Trochlear Nerve (CN IV) Definition Is the fourth paired cranial nerve. It is the smallest cranial nerve, has the longest intracranial course. It has a purely somatic motor function. Anatomical Course Arises from the trochlear nucleus of the brain, emerging from the posterior aspect of the midbrain. It runs anteriorly and inferiorly within the subarachnoid space before piercing the dura mater adjacent to the posterior clinoid process of the sphenoid bone. The nerve then moves along the lateral wall of the cavernous sinus before entering the orbit of the eye via the superior orbital fissure. Motor Function Innervates a single muscle – the superior oblique, which is a muscle of oculomotion. As the fibres from the trochlear nucleus cross in the midbrain before they exit, the trochlear neurones innervate the contralateral superior oblique. The tendon of the superior oblique is tethered by a fibrous structure known as the trochlea, giving the nerve its name. The overall action of the superior oblique is to depress and intort the eyeball. Trigeminal nerve, CN V Definition Is the fifth paired cranial nerve. It is also the largest cranial nerve. Is associated with derivatives of the 1st pharyngeal arch. Function Sensory: The three terminal branches of CN V innervate the skin, mucous membranes and sinuses of the face. Motor: Only the mandibular branch of CN V has motor fibres. It innervates the muscles of mastication: medial pterygoid, lateral pterygoid, masseter and temporalis. Mandibular nerve also supplies other 1st pharyngeal arch derivatives: 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. Anatomical Course Originates from 3 sensory nuclei (mesencephalic, principal sensory, spinal nuclei of trigeminal nerve) and 1 motor nucleus (motor nucleus of the trigeminal nerve) 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. These roots are analogous to the dorsal and ventral roots of the spinal cord. In the middle cranial fossa, the sensory root expands into the trigeminal ganglion, which is located lateral to the cavernous sinus, in a depression (trigeminal cave) of the temporal bone. 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 fibres 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. Divisions 1) 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 (from the facial nerve), travel with the zygomatic branch and then join the lacrimal branch. 2) Maxillary Nerve Gives rise to 14 terminal branches, which innervate the skin, mucous membranes and sinuses of derivatives of the maxillary prominence of the 1st pharyngeal arch: 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: 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. 3) Mandibular Nerve Gives rise to four terminal branches in the infra-temporal fossa: buccal nerve, inferior alveolar nerve, auriculotemporal nerve and lingual nerve. Innervate the skin, mucous membrane and striated muscle derivatives of the mandibular prominence of the 1st pharyngeal arch. Sensory supply: Mucous membranes and floor of the oral cavity External ear Lower lip Chin Anterior 2/3 of the tongue (General sensation) Lower molar, incisor and canine teeth and the associated gingiva Motor Supply: Muscles of mastication; medial pterygoid, lateral pterygoid, masseter, temporalis Anterior belly of digastric muscle and mylohyoid muscle (suprahyoid muscles) Tensor veli palatini Tensor tympani Parasympathetic Supply: Submandibular and Sublingual glands: Post-ganglionic fibres from submandibular ganglion (from facial nerve), travel with the lingual nerve to innervate it. Parotid gland: Post-ganglionic fibres from the otic ganglion (from glossopharyngeal nerve), travel with the auriculotemporal branch to innervate it. Abducens Nerve (CN VI) Definition Is the sixth paired cranial nerve. It has a purely somatic motor function – providing innervation to the lateral rectus muscle. Anatomical Course Arises from the abducens nucleus in the pons of the brainstem. It exits the brainstem at the junction of pons and medulla. It then enters the subarachnoid space and pierces the dura mater to travel in an area (Dorello’s canal). At the tip of petrous temporal bone, the abducens nerve leaves Dorello’s canal and enters the cavernous sinus, travels through it and enters the bony orbit via the superior orbital fissure. Within the bony orbit, the abducens nerve terminates by innervating the lateral rectus muscle. Motor Function Provides innervation to 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 (to rotate the gaze away from the midline). The Vestibulocochlear Nerve (CN VIII) Is the eighth paired cranial nerve. It is comprised of two parts – vestibular fibres and cochlear fibres. Both have a purely sensory function. Anatomical Course 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 in pons and medulla. Cochlear component – arises from the ventral and dorsal cochlear nuclei, situated in the inferior cerebellar peduncle. 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. The vestibulocochlear nerve splits, forming: 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. Sensory Functions The vestibulocochlear nerve primarily consists of bipolar neurones. It is responsible for the special senses of hearing (cochlear nerve), and balance (vestibular nerve). 1) 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. The magnitude of the sound determines how much the membrane vibrates and thereby how often action potentials are triggered, which travel from spiral ganglia. The frequency of the sound is coded by the position of the activated inner hair cells along the basilar membrane. 2) Equilibrium (Balance) The vestibular apparatus senses changes in position of head in relation to gravity. The vestibular hair cells are located in the otolith organs, 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, which allows images on the retina to be stabilized when the head is turning by moving the eyes in the opposite direction. Glossopharyngeal Nerve (CN IX) Definition Is the ninth paired cranial nerve. Embryologically, the glossopharyngeal nerve is associated with the derivatives of the 3rd pharyngeal arch. Anatomical Course Originates in the anterior aspect of medulla oblongata of the brain, moving laterally in the posterior cranial fossa. The nerve leaves the cranium via the jugular foramen. At this point, the tympanic nerve arises. It has a mixed sensory and parasympathetic composition. Immediately outside the jugular foramen lie two ganglia, superior and inferior (petrous ganglia)– they contain the cell bodies of the sensory fibres in the glossopharyngeal nerve. Extracranial, descends down the neck, anterolateral to the internal carotid artery. At the inferior margin of the stylopharyngeus muscle, several branches arise to provide motor innervation to the muscle. It also gives rise to the carotid sinus nerve. The nerve enters the pharynx by passing between the superior and middle pharyngeal constrictors. Terminates by dividing into branches – lingual, tonsil and pharyngeal. Sensory Functions Provides sensory innervation a variety of structures in the head and neck. Tympanic nerve Arises as the nerve traverses the jugular foramen. It penetrates the temporal bone and enters the cavity of the middle ear. Here, it forms the tympanic plexus – a network of nerves that provide sensory innervation to the middle ear, internal surface of the tympanic membrane and Eustachian tube. Carotid sinus nerve Arises at the level of the stylopharyngeus. It descends down the neck to innervate both the carotid sinus and carotid body, which provide information about blood pressure and oxygen saturation respectively. The glossopharyngeal nerve terminates by splitting into: Pharyngeal branch – combines with fibres of the Vagus nerve to form the pharyngeal plexus. It innervates the mucosa of the oropharynx. Lingual branch – provides posterior 1/3 of tongue with general and taste sensation. Tonsillar branch – forms a network of nerves, tonsillar plexus, which innervates the palatine tonsils. Special Sensory Provides taste sensation to the posterior 1/3 of the tongue, via its lingual branch. Motor Functions The stylopharyngeus muscle of the pharynx is innervated by it. This muscle acts to shorten and widen the pharynx and elevate the larynx during swallowing. Parasympathetic Functions Provides parasympathetic innervation to the parotid gland. These fibres originate in the inferior salivatory nucleus and travel with the tympanic nerve to the middle ear, from which continue as the lesser petrosal nerve, before synapsing at the otic ganglion. The fibres then stop on the auriculotemporal nerve to the parotid gland, where they have a secretomotor effect. Accessory Nerve (CN XI) Definition Is the eleventh paired cranial nerve. It has a purely somatic motor function, innervating the sternocleidomastoid and trapezius muscles. Anatomical Course Traditionally, is divided into spinal and cranial parts. Spinal Component Arises from neurones of the upper spinal cord, specifically C1-C5/C6 spinal nerve roots. These fibres coalesce to form the spinal part of the accessory nerve, which then runs superiorly to enter the cranial cavity via the foramen magnum. The nerve traverses the posterior cranial fossa to reach the jugular foramen. It briefly meets the cranial portion of the accessory nerve, before exiting the skull. Outside the cranium, the spinal part descends along the internal carotid artery to reach the sternocleidomastoid muscle, which it innervates. It then moves across the posterior triangle of the neck to supply motor fibres to the trapezius muscle. Cranial Component Is much smaller and arises from the lateral aspect of the medulla oblongata. It leaves the cranium via the jugular foramen. Immediately after leaving the skull, cranial part combines with the Vagus nerve at the inferior ganglion of vagus nerve. The fibres from the cranial part are then distributed through the vagus nerve. Motor Function Innervates two muscles – the sternocleidomastoid and trapezius. Sternocleidomastoid Attachments – Runs from the mastoid process of the temporal bone to the manubrium (sternal head) and the medial third of the clavicle (clavicular head). Actions – Lateral flexion and rotation of the neck when acting unilaterally Extension of the neck at the atlanto-occipital joints when acting bilaterally. Trapezius Attachments – Runs from the base of the skull and the spinous processes of the C7-T12 vertebrae to lateral third of the clavicle and the acromion of the scapula. Actions – It is made up of upper, middle, and lower fibres. Upper fibres of trapezius elevate scapula and rotate it during abduction of arm. Middle fibres retract the scapula Lower fibres pull the scapula inferiorly. Hypoglossal Nerve (CN XII) Definition Is the twelfth paired cranial nerve. The nerve has a purely somatic motor function, innervating all the extrinsic and intrinsic muscles of the tongue (except the palatoglossus, innervated by vagus nerve). Anatomical Course Arises from the hypoglossal nucleus in the medulla oblongata of the brainstem. It then passes laterally across the posterior cranial fossa, within the subarachnoid space. The nerve exits the cranium via the hypoglossal canal. Extracranial, the nerve receives a branch of the cervical plexus that conducts fibres from C1/C2 spinal nerve roots. These fibres do not combine with the hypoglossal nerve – they merely travel within its sheath. It then passes inferiorly to the angle of the mandible, crossing the internal and external carotid arteries, and moving in an anterior direction to enter the tongue. Motor Function Is a motor nerve; it doesn't send any sensory information to and from the brain, has important functions, including: Talking and singing Chewing Swallowing Supplies movements that help you clear your mouth of saliva, Aid unconscious movements involved in speech Automatic and reflexive motions. Is responsible for motor innervation of the vast majority of the muscles of the tongue. These muscles can be subdivided into two groups: Extrinsic muscles (on the exterior of the tongue) include: • Genioglossus: Makes up the bulk of the tongue and allows to stick tongue out and move it side to side. • Hyoglossus: Comes up from the neck, depresses and retracts the tongue and is important for singing. • Styloglossus: Above and on both sides of the tongue, allows to retract and lift tongue. Intrinsic muscles (fully contained within the tongue) include: • Superior longitudinal: A thin muscle right underneath the mucous membranes in the back of the tongue; to retract the tongue and make it short and thick. • Inferior longitudinal: A narrow band under the surface of the tongue between the genioglossus and the hyoglossus muscles; allows the tongue to be retracted. • Transverse: Along the sides; allows to narrow and elongate tongue. • Vertical: At the borders of the forepart of the tongue; allows to flatten and broaden tongue. Palatoglossus, which raises the back part of your tongue is the only muscle of the tongue not innervated by the hypoglossal nerve, but by Vagus Nerve, 10th CN. Roots The C1/C2 roots that travel with the hypoglossal nerve also have a motor function. They branch off to innervate the geniohyoid (elevates the hyoid bone) and thyrohyoid (depresses the hyoid bone) muscles. Another branch containing C1/C2 fibres descends to supply the ansa cervicalis. From which, nerves arise to innervate the omohyoid, sternohyoid and sternothyroid muscles. (depress the hyoid bone). Vagus Nerve (CN X) Definition Is the 10th cranial nerve (CN X). It is a functionally diverse nerve, offering many different modalities of innervation. It is associated with the derivatives of the 4th and 6th pharyngeal arches. Anatomical Course The vagus nerve (wandering nerve), has the longest course of all the cranial nerves, extending from the head to the abdomen. In the Head Originates from the medulla of brainstem. It exits the cranium via the jugular foramen, with the glossopharyngeal and accessory nerves. Within the cranium, the Auricular branch arises. This supplies sensation to the posterior part of the external auditory canal and external ear. In the Neck Passes into the carotid sheath, travelling inferiorly with the internal jugular vein and common carotid artery. At the base of the neck, the right and left nerves have differing pathways: 1) Right vagus nerve passes anterior to the subclavian artery and posterior to the sternoclavicular joint, entering the thorax. 2) Left vagus nerve passes inferiorly between the left common carotid and left subclavian arteries, posterior to the sternoclavicular joint, entering the thorax. Several branches arise in the neck: 1) Pharyngeal branches – Provides motor innervation to the majority of the muscles of the pharynx and soft palate. 2) Superior laryngeal nerve – Splits into Internal and External branches. The external laryngeal nerve innervates the cricothyroid muscle of the larynx. The internal laryngeal provides sensory innervation to the laryngopharynx and superior part of the larynx. 3) Recurrent laryngeal nerve– Hooks underneath right subclavian artery, then ascends towards to larynx. It innervates majority of intrinsic muscles of larynx. In the Thorax Right vagus nerve forms posterior vagal trunk, and left forms anterior vagal trunk. Branches from the vagal trunks contribute to the formation of the oesophageal plexus, which innervates the smooth muscle of the oesophagus. Two other branches arise in the thorax: Left recurrent laryngeal nerve – it hooks under the arch of the aorta, ascending to innervate the majority of the intrinsic muscles of the larynx. Cardiac branches – these innervate regulate heart rate and provide visceral sensation to the organ. Enter the abdomen via the oesophageal hiatus, an opening in the diaphragm. Sensory Functions There are somatic and visceral components to the sensory function of the vagus nerve. Somatic sensation refers from skin and muscles, provided by the auricular nerve, which innervates the skin of the posterior part of the external auditory canal and external ear. Viscera sensation is that from the organs of the body. The vagus nerve innervates: Laryngopharynx – via the internal laryngeal nerve. Superior aspect of larynx (above vocal folds) – via the internal laryngeal nerve. Heart – via cardiac branches of the vagus nerve. Gastro-intestinal tract – via terminal branches of Vagus nerve. Special Sensory Functions Minor role in taste sensation. It carries afferent fibres from the root of the tongue and epiglottis. Motor Functions Innervates the majority of the muscles associated with the pharynx and larynx, which are responsible for the initiation of swallowing and phonation. Pharynx Muscles of the pharynx are innervated by the pharyngeal branches of the vagus nerve: Pharyngeal constrictor muscles (Superior, middle and inferior) Palatopharyngeus Salpingopharyngeus Stylopharyngeus, is innervated by the glossopharyngeal nerve. Larynx Innervation to the intrinsic muscles of the larynx is achieved via: Recurrent laryngeal nerve: Thyro-arytenoid Posterior crico-arytenoid Lateral crico-arytenoid Transverse and oblique arytenoids Vocalis External laryngeal nerve: Cricothyroid Other Muscles Innervates palatoglossus of the tongue, and majority of the muscles of the soft palate. Parasympathetic Functions In the thorax and abdomen, the vagus nerve is the main parasympathetic outflow to the heart and gastro-intestinal organs. 1) The Heart Cardiac branches arise in the thorax, conveying parasympathetic innervation to the sino-atrial and atrio-ventricular nodes of the heart. Stimulate a reduction in resting heart rate (producing a rhythm of 60 – 80 b/per minute). 2) Gastro-Intestinal System Provides parasympathetic innervation to the majority of the abdominal organs. It sends branches to the oesophagus, stomach and most of the intestinal tract. Stimulate smooth muscle contraction and glandular secretions in these organs. In the stomach, increases the rate of gastric emptying, and stimulates acid production. Facial Nerve (CN VII) Definition Is the seventh paired cranial nerve. Anatomical Course The course of the facial nerve is very complex. There are many branches, which transmit a combination of sensory, motor and parasympathetic fibres. Can be divided into two parts: Intracranial – the course of the nerve through the cranial cavity, and the cranium itself. 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, in the petrous part of the temporal bone, in very close proximity to the inner ear. The roots leave the internal acoustic meatus, and enter into facial canal. Within them: 1) 2) 3) The two roots fuse to form the facial nerve. The nerve forms the geniculate ganglion. Lastly, the nerve gives rise to: Greater petrosal nerve – parasympathetic fibres to mucous and lacrimal glands. 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 cranium via the stylomastoid foramen, located posterior to the styloid process of the temporal bone. Extracranial – the course of the nerve outside the cranium, through the face and neck. After exiting the skull, the facial nerve turns superiorly to run just anterior to outer ear. The first 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. Main trunk of nerve, motor root, continues anteriorly and inferiorly into parotid gland. Within it, the nerve terminates by splitting into 5 branches: Temporal branch Zygomatic branch Buccal branch Marginal mandibular branch Cervical branch Responsible for innervating the muscles of facial expression. Motor Functions Innervating many of the muscles of the head and neck. All these muscles are derivatives of the 2nd pharyngeal arch. The first motor branch arises within the facial canal; the nerve to stapedius, which passes through the pyramidal eminence to supply the stapedius muscle (middle ear). Between stylomastoid foramen, and parotid gland, three motor branches are given off: Posterior auricular nerve – Ascends in front of the mastoid process, and innervates the intrinsic, extrinsic muscles of the outer ear and occipital part of the occipitofrontalis muscle. Nerve to the posterior belly of the digastric muscle – Innervates the posterior belly of the digastric muscle. It is responsible for raising the hyoid bone. Nerve to the stylohyoid muscle – Innervates the stylohyoid muscle. It is responsible for raising the hyoid bone. 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 depressor labii inferioris, depressor anguli oris and mentalis. Cervical – Innervates the platysma. Special 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. Arises in the facial canal, and travels across the bones of the middle ear, exiting via the petrotympanic fissure, and entering the infratemporal fossa and meets with lingual nerve, and leave it to innervate the tongue. Parasympathetic Function 1) Greater Petrosal Nerve Arises distal to the geniculate ganglion within the facial canal. It then moves exiting the temporal bone into the middle cranial fossa, and travels across the foramen lacerum, combining with the Deep petrosal nerve to form the Nerve of the pterygoid canal. Nerve of pterygoid canal then passes through the pterygoid canal to enter the pterygopalatine fossa, and synapses with the pterygopalatine ganglion. Branches from it provide parasympathetic innervation to the mucous glands of the oral cavity, nose and pharynx, and the lacrimal gland. 2) Chorda Tympani Carries some parasympathetic fibres that combine with the lingual nerve in the infratemporal fossa and form the submandibular ganglion. Branches from it travel to the submandibular and sublingual salivary glands. Olfactory Nerve (CN I) Definition Is the first and shortest cranial nerve. It is a special visceral afferent nerve, which transmits information relating to smell. Embryologically, the olfactory nerve is derived from the olfactory placode (a thickening of the ectoderm layer), which also give rise to the glial cells which support the nerve. Anatomical Course Describes the transmission of special sensory information from the nasal epithelium to the primary olfactory cortex of the brain. Nasal Epithelium Sense of smell is detected by olfactory receptors located within the nasal epithelium. Their axons (fila olfactoria) assemble into small bundles of olfactory nerves, which penetrate the small foramina in the cribriform plate of the ethmoid bone and enter the cranial cavity. Olfactory Bulb Once in it, the olfactory nerve fibres enter the olfactory bulb, which lies in olfactory groove within anterior cranial fossa, is an ovoid structure which contains specialized neurones, called mitral cells, and synapse with it, forming collections (synaptic glomeruli). From which, II order nerves then pass posteriorly into the olfactory tract. Olfactory Tract The olfactory tract travels posteriorly on the inferior surface of the frontal lobe. As the tract reaches the anterior perforated substance it divides into: Lateral stria – carries the axons to the primary olfactory cortex, located within the uncus of temporal lobe. Medial stria – carries the axons across the medial plane of the anterior commissure, where they meet the olfactory bulb of the opposite side. The primary olfactory cortex sends nerve fibres to other areas of the brain, the piriform cortex, the amygdala, olfactory tubercle and the secondary olfactory cortex. These areas are involved in the memory and appreciation of olfactory sensations. Sensory Function Via olfactory mucosa layer that senses smell and detects more advanced aspects of taste. Located in the roof of nasal cavity, composed of pseudostratified columnar epithelium which contains: Basal cells – form new stem cells from which new olfactory cells can develop. Sustentacular cells – tall cells for structural support, are analogous to glial cells. Olfactory receptor cells – bipolar neurons which consist of two processes: Dendritic process projects to surface of the epithelium, where project olfactory hairs, into mucous membrane, react to odors in the air and stimulate olfactory cells. Central process (axon) projects in the opposite direction through basement membrane. In addition to epithelium, Bowman’s glands in the mucosa secrete mucus. Optic Nerve (CN II) Definition Is the second cranial nerve, responsible for transmitting special sensory information for vision. It is developed from the optic vesicle, can therefore be considered part of the CNS. Due to its anatomical relation to brain, optic nerve is surrounded by cranial meninges. Anatomical Course Describes the transmission of special sensory information from the retina of the eye to the primary visual cortex of the brain. It can be divided into: Extracranial The optic nerve is formed by the convergence of axons from the retinal ganglion cells, which receive impulses from the photoreceptors of the eye (the rods and cones). After its formation, the nerve leaves the bony orbit via the optic canal, a passageway through the sphenoid bone, and enters the cranial cavity. Intracranial (The Visual Pathway) Within the middle cranial fossa, the optic nerves from each eye unite to form the optic chiasm, where fibres from the nasal half of each retina cross over to the contralateral optic tract, while fibres from the temporal halves remain ipsilateral: Left optic tract – contains fibres from the left temporal retina, and the right nasal retina. Right optic tract – contains fibres from right temporal retina, and left nasal retina. Each optic tract travels to its corresponding cerebral hemisphere to reach the lateral geniculate nucleus, a relay system located in the thalamus; the fibres synapse here. Axons from the LGN then carry visual information via a pathway (optic radiation), divided into: Upper optic radiation – carries fibres from the superior retinal quadrants (inferior visual field). It travels through the parietal lobe to reach the visual cortex. Lower optic radiation – carries fibres from the inferior retinal quadrants (superior visual field). It travels through the temporal lobe, via a pathway (Meyers’ loop), to reach the visual cortex. Once at it, the brain processes the sensory data and responds appropriately. Oculomotor Nerve (CN III) Is the third cranial nerve. It provides motor and parasympathetic innervation to some of the structures within the bony orbit. Anatomical Course Originates from the oculomotor nucleus – located in anterior aspect of the midbrain, ventral to the cerebral aqueduct, passing inferiorly to the posterior cerebral artery and superiorly to the superior cerebellar artery. The nerve then pierces the dura mater and enters in cavernous sinus, within receives sympathetic branches from the internal carotid plexus, but do not combined. The nerve leaves the cranial cavity via superior orbital fissure and it divides into: Superior branch – provides motor innervation to superior rectus and levator palpabrae superioris. Supplies Sympathetic fibres innervate the superior tarsal muscle. Inferior branch – provides motor innervation to the inferior rectus, medial rectus and inferior oblique. Supplies Parasympathetic fibres to ciliary ganglion, which innervates sphincter pupillae and ciliary muscles. Motor Functions Innervates many of the extraocular muscles, that move the eyeball and upper eyelid. Superior Branch Superior rectus – elevates the eyeball Levator palpabrae superioris – raises the upper eyelid. Superior tarsal muscle – acts to keep the eyelid elevated Inferior Branch: Inferior rectus – depresses the eyeball Medial rectus – adducts the eyeball Inferior oblique – elevates, abducts and laterally rotates the eyeball Parasympathetic Functions There are two structures in the eye that receive parasympathetic innervation: Sphincter pupillae – constricts pupil, reducing amount of light entering the eye. Ciliary muscles – contracts, causes the lens to become more spherical and adapted to short range vision. The pre-ganglionic fibres within the orbit branch off and synapse in the ciliary ganglion. The post-ganglionic fibres are carried to the eye via the short ciliary nerves. ORGAN OF SMELL Definition: The nose is an olfactory and respiratory organ. It consists of nasal skeleton, which houses the nasal cavity. Functions: Warms and humidifies the inspired air. Removes and traps pathogens and particulate matter from the inspired air. Responsible for sense of smell. Drains and clears the paranasal sinuses and lacrimal ducts. Divisions The nasal cavity is the most superior part of the respiratory tract. It extends from the vestibule of the nose to the nasopharynx, and has three divisions: Vestibule –area surrounding the anterior external opening to the nasal cavity. Respiratory region – lined by a ciliated psudeostratified epithelium, interspersed with mucus-secreting goblet cells. Olfactory region – located at the apex of the nasal cavity. It is lined by olfactory cells with olfactory receptors. Nasal Conchae Projecting out of the lateral walls of the nasal cavity are curved shelves of bone. They are called conchae, and are three– inferior, middle and superior. Create four pathways for the air to flow, called meatuses: Inferior meatus – between the inferior concha and floor of the nasal cavity. Middle meatus – between the inferior and middle concha. Superior meatus – between the middle and superior concha. Spheno-ethmoidal recess – superiorly and posteriorly to the superior concha. 1) Increase the surface area of the nasal cavity – this increases the amount of inspired air that can come into contact with the cavity walls. 2) Disrupt the fast, laminar flow of the air, making it slow and turbulent. The air spends longer in the nasal cavity, so that it can be humidified. +
