Trigeminal, Abducens, Facial & Vestibulocochlear Nerves - PDF

TRIGEMINAL NERVE (V) ABDUCENS NERVE (VI) FACIAL NERVE (VII) VESTIBULOCOCHLEAR NERVE(VIII) Mariam Tetradze MD  TRIGEMINAL NERVE The trigeminal nerve, CN V, is the fifth paired cranial nerve. It is also the largest cranial nerve. ...

TRIGEMINAL NERVE (V) ABDUCENS NERVE (VI) FACIAL NERVE (VII) VESTIBULOCOCHLEAR NERVE(VIII) Mariam Tetradze MD  TRIGEMINAL NERVE The trigeminal nerve, CN V, is the fifth paired cranial nerve. It is also the largest cranial nerve.  TRIGEMINAL NERVE Sensory: The three terminal branches of CN V innervate the skin, mucous membranes and sinuses of the face. Their distribution pattern is similar to the dermatome supply of spinal nerves (except there is little overlap in the supply of the divisions). Motor: Only the mandibular branch of CN V has motor fibres. It innervates the muscles of mastication: medial pterygoid, lateral pterygoid, masseter and temporalis. The mandibular nerve also supplies anterior belly of digastric, mylohyoid, tensor veli palatini and tensor tympani. Parasympathetic Supply: The post-ganglionic neurones of parasympathetic ganglia travel with branches of the trigeminal nerve. (But note that CN V is NOT part of the cranial outflow of PNS supply)  TRIGEMINAL NERVE The trigeminal nerve originates from three sensory nuclei and one motor nucleus extending from the midbrain to the medulla. At the level of the pons, the sensory nuclei merge to form a sensory root. The motor nucleus continues to form a motor root. In the middle cranial fossa, the sensory root expands into the trigeminal ganglion. The trigeminal ganglion is located lateral to the cavernous sinus, in a depression of the temporal bone  TRIGEMINAL NERVE The peripheral aspect of the trigeminal ganglion gives rise to 3 divisions: ophthalmic (V1), maxillary (V2) and mandibular (V3). The motor root passes inferiorly to the sensory root, along the floor of the trigeminal cave. Its fibers are only distributed to the mandibular division. The ophthalmic nerve and maxillary nerve travel lateral to the cavernous sinus exiting the cranium via the superior orbital fissure and foramen rotundum respectively. The mandibular nerve exits via the foramen ovale entering the infra-temporal fossa.  TRIGEMINAL NERVE DIVISIONS Ophthalmic Nerve Ophthalmic nerve gives rise to 3 terminal branches: frontal, lacrimal and nasociliary, which innervate the skin and mucous membrane of derivatives of the frontonasal prominence derivatives: Forehead and scalp Frontal and ethmoidal sinus Upper eyelid and its conjunctiva Cornea Dorsum of the nose Parasympathetic Supply: Lacrimal gland: Post ganglionic fibres from the pterygopalatine ganglion (derived from the facial nerve), travel with the zygomatic branch of V2 and then join the lacrimal branch of V1. The fibres supply parasympathetic innervation to the lacrimal gland.  CORNEAL REFLEX The corneal reflex is the involuntary blinking of the eyelids – stimulated by tactile, thermal or painful stimulation of the cornea. In the corneal reflex, the ophthalmic nerve acts as the afferent limb – detecting the stimuli. The facial nerve is the efferent limb, causing contraction of the orbicularis oculi muscle. If the corneal reflex is absent, it is a sign of damage to the trigeminal/ophthalmic nerve, or the facial nerve. CORNEAL REFLEX  TRIGEMINAL NERVE DIVISIONS Maxillary Nerve Maxillary nerve gives rise to 14 terminal branches, which innervate the skin, mucous membranes and sinuses : Lower eyelid and its conjunctiva Cheeks and maxillary sinus Nasal cavity and lateral nose Upper lip Upper molar, incisor and canine teeth and the associated gingiva Superior palate Parasympathetic Supply: Lacrimal gland: Post ganglionic fibres from the pterygopalatine ganglion (derived from the facial nerve), travel with the zygomatic branch of V2 and then join the lacrimal branch of V1. The fibres supply parasympathetic innervation to the lacrimal gland. Nasal glands: Parasympathetic fibres are also carried to the mucous glands of the nasal mucosa. Post-ganglionic fibres travel with the nasopalatine and greater palatine nerves (branches of V2)  TRIGEMINAL NERVE DIVISIONS Mandibular nerve Mandibular nerve gives rise to four terminal branches in the infra-temporal fossa: buccal nerve, inferior alveolar nerve, auriculotemporal nerve and lingual nerve. These branches innervate the skin, mucous membrane and striated muscle Sensory supply: Mucous membranes and floor of the oral cavity External ear Lower lip Chin Anterior 2/3 of the tongue (only general sensation; special taste sensation supplied by the chorda tympani, a branch of the facial nerve) Lower molar, incisor and canine teeth and the associated gingiva  TRIGEMINAL NERVE DIVISIONS Mandibular nerve Motor Supply: Muscles of mastication: medial pterygoid, lateral pterygoid, masseter, temporalis Anterior belly of the digastric muscle and the mylohyoid muscle Tensor veli palatini Tensor tympani Parasympathetic Supply: Submandibular and Sublingual glands: Post-ganglionic fibres from the submandibular ganglion (derived from the facial nerve), travel with the lingual nerve to innervate these glands. Parotid gland: Post-ganglionic fibres from the otic ganglion (derived from the glossopharyngeal nerve, CN IX), travel with the auriculotemporal branch of the V3 to innervate the parotid gland.  ABDUCENS NERVE (VI) The abducens nerve is the sixth paired cranial nerve. It has a purely somatic motor function – providing innervation to the lateral rectus muscle.  ABDUCENS NERVE The abducens nerve arises from the abducens nucleus in the pons of the brainstem. It exits the brainstem at the junction of the pons and the medulla.  ABDUCENS NERVE At the tip of petrous temporal bone, the abducens nerve enters the cavernous sinus (a dural venous sinus). It travels through the cavernous sinus and enters the bony orbit via the superior orbital fissure. Within the bony orbit, the abducens nerve terminates by innervating the lateral rectus muscle.  ABDUCENS NERVE The abducens nerve provides innervation to the lateral rectus muscle – one of the extraocular muscles. The lateral rectus originates from the lateral part of the common tendinous ring, and attaches to the anterolateral aspect of the sclera. It acts to abduct the eyeball (i.e. to rotate the gaze away from the midline).  ABDUCENS NERVE PALSY Abducens nerve palsy can be caused by any structural pathology which leads to downwards pressure on the brainstem (e.g. space-occupying lesion). This can stretch the nerve from its origin at the junction of the pons and medulla. Other causes include diabetic neuropathy and thrombophlebitis of the cavernous sinus (in these cases, it is rare for the abducens nerve to be affected in isolation). Clinical features of abducens nerve palsy include diplopia, the affected eye resting in adduction (due to unopposed activity of the medial rectus), and inability to abduct the eye. The patient may attempt to compensate by rotating their head to allow the eye to look sideways. ABDUCENS NERVE PALSY  FACIAL NERVE (VII) Motor – muscles of facial expression, posterior belly of the digastric, stylohyoid and stapedius muscles. Sensory – a small area around the concha of the external ear. Special Sensory – provides special taste sensation to the anterior 2/3 of the tongue via the chorda tympani Parasympathetic – supplies many of the glands of the head and neck, including: Submandibular and sublingual salivary glands. Nasal, palatine and pharyngeal mucous glands. Lacrimal glands.  FACIAL NERVE (VII) Anatomical Course The course of the facial nerve is very complex. There are many branches, which transmit a combination of sensory, motor and parasympathetic fibers. Anatomically, the course of the facial nerve can be divided into two parts: Intracranial – the course of the nerve through the cranial cavity, and the cranium itself. Extracranial – the course of the nerve outside the cranium, through the face and neck.  FACIAL NERVE Intracranial The nerve arises in the pons, an area of the brainstem. It begins as two roots; a large motor root, and a small sensory root. The two roots travel through the internal acoustic meatus, a 1cm long opening in the petrous part of the temporal bone. Still within the temporal bone, the roots leave the internal acoustic meatus, and enter into the facial canal. The canal is a ‘Z’ shaped structure. Within the facial canal, three important events occur: Firstly the two roots fuse to form the facial nerve. Next, the nerve forms the geniculate ganglion. Lastly, the nerve gives rise to: Greater petrosal nerve – parasympathetic fibres to mucous glands and lacrimal gland. Nerve to stapedius – motor fibres to stapedius muscle of the middle ear. Chorda tympani – special sensory fibres to the anterior 2/3 tongue and parasympathetic fibres to the submandibular and sublingual glands. The facial nerve then exits the facial canal (and the cranium) via the stylomastoid foramen. This is an exit located just posterior to the styloid process of the temporal bone.  FACIAL NERVE Extracranial The first extracranial branch to arise is the posterior auricular nerve. It provides motor innervation to the some of the muscles around the ear. Immediately distal to this, motor branches are sent to the posterior belly of the digastric muscle and to the stylohyoid muscle. The main trunk of the nerve, now termed the motor root of the facial nerve, continues anteriorly and inferiorly into the parotid gland (note – the facial nerve does not contribute towards the innervation of the parotid gland, which is innervated by the glossopharyngeal nerve). Within the parotid gland, the nerve terminates by splitting into five branches: Temporal branch Zygomatic branch Buccal branch Marginal mandibular branch Cervical branch These branches are responsible for innervating the muscles of  FACIAL NERVE MOTOR FUNCTIONS Branches of the facial nerve are responsible for innervating many of the muscles of the head and neck. The first motor branch arises within the facial canal; the nerve to stapedius. Between the stylomastoid foramen, and the parotid gland, three more motor branches are given off: Posterior auricular nerve –innervates the intrinsic and extrinsic muscles of the outer ear. It also supplies the occipital part of the occipitofrontalis muscle. Nerve to the posterior belly of the digastric muscle. Nerve to the stylohyoid muscle  FACIAL NERVE MOTOR FUNCTION Within the parotid gland, the facial nerve terminates by bifurcating into five motor branches. These innervate the muscles of facial expression: Temporal – Innervates the frontalis, orbicularis oculi and corrugator supercilii. Zygomatic – Innervates the orbicularis oculi. Buccal – Innervates the orbicularis oris, buccinator and zygomaticus. Marginal mandibular – Innervates the depressor labii inferioris, depressor anguli oris and mentalis. Cervical – Innervates the platysma.  FACIAL NERVE SENSORY FUNCTION The chorda tympani branch of the facial nerve is responsible for innervating the anterior 2/3 of the tongue with the special sense of taste. Within the infratemporal fossa, the chorda tympani ‘hitchhikes’ upon the lingual nerve. The parasympathetic fibres of the chorda tympani stay with the lingual nerve, but the main body of the nerve leaves to innervate the anterior 2/3 of the tongue.  FACIAL NERVE PARASYMPATHETIC FUNCTION The parasympathetic fibres of the facial nerve are carried by the greater petrosal and chorda tympani branches. Greater Petrosal Nerve The greater petrosal nerve combins with the deep petrosal nerve to form the nerve of the pterygoid canal. The nerve of pterygoid canal then passes through the pterygoid canal (Vidian canal) to enter the pterygopalatine fossa, and synapses with the pterygopalatine ganglion. Branches from this ganglion then go on to provide parasympathetic innervation to the mucous glands of the oral cavity, nose and pharynx, and the lacrimal gland. Chorda Tympani The chorda tympani also carries some parasympathetic fibres. These combine with the lingual nerve (a branch of the trigeminal nerve) in the infratemporal fossa and form the submandibular ganglion. Branches from this ganglion travel to the submandibular and sublingual salivary glands.  DAMAGE TO THE FACIAL NERVE Intracranial Lesions Intracranial lesions occur during the intracranial course of the facial nerve (proximal to the stylomastoid foramen). The muscles of facial expression will be paralysed or severely weakened. The other symptoms produced depend on the location of the lesion, and the branches that are affected: Chorda tympani – reduced salivation and loss of taste on the ipsilateral 2/3 of the tongue. Nerve to stapedius – ipsilateral hyperacusis (hypersensitive to sound). Greater petrosal nerve – ipsilateral reduced lacrimal fluid production. The most common cause of an intracranial lesion of the facial nerve is infection related to the external or middle ear. If no definitive cause can be found, the disease is termed Bell’s palsy.  DAMAGE TO THE FACIAL NERVE Extracranial lesions Extracranial lesions occur during the extracranial course of the facial nerve (distal to the stylomastoid foramen). Only the motor function of the facial nerve is affected, therefore resulting in paralysis or severe weakness of the muscles of facial expression. There are various causes of extracranial lesions of the facial nerve: Parotid gland pathology – e.g a tumour, parotitis, surgery. Infection of the nerve – particularly by the herpes virus. Compression during forceps delivery – the neonatal mastoid process is not fully developed and does not provide complete protection of the nerve. Idiopathic – If no definitive cause can be found then the disease is termed Bell’s palsy  VESTIBULOCOCHLEAR NERVE (VIII) The vestibular and cochlear portions of the vestibulocochlear nerve are functionally discrete, and so originate from different nuclei in the brain: Vestibular component – arises from the vestibular nuclei complex in the pons and medulla of the brainstem. Cochlear component – arises from the ventral and dorsal cochlear nuclei, situated within the medulla.  VESTIBULOCOCHLEAR NERVE Both sets of fibres combine in the pons to form the vestibulocochlear nerve. The nerve emerges from the brain at the cerebellopontine angle and exits the cranium via the internal acoustic meatus of the temporal bone. Within the distal aspect of the internal acoustic meatus, the vestibulocochlear nerve splits, forming the vestibular nerve and the cochlear nerve. The vestibular nerve innervates the vestibular system of the inner ear, which is responsible for detecting balance. The cochlear nerve travels to cochlea of the inner ear, forming the spiral ganglia which serve the sense of hearing.  VESTIBULOCOCHLEAR NERVE Special sensory functions Hearing The cochlea detects the magnitude and frequency of sound waves. The inner hair cells of the organ of Corti activate ion channels in response to vibrations of the basilar membrane. Action potentials travel from the spiral ganglia, which house the cell bodies of neurones of the cochlear nerve. The magnitude of the sound determines how much the membrane vibrates and thereby how often action potentials are triggered. Louder sounds cause the basilar membrane to vibrate more, resulting in action potentials being transmitted from the spiral ganglia more often, and vice versa. The frequency of the sound is coded by the position of the activated inner hair cells along the basilar membrane.  VESTIBULOCOCHLEAR NERVE Special sensory functions Equilibrium (Balance) The vestibular apparatus senses changes in the position of the head in relation to gravity. The vestibular hair cells are located in the otolith organs (the utricule and saccule), where they detect linear movements of the head, as well as in the three semicircular canals, where they detect rotational movements of the head Information about the position of the head is used to coordinate balance and the vestibulo-ocular reflex. The vestibulo-ocular reflex allows images on the retina to be stabilized when the head is turning by moving the eyes in the opposite direction. It can be demonstrated by holding one finger still at a comfortable distance in front of you and twisting your head from side to side whilst staying focused on the finger.