CH 12: CNs of the pons & midbrain (3,4,5,6,7)

Question 1

what is the functional component of the abducens nerve? what does it innervate?

Answer 1

GSE

innervates the lateral rectus muscle of the eye

Question 2

what is the course of the nerve?

Answer 2

arises from its nucleus beneath the 4th ventricle in the pons

leaves the BS anteriorly at the junction of the medulla and pons and passes along the floor of the posterior fossa of the skull between 2 layers of dura mater. enters the cavernous sinus, passes through the sinus and enters the orbit through the superior orbital fissure

Question 3

the abducens nerve can be damaged in the:

Answer 3

brainstem or more often in its intracranial course (has the longest intracranial course of the CNs)

prolonged elevation of intra-cranial pressure from any cause may damage the nerve

Question 4

complete loss of the abducens nerve :

Answer 4

makes it impossible voluntarily to turn the eye outward beyond the midline.

in addition, the unopposed pull of the medial rectus muscle causes the eye to turn inward (adduct), thereby producing and internal strabismus

Question 5

what is a strabismus? what is the result?

Answer 5

squint

abnormality of eye position and movement in which the axes of the eyes are not parallel

result is diplopia

Question 6

what is diplopia?

Answer 6

double vision- worsens with attempts to gaze to the side of the lesion

horizontal diplopia- when the 2 images appear side by side

patients try to minimize the diplopia by rotating the head towards the side of the lesion

Question 7

what happens with bilateral abducens nerve paralysis?

Answer 7

both eyes become turned inward, and neither eye can be moved in a lateral direction past the mid position

Question 8

where is the nucleus of the trochlear nerve? what is the course of the trochlear nerve?

Answer 8

lies anterior to the periaqueductal gray area in the midbrain in the region of the inferior colliculus

GSE fibers travel caudally a short distance, then curve posteriorly around the central gray area.

the fibers decussate in the anterior medullary velum and exit from the posterior surface of the tectum caudal to the inferior colliculus

the nerve then passes around the outside of the BS to its ventral surface, courses through a sheath in the lateral wall of the cavernous sinus, and enters the orbit through the superior orbital fissure

Question 9

what is the functional component of the trochlear nerve? what does it innervate?

Answer 9

GSE

innervates the superior oblique muscle on the OPPOSITE side of its nucleus – depresses the eye, especially when it is adducted

Question 10

the trochlear nerve is the only nerve with:

Answer 10

fibers emerging from the posterior aspect of the brain stem

Question 11

an isolated lesion of the trochlear nerve results in:

Answer 11

loss of downward ocular movement when the eye is turned toward the nose

patients complain of vertical diplopia (tilt the head to align their eyes to fix it)

Question 12

where is the nucleus of the oculomotor nerve? what is the course of the nerve?

Answer 12

nucleus lies anterior to the periaqueductal gray area in the midbrain, in the region of the superior colliculi

the fibers course ventrally, and some penetrate the medial portions of the red nucleus and the cerebral peduncle.

the nerve exits from the brain stem at the interpeduncular fossa, passes along the brain stem, courses through a sheath in the lateral wall of the cavernous sinus and enters the orbit through the superior orbital fissure

Question 13

shortly after the oculomotors exit from the BS, it passes close to the ___? why is this important?

Answer 13

circle of willis= an anastomotic group of arteries at the base of the brain

an aneurysm (saccular dilatation) in one of the arteries in this region may compress the oculomotor nerve

Question 14

what may compress the oculomotor nerve?

Answer 14

aneurysm at the circle of willis

mass lesions in the cavernous sinus or superior orbital fissure

Question 15

what is the functional component of the oculomotor nerve? what does it innervate?

Answer 15

GSE

innervates the:
medial, superior, and inferior recti
inferior oblique
levator palpebrae superioris

each of these muscles receive nerve fibers from its own subgroup of neurons in the oculomotor nuclear complex

Question 16

the medial rectus, inferior rectus and inferior oblique muscles receive input from:

Answer 16

neurons on the IPSILATERAL side of the brainstem

Question 17

the levitator palpebrae muscles receive input from neurons on:

Answer 17

both sides of the brain stem

Question 18

the superior rectus muscle receives innervation only from:

Answer 18

neurons on the CONTRALATERAL side of the brain stem

Question 19

what is the Edinger=Westphal nucleus?

Answer 19

special subgroup of GVE from the oculomotor complex that innervate the ciliary muscle for accommodation and the sphincter muscle of the iris for constriction of the pupil

Question 20

lesions of the oculomotor nerve cause:

Answer 20

ipsilateral motoneuron paralysis of the muscles supplied resulting in several abnormalities:

1- eye deviates outward –> external strabismus
(b/c of unopposed lateral rectus and inability to turn the eye vertically or inward)

2- ptosis (drooping of eyelid) (loss of levitator palpebrae)

3- pupil becomes dilated (mydriasis) -unopposed action of radial muscle fibers of iris which are supplied by sympathetic system

patient complains of drooping lid and double vision

Question 21

what is the functional component of the facial nerve?

Answer 21

SVE, GVE, SVA, GSA

Question 22

what does the motor division of the facial nerve innervate? what is its functional component?

Answer 22

muscles of facial expression
platysma
stylohyoid
stapedius
posterior belly of digastric

SVE

Question 23

what does the sensory/parasympathetic division of the facial nerve innervate? what is its functional component?

Answer 23

conveys parasympathetic secretory fibers to the salivary and lacrimal glands and to the mucous membranes of the oral and nasal cavities (GVE)

conveys taste sensation for the anterior 2/3 of the tongue (SVA)

sensation from the auricle and external auditory meatus (GSA)

Question 24

what is the nervus intermedius?

Answer 24

contains the sensory and parasympathetic fibers of the facial nerve

part of the vagal system

Question 25

what is the course of the facial nerve?

Answer 25

Motor division arises from nerve cell bodies in the facial nucleus of the pontine tegmentum→ fibers pass dorsally, encircle the nucleus of the abducens nerve and emerge at the lateral aspect of the cerebellopontine angle→ the nerve enters the internal auditory canal and then the facial canal→ leaves the skull by way of the stylomastoid foramen and courses through the substance of the parotid gland behind the ramus of the mandible→ fibers then divide into branches that fan to the face and scalp

the nervus intermedius courses together with the facial nerve from the brainstem to the internal auditory meatus and then into the facial canal where it splits from the facial nerve

Question 26

what happens if the facial nerve is lesioned at the stylomastoid foramen?

Answer 26

total paralysis of the muscles of facial expression on the SAME side
muscles sag
normal lines “iron out”
saliva oozes on same side
cheek may puff out
corneal reflex fails on same side
inability to close eye–> irritation –>infection

Bell’s palsy- swelling/compression - lose nerve function

Question 27

what are the consequences of a lesion to the facial nerve at the cerebellopontine angle, within the internal auditory canal or in the proximal parts of the facial canal?

Answer 27

may affect the facial nerve fibers to the stapedius muscle and the fibers of the nervus intermedius

3 symptoms in ADDITION to facial muscle paralysis:

1-hyperacusis (^ sensitivity to sounds) (paralysis of stapedius)

2-ipsilateral loss of taste sensation (ant 2/3) (injury to nervus int.)

3- decreased secretion of tears/saliva ipsilaterally (damage to nervus int.)