Cranial Nerves1

Cranial nerve nuclei

Olfactory nerve(I)

Functional component  It

has only one component  SVA(special visceral afferent)

Olfactory nerve The olfactory system consists of

the olfactory epithelium, bulb and tract olfactory areas of the brain collectively known as the rhinencephalon. 

Olfactory bulb

U

I. Olfactory: Sensory nerves for smell

The olfactory epithelium is located in the roof, superior conchae, and septum of the nasal cavity.

Olfactory receptor cells are bipolar neurons

The olfactory bulb is an enlargement of the rostral end of the olfactory tract.

the olfactory tract divides into the lateral, intermediate, and medial Striae

Fibers from the pyriform cortex project to the entorhinalcortex(Brodmann area 28), which contains an olfactory association area that sends projections to the hippocampus. This portion of the limbic system is undoubtedly associated with the learning of likes&dislikes of foods.

Olfactory projections

TEST 1.assessment

of the patency of the

nostrils. Each nostril is examined separately. The test involves occlusion of a single nostril while the eyes of the patient are closed.

The patient should inhale gently through the open nostril in close proximity to a common odorant (e.g., vanilla, ground coffee, fresh orange, etc.). The patient should be asked first whether the odorant can be detected. If detectable, the odorant should be identified by name.

If the patient fails at the task, the test should be repeated with at least one distinct odorant. The test should be repeated for the opposite nostril using entirely new odorants.

Optic nerve(II)

Functional component: Special somatic afferent(SSA)

Arises from the retina. Passes through the optic canals to enter the cranial cavity and decussates with its fellow to form the optic chiasm.

It then continues as optic tract to reach Lateral geniculate body and from here it continues as optic radiation to end in the visual cortex.

Each optic nerve contains nasal and temporal fibers. The nasal fibers decussate in the chiasm so that each optic tract contains temporal fibers of the same side and nasal fibers of the opposite side.

sphenoid

Arrows in optic canals

nerve chiasm tract

The Visual Pathway from Below

•Optic nerve •Optic chiasm •Optic tract •Lateral geniculate body •Optic radiation •Visual cortex •Superior colliculus of the midbrain •Putamen •Long association bundle inferior occipitofrontal fasciculus •Pulvinar of the thalamus •Calcarine fissure •Poster inferior horn of the lateral ventricle

Complete blind

hemianopia

Various hemianopias

The test is completed by asking the patient to read a Snellen eye chart viewed at 20 feet.

Oculomotor nerve(III)

Has a motor nucleus in the midbrain (at superior colliculus) which supplies all the extra ocular muscles except Lateral rectus and superior oblique.

It also has a parasympathetic nucleus(Edinger-westphal) Which supplies sphincter pupillae and ciliaris mucles.

Functional components;

1. Somatic efferent.(SE) 2.General visceral efferent(GVE)

Course;

Passes through the mid brain to emerge in the interpeduncular fossa. •Passes through the lateral wall of • the cavernous sinus •Enters the orbit through •superior orbital fissure’s middle compartment.

•Divides into upper and lower divisions •Supply •Levator palpebrae superioris,superior,inferior,medial recti • and inferior oblique muscles of the eye.

Nucleus of III nerve

Ts of midbrain at the level of superior colliculus

Middle cranial fossa

Arrow indicates cavernous sinus

III IV V1 V2 V3

Structures in the lateral wall of cavernous sinus

The Cavernous Sinuses A. Carotid Artery B. Trochlear Nerve C. Maxillary Nerve D. Abducens Nerve E. Sphenoid H. Ophthalmic Sinus Nerve F. Pituitary Gland I. Oculomotor G. Cavernous Nerve Sinus

Superior orbital fissure

Bony orbit(RT) viewed from front

Clinical aspects( ptosis) 

Trochlear nerve (IV)

Functional component: somatic efferent SE Supply: superior oblique muscle of the eye Nucleus: situated in the midbrain at the level of inferior colliculus

It’s the only cranial nerve to emerge from the dorsal aspect of the brain stem(rest all emerge from the ventral aspect).

It is the only cranial nerve that decussates with its fellow.

Passes through the lateral wall of the cavernous sinus below the III nerve and above the ophthalmic nerve. Enters the orbit through the lateral part of sup.orb.fissure.

It then passes medially to supply the superior oblique.

Nucleus of IVnerve in the midbrain (inferior collicular level)

D V

IVnerve in the interpeduncular fossa(dorsal view)

IV nerve

Cavernous sinus

Rt orbit(superior view)

Damage to the Trochlear nerve will present as: •Extorsion (outward rotation) of the affected eye due to the unopposed action of the inferior oblique muscle.

The patient will  . compensate by moving  his head to intort the  contra lateral eye.

•Vertical diplopia (double vision) due to the extorted eye. •Weakness of downward gaze most noticeable on medially-directed eye. • This is often reported as difficulty in descending stairs.

What is torsion?  It

is defined as rotation of an imaginary point at 12o clock position on the corneal margin. If it moves externally(temporally) it is called extorsion.  If it moves internally(nasally),it is called intorsion.

Extorsion of the left eye(IVnerve palsy)

Due to its long peripheral course around the midbrain CN IV is particularly susceptible to head trauma

Abducent nerve(VI)

Functional component: somatic efferent SE Supplies lateral rectus only. Nucleus: in the floor of the IV ventricle deep to facial colliculus Facial nerve fibers loop around this.

Facial colliculus

Course: emerges at the interpeduncular fossa in line with IIInerve. Passes through the cavernous sinus inferolateral to ICA.

Enters the orbit through the middle compartment,between the two divisions of IIInerve and ends by supplying lateral rectus.

VI

In the cranial cavity

At the interpeduncular fossa

III

III

Showing the relationship with circle of Wills IIInerve

VI nerve

Injury to Abducens nerve causes the paralysis of ipsilateral lateral rectus.

The eye is drawn medially due to unopposed action of medial rectus. This will result in diplopia(double vision)

Strabismus – the inability to direct both eyes to the same object.

Rt abducens palsy(no abduction)

Testing of III,IV,VI nerves

III

Oculomotor

Eye movement upward, downward, and inward; narrowing (constriction) or widening (dilation) of the pupil in response to changes in light

The ability to move each eye upward, downward, and inward is tested by asking the person to follow a target moved by the examiner. The upper eyelid is checked for drooping (ptosis).

IV

Trochlea Eye r movement downward and inward

The ability to move each eye downward and inward is tested by asking the person to follow a target moved by the examiner.

VI

Abduc Side-toens side (lateral) eye movement

The ability to move each eye outward beyond the midline is tested.