CNS - Cranial nerves I

CN I - Olfactory nerve

i) Course of tract   

ii) General features of CN I  Shortest cranial nerve  Does not join with brainstem  Has meningeal covering but is not myelinated - covered by Schwann cells iii) Primary olfactory cortex Sends nerve fibres to many areas of brain - important areas are: Piriform cortex Amygdala Olfactory tubercle  Secondary olfactory cortex  These areas control our memory & appreciation of smells  (See CNS physiology lecture - Taste & Smell)

iv) Clinical abnormalities  Anosmia Absence of sense of smell  Can be temporary, permanent, progressive or congenital: Temporary = typically infection or local nasal disorders (Headcolds, flu) Permanent = head injury, tumours in olfactory groove Progressive = neurodegenerative diseases like Parkinson's or Alzheimer's  Congenital = Kallmann syndrome (fail to start or finish puberty), Primary Ciliary dyskinesia (cilia don't move)  Unilateral olfactory loss can be compensated for by other side - therefore patient does not tend to notice loss 

CN II - Optic nerve

i) Course of tract  (See CNS anatomy Vision lecture + CNS vision physiology lec pg 4 i))

ii) General features of CN II Does not join with brainstem  Surrounded by cranial meninges  (See CNS vision physiology lecture for more detail) 

CN III - Oculomotor nerve

i) Anatomical course of tract 

ii) Motor & sensory functions  Motor Innervates most extra-ocular muscles - move eyeball & upper eyelid  Superior branch: Superior rectus - elevates eyeball Levator palpabrae superioris - raises upper eyelid  Inferior branch: Inferior rectus - depresses eyeball Medial rectus - adducts eyeball Inferior oblique - elevates, abducts & laterally rotates eyeball 

Sensory 2 structures - sphincter pupillae & ciliary muscles  Sphincter pupillae = constriction of pupil to reduce light entering  Ciliary muscles = contraction, causing lens to become 'fat & round' - short range vision Parasympathetic fibres travel in inferior branch CN III  Branch into ciliary ganglion 

iii) Clinical abnormalities  Oculomotor nerve lesion Causes of OcNL: Increase intracranial pressure - causes nerve to be compressed against temporal bone Aneurysm posterior cerebral artery  Cavernous sinus infection or trauma  Diseases like diabetes, MS, Myasthenia G  Clinical presentation of OcNL: Ptosis = drooping upper eyelid Due to paralysis of levator palpabrae sup. 'Down & Out' eyeball = eye rests in this position  Due to paralysis recti muscles + inf. oblique  Patient unable to elevate, depress or adduct eye  Dilated pupil = unopposed action dilator pupillae muscle 

CN IV - Trochlear nerve

i) Anatomical course of tract

ii) General features of CN IV Longest intracranial course of all CNs Fewest number of axons of all CNs Most fragile  Only CN to emerge from posterior midbrain 

iii) Motor function  Innervates superior oblique muscle -  depresses & intorts eye (allows us to look 'south-west' in field of vision) 

iv) Clinical abnormalities  Trochlear nerve palsy Vertical diplopia = 'Vertically - oriented double vision'  Made worse when looking downwards & inwards  Typically caused by microvascular damage due to diabetes or hypertension - also raised intracranial pressure  Tested for by asking patient to follow a moving point (moved in a H-shape) with their eyes without moving their head - patient asked if any double vision occurs

CN V - Trigeminal nerve

i) Anatomical course of tract 

ii) General features of CN V Largest cranial nerve  Has 3 branches: Opthalmic nerve Maxillary nerve Mandibular nerve 

iv) Divisions of CN V a) Opthalmic nerve  Innervates the skin & mucous membranes of: Forehead & scalp Frontal & ethmoidal sinus Upper eyelid & its conjunctiva Cornea Dorsum of nose  Involved in corneal reflex (involuntary blinking of eyelids)  - stimulated by tactile, thermal or pain stimulation  Opthalmic nerve detects stimulus  Absent reflex suggests damage 

b) Maxillary nerve  Innervates skin & mucous membranes of: Lower eyelid & its conjuctiva Cheeks & maxilary sinus Nasal cavity & lateral nose Upper lip Upper front teeth & gingiva  Superior palate 

c) Mandibular nerve  Gives sensory supply to: Mucous membranes & floor of oral cavity External ear Lower lip Chin Anterior 2/3rds tongue Lower fronal teeth & gingiva  Gives motor supply to: Muscles of mastication 

Useful mnemonic for remembering exit points of 3 branches! R - Foramen rotundum = Maxillary branch O - Foramen ovale = Mandibular branch S - Superior orbital fissure = Opthalmic branch 

iii) Clinical relevance  Would test sensory aspect using cotton bud on all 3 facial areas Test motor aspect asking patient to clench jaw & also move it left & right  Test for corneal reflex 

CN VI - Abducent nerve

i) Anatomical course of tract 

ii) General features of CN VI Has purely somatic function 

iii) Motor function  Innervates lateral rectus muscle  Allows eyeball to abduct 

iv) Clinical abnormalities  Abducens nerve palsy Diplopia  Medially rotated eye which cannot abduct past midline  Can be caused by variety of issues (diabetic neuropathy, thrombophlebitis among others)