Reading 2: Clinical Effects Of Cranial Nerve And Brainstem Damage

Question 1

Tumours or lesions near the brainstem

Answer 1

Tumors or lesions in the vicinity of the braistem can damage cranial nerves or cranial nerve nuclei.

Question 2

What nerves control the eye muscles?

Answer 2

• Nerves III, IV and VI
• The oculomotor (III), trochlear (IV) and abducens (VI) nerves control the six small muscles that move the eye.

Question 3

What muscles move the eye and in what direction?

Answer 3

• Medial rectus: rotates the eye medially (towards the nose).
• Lateral rectus: rotates the eye laterally (towards the side of the head)
• Superior and inferior recti + superior and inferior obliques: rotate the eye both vertically and torsionally (ie, third axis of rotation).

Question 4

What is different about the superior oblique?

Answer 4

The superior oblique, unlike the other 5 muscles, is not directly connected to the eye. It acts by pulling on a small tendon called the trochlea.

Question 5

Oculomotor nerve controls what muscles

Answer 5

• The oculomotor nerve controls the medial, superior, and inferior recti, and the inferior oblique.
• The motor neurons for these muscles originate in the oculomotor nucleus in the rostral midbrain.
• This nerve also contains axons that control muscles that raise the eyelid, constrict the pupil, and change the shape of the lens to focus light onto the retina.

Question 6

Damage to the oculomotor nerve

Answer 6

• Damage to the oculomotor nerve causes the ipsilateral eye to drift laterally because the medial rectus is paralyzed, and the lateral rectus is unopposed.
• This lateral deviation is called lateral strabismus
• Since the two eyes are not aligned, the patient may complain of double vision (diplopia).
• During voluntary eye movements, the affected eye cannot move medially beyond the mid-position. Vertical eye movements are also impaired due to paralysis of the superior and inferior recti.
• In addition to these effects on eye movement, the ipsilateral eyelid droops shut (ptosis), the pupil dilates and does not constrict in response to light, and the eye cannot focus on near objects.

Question 7

Trochlear Nerve controls which muscles

Answer 7

• The trochlear nerve innervates the superior oblique. This muscle is involved in moving the eye downward and medially (like when you’re reading a book).

Question 8

Damage to trochlear nerve

Answer 8

Damage to the trochlear nerve causes the
affected eye to drift upward, especially when the eye is looking medially

Question 9

What muscle innervates the Abducens Nerve

Answer 9

• The abducens nerve innervates the lateral rectus and controls lateral eye movements.

Question 10

Damage to the abducens nerve

Answer 10

• Damage to the abducens nerve causes the ipsilateral eye to deviate medially (medial strabismus).
• In addition, the eye cannot move laterally past the mid-position when the patient attempts to move the eyes toward the affected side.
• Damage to the abducens nucleus has a more complicated effect. That is because there are two different types of neurons in the abducens nucleus.

Question 11

What are the two types of neurons in the abducens nucleus?

Answer 11

• One type are motor neurons that activate the ipsilateral lateral rectus.
• The other type are interneurons that cross the midline to activate motor neurons in the contralateral oculomotor nucleus, activating the contralateral medial rectus.
• This might at first seem pointlessly complicated, but it makes sense when you consider that the eyes usually move together. So, for example, when you move your eyes to the right, the right lateral rectus contracts, as does the left medial rectus. Thus, this double innervation from the abducens nucleus helps the eyes to move horizontally in a coordinated fashion. If the abducens nucleus is damaged, both the ipsilateral and contralateral eyes cannot move past the mid-position when the patient attempts to move them toward
  the affected side

Question 12

Nerve VII

Answer 12

• The facial nerve (nerve VII) is a mixed nerve, with motor, somatic sensory and autonomic fibers.
• Most of the fibers that form the nerve are axons of motor neurons in the facial motor nucleus (Fig. 7).
• These fibers innervate ipsilateral facial muscles involved in facial expression as well as the stapedius muscle, the smallest muscle in the body (~ 1 mm in length) located in the middle ear.

Question 13

Damage to the facial nerve

Answer 13

• Damage to the facial nerve results in paralysis of the ipsilateral side of the face. This sometimes happens after viral infection, a condition called Bell’s palsy.
• Patients with facial nerve damage may also complain that sounds have an unpleasant booming quality.
• This occurs because of paralysis of the stapedius muscle. Normally, this muscle rapidly contracts in response to sudden loud noises, dampening vibrations of the stapes, a tiny bone in the middle ear that transmits sound vibrations to the cochlea.

Question 14

Corneal blink reflex

Answer 14

• The facial motor nucleus and facial nerve are involved in a clinically useful reflex called the corneal blink reflex. If either cornea is gently touched with a foreign object, such as a piece of cotton, both eyes reflexively blink.
• . The sensory input for the reflex flows through the trigeminal nerve, the somatic sensory nerve for the face. The sensory fibers from the cornea synapse in the spinal trigeminal nucleus (Fig 7).
• The neurons from the spinal trigeminal nucleus project bilaterally and through interneurons in the reticular formation to neurons in the two facial motor nuclei that close the eyelids.
• A neurologist,
  armed with a wisp of cotton, by touching each cornea and observing the reflex, can test the integrity of the trigeminal nerves, the facial nerves and the internal brainstem circuitry connecting them.
• For example, if the right cornea is touched and neither eye blinks, the damage is probably in the right trigeminal nerve, whereas if the left eye blinks but the right eye doesn’t, the damage is more likely to be in the right facial nerve.