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Cranial Nerve Notes

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22 Cranial Nerves

There are twelve bilateral pairs of cranial nerves, which have afferent and/or efferent fibres running between the brain and peripheral structures, mainly those in the head and neck.

The nerves are numbered according to the rostro-caudal sequence in which they join the brain. I = olfactory II = optic III = oculomotor IV = trochlear V = trigeminal - (i = ophthalmic, ii = maxillary, iii = mandibular) VI = abducens VII = facial VIII = vestibulo-cochlear (auditory) IX = glossopharyngeal X = vagus XI = (spinal) accessory XII = hypoglossal

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The first two pairs of nerves attach directly to the forebrain; the others attach to the brain stem.

Those that attach to the brain stem are associated with nuclei within the brainstem. These nuclei contain the cell bodies of cranial nerve efferent neurons and/or receive afferents from cranial nerves.

The nuclei are derived from derivatives of the alar (sensory) or basal (motor) plate.

During development, the alar plate is displaced laterally so the sensory nuclei tend to have a more lateral position than do the motor nuclei.

The motor nuclei have a rostro-caudal arrangement, which is related to the rhombomeres, segmental structures that develop in the developing brainstem.

Hox genes are thought to pattern the rhombomeres and specify the identity of the neurons in each of them.

22.1 Specific Cranial Nerves

For each cranial nerve, you need to know:
 Function
 Origin
 Emergence from the CNS
 Intra- and extra-cranial course
 Peripheral distribution

Testing of function Consequences of lesions at different levels Control and inter-relations of cranial nerve nuclei Classification of fibres into sensory fibres supplying somatic tissues or viscera, motor fibres to striated muscle and autonomic fibres to smooth muscle and glands. Also: the related sympathetic cervical ganglia; parasympathetic ganglia (ciliary, pterygopalatine, submandibular, otic), the control of sweating, lacrimation, salivation and eyelid and pupillary reflexes.

22.1.1 Olfactory (I) Function
 Sense of smell - conveys stimuli from the olfactory epithelium. Classification of fibres.
 Entirely sensory Origin, emergence from the CNS, intra- and extra-cranial course
 Olfactory neuroepithelial cells have unmyelinated axons that pass from the olfactory receptors of the nasal mucosa, entering the skull via the cribriform plate and synapse in the olfactory bulb.

The major output cells from this bulb are the mitral cells (second order projection neurons)

The mitral cells project to the cerebral cortex in the olfactory tracts both of which are visible on the ventral aspect of the forebrain.

This is the only one of the five main senses not to have a relay in the thalamus.

Peripheral distribution
 Sensory from nasal mucosa Testing of function
 To test the function of this nerve, the subject is asked to identify various smells impregnanted into cotton wool buds. Consequences of lesions at different levels

Fractures to the skull, particularly in the frontal bone, can disrupt the cribriform plate and damage the axons of the neuroepithelial cells as they pass to the olfactory bulb, leading to anosmia. As these cells are constantly replaced, the loss is often only transient.

Anosmia can also be produced by the congenital disorder Kallman's syndrome, in which the neurons releasing GnRH fail to migrate to the hypothalamus. The gene mutated in this condition is also responsible for the correct migration of cells to the olfactory epithelium and olfactory bulb.

22.1.2 Optic (II) Core: see 20.3.4 tests: visual field tests, perimetry, signs of raised intracranial pressure Function
 Vision Classification of fibres
 Sensory only Origin, emergence from the CNS, intra- and extra-cranial course
 Fibres from retinal ganglion cells join the optic nerve, which enters the skull through the optic foramina, along with the ophthalmic artery.

At the optic chiasm, the two optic nerves converge and information from the nasal parts of the retina decussates, so that this part of the visual field is processed in the contralateral hemisphere.

The nerves (now known as the optic tracts) then diverge again.

The optic tracts pass around the cerebral peduncle, terminating mainly in the lateral genticulate nucleus of the thalamus.

Third order neurons project from this region to the form the optic radiation, which terminates in the visual cortex in the occipital lobe.

Peripheral distribution
 Sensory from retina Testing of function
 Important tests for the function of the optic nerve include testing the visual fields, looking for signs of raised intra-cranial pressure and perimetry. Consequences of lesions at different levels
 Lesions of the optic nerve (which, when strictly defined, just runs between the retina and the optic chiasm) lead to complete loss of vision on the ipsilateral side.

Lesions at other points in the optic pathway cause different patterns of visual field defects due to the decussation of neurons from certain parts of the retina only.

Increases in intra-cranial pressure can lead to bulging of the optic nerve into the eye, which can be detected with an ophthalmoscope as a less clearly defined macula.

22.1.3 Oculomotor, trochlear, abducent (III, IV, VI) Core: see control of eye movement (21.7), tests: examine pupillary reflexes and eye movements Extension: position of nerves in cavernous sinus, relation of nerves to internal carotid artery Oculomotor (III) Core: damage to III: loss of upward, downward and medial rotation of eye; ptosis; pupillary dilation (see 21.7) Function/classification of fibres/peripheral distribution

Somatic motor to most of the extraocular muscles that move the eye (apart from those supplied by the trochlear and abducens nerves): superior, inferior and medial rectus and inferior oblique- elevate, depress, adduct the eyball, It also supplies striated muslce levator palpebrae, which is the muscle of the eyelid- elevates upper eyelid

Oculomotor nerve also contains preganglionic parasympathetic neurones that via the intermediary of ciliary ganglion control the smooth muscle within the eye- It has parasympathetic functions, containing neurons that synapse in the ciliary ganglion to form short ciliary nerves which innervate the smooth muscle of the eye- sphincter (constrictor) pupillae muscle of iris and ciliary muscle contained within the ciliary body

Pupillary light reflex: amount of light enetering eye regulated by size of pupilillumination of retina causes constriction of pupil through contraction of sphincter pupillae muscle of iris- direct light reflex. Constriction of the pupil of the non-illuminated eye- consensual light reflex

Accommodation reflex: Fixation on a nearby object- contraction of ciliary muscles to increase convexity of lens- focusing the image

Origin, emergence from the CNS, intra- and extra-cranial course

The somatic motor neurons originate in the oculomotor nucleus, which is at the base of the PAG of the midbrain at the level of the superior colliculus.

The preganglionic parasympathetic neurons originate in the Edinger-Westphal nucleus, which is located nearby and innervates the ciliary ganglion.

From both nuclei the fibres run ventrally through the midbrain tegmentum, many of them cross the red nucleus and exit on the medial side of the crus cerebri within the interpeduncular fossa.

Passes between the posterior cerebral and superior cerebellar arteries, then runs anteriorly lying the wall of the cavernous sinus It then runs in the lateral wall of the cavernous sinus with V1, V2, IV, VI and the internal carotid artery and leaves the skull via the superior orbital fissure to reach the orbit.

Testing of function
 Testing the function of this nerve therefore involves determining whether the pupil reflex is present and whether the patient can move the eye medially and upwards. Consequences of lesions at different levels

The oculmotor nerve can be damaged anywhere between the brainstem and the orbit. Causes of this include a posterior communicating artery aneurysm, an uncal herniation through the tentorium due to increased IC pressure or a diabetic neuropathy.

If there is a lesion of this nerve the eye deviates laterally due to unopposed actions of the lateral rectus, and downwards due to unopposed actions of the superior oblique.

Complete ptosis (drooping eyelid) is seen due to reduced innervation of levator palpebrae.

The pupil is fully dilated and the lens cannot accommodate because of a loss of parasympathetic supply.

Trochlear (IV) Core: damage to IV: diplopia on looking downward and medially Function/classification of fibres/peripheral distribution
 This has somatic motor function only, supplying the superior oblique muscle (SO4), which moves the eye downwards and out?
Origin, emergence from the CNS, intra- and extra-cranial course
 The neurons originate in the trochlear nucleus, which is in the PAG at the level of the inferior colliculus.

The axons run dorsally around the PAG and cross the midline, emerging from the dorsal aspect of the brainstem (all other cranial nerves emerge from the ventral aspect) just posterior to the inferior colliculus. Axons of each nerve cross the midline just before they leave the brain and therefore innervate the opposite SO muscle.

The nerve then runs around the cerebral peduncle to the ventral aspect of the brain, runs in the lateral wall of the cavernous sinus to enter the orbit through the superior orbital fissure.

Passes between the posterior cerebral and superior cerebellar arteries

Testing of function The main muscle for abduction is the lateral rectus, so although superior oblique contributes to a downwards and lateral eye movement, testing this motion would not be specific enough as inferior and lateral recti muscles would also be tested. Therefore, during neurological examinations, the superior oblique is tested by having the patient look inwards and downwards, testing only the depressing action of the muscle. This is a source of confusion on the subject as although clinical testing asks the patient to adduct and depress the eye, anatomically the muscle depresses and abducts it. Consequences of lesions at different levels

Lesions of the trochlear nerve alone are relatively rare and usually due to trauma

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