What is the blink reflex, and why is it important?

BRANCHIAL MOTOR (EFFERENT) COMPONENT

5. What is the blink reflex, and why is it important?

The blink reflex (see Figure V–18) is the closure of the eyes in response to certain stimuli (bright light, corneal irritation, and loud noise). The motor component of the blink reflex (closing the eyes) is mediated by the branchial motor fibers of the seventh cranial nerve (facial nerve). Several sensory pathways converge on the facial motor nucleus to activate the motor neurons. Initiating stimuli come from bright light via cranial nerve II, corneal stimulation via cranial nerve V (trigeminal), and loud sounds via cranial nerve VIII (auditory component).

Closing the eyes in response to intense light protects the retina from damage.

Blinking in response to corneal irritation protects the cornea from airborne parti-cles or other objects that could injure the eye. Blinking also prevents corneal dry-ness by moistening the cornea with tears. A dry cornea is painful and vulnerable to ulceration and infection. It is not certain why the eyes close in response to loud sounds, although presumably loud sounds could predict flying debris.

Trigeminal ganglion

Trigeminal nerve fibers stretched over neuroma

CN IX

Internal auditory meatus (cut) Acoustic neuroma

CN VII

CN XI CN X Internal jugular

vein and foramen

CN VIII

Petrous temporal bone (cut)

Junction between pons and midbrain Basilar artery

Internal carotid artery

Carotid sheath CN VI

Figure V–16 Acoustic neuroma: an enlarged tumor in the cerebellopontine angle compressing the root of the trigeminal nerve (sagittal section through the jugular foramen).

© L. Wilson-Pauwels

Trigeminal Nerve 101

CLINICAL TESTING

When testing the trigeminal nerve, it is important to remember to test both the sen-sory and motor components. (See also Cranial Nerves Examination on CD-ROM.) Sensory Component

The trigeminal nerve carries several sensory modalities, including discriminative touch, pain, temperature, and simple touch. All modalities are tested with the patient’s eyes closed. When testing, the examiner first must check bilaterally for the presence of each modality in the forehead (V₁), cheeks (V₂), and jaw (V₃), and deter-mine whether both sides of the face are equally sensitive (Figure V–19).

Internal capsule Thalamus

Pons

Medial lemniscus Sensory

neurons in the face region of the sensory cortex

Spinal lemniscus

to muscles of mastication Pain and temperature

B C

Motor neuron in the face region of the motor cortex

to muscles of mastication

Discriminative touch Pain and temperature

Figure V–17 Damage to the trigeminal pathways. A, Within the medulla; B, within the pons; and C, above the brain stem.

© L. Wilson-Pauwels

The discriminative touch pathway is tested by touching the skin (gently) with the sharp end of a pointed object (ie, toothpick) and asking the patient what he or she feels. The pain and temperature pathway is tested by holding warm or cool objects against the skin. This can be done easily at the bedside by using the flat cool end of a tuning fork. Simple touch pathways are tested by lightly touching the skin with a wisp of cotton (see Figure V–19). The territories supplied by each division of the nerve are less variable in the central part of the face; therefore, testing should be done close to the midline.

The corneal reflex should always be tested when examining the trigeminal nerve.

This test is particularly useful when assessing the integrity of V₁in an unconscious patient. The corneal reflex is tested by observing whether the patient blinks in response to a light touch with a wisp of cotton on the cornea, not the sclera (see Figure V–18).

Motor Component

To test the motor component of the trigeminal nerve, the examiner palpates the masseter and temporalis muscles on both sides and asks the patient to clamp the jaws tightly together. The examiner should feel the contraction of each muscle. The patient is then asked to open his or her mouth so that the examiner can look for jaw deviation. If the motor component is not functioning correctly, the jaw will devi-ate toward the weak side. The patient is then asked to move the jaw to one side while the examiner attempts to push it back to the midline position. The examiner should

V₁ nasociliary branch

Figure V–18 Schematic rendering of the blink reflex (brain stem is elevated in response to touch).

Cornea

Orbicularis oculi muscles

© L. Wilson-Pauwels

Trigeminal Nerve 103

not be able to overcome the strength of the pterygoid muscles. This is repeated on the other side.

To complete the motor examination, the jaw jerk should be tested. The mandibular (jaw jerk) reflex can be tested by tapping the middle of the chin with a reflex hammer while the patient’s mouth is slightly open. A sudden slight closing of the jaw constitutes a normal reflex.

ADDITIONAL RESOURCES

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V₁

V₂

V₃

Figure V–19 Clinical testing for sensation.

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p. 59–83.

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p. 209–10.

innervates the lateral rectus muscle of the eye

In document Wilson - Cranial Nerves (Page 104-109)