CN 1...6

Anatomy , function and examination

Cranial nerves The cranial nerves are composed of twelve pairs of nerves that emanate from the nervous tissue of the brain. In order reach" their targets they must ultimately exit/enter the cranium through openings in the skull. hence their name is derived from there association with the cranium. The function of the cranial nerves is for the most part similar to the spinal nerves, the nerves that are associated with the spinal cord.

Olfactory nerve

Summary of Function of Cranial Nerves

Cranial nerve I Olfactory nerve

CN I .Olfactory Nerve The olfactory nerve is a collection of sensory nerve rootlets that extend down from the olfactory bulb, and pass through the many openings of the cribriform plate in the ethmoid bone. These specialized sensory receptive parts of the olfactory nerve are then located in the olfactory mucosa of the upper parts of the nasal cavity.

The sense of smell (olfaction) arises from the stimulation of the olfactory receptors by activation from gas molecules that pass by the nose during respiration.
Function
The resulting electrical activity is transduced into the olfactory bulb which then transmits the electrical activity to other parts of the olfactory system and the rest of the central nervous system via the olfactory tract.



Testing the sense of smell Commonly used test substances include vials of pepperment, vanilla, coffee, almond oil or even stronger odours such as whisky or isopropyl alcohol used for skin cleaning. Should not used irritating material.
Examination sequence J. Check that the nasal passages are clear.. 2-Test the sense of smell for each nostril separately. 3. Occlude one nostril by digital pressure. 4. Ask the patient, with eyes closed, to sniff and identify in turn the test substances.

Cranial nerve IIOptic nerve

CN II. Optic nerve: • The optic nerve originates from the bipolar cells of the retina which are connected to the specialized receptors in the retina (rod and cone cells). Light strikes the rod and cone cells and electrical impulses are produced and transduced to the bipolar cells. The bipolar cells in turn transmit electrical activity to the central nervous system through the optic nerve. The optici nerve exits the back of the eye in the orbit and enters the optic canal and exits into the cranium. It enter the CNS at the optic chiasm (crossing) where the nerve fibers become in the optic tract just prior to entering the brain.

Optic nerve leaving the back of a calf eye ( from dissection )

Examination of optic nerve
1-Visual acuity. 2-pupillary reflex. 3-Visual field. 4-Colour vision. 5-Fundoscopy.


1-Visual acuityis measured by testing the ability of the retinal cones to distinguish test object.

Testing visual acuity

Distance visiual acuity should be measured using distance with a snellen chart at 6 metres. Distance is recorded as two numbers written one over the other. The number on top is the distance between the patient and the test chart. This is usually 6 meters ,but may be 3 meters or less when vision is poor. The number below record the smallest line of number read.



2-Pupillary reflexes If a light is directed at one eye, both pupils will normally constrict. The reaction of the pupil on the side stimulated is called the direct light reflex, and the constriction of the other pupil the consensual lighl reflex. when a near object (e.g. at .10cm) is viewed .convergence of the eyes is accompanied by bilateral papillary constriction referred to as the accommodation reflex.

Examination sequence 1. Examine the pupils in a dimly lit room for size and symmetry.

2. Check the direct and consensual reflex in each eye inturn by shining a bright pen torch from the side and from below with the patient looking into the distance in order to avoid an accommodation response. 3. To test the reaction to accommodation ask the patient first to look into the distance and then at an object held close to the face; observe any change in the pupil size.


3-Visual fields Visual fields can be estimated by the method of confrontation or measured accurately using perimeters. Both methods require a cooperative patient. If'a patient is unable to cooperate (because of an impaired conscious state or dysphasia) moving the hand quickly towards the patient's face can make a crude test of visual field integrity. This menacing stimulus will usually evoke reflex blinking if the incoming stimulus is detected.


Examination squence 1. Sit directly opposite and facing the patient. 2. Examine each eye separately first. 3. Ask the patient to cover one eye and look at the examiner's opposing eye. 4- Examine the outer aspects of the visual fields with waggling finger or preferably a fine probe with a large red or while head (e.g. a hat-pin). 5. Bring the test object into the field of vision in a curve, not a straight line. Approach from the periphery at several points on the circumference of the upper and lower, nasal and temporal quadrants of the visual fields.

6. Ask the patient to respond as soon as the movement of the test object is observed. 7. Map out central field defects by moving the test object across the visual field. 8. When using a red test object instruct the patient to report as soon as the colour is perceived. 9. Test for visual inattention by asking the patient to report when the finger tip is moved on one or both sides simultaneously.

Colour vision-4

Tests of color vision assess the function of the retinal cones and optic nerve. Ishihara plates are the most commonly used test. they consist of a series of plate in which colored spots contain shapes which the patient is asked to pick out. Errors are made when color vision is defective.

Abnormalities are found in: 

1-congenital red\green color blindness, which is carried on the X chromosome and affect 70% of male.2-age –related macular disease.3-optic nerve disease.


5- FundoscopyFor checking optic disk

Cranial nerve IIIOculomotor

C/V ///. Oculomotor nerve
The oculomotor nerve originates from motor neurons in the oculomotor (somatomotor) and Edinger-Westphal (visceral motor) nuclei in the brainstem.
Nerve cell bodies in this region give rise to axons that exit the ventral surface of the brain stem as the oculomotor nerve. The nerve passes through the two layers of the dura mater including the lateral wall of the cavernous sinus and then enters the superior orbital fissure to access the orbit. The somatomotor component of the nerve divides into a superior and inferior division.

The superior division supplies the levator palpebrae superioris and.superior rectus muscles. The inferior division supplies the medial rectus, inferior rectus and inferior oblique muscles. The visceromotor or parasympathetic component of the oculomotor nerve travels with inferior division. In the orbit the inferior division sends braches that enter the ciliary ganglion where they form functional contacts (synapses) with the ganglion cells. The ganglion cells send nerve fibers into the back of the eye where they travel to ultimately innervate the ciliary muscle and the constrictor pupillae muscle.

The oculomotor nerve is the third of twelve paired cranial nerves. It controls most of the eye movements (cranial nerves IV and VI also do some), constriction of the pupil, and holding the eyelid open.
Function

Cranial nerve IV

Tochlear nerve


CN IV. Trochlear Nerve The trochlear nerve is purely a motor nerve. The trochlear nerve supplies one muscle the superior oblique. the cell bodies that originate the fourth cranial nerve are located in ventral part of the brainstem in the trochlear nucleus. the trochlear nucleus gives rise to nerves that cross (decussate) to the other side of the brainstem just prior to exiting the brainstem. Thus, each superior oblique muscle is supplied by nerve fibers from the trochlear nucleus of the opposite side.


Test of trochlear nerve Movement of eye downward .inward

C/N V. Trigeminal Nerve The trigeminal nerve as the name indicates is composed of three large branches. They are the ophthalmic (V1 sensory), maxillary (V2 sensory) and mandibuIar (V3, motor and sensory) branches. The large sensory root and smaller is motor root.
The motor root originates from cells located in the masticator motor nucleus of trigeminal nerve located in the midpons of the brainstem. The motor root passes through the trigeminal ganglion and combines with the corresponding sensory root to become the mandibular nerve


.. . It is distributed to the muscles of mastication, the mylohyoid muscle and the anterior belly of the digastric. The mandibular nerve also innervates the tensor veli palatini and tensor tympani muscles the three sensory branches of the trigeminal nerve emanate from The ganglia to form the three branches of the trigeminal nerve. The ophthalmic and maxillary branches travel in the wall of the cavernous sinus just prior to leaving the cranium.the ophthalmic branch travels through the superior orbital fissure and passes through the orbit to reach the skin of the forehead and top of the head.

. "the maxillary nerve enters the cranium through the foramen rotundum via the pterygopalatine fossa. Its sensory branches reach the pterygopalatine fossa via the inferior orbital fissure (face, cheeck and upper teeth) and pterygopalatine canal (soft and hard palate, nasal cavity and pharynx), there are also meningeal sensory branches that enter the trigeminal ganglion within the cranium. The sensory part of the mandibular nerve is composed of branches that carry general sensory information from the mucous membranes of the mouth and cheek,anterior two-thirds of the tongue, lower teeth, skin of the lower Jaw, side of the head and scalp and meninges of the- anterior and middle cranial fossae.


Examination In addition to testing the sensory and motor function, examination of the V nerve includes testing two reflexes 1-The cornel reflex. Touching the cornea; which. .is very sensetive. evokes a brisk contraction of the orbicularis oculi (blinking). Because of the presence of inteneurons. A unilateral corneal stimulus produces a bilateral reflex blink.i.e the reflex has direct and
consensual responses. The afferent part of the corneal reflex is the nasociliary branch of V1 and the efferent limb is the facial nerve


2-the jaw jerk. this is analogous to the tendon reflexes in the limbs. The afferent and efferent pathways are subserved by the V cranial nerve. The reflex response is a brisk contraction of the jaw muscles producing closure of the jaw. This is often not visible in young people but is commonly present above the age of 50.

Examination sequenceSensory1.Test light touch and pain sensation in the territory of the three sensory regions using cotton wool and pin prick respectively. (Temperature sensation may also be tested.) . 2.Test two-point discrimination on the upper and lower lips using calipers. Normally a separation of 3—1 mm can be detected.3.Check sensation in each division of the trigeminal nerve independently, comparing the right with the left.

Motor 1-Inspect the muscle of mastication for wasting, most easily seen in the temporalis muscle above the zygomatic arch. : 2-Ask the patient to open the jaw against resistance (pterygoids .mylohyoid. anterior belly of the digastric). . 3-Palpate the masseter and estimate their bulk and symmetry as the patient clenches the teeth.



Corneal Reflex 1-Approach from the side as the patient looks in the opposite direction, very lightly touch the cornea, not the conjunctiva, with wisp of damp cotton wool. . 2-Observe the presence of both direct and consensual corneal reflexes.


Jaw jerk 1. Place the thumb or forefinger in the midline over the tip of the patient's mandible, with the mouth slightly open. 2. Tap the examiner's finger downwards with a tendon hammer.

CN VI. Abducens Nerve

The abducens nerve originates from neuronal cell bodies located in the ventral pons. These cells give rise to axons that course ventrally and exit the brain at the junction of the pons and the pyramid of the medulla. the nerve of each side then travels anteriorly where it pierces the dura lateral to the dorsum sellae then nerve continues forward and bends over the ridge of the petrous part of the temporal bone and enters the cavernous sinus. The nerve passes lateral to the carotid artery prior to entering superior orbital fissure. The abducens nerve passes through the common tendonous ring of the four rectus muscles and then enters the deep surface of the lateral rectus muscle. The function of the abducens nerve is to contract the lateral rectus which results in abduction of the eye. The abducens nerve in humans is solely and somatomotor nerve.

(eye movements (Examination sequence 1. Inspect the eyes for any abnormality. 2. With both eyes open. test for ocular movements with the patient's head in the neutral position and, if necessary, held there by one of the examiner's hands on the crown of the head. 3. Examine carefully for any squint (strabismus) or other abnormality including nystagmus. 4-. First ask the patient to look up and down, and to the right and the left. 5. Then ask the patient to fix the gaze on the examiner' finger and to report if double vision occurs while following the movement of the finger held about 60 cm. 6. Move the finger up and down, then to the right and up and down, and then to the left and up and down. If necessary repeat the examination, one eye at a time, to distinguish muscle and gaze palsies.

7. Record the direction in which double vision is present and where maximal separation of the images occurs. 8. If diplopia is reported ask the patient to close one eye at a time to identify which eye is producing the false image. 9. To test convergence, ask the patient to focus on the finger as it is brought from a distance towards the tip of the nose. 10. Look for nystagmus while examining eye movements. 11. Record the presence of vertical, horizontal or rotatory nystagmus and the direction of gaze in which it is most marked. 12. Note the direction of the fast component of nystagmus, whether it changes direction with the direction of gaze and whether the degree of nystagmus is different in each eyes.