1. Seventh Cranial Nerve Disorder

Key Takeaways Key Points. The cranial nerves serve functions such as smell, sight, eye movement, and feeling in the face. The olfactory nerve, or cranial nerve I, is the first of the 12 cranial nerves. It is instrumental in the sense of smell.

If there is cranial nerve VII nerve damage, this muscle is paralyzed. Because the branch of the seventh cranial nerve that goes to the stapedius muscle begins very proximally, hyperacusis due to seventh cranial nerve lesions indicates a lesion close to the nerve’s origin in the brainstem rather than more peripheral. The 7th cranial nerve is mixed nerve containing both sensory and motor components. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity. 7th cranial nerve also supplies preganglionic parasympathetic fibers to several head and neck ganglia.

The olfactory nerve is the shortest of the 12 cranial nerves and only one of two cranial nerves (the other being the optic nerve) that do not join with the brainstem.The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity. The olfactory nerves consist of a collection of many sensory nerve fibers that extend from the olfactory epithelium to the olfactory bulb, passing through the many openings of the cribriform plate of the ethmoid bone.Olfactory receptor neurons continue to emerge throughout life and extend new axons to the olfactory bulb.

Olfactory-ensheathing glia wrap bundles of these axons and are thought to facilitate their passage into the central nervous system.The sense of smell (olfaction) arises from the stimulation of olfactory (or odorant) receptors by small molecules of different spatial, chemical, and electrical properties that pass over the nasal epithelium in the nasal cavity during inhalation. These interactions are transduced into electrical activity in the olfactory bulb, which then transmits the electrical activity to other parts of the olfactory system and the rest of the central nervous system via the olfactory tract.

Optic (II) NerveThe optic nerve (cranial nerve II) receives visual information from photoreceptors in the retina and transmits it to the brain. Key Takeaways Key Points. The optic nerve is considered part of the central nervous system. Optic nerve: An illustration of the brain highlighting the optic nerve and optic tract.The optic nerve is the second of twelve paired cranial nerves. It is considered by physiologists to be part of the central nervous system, as it is derived from an outpouching of the diencephalon during embryonic development.As a consequence, the fibers are covered with myelin produced by oligodendrocytes, rather than Schwann cells that are found in the peripheral nervous system. The optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in the peripheral nerves.The fiber tracks of the mammalian central nervous system are incapable of regeneration. Key Takeaways Key Points.

The trochlear nerve innervates the superior oblique muscle of the eye. The trochlear nerve contains the smallest number of axons of all the cranial nerves and has the greatest intracranial length. The two major clinical syndromes that can arise from damage to the trochlear nerve are vertical and torsional diplopia.Key Terms. contralateral: On the opposite side of the body. cerebral aqueduct: The channel in the brain that connects the third ventricle to the fourth ventricle. Also called the aqueduct of Sylvius, it is surrounded by periaqueductal gray matter.The trochlear nerve (cranial nerve IV) is a motor nerve that innervates a single muscle: the superior oblique muscle of the eye.

The trochlear nerve is unique among the cranial nerves in several respects. It is the smallest nerve in terms of the number of axons it contains and it has the greatest intracranial length. Other than the optic nerve (cranial nerve II), it is the only cranial nerve that decussates (crosses to the other side) before innervating its target. It is the only cranial nerve that exits from the dorsal aspect of the brainstem.The nucleus of the trochlear nerve is located in the caudal mesencephalon beneath the cerebral aqueduct. It is immediately below the nucleus of the oculomotor nerve (III) in the rostral mesencephalon.The trochlear nucleus is unique in that its axons run dorsally and cross the midline before emerging from the brainstem—so a lesion of the trochlear nucleus affects the contralateral eye.

Lesions of all other cranial nuclei affect the ipsilateral side (except of course the optic nerve, cranial nerve II, which innervates both eyes).Homologous trochlear nerves are found in all jawed vertebrates. The unique features of the trochlear nerve, including its dorsal exit from the brainstem and its contralateral innervation, are seen in the primitive brains of sharks.The human trochlear nerve is derived from the basal plate of the embryonic midbrain.

Clinical SyndromesThere are two major clinical syndromes that can manifest through damage to the trochlear nerve:. Vertical diplopia: Injury to the trochlear nerve causes weakness of downward eye movement with consequent vertical diplopia (double vision). Torsional diplopia: Weakness of intorsion results in torsional diplopia, in which two different visual fields, tilted with respect to each other, are seen at the same time. To compensate for this, patients with trochlear nerve palsies tilt their heads to the opposite side, in order to fuse the two images into a single visual field.The clinical syndromes can originate from both peripheral and central lesions. A peripheral lesion is damage to the bundle of nerves, in contrast to a central lesion, which is damage to the trochlear nucleus. Trigeminal (V) NerveThe trigeminal nerve is the fifth cranial nerve and it is responsible for sensation and motor function in the face and mouth.

StructureThe trigeminal nerve is the largest of the cranial nerves. Its name, trigeminal, means three twins. It is derived from the fact that each nerve, one on each side of the pons, has three major branches: the ophthalmic nerve (V1 in the illustration below), the maxillary nerve (V2), and the mandibular nerve (V3).The ophthalmic and maxillary nerves are purely sensory. The mandibular nerve has both sensory and motor functions.The three branches converge on the trigeminal ganglion that is located within the trigeminal cave in the brain; it contains the cell bodies of incoming sensory nerve fibers. The trigeminal ganglion is analogous to the dorsal root ganglia of the spinal cord, which contain the cell bodies of incoming sensory fibers from the rest of the body. Areas of the face innervated by the trigeminal nerve: The ophthalmic nerve branch (V1) innervates the bright red area, the maxillary nerve branch (V2) innervates the light red area, and the mandibular nerve branch (V3) innervates the yellow area.From the trigeminal ganglion, a single large sensory root enters the brainstem at the level of the pons. Immediately adjacent to the sensory root, a smaller motor root emerges from the pons at the same level.Motor fibers pass through the trigeminal ganglion on their way to peripheral muscles, but their cell bodies are located in the nucleus of the trigeminal nerve, deep within the pons.

FunctionThe sensory function of the trigeminal nerve is to provide tactile, proprioceptive, and nociceptive afferents to the face and mouth. The motor component of the mandibular division (V3) of the trigeminal nerve controls the movement of eight muscles, including the four muscles of mastication: the masseter, the temporal, and the medial and lateral pterygoids.The other four muscles are the tensor veli palatini, the mylohyoid, the anterior belly of the digastric, and the tensor tympani. With the exception of the tensor tympani, all of these muscles are involved in biting, chewing and swallowing, and all have bilateral cortical representation. Key Takeaways Key Points. The abducens nerve exits the brainstem at the junction of the pons and the medulla and runs upward to reach the eye, traveling between the dura and the skull. The long course of the abducens nerve between the brainstem and the eye makes it vulnerable to injury at many levels.

In most mammals besides humans, it also innervates the musculus retractor bulbi, which can retract the eye for protection.Key Terms. abducens nerve: A nerve that controls the lateral rectus muscle in the eye. clivus: A part of the cranium at the base of the skull. It forms a gradual sloping process at the anterior-most portion of the basilar occipital bone at its junction with the sphenoid bone. petrous temporal bone: A pyramid-shaped bone that is wedged in at the base of the skull between the sphenoid and occipital bones and is part of the endocranium. lateral rectus muscle: A muscle in the orbit. It is one of six extraocular muscles that control the movements of the eye (abduction in this case) and the only muscle innervated by the abducens nerve, cranial nerve VI, functioning to bring the pupil away from the midline of the body.The abducens nerve (cranial nerve VI) is a somatic efferent nerve that, in humans, controls the movement of a single muscle: the lateral rectus muscle of the eye that moves the eye horizontally.

In most other mammals it also innervates the musculus retractor bulbi, which can retract the eye for protection. Homologous abducens nerves are found in all vertebrates except lampreys and hagfishes. Abducens nerve: Schematic of cranial nerves showing cranial nerve VI, the abducens nerve.The abducens nerve leaves the brainstem at the junction of the pons and the medulla, medial to the facial nerve. In order to reach the eye, it runs upward (superiorly) and then bends forward (anteriorly).The nerve enters the subarachnoid space when it emerges from the brainstem. It runs upward between the pons and the clivus, and then pierces the dura mater to run between the dura and the skull.At the tip of the petrous temporal bone, it makes a sharp turn forward to enter the cavernous sinus.

In the cavernous sinus it runs alongside the internal carotid artery. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle of the eye.The long course of the abducens nerve between the brainstem and the eye makes it vulnerable to injury at many levels. For example, fractures of the petrous temporal bone can selectively damage the nerve, as can aneurysms of the intracavernous carotid artery.Mass lesions that push the brainstem downward can damage the nerve by stretching it between the point where it emerges from the pons and the point where it hooks over the petrous temporal bone. Key Takeaways Key Points. The facial nerve (cranial nerve VII) is responsible for the muscles that determine facial expression, as well as the sensation of taste in the front of the tongue and oral cavity.

The facial nerve’s motor component begins in the facial nerve nucleus in the pons, and the sensory component begins in the nervus intermedius. The nerve then runs through the facial canal, passes through the parotid gland, and divides into five branches.

Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the eyes tightly (inability to do so is called lagophthalmos), pursing the lips, and puffing out the cheeks, all test the facial nerve.Key Terms. nervus intermedius: A part of the facial nerve (cranial nerve VII) located between the motor component of the facial nerve and the vestibulocochlear nerve (cranial nerve VIII). It contains the sensory and parasympathetic fibers of the facial nerve. Bell’s Palsy: Bell’s palsy is a form of facial paralysis resulting from a dysfunction of the cranial nerve VII (the facial nerve) that results in the inability to control facial muscles on the affected side.

Seventh Cranial Nerve Disorder

The facial nerve: Illustration of the facial nerve and its branches.The facial nerve is the seventh (cranial nerve VII) of the 12, paired cranial nerves. It emerges from the brainstem between the pons and the medulla and controls the muscles of facial expression.It also functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity, and it supplies preganglionic parasympathetic fibers to several head and neck ganglia.

LocationThe motor part of the facial nerve arises from the facial nerve nucleus in the pons, while the sensory part of the facial nerve arises from the nervus intermedius. The motor and sensory parts of the facial nerve enter the petrous temporal bone into the internal auditory meatus (intimately close to the inner ear), then runs a tortuous course (including two tight turns) through the facial canal, emerges from the stylomastoid foramen, and passes through the parotid gland, where it divides into five major branches.Although it passes through the parotid gland, it does not innervate the gland (this is the responsibility of cranial nerve IX, the glossopharyngeal nerve). The facial nerve forms the geniculate ganglion prior to entering the facial canal.The path of the facial nerve can be divided into six segments.

The intracranial (cisternal) segment. The meatal segment (brainstem to internal auditory canal). The labyrinthine segment (internal auditory canal to geniculate ganglion),. The tympanic segment (from geniculate ganglion to pyramidal eminence). The mastoid segment (from pyramidal eminence to stylomastoid foramen). The extratemporal segment (from stylomastoid foramen to post parotid branches).Function.

Bell’s Palsy: A person attempting to show his teeth and raise his eyebrows with Bell’s palsy on his right side (left side of the image).Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the eyes tightly (inability to do so is called lagophthalmos), pursing the lips, and puffing out the cheeks, all test the facial nerve. There should be no noticeable asymmetry.In an upper motor neuron lesion, called central seven (central facial palsy ), only the lower part of the face on the contralateral side will be affected due to the bilateral control to the upper facial muscles (frontalis and orbicularis oculi).Lower motor neuron lesions can result in a cranial nerve VII palsy (Bell’s palsy is the idiopathic form of facial nerve palsy), manifested as both upper and lower facial weakness on the same side of the lesion.Taste can be tested on the anterior 2/3 of the tongue.

This can be tested with a swab dipped in a flavored solution, or with electronic stimulation (similar to putting your tongue on a battery).In regards to the corneal reflex, the afferent arc is mediated by the general sensory afferents of the trigeminal nerve. The efferent arc occurs via the facial nerve.The reflex involves the consensual blinking of both eyes in response to stimulation of one eye.

This is due to the facial nerve’s innervation of the muscles of facial expression, namely the orbicularis oculi, responsible for blinking. Thus, the corneal reflex effectively tests the proper functioning of both cranial nerves V and VII. Key Takeaways Key Points.

The vestibulocochlear nerve comprises the cochlear nerve that transmits hearing information, and the vestibular nerve that transmits balance information. The cochlear nerve travels away from the cochlea of the inner ear where it starts as the spiral ganglia.

The vestibular nerve travels from the vestibular system of the inner ear.Key Terms. cochlear nerve: A sensory nerve that conducts information about the environment to the brain, in this case acoustic energy impinging on the tympanic membrane (sound waves reaching the ear drum). The cochlear nerve arises from within the cochlea and extends to the brainstem where its fibers make contact with the cochlear nucleus, the next stage of neural processing in the auditory system.

vestibulocochlear nerve: Also known as the auditory vestibular nerve, this is the eighth of twelve cranial nerves, and it is responsible for transmitting sound and equilibrium (balance) information from the inner ear to the brain. vestibular nerve: One of the two branches of the vestibulocochlear nerve (the cochlear nerve being the other). It connects to the semicircular canals via the vestibular ganglion and receives positional information.The vestibulocochlear nerve (also known as the auditory vestibular nerve and cranial nerve VIII) has axons that carry the modalities of hearing and equilibrium.It consists of the cochlear nerve that carries information about hearing, and the vestibular nerve that carries information about balance.This is the nerve along which the sensory cells (the hair cells) of the inner ear transmit information to the brain. It emerges from the pons and exits the inner skull via the internal acoustic meatus (or internal auditory meatus) in the temporal bone. Vestibular system’s semicircular canal: An illustration of the inner ear showing its semicircular canal, hair cells, ampulla, cupula, vestibular nerve, and fluid.The vestibulocochlear nerve consists mostly of bipolar neurons and splits into two large divisions: the cochlear nerve and the vestibular nerve. The cochlear nerve travels away from the cochlea of the inner ear where it starts as the spiral ganglia.Processes from the organ of Corti (the receptor organ for hearing) conduct afferent transmission to the spiral ganglia.

It is the inner hair cells of the organ of Corti that are responsible for activating the afferent receptors in response to pressure waves reaching the basilar membrane through the transduction of sound.The vestibular nerve travels from the vestibular system of the inner ear. The vestibular ganglion houses the cell bodies of the bipolar neurons and extends processes to five sensory organs.Three of these are the cristae, located in the ampullae of the semicircular canals. Hair cells of the cristae activate afferent receptors in response to rotational acceleration.The other two sensory organs supplied by the vestibular neurons are the maculae of the saccule and utricle. Hair cells of the maculae activate afferent receptors in response to linear acceleration.The vestibulocochlear nerve has axons that carry the modalities of hearing and equilibrium. Damage to the vestibulocochlear nerve may cause hearing loss, vertigo, a false sense of motion, loss of equilibrium in dark places, nystagmus, motion sickness, and gaze-evoked tinnitus.A benign primary intracranial tumor of vestibulocochlear nerve is called a vestibular schwannoma (also called acoustic neuroma). FunctionThere are a number of functions of the glossopharyngeal nerve.

It controls muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva.Along with taste, the glossopharyngeal nerve relays general sensations from the pharyngeal walls. Vagus nerve: Diagram demonstrating the course of the vagus nerve.Besides output to the various organs in the body, the vagus nerve conveys sensory information about the state of the body’s organs to the central nervous system. Key Takeaways Key Points. Cranial nerve XI is responsible for tilting and rotating the head, elevating the shoulders, and adducting the scapula. Most of the fibers of the accessory nerve originate in neurons situated in the upper spinal cord. The fibers that make up the accessory nerve enter the skull through the foramen magnum and proceed to exit the jugular foramen with cranial nerves IX and X.

Due to its unusual course, the accessory nerve is the only nerve that enters and exits the skull.Key Terms. sternocleidomastoid: A paired muscle in the superficial layers of the side of the neck. The accessory nerve: Upon exiting the skull via the jugular foramen, the spinal accessory nerve pierces the sternocleidomastoid muscle before terminating on the trapezius muscle.The fibers that make up the accessory nerve enter the skull through the foramen magnum and proceed to exit the jugular foramen with cranial nerves IX and X. Due to its unusual course, the accessory nerve is the only nerve that enters and exits the skull.Traditional descriptions of the accessory nerve divide it into two components: a spinal component and a cranial component. However, more modern characterizations of the nerve regard the cranial component as separate and part of the vagus nerve.Therefore, in contemporary discussions of the accessory nerve, it is common to disregard the cranial component when referencing the accessory nerve and assume reference to the spinal accessory nerve.

Accessory Nerve FunctionThe accessory nerve provides motor innervation from the CNS to the sternocleidomastoid and trapezius muscles of the neck. The sternocleidomastoid muscle tilts and rotates the head, while the trapezius muscle has several actions on the scapula, including shoulder elevation and adduction of the scapula.During neurological examinations, the function of the spinal accessory nerve is often measured by testing the range of motion and strength of the aforementioned muscles.

Limited range of motion or diminished muscle strength often indicates injury of the accessory nerve.Patients with spinal accessory nerve palsy may exhibit signs of lower motor neuron disease, such as atrophy and fasciculations of both the sternocleidomastoid and trapezius muscles. Key Takeaways Key Points. It controls tongue movements of speech, food manipulation, and swallowing.

While the hypoglossal nerve controls the tongue’s involuntary activities of swallowing to clear the mouth of saliva, most of the functions it controls are voluntary, meaning that the execution of these activities requires conscious thought. Proper function of the hypoglossal nerve is important for executing tongue movements associated with speech.

Hypoglossal nerve: Schematic image of the hypoglossal nerve and the structures it innervates. FunctionThe hypoglossal nerve controls tongue movements of speech, food manipulation, and swallowing. CC licensed content, Specific attribution. Cranial Nerves.

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The cranial nerves are nerves that originate in the rather than the. There are 12 paired nerves. The seventh cranial nerve, the, originates in the brainstem between the pons and the medulla. It controls movement of the facial expression muscles. It is involved in the conveyance of taste from the sensors on the tongue to the brain, and it supplies preganglionic parasympathetic fibers to the head and neck region.The largest portion of the seventh cranial nerve is made up of branchial motor fibers.

These fibers originate in the facial nerve nucleus in the pons, a part of the brainstem. The branchial motor fibers run from the pons to the muscles of the face, where they control the movements involved in facial expressions such as smiling and frowning. Blinking is also controlled in part by this nerve. It also enervates the digastric and stylohyoid muscles, which are located beneath the chin.The visceral motor portion of the seventh cranial nerve enervates the submandibular and sublingual glands, two of the mouth’s salivary glands. The facial nerve controls production of saliva from these glands. It also controls tear production through the lacrimal gland in the eye. The special sensory branch of the facial nerve conveys taste sensations from the back part of the tongue to the brain.

It also innervates the oropharynx, the part of the throat that begins at the base of the tongue and includes the tonsil and soft palate. The general sensory branch, the other part of the seventh cranial nerve involved in sensory stimuli, carries sensory signals from a small area of skin behind the ear.If a person is able to voluntarily move the face in typical expressions such as a frown, raised eyebrows or, the facial nerve is functioning normally. Unintentional asymmetries such as only one side of the mouth raising in a smile can indicate damage to the seventh cranial nerve. Taste generally is tested by swabbing the tongue with a flavored substance, and if the anterior part of the tongue is unable to taste, it also can be a sign of potential facial.on the seventh cranial nerve, such as those that occur in facial, can cause in the.

Facial paralysis is another possible disorder involving this nerve. Facial paralysis can be caused by or some types of viruses.