Olfactory, Optic, Oculomotor, Trochlear Nerves - PDF

OLFACTORY NERVE OPTIC NERVE OCULOMOTOR NERVE TROCHLEAR NERVE Mariam Tetradze MD  OLFACTORY NERVE The olfactory nerve (CN I) is the first and shortest cranial nerve. It is a special visceral afferent nerve, which transmits information relating to s...

OLFACTORY NERVE OPTIC NERVE OCULOMOTOR NERVE TROCHLEAR NERVE Mariam Tetradze MD  OLFACTORY NERVE The olfactory nerve (CN I) is the first and shortest cranial nerve. It is a special visceral afferent nerve, which transmits information relating to smell. The anatomical course of the olfactory nerve describes the transmission of special sensory information from the nasal epithelium to the primary olfactory cortex of the brain.  OLFACTORY NERVE Nasal Epithelium The sense of smell is detected by olfactory receptors located within the nasal epithelium. Their axons (fila olfactoria) assemble into small bundles of true olfactory nerves, which penetrate the small foramina in the cribriform plate of the ethmoid bone and enter the cranial cavity.  OLFACTORY NERVE Olfactory Bulb Once in the cranial cavity, the fibres enter the olfactory bulb, which lies in the olfactory groove within the anterior cranial fossa.  OLFACTORY NERVE The olfactory bulb is an ovoid structure which contains specialised neurones, called mitral cells. The olfactory nerve fibres synapse with the mitral cells, forming collections known as synaptic glomeruli. From the glomeruli, second order nerves then pass posteriorly into the olfactory tract.  OLFACTORY NERVE Olfactory Tract The olfactory tract travels posteriorly on the inferior surface of the frontal lobe. As the tract reaches the anterior perforated substance (an area at the level of the optic chiasm) it divides into medial and lateral stria: 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 many other areas of the brain, notably the piriform cortex, the amygdala, olfactory tubercle and the secondary olfactory cortex. These areas are involved in the memory and appreciation of olfactory sensations.  OLFACTORY NERVE SENSORY FUNCTION The sensory function of the olfactory nerve is achieved via the olfactory mucosa. This mucosal layer not only senses smell, but it also detects the more advanced aspects of taste. It is located in the roof of the nasal cavity and is composed of pseudostratified columnar epithelium which contains a number of cells: Basal cells – form the new stem cells Sustentacular cells – tall cells for structural support. Olfactory receptor cells – bipolar neurons which consist of two processes: Dendritic process projects to the surface of the epithelium, where they project a number of short cilia, the olfactory hairs, into the mucous membrane. These cilia react to odors in the air and stimulate the olfactory cells. Central process (also known as the axon) projects in the opposite direction In addition to the epithelium, there are Bowman’s glands present in the mucosa, which secrete mucus.  ANOSMIA Anosmia is defined as the absence of the sense of smell. It can be temporary, permanent, progressive or congenital. Temporary anosmia can be caused by infection (e.g. meningitis) or by local disorders of the nose (e.g. common cold) Permanent anosmia can be caused by head injury, or tumors which occur in the olfactory groove (e.g. meningioma). Anosmia can also occur as a result of neurodegenerative conditions, such as Parkinson’s or Alzheimer’s disease. In these conditions, the anosmia is progressive and precedes motor symptoms but it is not often noticed by the patient. Anosmia is also a feature of a number of genetic conditions such as Kallmann syndrome (failure to start or finish puberty) and Primary Ciliary Dyskinesia (defect in cilia causing it to be immobile)  OPTIC NERVE The optic nerve (CN II) is the second cranial nerve, responsible for transmitting the special sensory information for vision. The anatomical course of the optic nerve 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 (outside the cranial cavity) and intracranial components.  OPTIC NERVE Extracranial The optic nerve is formed by the convergence of axons from the retinal ganglion cells. These cells in turn 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. It enters the cranial cavity, running along the surface of the middle cranial fossa (in close proximity to the pituitary gland).  OPTIC NERVE Intracranial (The Visual Pathway) Within the middle cranial fossa, the optic nerves from each eye unite to form the optic chiasm. At the chiasm, fibres from the nasal (medial) half of each retina cross over to the contralateral optic tract, while fibres from the temporal (lateral) halves remain ipsilateral: Left optic tract – contains fibres from the left temporal (lateral) retina, and the right nasal (medial) retina. Right optic tract – contains fibres from the right temporal retina, and the left nasal retina.  OPTIC NERVE Each optic tract travels to its corresponding cerebral hemisphere to reach the lateral geniculate nucleus (LGN), a relay system located in the thalamus; the fibres synapse here. Axons from the LGN then carry visual information via a pathway known as the optic radiation. The pathway itself can be divided into: Upper optic radiation – carries fibres from the superior retinal quadrants (corresponding to the inferior visual field quadrants). It travels through the parietal lobe to reach the visual cortex. Lower optic radiation – carries fibres from the inferior retinal quadrants (corresponding to the superior visual field quadrants). It travels through the temporal lobe, via a pathway known as Meyers’ loop, to reach the visual cortex. Once at the visual cortex, the brain processes the sensory data and responds appropriately. OPTIC PATHWAY  VISUAL FIELD DEFECTS  OCULOMOTOR NERVE The oculomotor nerve is the third cranial nerve (CN III). It provides motor and parasympathetic innervation to some of the structures within the bony orbit. Motor – Innervates the majority of the extraocular muscles (levator palpebrae superioris, superior rectus, inferior rectus, medial rectus and inferior oblique). Parasympathetic – Supplies the sphincter pupillae and the ciliary muscles of the eye. Sympathetic – No direct function, but sympathetic fibres run with the oculomotor nerve to innervate the superior tarsal muscle (helps to raise the eyelid).  OCULOMOTOR NERVE The oculomotor nerve originates from the oculomotor nucleus – located within the midbrain of the brainstem, ventral to the cerebral aqueduct. It emerges from the anterior aspect of the midbrain  OCULOMOTOR NERVE The nerve then pierces the dura mater and enters the lateral aspect of the cavernous sinus. Within the cavernous sinus, it receives sympathetic branches from the internal carotid plexus. These fibres do not combine with the oculomotor nerve – they merely travel within its sheath.  OCULOMOTOR NERVE The nerve leaves the cranial cavity via the superior orbital fissure. At this point, it divides into superior and inferior branches: Superior branch – provides motor innervation to the superior rectus and levator palpabrae superioris. Sympathetic fibres run with the superior branch to innervate the superior tarsal muscle. Inferior branch – provides motor innervation to the inferior rectus, medial rectus and inferior oblique. Also supplies pre-ganglionic parasympathetic fibres to the ciliary ganglion, which ultimately innervates the sphincter pupillae and ciliary muscles.  MOTOR FUNCTION OF OCULOMOTOR NERVE The oculomotor nerve innervates many of the extraocular muscles. These muscles move the eyeball and upper eyelid. Superior Branch Superior rectus – elevates the eyeball Levator palpabrae superioris – raises the upper eyelid. Additionally, there are sympathetic fibres that travel with the superior branch of the oculomotor nerve. They innervate the superior tarsal muscle, which acts to keep the eyelid elevated after the levator palpabrae superioris has raised it. Inferior Branch: Inferior rectus – depresses the eyeball Medial rectus – adducts the eyeball Inferior oblique – elevates, abducts and laterally rotates the eyeball  OCULOMOTOR NERVE Parasympathetic Functions There are two structures in the eye that receive parasympathetic innervation from the oculomotor nerve: 1) Sphincter pupillae – constricts the pupil, reducing the amount of light entering the eye.  OCULOMOTOR NERVE Parasympathetic function 2)Ciliary muscles – contracts, causes the lens to become more spherical, and thus more adapted to short range vision. The pre-ganglionic parasympathetic fibres travel in the inferior branch of the oculomotor nerve. Within the orbit, they branch off and synapse in the ciliary ganglion. The post-ganglionic fibres are carried to the eye via the short ciliary nerves. Lens accom modation  OCULOMOTOR NERVE PALSY Oculomotor nerve palsy is a condition resulting from damage to the oculomotor nerve. The most common structural causes include: Raised intracranial pressure (compresses the nerve against the temporal bone). Posterior communicating artery aneurysm Cavernous sinus infection or trauma. Clinical features of CN III injury are associated with the eye: Ptosis (drooping upper eyelid) – due to paralysis of the levator palpabrae superioris and unopposed activity of the orbicularis oculi muscle. ‘Down and out‘ position of the eye at rest – due to paralysis of the superior, inferior and medial rectus, and the inferior oblique (and therefore the unopposed activity of the lateral rectus and superior oblique). The patient is unable to elevate, depress or adduct the eye. Dilated pupil – due to the unopposed action of the dilator pupillae muscle.  TROCHLEAR NERVE The trochlear nerve is the fourth paired cranial nerve. It is the smallest cranial nerve (by number of axons), yet has the longest intracranial course. It has a purely somatic motor function. The trochlear nerve arises from the trochlear nucleus of the brain, emerging from the posterior aspect of the midbrain (it is the only cranial nerve to exit from the posterior midbrain).  TROCHLEAR NERVE The nerve then moves along the lateral wall of the cavernous sinus (along with the oculomotor nerve, the abducens nerve, the ophthalmic and maxillary branches of the trigeminal nerve and the internal carotid artery) before entering the orbit of the eye via the superior orbital fissure.  TROCHLEAR NERVE MOTOR FUNCTION The trochlear nerve 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. Although the mechanism of action of the superior oblique is complex, in clinical practice it is sufficient to understand that the overall action of the superior oblique is to depress and intort the eyeball.  TROCHLEAR NERVE PALSY Trochlear nerve palsy commonly presents with vertical diplopia, exacerbated when looking downwards and inwards (such as when reading or walking down the stairs). Patients can also develop a head tilt away from the affected side. They are commonly caused by microvascular damage from diabetes mellitus or hypertensive disease. Other causes include congenital malformation, thrombophlebitis of the cavernous sinus, and raised intracranial pressure.