Cranial nerves

Question 1

Which cranial nerve is the only nerve to decussate within the brainstem? What type of deficit would occur if there was a lesion of the nucleus?

Answer 1

Trochlear nerve exits its nucleus and crosses the midline before emerging from the brainstem.
A contralateral deficit would occur. A lack of innervation of the superior oblique muscle causes the affected eye to go “up and out” and the patient will get double vision

Question 2

Which cranial nerve is the only nerve to emerge from the dorsal brainstem?

Answer 2

The trochlear nerve

Question 3

Which cranial nerves emerge from the midbrain? Pons? Medulla?

Answer 3

midbrain - III, IV
Pons - V
Pontinemedullary junction - VI, VII, VIII
Medulla - IX, X, XI, XII

Question 4

What is the difference between pseudo bulbar palsy and bulbar palsy? What are the clinical manifestations of pseudobulbar palsy?

Answer 4

Pseudobulbar palsy = upper motor neuron lesions to cranial nerves, because CN nuclei usually receive innervation from both hemispheres, this requires bilateral damage. Clinical features = impaired swallowing, spastic speech, random crying and laughing spells without emotion, inability to control facial movements
bulbar palsy = lower motor neuron lesions to cranial nerves

Question 5

Cranial nerve nuclei usually receive motor innervation from both hemispheres. Which cranial nerves receive motor innervation from only one hemisphere? Which hemisphere innervates the nuclei?

Answer 5

The lower portion of the facial nerve and the hypoglossal nerve are innervated by only the contralateral UMN. The upper portion of the facial nerve (innervating the forehead) is innervated by both hemispheres

Question 6

If there is a lower motor neuron lesion in the facial nerve, what part of the face would be affected? If there is an upper motor neuron lesion?

Answer 6

LMN lesion - entire half of face is paralyzed (forehead, cheek and chin)
UMN lesion - cheek and chin is paralyzed but forehead is unaffected

Question 7

If there is a lower motor neuron lesion in the hypoglossal nerve, what would happen to the tongue? If there is an upper motor neuron lesion?

Answer 7

The tongue always deviates towards the side that is weak/paralyzed.
LMN lesion - tongue deviates toward the side of the lesion
UMN lesion - tongue deviates away from the side of the lesion