Session 4: Cranial Nerves I-VI - Origin, Route and Function Flashcards
Where do the cranial nerves arise from?
From the central nervous system at the level of the brainstem. However there are two exceptions which do not arise from the brainstem.
What type of cranial nerves are there?
Purely special sensory Purely motor Mixed sensor and motor Some with autonomics
Give some functions of the brainstem.
Adjoins the brain to the spinal cord and is continuous with the spinal cord caudally. Regulation of cardio-respiratory functions and maintaining consciousness. Ascending sensory and descending motor fibres between brain and rest of body run through the brainstem. Location of majority of cranial nerve nuclei.
State the first six cranial nerve with respective roman numerals.
CN I - Olfactory nerve CN II - Optic nerve CN III - Oculomotor nerve CN IV - Trochlear nerve CN V - Trigeminal nerve CN VI - Abducens nerve
There are two exceptions of cranial nerves which do not arise from the brainstem, which?
CN I - Olfactory nerve CN II - Optic nerve
Where do the olfactory nerve and the optic nerve arise from?
The forebrain
Where do the other nerves arise from? (Broadly speaking)
(2 from forebrain) 2 from midbrain 4 from pons 4 from medulla
Explain the route of the olfactory nerve.
Numerous of olfactory nerves nestled in the roof of the nose. They then run vertically to enter the anterior cranial fossa via the cribiform foramina. It then goes into the olfactory bulb, turns into the olfactory tract and lastly into the uncus of the temporal lobe.
What type of cranial nerve is the olfactory nerve?
Special sensory
They are also more of a paired anterior extensions rather than a ‘true’ cranial nerve.
Function of the olfactory nerve.
Olfaction (sense of smell)
Clinical significance of the olfactory nerve.
It is not often tested but if it has to be done, test one nostril at a time.
What is anosmia?
Loss of sense of smell
Most common cause of anosmia
Common cold (upper resp tract infections)
What can cause anosmia other than infection?
Head injury secondary to shearing forces and/or basilar skull fracture.
Tumours at base of frontal lobes within the anterior cranial fossa. (Can pinch on CN I)
Explain the route of the optic nerve.
From retinal ganglion cells axons will form the optic nerve. The optic nerve will then exit back of orbit via the optic canal. The fibres cross (from both eyes) and merge at the optic chiasm and then into the optic tracts.
Function of the optic nerve.
Part of the visual pathway. There are impulses generated by cells within the retina in response to light (action potentials) which will propagate along optic nerve.
Via other components of the visual pathway they reach primary visual cortex where they are perceived as vision.
Just like the olfactory nerve the optic nerve is just paired extensions of forebrain rather than a true cranial nerve.
Explain the route of the sensory of the optic nerve.
Impulses are generated by cells within the retina in response to light. These action potentials are sent down the optic nerve (which carries sensory fibres from one of the eyes). At the optic chiasm the optic nerves cross and mix the sensory fibres from the right and left optic nerves. This means that information gets mixed here. The sensory information then exits via the optic tracts and each optic tract carries information of part of the right eye, and also part of the left eye.
The optic tract will fuse in with the laterial geniculate nucleus and continue via the optic radiation to finally reach the primary visual cortex.
However at the optic tract there is a branch going into the superior colliculus. This is a communication from the optic tracts with the brainstem (midbrain).
What is the clinical significance of the visual pathway and its mix of sensory information?
That depending where a lesion would sit in the visual pathway, a patient would present accordingly.
E.g. retinal detachment would only affect one eye. Optic neuritis which is inflammation of the optic nerve usually only affects on eye.
Where as pituitary tumours which compress the optic chiasm can cause bilateral visual symptoms like bitemporal hemianopia.
What tests are common to test the optic nerve?
Visual acuity tests
Visual fields
Pupillary light responses
Opthalmoscope to study the optic disc
What is the optic disc?
Point at which nerve enters the retina.
The optic nerve carry extension of meninges. What is the clinical significance of this?
The nerve will be affected by raised intracranial pressure meaning a raised ICP can squash the optic nerve.
What is papilloedema?
A swollen optic disc
What type of nerve is the optic nerve?
Special sensory
What is the purpose of the nerve extension from optic tract reaching the superior colliculus?
It allows the optic nerve to communicate with the brainstem and to allow for certain visual reflexes like pupillary reflexes to light.
Which is the third cranial nerve?
Oculomotor
What type of nerve is the oculomotor nerve?
Motor + autonomic
It carries autonomic parasympathetic fibres.
Function of the oculomotor nerve.
Most of the muscles that move the eyeball called extra-ocular muscles
Levator Palpebrae Superioris which is the main muscle (only muscle??) to open the eyelid.
Innervates the sphincter pupillae muscle as well which constrict the pupil.
Explain the route of the oculomotor nerve.
Arise from the midbrain, then runs through the lateral wall of the cavernous sinus, through the superior orbital fissure and onto the eye.
Why is the oculomotor nerve vulnerable to raised intracranial pressure?
Because it can become compressed between tentorium cerebelli and part of temporal lobe as there is a herniation of the uncus (uncal herniation) which compress the oculomotor nerve.
Which part of the oculomotor nerve is especially vulnerable to uncal herniation?
The parasympathetic fibres of the oculomotor nerve as they are found outside of the nerve itself.
How to test the oculomotor nerve.
Inspect eyelids and pupil size
Test eye movent and pupillary reflexes.
Pathology can cause pupillary dilation and/or diplopia.
Also ‘down and out’ position can occur with severe ptosis.
What is special about the origin of the trochlear nerve?
It is the only nerve to emerge from the dorsal aspect of the brainstem and has the longest intracranial course of any of the cranial nerves.
Clinical presentation of trochlear nerve palsy.
Diplopia due to misaligned eyes. It is rare and often subtle. The patient usually is aware of the misalignment and correct the diplopia with a tilt of the head.
How do shingles of the trigeminal nerve usually present?
It is usually restricted to one of the dermatomes.
Ophthalmic shingles is especially important to look out for because it can cause blindness.
Why is the frontal branch of Va important?
It exits the front of orbit as supraorbital and supratrochlear to carry sensory information from forehead.
Why is the infraorbital nerve of the maxillary nerve important?
Because it runs through floor of orbit and carry sensory from area of cheek and lower eye lid. It is susceptible to injury in orbital floor fractures.
What is the significance of the inferior alveolar nerve and the mental nerve?
Carrying sensory from area of mental protuberance, lower lip and gum. They are susceptible to injury in mandibular fractures.
Clinical presentation of abducens conditions.
Diplopia due to misalignment of eyes
