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|Brain: Cranial nerves|
Cranial nerves as they pass through the skull base to the brain.
(pl: nervi craniales)
|Brain: Cranial nerves|
Cranial nerves as they pass through the skull base to the brain.
(pl: nervi craniales)
Cranial nerves (sometimes termed cerebral nerves), are nerves that emerge directly from the brain and the brainstem, in contrast to spinal nerves (which emerge from various segments of the spinal cord). Information is exchanged between the brain and various regions, primarily of the head and neck, via the cranial nerves.
Spinal nerves reach as far as the first cervical vertebra, and the cranial nerves fill a corresponding role above this level. Each cranial nerve is paired and is present on both sides. Depending on source there are in humans twelve or thirteen pairs of cranial nerves, which are assigned Roman numerals I–XII, and zero assigned to cranial nerve zero, (or the terminal nerve), according to the order in which they originate from the forebrain to the back of the brain and the brainstem.
The cranial nerves are components of the peripheral nervous system, with the exception of cranial nerve II (the optic nerve), which is not a true peripheral nerve but a neural tract of the diencephalon connecting the retina with the lateral geniculate nucleus; hence both the optic nerve and the retina are part of the central nervous system (CNS). The axons of the remaining twelve nerves extend beyond the brain and are considered part of the peripheral nervous system. The central ganglia of the cranial nerves or cranial nerve nuclei originate in the CNS, preferentially from the brainstem.
Traditionally, humans are considered to have twelve pairs of cranial nerves, which are numbered I–XII or 1–12. They are the following: the olfactory nerves (I), the optic nerves (II), the oculomotor nerves (III), the trochlear nerves (IV), the trigeminal nerves (V), the abducens nerves (VI), the facial nerves (VII), the vestibulocochlear nerves (VIII), the glossopharyngeal nerves (IX), the vagus nerves (X), the accessory nerves (XI), and the hypoglossal nerves (XII).
Nerves are generally named according to their structure or function. For example, the olfactory nerve (I) supplies smell, and the facial nerve (VII) supplies motor innervation to the face. As Latin was the lingua franca of the study of Anatomy when the nerves were first documented, recorded, and discussed, many nerves hold Latin, including the trochlear nerve (IV), named according to its structure, as it supplies a muscle that attaches to a pulley (Greek: trochlea), the trigeminal nerve (V) named according to its three heads (Latin: tri-geminus meaning triplets), and the vagus nerve (X), named for its wandering course (Latin: vagus).
Cranial nerves are numbered based on their rostral-caudal orientation, as, when viewing the brain and brainstem from below, they are often visible in their numeric order. For example, the olfactory nerves (I) and optic nerves (II) arise from the base of the forebrain, and the other nerves, III to XII, arise from the brainstem.
Unique anatomical terminology is used to describe the course of the cranial nerves. Like all nerves, the nerves have a nucleus, and a course within and outside of the brain. The course within the brain is known as the central course of the nerve, and the course after it has emerged from the brain as the peripheral course. The nerves are paired, which means that they occur on both the right and left sides. Some nerves cross from the right side to the left side, and this is known anatomically as decussation. If a nerve supplies a muscle, skin, or has another function on the same side of the body as where it originates, this is called an ipsilateral course. If the course is opposite to the nucleus of the nerve, this is known as a contralateral course.
Specifically, the midbrain of the brainstem has the nuclei of the oculomotor nerve (III) and trochlear nerve (IV); the pons has the nuclei of the trigeminal nerve (V), abducens nerve (VI), facial nerve (VII) and vestibulocochlear nerve (VIII); and the medulla has the nuclei of the glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI) and hypoglossal nerve (XII). The olfactory nerve (I) emerges from the olfactory bulb, and depending slightly on division the optic nerve (II) is deemed to emerge from the lateral geniculate nuclei.
Because each nerve may have several functions, the nerve fibres that make up the nerve may collect in more than one nucleus. For example, the trigeminal nerve (V), which has a sensory and a motor role, has at least four nuclei.
With the exception of the olfactory nerve (I) and optic nerve (II), the cranial nerves emerge from the brainstem. The oculomotor nerve (III) and trochlear nerve (IV) emerge from the midbrain, the trigeminal (V), abducens (VI), facial (VII) and vestibulocochlea (VIII) from the pons, and the glossopharyngeal (IX), vagus (X), accessory (XI) and hypoglossal (XII) emerge from the medulla.
The olfactory nerve (I) and optic nerve (II) emerge separately. The olfactory nerves emerge from the olfactory bulbs on either side of the crista galli, a bony projection below the frontal lobe, and the optic nerves (II) emerge from the lateral colliculus, swellings on either side of the temporal lobes of the brain.
The cranial nerves give rise to a number of ganglia, collections of the cell bodies of neurons in the nerves that are outside of the brain. These ganglia are both parasympathetic and sensory ganglia.
Similar to the dorsal root ganglia of the spinal nerves and parasympathetic ganglia of the sacral parasympathetic system, the sensory cranial nerves have a number of ganglia outside the central nervous system. The sensory ganglia are directly correspondent to dorsal root ganglia and are as known as cranial sensory ganglia, and found along the course of the cranial nerves, outside the brain and skull. Sensory ganglia exist for nerves with sensory function: V, VII, VIII, IX, X. There are also a number of parasympathetic cranial nerve ganglia. Sympathetic ganglia innervating the head and neck reside in the upper regions of the sympathetic trunk, and do not belong to the cranial nerves.
Additional ganglia for nerves with parasympathetic function exist, and include the Ciliary ganglion of the oculomotor nerve (III), the pterygopalatine ganglion of the maxillary nerve (V2), the submandibular ganglion of the lingual nerve, a branch of the facial nerve (VII), and the otic ganglion of the glossopharyngeal nerve (IX).
|cribiform plate||Olfactory nerve (I)|
|optic foramen||Optic nerve (II)|
|superior orbital fissure||Oculomotor (III)|
|round foramen||Trigeminal V2|
|oval foramen||Trigeminal V3|
|internal auditory canal||Facial (VII)|
|jugular foramen||Glossopharyngeal (IX)|
|hypoglossal canal||Hypoglossal (XII)|
After emerging from the brainstem, the cranial nerves travel within the skull, but must leave this bony compartment in order to reach their destinations. Some nerves pass through holes in the skull, called foramina, as they travel to their destination. Other nerves pass through bony canals, longer pathways enclosed by bone. These foramina and canals may contain more than one cranial nerve, and may also contain additional blood vessels.
The facial nerve enters the temporal bone at the internal acoustic meatus, but exits the skull via the stylomastoid foramen while the vestibulocochlear nerve never actually exits the skull.
The following images show the cranial nerves schematically showing their respective exits from the CNS or brain-stem (not including the optic nerve, which, being part of the CNS, does not leave it), and their path, as well as conceptual innervation targets.
3 The oculomotor controls a number of motor function of the eye, both somatic and autonomic.
5 The trigeminal innervates a large number of structures, both motor and sensory.
7 The facial nerve stands for innervation of a large number of structures both motor and sensory.
9 The glossopharyngeal innervates a number of motor and sensory structures of the tongue and pharynx.
12 The hypoglossal nerve innervates the musculature of the tongue and pharynx.
Brainstem nuclei with associated functions are often found in similar areas of the brainstem. These are also known as functional columns. Functional columns are a result of the development of the spinal cord. Four columns of gray matter are present in the spinal cord during embryological development. Each column represents a different function, and contributes neurons to different nerves. Each nerve is innervated by neurons from one or more of the columns.
As the spinal cord develops, there are four columns. These are the general somatic efferent column, the general visceral efferent column, the general visceral afferent column and general somatic afferent column. These columns also extend into the brainstem, but are divided into smaller pieces. In the brainstem there are seven columns.
Four 'general' columns contain fibres that supply sensation or control muscles:
The cranial nerves innervate the head and neck area, including both somatic and autonomic motor innervation as well as sensory innervation. Together the cranial nerves supply sensory innervation of the special senses, such as taste, vision, smell, and hearing. They also supply afferents of the somatic senses: visceral sensation of the head and neck, balance, and proprioception, combining vestibular perception with proprioceptive information from the head and neck.
Two nerves, the glossopharyngeal nerve (IX) and vagus nerve (X), innervate both motor and sensory synapses pertaining to abdominal organs (though not pelvic), as well as structures of the neck and chest.
Unlike the spinal nerves, the cranial nerves are not strictly bound to certain segments of the body (as in dermatomes), but rather organize by function; hence, the innervated areas overlap significantly more than those of spinal nerves.
The olfactory nerve (I) conveys the sense of smell.
Damage to the olfactory nerve (I) can cause an inability to smell (anosmia), a distortion in the sense of smell (parosmia), or a distortion or lack of taste. Specific testing is performed when an individual perceives lack of taste or affected taste. The smell from each nostril is tested individually, and with consideration of airflow. Different substances are used, and these include coffee or soap. Using stronger smelling substances, for example ammonia, may lead to the activation of nociceptors of the trigeminal nerve.
Damage to the optic nerve (II) affects vision. Vision is affected depending on the location of the lesion. A person may not be able to see things on their left or right side (homonymous hemianopsia), or may have difficulty seeing things on their outer visual fields (bitemporal hemianopsia) if the optic chiasm is involved.:82 Vision may be tested using a number of different tests, examining the visual field, or by examining the retina with an ophthalmoscope, using a process known as funduscopy. Visual field testing may be used to pin-point structural lesions in optic nerve, or further along the visual pathways.
The oculomotor nerve (III), trochlear nerve (IV) and abducens nerve (VI) coordinate eye movement.
Damage or lesion of nerves III, IV, or VI may affect the movement of the eye or pupil. Either both or one eye may be affected, and if both eyes are affected no double vision (diplopia) will occur. These nerves might be examined by observing how the eye follows an object in different directions. This object may be a finger or a pin, and may be moved at different directions to test for pursuit velocity. If the eyes do not work together, the most likely cause is damage to a specific cranial nerve or nuclei.
Damage to the oculomotor nerve (III), such as from a palsy can cause double vision (diplopia) with lateral strabismus, and also ptosis and mydriasis.:84 All but specific deviations may be due to damage in this nerve or any of the muscles it innervates, (though not internuclear ophthalmoplegia). Lesion may also lead to inability to open the eye due to disrupted innervation of the levator palpebrae (unlike in Horner syndrome, which only results in a droopy eyelid). Individuals suffering from lesion or damage to the oculomotor nerve may compensate by tilting their heads to alleviate symptoms due to lack of control from oblique muscles when the eye is not adducted.
Damage to the trochlear nerve (IV) can also cause diplopia with the eye adducted and elevated.:84 The result will be an eye which can not move downwards or inwards properly (especially downwards when in an inward position). This is due to impairment in the superior oblique muscle, which is innervated by the trochlear nerve.
Conditions affecting the trigeminal nerve (V) include trigeminal neuralgia, cluster headache, and trigeminal zoster. Trigeminal neuraliga occurs later in life, from middle age onwards, most often after an age of 60, and is a condition associated with very strong pain distributed over the area innervated by the trigeminal nerve. Often, the pain follows the distribution of the maxillary or mandibular nerve (branches V2 and V3). The trigeminal nerve is also present in the tendon reflexive jaw jerk. The reflex gives rise to twitch in some of the muscles involved in closing the jaw, and occurs when the jaw is tapped from a precise angle. A stronger reflex may be present if there is a supranuclear lesion of the trigeminal nerves motor nucleus, for example in pseudobulbar palsy. In Parkinson's disease, the trigeminal nerve is involved in the glabellar reflex which causes involuntary eye-blinking.
Lesions of the facial nerve (VII) may manifest as facial palsy. This is where a person is unable to move the muscles on one or both sides of their face. If only the peripheral nerve itself is affected, this may cause Bell's palsy. Palsy that occurs is on the same side of the affected nerve. Central facial palsy will manifest in a similar fashion. If the nerve is damaged only on one side, a person will still be able to raise the eyebrows and crease the forehead on that side. That is because the frontalis muscle is innervated by both the left and the right cranial nerve. The effect is most often unilateral, and indicates contralateral damage or engagement of the cerebrum.
The vestibulocochlear nerve (VIII) splits into the vestibular and cochlear nerve. The vestibular part is responsible for innervating the vestibules and semicircular canal of the inner ear; this structure transmits information about balance, and is an important component of the vestibuloocular reflex, which keeps the head stable and allows the eyes to track moving objects. The cochlear nerve transmits information from the cochlea, allowing sound to be heard.
When damaged, the vestibular nerve may give rise to the sensation of spinning and dizziness, and may cause rotatory nystagmus. Function of the vestibular nerve may be tested through caloric stimulation. Damage to the vestibulocochlear nerve can also present as repetitive and involuntary eye movements (nystagmus), particularly when looking in a horizontal plane. The cochlear nerve will cause partial or complete deafness in the affected ear.
The glossopharyngeal nerve (IX) is almost exclusively sensory and supplies five afferent nuclei of the brainstem, providing sensory innervation to the oropharynx and back of the tongue. The glossopharyngeal nerve also provides parasympathetic innervation to the parotid gland (though the submandibular and sublingual glands are innervated by the facial nerve).
Loss of function of the vagus nerve (X) will lead to a loss of parasympathetic innervation to a very large number of structures. Major effects of damage to the vagus nerve may include a rise in blood pressure and heart rate. Isolated dysfunction of only the vagus nerve is rare, but can be diagnosed by a hoarse voice, due to dysfunction of the superior laryngeal nerve.
Testing of function may be performed by assessing ability to drink liquids. Choking on either saliva or liquids may indicated neurological damage to the vagus nerve (X). Damage to this nerve may result in difficulties swallowing. Damage to the glossopharyngeal nerve (IX) can be assessed by asking the subject to say "Ah", and observing if during phonation the uvula deviates. A positive sign that is indicative of unilateral damage is a finding of an asymmetrically deviating uvula, deviating towards the side, with an intact or healthy nerve.
Damage to the accessory nerve (XI) may lead to contralateral weakness in the trapezius. This can be tested by asking the subject to raise their shoulders or shrug, upon which the scapula will move out into a winged position if the nerve is damaged. Weakness or an inability to elevate the scapula may be present, since the levator scapulae is alone in providing this function. There may also be weakness present of the sternocleidomastoid muscle, but as it received cortical innervation from the ipsilateral side, any damage will give rise to ipsilateral weakness.
The hypoglossal nerve (XII) is unique in that it is innervated bilaterally from both hemispheres motor cortex. Damage to the nerve at lower motor neuron level may lead to fasciculations or atrophy of the musculature of the tongue. The fasciculations of the tongue are sometimes said to look like a "bag of worms". Upper motor neuron damage will not lead to atrophy or fasciculations, but only weakness of the innervated muscles.
When the nerve is damaged, it will lead to unilateral weakness and the tongue, when extended, will move towards the weaker or damaged side, as shown in the image.
Doctors, neurologists and other medical professionals may conduct a cranial nerve examination as part of a neurological examination to examine the cranial nerves. This is a highly formalised series of steps involving specific tests for each nerve, testing the function of the olfactory nerve (I) first, and progressing sequentially for each nerve. Knowledge of cranial nerve function is an important, as it may indicate which portion of the brainstem is damaged. It is of clinical importance to know the path and origin of the cranial nerves, both intracranially as well as extracranially.
A cranial nerve exam starts with observation of the patient, as some cranial nerve lesions may affect the symmetry of the eyes or face. The eyes are examined and the visual acuity is tested through reading a Snellen chart. The visual fields are tested for nerve lesions or nystagmus via a task to perform specific eye movements . The sensation of the face is tested, and patients are asked to perform different facial movements, such as puffing out of the cheeks. Hearing is checked by voice and tuning forks. The patient's uvula is examined. After performing a shrug and head turn, the patient's tongue function assessed by various tongue movements.
Nerves may be compressed because of increased intercranial pressure, a mass effect of an intracerebral haemorrhage, or tumour that presses against the nerves. The cranial nerves are often the first structures to be affected by different forms of brain injury, such as hemorrhaging or tumors, partly because they are sensitive to compression. Mononeuropathy of a cranial nerve may sometimes be the first symptom of an intracranial or skull base cancer.
An increase in intercranial pressure may lead to swelling of the optic nerves (II) and compression of the surrounding veins and capillaries, causing papilloedema. A glioma, such as an optic glioma, may also impact on the optic nerve (II). A pituitary tumour may compress the optic tracts or the optic chiasm of the optic nerve (II), leading to visual loss. A pituitary tumour may also extend into the cavernous sinus, compressing the oculuomotor nerve (III), trochlear nerve (IV) and abducens nerve (VI), leading to double-vision and strabismus. These nerves may also be affected by herniation of the temporal lobes of the brain through the falx cerebri.
The cause for trigeminal neuralgia, in which one side of the face is exquisitely tender, is thought to be compression of the nerve by a the superior cerebellar artery, one of the arteries supplying the cerebellum. An acoustic neuroma, particularly at the junction between the pons and medulla, may compress the facial nerve (VII) and vestibulocochlear nerve (VIII), leading to hearing and sensory loss on the affected side.
Occlusion of blood vessels that supply the nerves or their nuclei, an ischemic stroke, may cause specific signs and symptoms that can localise where the occlusion occurred. If there is a stroke of the midbrain, pons or medulla, various cranial nerves may be damaged, resulting in dysfunction and symptoms of a number of different syndromes. Thrombosis, such as a cavernous sinus thrombosis, refers to a thrombus affecting the venous drainage from the cavernous sinus, affects the optic (II), oculomotor (III), trochlear (IV), opthalamic branch of the trigeminal nerve (V1) and the abducens nerve (VI).
Inflammation can be a result of infection, such as viral causes like reactivated herpes simplex virus, or can occur spontaneously. Inflammation of the facial nerve (VII) may result in Bell's palsy.
Multiple sclerosis, an inflammatory process resulting in a loss of the myelin sheathes which surround the cranial nerves, may cause a variety of shifting symptoms affecting multiple cranial nerves. Inflammation may also affect other cranial nerves. Other rarer inflammatory causes affecting the function of multiple cranial nerves include sarcoidosis, miliary tuberculosis, and inflammation of arteries, such as Wegener's granulomatosis.
The terminal nerve, often called cranial nerve zero, CN 0 (or cranial nerve nulla or N, since there is no Roman numeral for zero), has been largely neglected from textbooks, even though it was first clearly identified over a century ago. It was first shown to be present in the shark, but its presence in humans (and other mammals) remained somewhat controversial. More recent studies have shown the nerve to be quite distinct in human fetuses and infants, and has also regularly been seen in the adult brain. The nerve axons are unmyelinated and arise from ganglia. The terminal nerve has also been shown to release luteinising hormone. Another study has shown the terminal nerve to be a microscopic plexus of unmyelinated fibres in the frontal lobes. It was concluded in the study, confirming earlier findings by light microscope, that this nerve is a common finding in the human brain.
Cranial nerves are also present in other vertebrates. Other amniotes (non-amphibian tetrapods) have cranial nerves similar to those of humans. In anamniotes (fishes and amphibians), the accessory nerve (XI) and hypoglossal nerve (XII) do not exist, with the accessory nerve (XI) being an integral part of the vagus nerve (X); the hypoglossal nerve (XII) is represented by a variable number of spinal nerves emerging from vertebral segments fused into the occiput. These two nerves only became discrete nerves in the ancestors of amniotes (non-amphibian tetrapods).
Dorsal view of the sheep's brain. The exits of the various cranial nerves are marked with red.
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