The Cranial Nerves
Twelve pairs of cranial nerves transmit information to and receive commands from the brain. The complex receptor organs associated with these nerves in the head and neck are not found in the rest of the body. Cranial nerves are referred to in Roman numerical order (I through XII) with I at the rostral end of the brain and XII at the caudal end near the medulla-spinal cord junction. The name of a cranial nerve may be written in three ways: for example, cranial nerve II, second (cranial) nerve, or optic nerve.Note that the majority of cranial nerves are related to the hindbrain and that cranial nerve XI has both cranial and spinal origins.
Examination of the cranial nerves allows one to "view" the brainstem from rostral to caudal. The associated cranial nerves of the brainstem, which can be divided into three levels, the midbrain, the pons and the medulla, are: 2 for the midbrain (CN 3 & 4), 4 for the pons (CN 5-8), and 4 for the medulla (CN 9-12). Cranial nerves I and II are situated in the forebrain and convey olfactory and visual information to the thalamus and ventral telencephalon respectively. Cranial nerves II, IV and VI are found in the mibrain and control eye movements. Cranial nerves V and VII are found in the pons and are associated with facial sensation and musculature. Cranial Nerves VIII-XII are located in the medulla and are associated with the balance and hearing (VIII), oropharyngeal cavity (IX), viscera (X), neck and shoulder muscles (XI) and tongue (XII) respectively.
We can see similarities in the functional organization of the cranial and spinal nerves and of the parts of the central nervous system with which they directly connect. For example, sensory axons in cranial nerves synapse in sensory cranial nerve nuclei. Similarly, sensory axons in spinal nerves synapse on neurons of the dorsal horn of the spinal cord and the dorsal column nuclei. The motor cranial nerve nuclei, like the motor nuclei of the ventral horn, contain the motor neurons whose axons project to the periphery. The segmental representation of the head is conserved in the cranial nerves; this organization is roughly analogous to the segmental configuration of the spinal cord.
The primary sensory neurons in cranial nerves that enter the brain stem synapse in sensory cranial nerve nuclei; the cell bodies of motor axons in cranial nerves are located in motor cranial nerve nuclei. This arrangement is similar to the dorsal roots, which synapse in the dorsal horn and dorsal column nuclei and the skeletal motor axons of the ventral roots, whose cell bodies are found in the spinal motor nuclei. Each cranial nerve nucleus subserves a single sensory or motor function. This is in contrast to many of the cranial nerves themselves, which contain a mixture of axons that have different functions.
The cranial nerve nuclei follow a logical plan of spatial organization within the brain stem and are organized into rostro-caudal columns. The nuclei form four discrete columns, sensory: somatic and visceral and motor: somatic and visceral. The general organization of the cranial nerves and the columnar organization of the cranial nerve nuclei helps explain the functional organization of the cranial nerves and nuclei; the location of the column provides important information about function. Cranial nerve findings when combined with long tract findings (corticospinal and somatosensory) are powerful for localizing lesions in the brainstem.
There are seven functional categories of cranial nerves and seven categories of cranial nerve nuclei. Nuclei of each of these categories form discontinuous columns that extend rostrocaudally through the brain stem. The seven functional categories are distributed through only six discrete columns, however, because two of the sensory categories synapse on neurons in a single column but at separate rostrocaucal locations.
A systematic relationship exists between the function of each column and its location with respect to the midline. The motor columns are located medial to the sensory columns. In the pons and medulla, nucleus on the ventricular floor are separated by the sulcus limitans. In the embryonic brain and spinal cord the sulcus limitans separates the developing alar and basal plates.
Examination of the cranial nerves allows one to "view" the brainstem from rostral to caudal. The associated cranial nerves of the brainstem, which can be divided into three levels, the midbrain, the pons and the medulla, are: 2 for the midbrain (CN 3 & 4), 4 for the pons (CN 5-8), and 4 for the medulla (CN 9-12). Cranial nerves I and II are situated in the forebrain and convey olfactory and visual information to the thalamus and ventral telencephalon respectively. Cranial nerves II, IV and VI are found in the mibrain and control eye movements. Cranial nerves V and VII are found in the pons and are associated with facial sensation and musculature. Cranial Nerves VIII-XII are located in the medulla and are associated with the balance and hearing (VIII), oropharyngeal cavity (IX), viscera (X), neck and shoulder muscles (XI) and tongue (XII) respectively.
We can see similarities in the functional organization of the cranial and spinal nerves and of the parts of the central nervous system with which they directly connect. For example, sensory axons in cranial nerves synapse in sensory cranial nerve nuclei. Similarly, sensory axons in spinal nerves synapse on neurons of the dorsal horn of the spinal cord and the dorsal column nuclei. The motor cranial nerve nuclei, like the motor nuclei of the ventral horn, contain the motor neurons whose axons project to the periphery. The segmental representation of the head is conserved in the cranial nerves; this organization is roughly analogous to the segmental configuration of the spinal cord.
The primary sensory neurons in cranial nerves that enter the brain stem synapse in sensory cranial nerve nuclei; the cell bodies of motor axons in cranial nerves are located in motor cranial nerve nuclei. This arrangement is similar to the dorsal roots, which synapse in the dorsal horn and dorsal column nuclei and the skeletal motor axons of the ventral roots, whose cell bodies are found in the spinal motor nuclei. Each cranial nerve nucleus subserves a single sensory or motor function. This is in contrast to many of the cranial nerves themselves, which contain a mixture of axons that have different functions.
The cranial nerve nuclei follow a logical plan of spatial organization within the brain stem and are organized into rostro-caudal columns. The nuclei form four discrete columns, sensory: somatic and visceral and motor: somatic and visceral. The general organization of the cranial nerves and the columnar organization of the cranial nerve nuclei helps explain the functional organization of the cranial nerves and nuclei; the location of the column provides important information about function. Cranial nerve findings when combined with long tract findings (corticospinal and somatosensory) are powerful for localizing lesions in the brainstem.
There are seven functional categories of cranial nerves and seven categories of cranial nerve nuclei. Nuclei of each of these categories form discontinuous columns that extend rostrocaudally through the brain stem. The seven functional categories are distributed through only six discrete columns, however, because two of the sensory categories synapse on neurons in a single column but at separate rostrocaucal locations.
A systematic relationship exists between the function of each column and its location with respect to the midline. The motor columns are located medial to the sensory columns. In the pons and medulla, nucleus on the ventricular floor are separated by the sulcus limitans. In the embryonic brain and spinal cord the sulcus limitans separates the developing alar and basal plates.
Cranial Nerves and Nuclei
Among the 12 pairs of cranial nerves, the first two - olfactory I and optic II are purely sensory. The olfactory nerve, which mediates the sense of smell, directly enters the telencephalon, and the optic nerve, for vision, enters the diencephalon. The other 10 cranial nerves enter and leave the brain stem.
The oculomotor III and trochlear IV nerves, which are motor nerves, exit from the midbrain. They innervate muscles that move the eyes. The trochlear nerve is further distinguished as the only cranial nerve found on the dorsal brain stem surface.
The pons contains four cranial nerves. The trigeminal V nerve is located at the middle of the pons. It is termed a mixed nerve because it has both sensory and motor functions and it consists of separate sensory and motor roots. This separation is reminiscent of the segregation of function in the dorsal and ventral spinal roots. The sensory root provides the somatic sensory innervation of the facial skin and mucous membranes of parts of the oral and nasal cavities and the teeth. The motor root contains axons that innervate jaw muscles.
The remaining pontine nerves are found at the pontomedullary junction. The abducens VI nerve is a motor nerve that, like the oculomotor and trochlear nerves, innervates eye muscles. The facial VII nerve is a mixed nerve and has separate sensory and motor roots. The motor root innervates the facial muscles that determine our expressions, whereas the sensory root primarily innervates taste buds and mediates taste. The facial sensory root is sometimes called the intermediate nerve. (The intermediate nerve also contains axons that innervate various cranial autonomic ganglia. The vestibulocochlear VIII nerve is a sensory nerve and has two separate components. The vestibular component innervates the semicircular canals, saccule, and utricle and mediates balance, whereas the cochlear component innervates the organ of Corti and serves hearing.
The medulla has four cranial nerves, each of which contain numerous roots that leave from different rostrocaudal locations. Although the glossopharyngeal IX nerve is a mixed nerve, it's major function is to provide the sensory innervation of the pharynx and to innervate taste buds of the posterior one third of the tongue. The motor function of the glossopharyngeal nerve is to innervate a single pharyngeal muscle and peripheral autonomic ganglion. The vagus X nerve, a mixed nerve, has myriad sensory and motor functions that include somatic and visceral sensation, innervation of pharyngeal muscles, and much of the visceral autonomic innervation. The spinal accessory XI and hypoglossal XII nerves subserve motor function, innervating neck and tongue muscles, respectively.
The Olfactory Nerve I consists of short, fine fibers in the olfactory mucous membrane at the roof of the nasal cavity and are receptive to smell (olfaction). These fibers pass through a perforated bony plate to enter the olfactory bulb. The bulb and contiguous olfactory tract are extended parts of the telencephalon; they are not olfactory nerves and therefore are not colored.
Olfactory - S - Smell
The Optic Nerve II arises in the retina (visual photoreceptor layer) in the posterior aspect of the eyeball. The fibers leave the eyeball and surrounding orbit to enter the middle cranial fossa of the skull and merge with contralateral fibers at the optic chiasm. The optic chiasm and contigious tract are extended parts of the diencephalon and therefore are not colored.
Optic - S - Vision
The Optic Nerve (II) rods and cones encode brightness and form information that is used to compute object speed and distance.
The Oculomotor III nerve projects from the midbrain, enters the orbit, and innervates four of the six extrinsic eye muscles as well as the intrinsic eye muscles and the levator muscle of the upper eyelid.
Oculomotor - M - Eye Movement (superior/inferior/medial rectus & inferior oblique muscles)
The Oculomotor Nerve (III) projects to all extraocular muscles except the superior oblique (IV) and lateral rectus (abducens VI)
The Trochlear Nerve IV projects from the midbrain, arising from the posterior surface (the only one to do so) and enters the orbit close to the oculomotor nerve to innervate one (the superior oblique) of the six extrinsic eye muscles. *It is important to remember that cranial nerves never cross (except for one exception, the 4th CN) and clinical findings are always on the same side as the cranial nerve involved.*
Trochlear - M - Eye Movement (Superior Oblique)
The Trochlear Nerve (IV) lower motoneurons projecting to superior oblique muscles
Trigeminal Complex
The Trigeminal Nerve V arises from the pons as three large branches (V1, V2 and V3) each of which passes through different skull foramina to provide sensory fibers to the face, nose, and mouth. The V1 branch contains a motor component to the muscles of mastication and the tensor tympani muscle in the middle ear.
Trigeminal - B/S - Face, Sinus, Teeth, Muscles of mastication - Temporalis & Masseter
Organization of the trigeminal system; complex serves dorsal column, spinothalamic and proprioceptive sensibility for the head. Opthalmic branch supplies the regions of the eyes, maxillary branch supplies the infraorbital area and the upper jaw, mandibular branch innervates the caudal part of the jaw just in front of the ear. Main sensory nucleus of V, spinal trigeminal nucleus and tract of V, mesencephalic nucleus of V, motor nucleus of V; mastication and deglutition; palatal and auditory tube movement, motion of tympanic membrane and ossicles. Trigeminal neuralgia, Sjoqvist's tractotomy can relieve tic douloureux while leaving facial sensibility intact
The Abducens Nerve VI arises at the pontine medullary junction and enters the orbit with the oculomotor and trochlear nerves to innervate one (the lateral rectus) of the six extrinsic eye muscles.
Abducens - M - Eye Movement (Lateral Rectus)
The Abducens Nerve (VI) lower motoneurons innervating lateral rectus muscles.
The Facial Nerve VII consists of two branches as it leaves the cerebellopontine angle and divides complexly. It supplies the muscle of facial expression, the stapedius muscle in the middle ear, two pairs of salivary glands, the soft palate, and the taste buds on the anterior two-thirds of the tongue.
Facial - VII - B/M - Facial Movement (Muscles of the face and anterior 2/3 of tongue - taste)
The Facial Nerve
Special Visceral Motor Component
The Facial Nerve (VII), the motor nucleus of VII is a mixed nerve, innervates muscles of facial expression. The movement of stapes for activation of middle ear reflexes that regulate reflex responses that reduce the loudness of potentially damaging sounds. The elevation of the hyoid has a role in controlling the pitch of speech.
The facial nerve VII contains axons from several cell columns, including general somatic afferent (skin behind the ear), special visceral afferent (taste receptors) and general visceral efferent (salivary glands, lacrimal gland). Also present are special visceral efferent axons to the muscles of facial expression, certain muscles of the floor of the mouth, and a muscle of the middle ear. The special visceral efferent component is the subject of this page.
The motor nucleus of cranial nerve VII is immediately posterior to the superior olivary nucleus and lateral to the reticular formation. collaterals from the corticobulbar tracts synapse directly or indirectly, via neurons of the reticular formation, with the motor nucleus. The rostral part of the nucleus, supplying the more superior facial muscles (in the forehead and around the eyes), is supplied by both crossed and uncrossed corticobulbar collaterals; thus a lesion of the corticobulbar fibers or upper motor neurons seldom produces weakness or paralysis of these muscles, whereas the facial muscles in the lower half of the face on the contralateral side may be paralyzed.
After leaving the nucleus of the seventh nerve, the motor root travels rostrally, posteriorly and medially toward the midline, then swings around the abducens nucleus forming the facial colliculus. The motor root proceeds through the pons laterally and anteriorly to exit the brain stem, with the sensory root at the cerebellopontine angle.
Passing a short distance through the posterior cranial fossa, the motor and sensory roots, in company with the VIII nerve, enter the internal auditory meatus in the petrous portion of the temporal bone which contains the internal and middle ear. Emerging laterally from the meatus, the two roots (motor and sensory) now called the facial nerve and enclosed within the facial canal proceed laterally in the roof of the internal ear and then bend posteriorly at a sharp angle. It is here that the cell bodies of the sensory neurons of VII are located, appropriately called the geniculate ganglion. The axons responsible for different functions are routed in either anterior or posterior directions at the ganglion. The anteriorly directed fibers are presented in plate 6-15; here we are concerned with the posteriorly directed fibers (facial nerve) which contained within the facial canal bend caudally in an arc to pass through the posterior wall of the middle ear cavity. In this caudally directed canal, a nerve is given off to the stapedius muscle a dampener of excess movement of the stapes.
Emerging from the sylomastoid foramen, (inferior opening of the facial canal) the facial nerve turns anteriorly to enter the parotid (salivary gland), within which divides into a number of branches including (from superior to inferior) the temporal, innervating the frontalis muscle of the forehead, the zygomatic innervating the muscles of the eye and cheek, the buccal innervating the muscles of mastication or buccinator muscles, marginal mandibular innervating the chin or mandible and cervical branch innervating the platysma. Each of these branches forms smaller filaments, which supply the muscles of facial expression. The muscles of the scalp as well as a muscle of the neck are innervated by this nerve (as are two muscles in the floor of the mouth, the stylohyoid and the posterior belly of the digastric.
The facial nerve is vulnerable to injury at several points along its path - for example, at the entrance to the internal auditory auditory meatus (due to a tumor of the VIII nerve or its sheath), in the facial canal (infection of the middle ear cavity), and in the substance of the parotid gland (tumor or surgery). The signs of a facial nerve lesion vary according to the site of the lesion; peripheral lesions are characterized by facial muscle paralysis (Bell's palsy) on the affected side (drooping of the corners of the eye and mouth). Central lesions are more complicated, often involve major descending tracts and one or more neighboring cranial nerve nuclei.
When monitoring the facial nerve, it is important to realize that:
This cranial nerve has a motor component for muscles of facial expression (and, don't forget, the stapedius muscle which is important for the acoustic reflex), parasympathetics for tear and salivary glands, and sensory for taste (anterior two-thirds of the tongue).
Central (upper motor neuron-UMN) versus Peripheral (lower motor neuron-LMN) 7th nerve weakness- with a peripheral 7th nerve lesion all of the muscles ipsilateral to the affected nerve will be weak whereas with a "central 7th ", only the muscles of the lower half of the face contralateral to the lesion will be weak because the portion of the 7th nerve nucleus that supplies the upper face receives bilateral corticobulbar (UMN) input.
The Vestibulocochlear VIII nerve leaves the medulla at the cerebellopontine angle to enter the inner ear as two parts, the cochlear division for hearing and the vestibular division for maintenance of equilibrium.
Vestibulocochlear - S - Auditory - Hearing and Balance
The Vestibulocochlear nerve (VIII) (Note: the only purely sensory cranial nerve subserving two modalities); the vestibular nuclei receive input from vestibular hair cells via vestibular ganglion cells for equilibrium analysis and project to the forebrain, the oculomotor system and the spinal cord. The cochlear nuclei receive input from spiral ganglion cells that innervate cochlear hair cells.
The Glossopharyngeal Nerve IX leaves the medulla and skull to supply the taste buds of the posterior third of the tongue, the parotid salivary gland, and the muscles of the pharynx; it is sensory to the posterior mouth and pharynx.
Glossopharyngeal B - Innervates posterior 1/3 of tongue
The Glossopharyngeal nerve (IX), the solitary nucleus is recipient of pain, touch and temperature from these receptors on the posterior one-third of the tongue, via the petrosal ganglion. The spinal trigeminal nucleus, general somatic afferents, pain and temperature from skin around the ear through the jugular ganglion of IX. The inferior salvatory nucleus parasympathetic motor supply to the parotid salivary gland. The nucleus ambiguus; motor neurons controlling pharyngeal muscle.
The Vagus Nerve leaves the medulla and skull to supply motor and sensory innervation to the pharynx, larynx, and thoracic and abdominal viscera, as well as part of the ear.
Vagus - B - Innervates internal organs and external ear (heart, lungs, vocal chords)
Vagus nerve (X) spinal trigeminal nucleus, touch pain and temperature from skin of ear and external auditory meatus. Solitary nucleus, touch pain and temperature from larynx, pharynx and thoracic and abdominal viscera via the inferior ganglion of X. Nucleus ambiguus, motor to larynx and pharynx and to branchial arch muscles. Dorsal motor nucleus supplies thoracic and abdominal viscera via preganglionic neurons situated near the target organs. solitary nucleus.
The Accessory Nerve XI has both a spinal component and a cranial componenet, of which the cranial part is associated with (accesory to) the vagus nerve. The spinal (root) supplies the large neck muscles, sternocleidomastoid (laterally) and trapezius (posteriorly).
Spinal Accessory - M - Neck and Shoulder Movement (sternocleidomastoid and trapezius muscles)
Spinal Accessory Nerve (XI) control of sternocleidomastoid and trapezius muscles
The Hypoglossal Nerve XII leaves the medulla and skull to supply the muscles of the tongue.
Hypoglossal - M - innervates muscles of the tongue Mnemonic Device for memorization
Hypoglossal Nerve (XII), hypoglossal nucleus contains lower motoneurons innervating the tongue
Among the 12 pairs of cranial nerves, the first two - olfactory I and optic II are purely sensory. The olfactory nerve, which mediates the sense of smell, directly enters the telencephalon, and the optic nerve, for vision, enters the diencephalon. The other 10 cranial nerves enter and leave the brain stem.
The oculomotor III and trochlear IV nerves, which are motor nerves, exit from the midbrain. They innervate muscles that move the eyes. The trochlear nerve is further distinguished as the only cranial nerve found on the dorsal brain stem surface.
The pons contains four cranial nerves. The trigeminal V nerve is located at the middle of the pons. It is termed a mixed nerve because it has both sensory and motor functions and it consists of separate sensory and motor roots. This separation is reminiscent of the segregation of function in the dorsal and ventral spinal roots. The sensory root provides the somatic sensory innervation of the facial skin and mucous membranes of parts of the oral and nasal cavities and the teeth. The motor root contains axons that innervate jaw muscles.
The remaining pontine nerves are found at the pontomedullary junction. The abducens VI nerve is a motor nerve that, like the oculomotor and trochlear nerves, innervates eye muscles. The facial VII nerve is a mixed nerve and has separate sensory and motor roots. The motor root innervates the facial muscles that determine our expressions, whereas the sensory root primarily innervates taste buds and mediates taste. The facial sensory root is sometimes called the intermediate nerve. (The intermediate nerve also contains axons that innervate various cranial autonomic ganglia. The vestibulocochlear VIII nerve is a sensory nerve and has two separate components. The vestibular component innervates the semicircular canals, saccule, and utricle and mediates balance, whereas the cochlear component innervates the organ of Corti and serves hearing.
The medulla has four cranial nerves, each of which contain numerous roots that leave from different rostrocaudal locations. Although the glossopharyngeal IX nerve is a mixed nerve, it's major function is to provide the sensory innervation of the pharynx and to innervate taste buds of the posterior one third of the tongue. The motor function of the glossopharyngeal nerve is to innervate a single pharyngeal muscle and peripheral autonomic ganglion. The vagus X nerve, a mixed nerve, has myriad sensory and motor functions that include somatic and visceral sensation, innervation of pharyngeal muscles, and much of the visceral autonomic innervation. The spinal accessory XI and hypoglossal XII nerves subserve motor function, innervating neck and tongue muscles, respectively.
The Olfactory Nerve I consists of short, fine fibers in the olfactory mucous membrane at the roof of the nasal cavity and are receptive to smell (olfaction). These fibers pass through a perforated bony plate to enter the olfactory bulb. The bulb and contiguous olfactory tract are extended parts of the telencephalon; they are not olfactory nerves and therefore are not colored.
Olfactory - S - Smell
The Optic Nerve II arises in the retina (visual photoreceptor layer) in the posterior aspect of the eyeball. The fibers leave the eyeball and surrounding orbit to enter the middle cranial fossa of the skull and merge with contralateral fibers at the optic chiasm. The optic chiasm and contigious tract are extended parts of the diencephalon and therefore are not colored.
Optic - S - Vision
The Optic Nerve (II) rods and cones encode brightness and form information that is used to compute object speed and distance.
The Oculomotor III nerve projects from the midbrain, enters the orbit, and innervates four of the six extrinsic eye muscles as well as the intrinsic eye muscles and the levator muscle of the upper eyelid.
Oculomotor - M - Eye Movement (superior/inferior/medial rectus & inferior oblique muscles)
The Oculomotor Nerve (III) projects to all extraocular muscles except the superior oblique (IV) and lateral rectus (abducens VI)
The Trochlear Nerve IV projects from the midbrain, arising from the posterior surface (the only one to do so) and enters the orbit close to the oculomotor nerve to innervate one (the superior oblique) of the six extrinsic eye muscles. *It is important to remember that cranial nerves never cross (except for one exception, the 4th CN) and clinical findings are always on the same side as the cranial nerve involved.*
Trochlear - M - Eye Movement (Superior Oblique)
The Trochlear Nerve (IV) lower motoneurons projecting to superior oblique muscles
Trigeminal Complex
The Trigeminal Nerve V arises from the pons as three large branches (V1, V2 and V3) each of which passes through different skull foramina to provide sensory fibers to the face, nose, and mouth. The V1 branch contains a motor component to the muscles of mastication and the tensor tympani muscle in the middle ear.
Trigeminal - B/S - Face, Sinus, Teeth, Muscles of mastication - Temporalis & Masseter
Organization of the trigeminal system; complex serves dorsal column, spinothalamic and proprioceptive sensibility for the head. Opthalmic branch supplies the regions of the eyes, maxillary branch supplies the infraorbital area and the upper jaw, mandibular branch innervates the caudal part of the jaw just in front of the ear. Main sensory nucleus of V, spinal trigeminal nucleus and tract of V, mesencephalic nucleus of V, motor nucleus of V; mastication and deglutition; palatal and auditory tube movement, motion of tympanic membrane and ossicles. Trigeminal neuralgia, Sjoqvist's tractotomy can relieve tic douloureux while leaving facial sensibility intact
The Abducens Nerve VI arises at the pontine medullary junction and enters the orbit with the oculomotor and trochlear nerves to innervate one (the lateral rectus) of the six extrinsic eye muscles.
Abducens - M - Eye Movement (Lateral Rectus)
The Abducens Nerve (VI) lower motoneurons innervating lateral rectus muscles.
The Facial Nerve VII consists of two branches as it leaves the cerebellopontine angle and divides complexly. It supplies the muscle of facial expression, the stapedius muscle in the middle ear, two pairs of salivary glands, the soft palate, and the taste buds on the anterior two-thirds of the tongue.
Facial - VII - B/M - Facial Movement (Muscles of the face and anterior 2/3 of tongue - taste)
The Facial Nerve
Special Visceral Motor Component
The Facial Nerve (VII), the motor nucleus of VII is a mixed nerve, innervates muscles of facial expression. The movement of stapes for activation of middle ear reflexes that regulate reflex responses that reduce the loudness of potentially damaging sounds. The elevation of the hyoid has a role in controlling the pitch of speech.
The facial nerve VII contains axons from several cell columns, including general somatic afferent (skin behind the ear), special visceral afferent (taste receptors) and general visceral efferent (salivary glands, lacrimal gland). Also present are special visceral efferent axons to the muscles of facial expression, certain muscles of the floor of the mouth, and a muscle of the middle ear. The special visceral efferent component is the subject of this page.
The motor nucleus of cranial nerve VII is immediately posterior to the superior olivary nucleus and lateral to the reticular formation. collaterals from the corticobulbar tracts synapse directly or indirectly, via neurons of the reticular formation, with the motor nucleus. The rostral part of the nucleus, supplying the more superior facial muscles (in the forehead and around the eyes), is supplied by both crossed and uncrossed corticobulbar collaterals; thus a lesion of the corticobulbar fibers or upper motor neurons seldom produces weakness or paralysis of these muscles, whereas the facial muscles in the lower half of the face on the contralateral side may be paralyzed.
After leaving the nucleus of the seventh nerve, the motor root travels rostrally, posteriorly and medially toward the midline, then swings around the abducens nucleus forming the facial colliculus. The motor root proceeds through the pons laterally and anteriorly to exit the brain stem, with the sensory root at the cerebellopontine angle.
Passing a short distance through the posterior cranial fossa, the motor and sensory roots, in company with the VIII nerve, enter the internal auditory meatus in the petrous portion of the temporal bone which contains the internal and middle ear. Emerging laterally from the meatus, the two roots (motor and sensory) now called the facial nerve and enclosed within the facial canal proceed laterally in the roof of the internal ear and then bend posteriorly at a sharp angle. It is here that the cell bodies of the sensory neurons of VII are located, appropriately called the geniculate ganglion. The axons responsible for different functions are routed in either anterior or posterior directions at the ganglion. The anteriorly directed fibers are presented in plate 6-15; here we are concerned with the posteriorly directed fibers (facial nerve) which contained within the facial canal bend caudally in an arc to pass through the posterior wall of the middle ear cavity. In this caudally directed canal, a nerve is given off to the stapedius muscle a dampener of excess movement of the stapes.
Emerging from the sylomastoid foramen, (inferior opening of the facial canal) the facial nerve turns anteriorly to enter the parotid (salivary gland), within which divides into a number of branches including (from superior to inferior) the temporal, innervating the frontalis muscle of the forehead, the zygomatic innervating the muscles of the eye and cheek, the buccal innervating the muscles of mastication or buccinator muscles, marginal mandibular innervating the chin or mandible and cervical branch innervating the platysma. Each of these branches forms smaller filaments, which supply the muscles of facial expression. The muscles of the scalp as well as a muscle of the neck are innervated by this nerve (as are two muscles in the floor of the mouth, the stylohyoid and the posterior belly of the digastric.
The facial nerve is vulnerable to injury at several points along its path - for example, at the entrance to the internal auditory auditory meatus (due to a tumor of the VIII nerve or its sheath), in the facial canal (infection of the middle ear cavity), and in the substance of the parotid gland (tumor or surgery). The signs of a facial nerve lesion vary according to the site of the lesion; peripheral lesions are characterized by facial muscle paralysis (Bell's palsy) on the affected side (drooping of the corners of the eye and mouth). Central lesions are more complicated, often involve major descending tracts and one or more neighboring cranial nerve nuclei.
When monitoring the facial nerve, it is important to realize that:
This cranial nerve has a motor component for muscles of facial expression (and, don't forget, the stapedius muscle which is important for the acoustic reflex), parasympathetics for tear and salivary glands, and sensory for taste (anterior two-thirds of the tongue).
Central (upper motor neuron-UMN) versus Peripheral (lower motor neuron-LMN) 7th nerve weakness- with a peripheral 7th nerve lesion all of the muscles ipsilateral to the affected nerve will be weak whereas with a "central 7th ", only the muscles of the lower half of the face contralateral to the lesion will be weak because the portion of the 7th nerve nucleus that supplies the upper face receives bilateral corticobulbar (UMN) input.
The Vestibulocochlear VIII nerve leaves the medulla at the cerebellopontine angle to enter the inner ear as two parts, the cochlear division for hearing and the vestibular division for maintenance of equilibrium.
Vestibulocochlear - S - Auditory - Hearing and Balance
The Vestibulocochlear nerve (VIII) (Note: the only purely sensory cranial nerve subserving two modalities); the vestibular nuclei receive input from vestibular hair cells via vestibular ganglion cells for equilibrium analysis and project to the forebrain, the oculomotor system and the spinal cord. The cochlear nuclei receive input from spiral ganglion cells that innervate cochlear hair cells.
The Glossopharyngeal Nerve IX leaves the medulla and skull to supply the taste buds of the posterior third of the tongue, the parotid salivary gland, and the muscles of the pharynx; it is sensory to the posterior mouth and pharynx.
Glossopharyngeal B - Innervates posterior 1/3 of tongue
The Glossopharyngeal nerve (IX), the solitary nucleus is recipient of pain, touch and temperature from these receptors on the posterior one-third of the tongue, via the petrosal ganglion. The spinal trigeminal nucleus, general somatic afferents, pain and temperature from skin around the ear through the jugular ganglion of IX. The inferior salvatory nucleus parasympathetic motor supply to the parotid salivary gland. The nucleus ambiguus; motor neurons controlling pharyngeal muscle.
The Vagus Nerve leaves the medulla and skull to supply motor and sensory innervation to the pharynx, larynx, and thoracic and abdominal viscera, as well as part of the ear.
Vagus - B - Innervates internal organs and external ear (heart, lungs, vocal chords)
Vagus nerve (X) spinal trigeminal nucleus, touch pain and temperature from skin of ear and external auditory meatus. Solitary nucleus, touch pain and temperature from larynx, pharynx and thoracic and abdominal viscera via the inferior ganglion of X. Nucleus ambiguus, motor to larynx and pharynx and to branchial arch muscles. Dorsal motor nucleus supplies thoracic and abdominal viscera via preganglionic neurons situated near the target organs. solitary nucleus.
The Accessory Nerve XI has both a spinal component and a cranial componenet, of which the cranial part is associated with (accesory to) the vagus nerve. The spinal (root) supplies the large neck muscles, sternocleidomastoid (laterally) and trapezius (posteriorly).
Spinal Accessory - M - Neck and Shoulder Movement (sternocleidomastoid and trapezius muscles)
Spinal Accessory Nerve (XI) control of sternocleidomastoid and trapezius muscles
The Hypoglossal Nerve XII leaves the medulla and skull to supply the muscles of the tongue.
Hypoglossal - M - innervates muscles of the tongue Mnemonic Device for memorization
Hypoglossal Nerve (XII), hypoglossal nucleus contains lower motoneurons innervating the tongue
Which cranial nerves lie at the base of the cerebellar peduncles?
Name the important cranial nerve that is the largest one and lies midway along the pons.
What structures would be affected by cutting it?
What nerves is it comparable to in the spinal cord?
Name the important cranial nerve that is the largest one and lies midway along the pons.
What structures would be affected by cutting it?
What nerves is it comparable to in the spinal cord?