduction of the sense of smell, and anosmia , or total reduction of the sense of smell. Qualitative alteration
pathologies of the sense of smell are represented by parosmia , a disorder characterized by the alteration of the sense of smell, and cacosmia , that is the perception of unpleasant odor.
The underlying causes of these disorders can be different: rhinitis, sinusitis, neoplasms, lesions of the mucosa of various origins (viral, traumatic or inflammatory). A damaged olfactory nerve can also compromise, alter or determine alteration or absence of the sense of taste ( dysgeusia and ageusia ). If there is a suspicion of a change in the sense of smell, each nostril is tested with substances of known odors, such as coffee or soap. Substances that smell strongly, for example ammonia, can lead to the activation of the pain receptors (nociceptors) of the trigeminal nerve that are located in the nasal cavity and this can confuse the olfactory tests.
The optic nerve (II) transmits visual information from the retina to the brain.
Damage to the optic nerve (II) compromises specific aspects of vision that depend on the location of the lesion. A person may not be able to see objects on the left or right side (homonymous homianopsia), or may have difficulty seeing objects in their external visual fields (homonymous hemianopsia) if the optical chiasm is also involved. Vision can be tested by examining the visual field or the retina by means of an ophthalmoscope. Visual field examination can be used to locate structural lesions in the optic nerve or, later on, along visual courses.
Eye movement (III, IV, VI)
The oculomotor nerve (III), the trochlear nerve (IV) and the abducent nerve (VI) coordinate eye movements.
Nerve damage III, IV or VI can affect the movement of the eyeball (globe). One or both eyes can be compromised; in both cases a double vision (diplopia) could occur because the eye movements are no longer synchronized. These nerves can be examined by observing how the eye follows an object in different directions. This object can be a finger or a pin and can be moved in different directions to evaluate the tracking speed. If the eyes don’t work together, the most likely cause is damage to a specific cranial nerve or its nuclei.
Damage to the oculomotor nerve (III) can cause double vision and the inability to coordinate the movements of both eyes (strabismus), as well as an eyelid fall (ptosis) and dilation of the pupil (mydriasis). Injuries can also lead to the inability to open the eye due to paralysis of the levator muscle of the upper eyelid. Individuals suffering from an injury to the oculomotor nerve can compensate for this by tilting their heads to relieve symptoms of paralysis.
Damage to the trochlear nerve (IV) can cause diplopia. The result will be an eye that cannot move properly downward. This is due to the impaired functioning of the superior oblique muscle which is innervated by the trochlear nerve.
Damage to the abducent (VI) nerve can also lead to diplopia. This is due to the impairment of the lateral rectus muscle which is innervated by the abducent nerve.
Trigeminal nerve (V)
The trigeminal nerve (V) comprises three distinct parts: the ophthalmic nerve (V1), the maxillary nerve (V2) and the mandibular nerve (V3). Together, these nerves provide sensations to the skin of the face and control the muscles that allow the chewing of food.
The conditions and pathologies that affect the trigeminal nerve (V) include trigeminal neuralgia, cluster headache and trigeminal zoster. Trigeminal neuralgia occurs later in life than other conditions, from middle age onwards, more often after 60 years, and is a pathology typically related to very severe pain distributed over the area innervated by the maxillary or mandibular branches of the trigeminal nerve (V2 and V3).
Facial expression (VII)
The facial or “facial” nerve (VII) comprises two distinct nerves: the facial nerve itself , which contains somatic motor fibers for the innervation of the mimic muscles and other derived muscles of the second branchial arch; the intermediate nerve (of Wrisberg) , which includes somatic and visceral sensitive fibers that have a common origin in the geniculate ganglion and go to the anterior 2/3 of the tongue and to a restricted area of the auricle, and parasympathetic preganglionic fibers for the lacrimal glands, for the submandibular and sublingual salivary glands and for the glands of the mucous membrane of the nose and palate.
Lesions of the facial nerve can manifest as facial paralysis, a situation where a person is unable to move their muscles on one or both sides of their face. In blunt trauma, the facial nerve is the cranial nerve that is most frequently damaged. A very common and generally temporary facial paralysis is known as Bell’s palsy which is the result of an idiopathic injury (unknown cause), one-sided lower motor neuron of the facial nerve and is characterized by an inability to move the ipsilateral muscles of facial expression, including raising the eyebrow and furrowing the forehead. Bell’s palsy patients often have a sagging mouth on the affected side and often have difficulty chewing as the buccinator muscle is affected. Bell’s palsy occurs very rarely and affects around 40,000 Americans every year. Herpesviridae are capable of producing Bell’s palsy. Facial paralysis can be caused by other conditions including stroke , and similar conditions are occasionally incorrectly diagnosed as Bell’s palsy. It is a temporary condition that usually lasts 2 to 6 months, but can have life-changing after-effects, while symptoms can also recur.
Hearing and balance (VIII)
The vestibulocochlear nerve (VIII) divides into the vestibular and cochlear nerve. The vestibular part is responsible for the innervation of the vestibules and the semicircular canal of the inner ear; this structure transmits information on balance and is an important component of the vestibulocular reflex, which keeps the head stable and allows the eyes to follow moving objects. The cochlear nerve transmits information from the cochlea, allowing sound to be perceived.
If damaged, the vestibular nerve can give rise to spinning sensations and dizziness. Damage can also present as repetitive and involuntary eye movements (nystagmus) or partial or total deafness in the affected ear. In the vestibulocochlear nerve it can be affected by a benign tumor called “acoustic neuroma”.
Oral sensation, taste and salivation (IX)
The glossopharyngeal nerve (IX) innervates the styopharyngeal muscle and provides sensory innervation to the oropharynx and the back of the tongue. The glossopharyngeal nerve also provides parasympathetic innervation to the parotid gland.
The unilateral absence of a vomiting reflex suggests a lesion of the glossopharyngeal nerve (IX) and probably of the vagus nerve (X).
Vagus nerve (X)
The vagus nerve (X) starts from the medulla oblongata and goes, through the jugular hole, down to the thorax and abdomen. The 2 left and right vagus nerves are among the most important in the body as well as the longest and most branched of the cranial nerves. One of its main purposes is to stimulate the production of gastric acid and regulate the movements made by the stomach and intestines during the digestion phase, it also provides innervation by the parasympathetic autonomic nervous system of the heart; the right vagus innervates the sinoatrial node and its stimulation induces bradycardia (slowing of the heart rate).
Loss of vagus nerve function will result in a loss of parasympathetic innervation for a very large number of structures. The main effects of vagus nerve damage can include an increase in blood pressure and heart rate . Vagal hyperstimulation can lead to severe bradycardia and even death to the patient. Isolated vagus nerve dysfunction is rare but, if the conflict or injury is above the point where the vagus branches out for the first time, it can be diagnosed by a hoarse voice, due to the dysfunction of one of its branches , the recurrent laryngeal nerve. Damage to this nerve can cause difficulty in swallowing.
Shoulder elevation and head movement (XI)
The accessory nerve (XI) is a motor nerve that has the particularity of being equipped with both a spinal root (which originates from the spinal cord) and a cranial root (which originates from the medulla oblongata). Injuries of this nerve lead to the establishment of an ipsilateral weakness in the trapezius muscle, difficulty in the rotation of the head and elevation of the scapula.
Tongue movement (XII)
The hypoglossal nerve (XII) is the motor nerve of the tongue and allows its movements.
Nerve damage at the level of the lower motor neuron can lead to atrophy of the muscles of the tongue or fasciculations of the tongue (“bag of worms”). Damage to the upper motor neuron will not lead to atrophy or fasciculations, but only weakness of the innervated muscles. When the nerve is damaged, weakness of the movement of the tongue on one side will occur. If the damage is particularly extensive, the tongue will move to the weaker side. Hypoglossal nerve damage can ultimately result in difficulty speaking, chewing, and swallowing.
Doctors, neurologists and other medical professionals can conduct a cranial nerve exam as part of a general neurological exam. A cranial nerve exam begins with observation of the patient as some lesions can affect the symmetry of the eyes or face. The visual fields are analyzed for any nerve injury or presence of nystagmus, through the analysis of specific eye movements. Face sensitivity is tested and patients are asked to perform different facial movements. Hearing is also controlled by voice and a tuning fork. The position of the patient’s uvula is examined because asymmetry in the position could indicate a lesion of the glossopharyngeal nerve. The patient’s ability to use the shoulder to evaluate the accessory nerve (XI) can be observed,
The cranial nerves can be compressed due to the increase in intracranial pressure, a mass effect due to an intracerebral hemorrhage or due to a tumor that presses against the nerves and interferes with the transmission of impulses along it. A loss of function of a single cranial nerve can sometimes be the first symptom of intracranial or skull base cancer.
An increase in intracranial pressure can lead to impairment of the optic nerves (II) due to the compression of the surrounding veins and capillaries, causing swelling of the eyeball (papilledema). A tumor, such as an optic glioma, can also have an impact on the optic nerve (II). A pituitary adenoma can compress the optic tracts or the optic nerve chiasm (II), leading to the loss of the visual field. A pituitary tumor can also extend into the cavernous sinus, compressing the oculomotor nerve (III), the trochlear nerve (IV) and the abducent nerve (VI), leading to double vision and strabismus. These nerves can also be affected by the herniation of the brain’s temporal lobes through the brain sickle.
The cause of trigeminal neuralgia, in which one side of the face appears painful, is believed to be compression of the nerve by an artery at the point where the nerve emerges from the brain stem. An acoustic neuroma, particularly at the junction between the Varolio bridge and the medulla, can compress the facial nerve (VII) and the vestibulocochlear nerve (VIII), leading to hearing loss and sensory capacity on the affected side.
The stroke can be due to the occlusion of the blood vessels that perforate the nerves or their nucleus. It causes specific signs and symptoms which may indicate where the occlusion occurred. A clot in a blood vessel that drains the cavernous sinus (thrombosis of the cavernous sinus) affects the oculomotor (III), the trochlear (IV), the ophthalmic branch of the trigeminal nerve (V1) and the abducent nerve (VI).
An inflammation resulting from an infection can impair the function of any cranial nerve. Inflammation of the facial nerve (VII) can cause Bell’s palsy. Multiple sclerosis, an inflammatory process that can produce a loss of the myelin sheaths that surround the cranial nerves, can cause a variety of mutant symptoms that affect multiple cranial nerves.
Head injuries, bone diseases such as Paget’s disease and nerve injuries that occurred during a neurosurgery (such as removing a tumor) are other possible causes of cranial nerve damage