Cranial Nerves 1-6 Flashcards
What are the cranial nerves?
Twelve pairs numbered I to XII from anterior to posterior according to attachment to the brain
What is the function of olfactory nerve 1?
special sensory (sense of smell)
What is the Origin, Course and Relations of Olfactory CN 1?
Olfactory epithelium in the roof of the nasal cavity
Central processes of olfactory cells form the olfactory nerve
Pass through the foramina in the cribriform plate of the ethmoid bone, pierce the dura and arachnoid
Enter the olfactory bulb in the anterior cranial fossa
Axons form the olfactory bulb then form the olfactory tract
Olfactory tract is a narrow band of white matter that runs from the posterior end of the olfactory bulb, which divides into medial and lateral olfactory striae
Lateral stria carries the axons to the primary olfactory cortex (periamygdaloid and prepiriform areas in the uncus)
Medial stria carries the fibres that cross the median plane and pass to the olfactory bulb of the opposite side
What causes Anosmia = Loss of smell?
Damage to the receptor cells, Atrophy of Olfactory bulbs or the olfactory tract - smoking, aging, use of cocaine
Acute and Chronic Inflammatory nasal diseases
Parkinson disease (Lewis body formation)
Intracranial tumours
Inferior frontal glioma, Olfactory glioma / Meningioma
Basal skull / Maxillary fracture
What is the function of CN2 - Optic?
special sensory (vision)
What is the Origin, Course and Relations of the Optic nerve - CN2?
Retinal ganglion cells in the neural retina to optic disc
Optic nerve passes posteromedially to exit the orbit through the optic canal
Enters middle cranial fossa where it forms the optic chiasm
Changes name to optic tract when the fibres have passed through the optic chiasm
Fibres from the medial (nasal) halves of each retina decussate in the chiasm and then join uncrossed fibres from the lateral halves of each retina to form the right and left optic tracts
Most fibres terminate in the lateral geniculate bodies of the thalamus
From these nuclei axons relay via the optic radiation to the visual cortices on the medial surface the occipital lobes of the brain
From the optic nerve the nerve fibres also pass beyond the thalamus to the superior colliculus and pretectal nucleus/region in the midbrain
From the colliculi, impulses pass down the cord in the tectospinal tracts
What is the tectospinal tract?
Tectospinal tract is in the superior colliculus of the midbrain. As this area recieves information regarding visual input, this tract is primarily responsible for mediating reflex responses to visual stimuli.
What causes loss of visual fields?
Damage to the left optic nerve (1) results in blindness in the left eye
E.g. raised intracranial pressure causing blockage of axoplasmic flow of CSF or loss of blood supply from the central retinal artery or a berry aneurysm in the Circle of Willis
Damage to the optic chiasma (2) results in loss of all visual fields and therefore causes blindness in both eyes
E.g. pituitary gland tumours or pituitary hypertrophy during pregnancy
If only the middle crossing fibres of the optic chiasma is affected, then only the nasal visual fields are lost
Damage to the left optic tract (3) or left lateral geniculate bodies (4) results in right homonymous hemianopia (loss of the right visual fields)
E.g. loss of blood supply from the anterior choroidal arteries
Damage to the left optic radiations (5) results in right homonymous hemianopia
E.g. loss of blood supply from the posterior and middle cerebral arteries
What is the function of Oculomotor Nerve (CN III)?
somatic motor and visceral motor
What is the origin of Oculomotor Nerve (CN III)?
Origin
Main oculomotor nucleus
Lies in the periaqueductal gray matter of the midbrain ventral to the cerebral aqueduct at the level of the superior colliculus
Supplies the superior rectus, medial rectus, inferior rectus, inferior oblique and levator palpebrae superioris muscles
Outgoing nerve fibres pass through the red nucleus and emerge on the anterior surface of the midbrain in the interpeduncular fossa
Receives corticonuclear fibres from both cerebral hemispheres
Receives tectobulbar fibres from the superior colliculus
Receives fibres from the medial longitudinal fasciculus, which connects it to the 4th, 6th, and 8th cranial nerves
What is the Accessory parasympathetic nucleus
(Edinger Westphal nucleus) of Oculomotor Nerve (CN III)?
Parasympathetic, through the ciliary ganglion, to the sphincter of the pupil (causing constriction), and to ciliary muscles of lens (accommodation)
What is the course and relations to Oculomotor Nerve (CN III)?
Fibres from both nuclei course ventrally
Pierces dura and runs in lateral wall of cavernous sinus
Leaves cranial cavity through superior orbital fissure to enter orbit
Preganglionic parasympathetic neurones terminate in ciliary ganglion
Postganglionic neurones run in short ciliary nerves to sphincter pupillae muscle and ciliary muscle
What are the clinical correlations of Oculomotor Nerve (CN III)?
Complete paralysis prevents the eye moving upward, downward or inward
Eye looks downward and laterally due to the unopposed lateral rectus and superior oblique muscles
Patient has diplopia diplopia
Ptosis of upper eyelid due to paralysis of levator palpebrae superioris
Pupil is dilated and non-reactive to light due to the paralysis of the sphincter pupillae and the unopposed action of the dilator pupillae (supplied by the sympathetics)
Accommodation is impossible
What are Direct and Consensual Light Reflexes?
If a light is shone into one eye, the pupils of both eyes normally constrict
The constriction of the pupil on which the light is shone is called the direct light reflex
The constriction of the opposite pupil even though no light fell on it is called the consensual light reflex
Retinal ganglion cells Optic nerve optic chiasma optic tract
Some fibres leave optic tract pretectal nucleus (near superior colliculus of midbrain) parasympathetic Edinger-Westphal nuclei of CN III (on both sides) travel through CN III ciliary ganglion in the orbit
Postganglionic parasympathetic fibres pass via short ciliary nerves to constrictor pupillae muscle, thus causing the pupil to constrict in response to light
Both pupils constrict in the consensual light reflex because the pretectal nucleus sends fibres to the parasympathetic nuclei on both sides of the midbrain
What is accommodation reflex?
When the eyes are directed from a distant object to a near object:
Contraction of the medial recti brings about convergence of the ocular axes
Lens thickens to increase it refractive power by contraction of the ciliary muscle
Pupils constrict to restrict light waves to the thickest central part of the lens
Retinal ganglion cells > optic nerve > optic chiasma >optic tract > lateral geniculate body > optic radiation > visual cortex
Visual cortex > frontal eye field of frontal lobe cortical fibres > internal capsule main oculomotor nuclei > medial recti muscles
Cortical fibres > parasympathetic Edinger Westphal nuclei on both sides > CN III > ciliary ganglion > postganglionic parasympathetic fibres > short ciliary nerves > ciliary and constrictor pupillae muscles of the iris
Pupil constricts, lens more convex & eyes converge
What is the function of Trochlear Nerve (CN IV)?
Somatic motor to superior oblique muscle
