Cranial Nerves Flashcards
What part of the nervous system are cranial nerves and which part do they arise from?
Cranial nerves are part of the peripheral nervous system but arise from the central nervous system at the level of the brainstem (or forebrain in the case of olfactory and optic nerves).
What type of signals does each cranial nerve carry? (i.e. motor, sensory, both or autonomic)
All the nerves are paired. Only 4 of the nerves are both sensory and motor (CN V, CN X, CN IX and CN X)
3 are purely sensory (hearing and balance CN VIII, vision CN II and smell CN I)
5 are purely motor (CN III, CN IV, CN VI, CN XI and CN XII). Taste is carried in 2 mixed cranial nerves CN VII and CN IX.
4 of the cranial nerves also carry autonomic fibres (visceral motor) these are CN III, CN VII, CN IX and CN X.
What does the numbering relate to in cranial nerves?
The numbering of the cranial nerves follows the order in which they enter/arise from the brain from rostral to caudal.
Describe the brain stem and its relation to the spinal cord
The brainstem adjoins the brain to the spinal cord. It has a vital role in regulation of the cardio-respiratory functions and maintaining consciousness. The brainstem contains many of the cranial nerve nuclei (collection of nerve bodies) as it is the structure that many of the cranial nerves enter/leave. Even though it is continuous with the spinal cord it is structurally and functionally very different to it.
Where do all the cranial nerves come from in the brain?
2 cranial nerves come from the forebrain itself
There are three parts to the brainstem:
• Midbrain – 2 cranial nerves come from here
• Pons – 4 cranial nerves from here
• Medulla – 4 cranial nerves from here
What is the 1st cranial nerve, what is it’s function, how do we test it and how can it be damaged?
Olfactory. Sense of smell. To test we check one nostril at a time using smelling salts. These are paired anterior extensions of the forebrain. The loss of sense of smell is called anosmia. The most common cause of anosmia is a URTI, head injury can also cause anosmia through shearing of the nerves or basilar skull fractures.
What is the route of the 1st cranial nerve?
Olfactory nerves hang down though the cribriform plate into the olfactory mucosa. They then pass up through the olfactory bulb and then along the olfactory tract
What is the 2nd cranial nerve, what is it’s function and how do we test it?
Optic. Sense of vision. Anterior paired extension rather than a true cranial nerve. This is tested one eye at a time using visual and pupil tests.
What is the route of the 2nd cranial nerve?
Route from the retina passing through the optic canal and then the pairs cross over each other (pituitary gland on posterior side of this cross over), note not all nerves actually mix. This crossover is called the optic chiasm. After cross over the paired nerves become known as optic tracts.
What is the 3rd cranial nerve, what is it’s function, how do we test it, how can it be damaged and how would this present?
Oculomotor. Motor: Ciliary muscles, sphincter of pupil and all extrinsic muscles of the eye except those under CM IV and CN VI. Also, contains some autonomic nerves for the pupil. Tested through inspection of eyelid, pupils and eye movements. Causes of lesions include: raised intracranial pressure, aneurysms (internal carotid artery), cavernous sinus thrombosis from infection and diabetes/hypertension. Damage to this nerve will present with double vison and the eye will be in a down and out (lateral) position.
What is the route of the 3rd cranial nerve?
Route from the midbrain through the superior orbital fissure to the orbit.
Where do the autonomic fibers lie on the occulomotor cranial nerve?
Note parasympathetic fibres line the outside of the nerve so pressure on the nerve will first present with blown pupil (dilates)
What is the 4th cranial nerve, what is it’s function, how do we test it and how can it be damaged?
Trochlear. Motor control of the superior oblique muscle of the eye. Tested with eye movements. Rare to see isolated injury, very subtle change when it is damaged. Patients can compensate by tilting the head. Most commonly damage with head injury or raised ICP, can also get congenital palsies in children.
What is the route of the 4th cranial nerve?
Longest route of all intracranial nerves this is because it is the only nerve to arise from the dorsal aspect of the brainstem. Passes through the superior orbital fissure.
What is the 5th cranial nerve, what is it’s function, how do we test it and how can it be damaged? Which important reflex is this nerve involved in?
Trigeminal. Sensory: face, sinuses, nasal cavities, anterior 2/3 of the tongue and teeth. Motor: muscles for mastication. Tested through sensation of the face and mastication. Trigeminal neuralgia and shingles can affect this nerve . The trigeminal nerve is involved in the corneal reflex (afferent limb of this reflex) which makes us blink when the eye is irritated.
