CH 12: CNs of the pons & midbrain (3,4,5,6,7) Flashcards
what is the functional component of the abducens nerve? what does it innervate?
GSE
innervates the lateral rectus muscle of the eye
what is the course of the nerve?
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
the abducens nerve can be damaged in the:
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
complete loss of the abducens nerve :
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
what is a strabismus? what is the result?
squint
abnormality of eye position and movement in which the axes of the eyes are not parallel
result is diplopia
what is diplopia?
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
what happens with bilateral abducens nerve paralysis?
both eyes become turned inward, and neither eye can be moved in a lateral direction past the mid position
where is the nucleus of the trochlear nerve? what is the course of the trochlear nerve?
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
what is the functional component of the trochlear nerve? what does it innervate?
GSE
innervates the superior oblique muscle on the OPPOSITE side of its nucleus – depresses the eye, especially when it is adducted
the trochlear nerve is the only nerve with:
fibers emerging from the posterior aspect of the brain stem
an isolated lesion of the trochlear nerve results in:
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)
where is the nucleus of the oculomotor nerve? what is the course of the nerve?
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
shortly after the oculomotors exit from the BS, it passes close to the ___? why is this important?
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
what may compress the oculomotor nerve?
aneurysm at the circle of willis
mass lesions in the cavernous sinus or superior orbital fissure
what is the functional component of the oculomotor nerve? what does it innervate?
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
the medial rectus, inferior rectus and inferior oblique muscles receive input from:
neurons on the IPSILATERAL side of the brainstem
the levitator palpebrae muscles receive input from neurons on:
both sides of the brain stem
the superior rectus muscle receives innervation only from:
neurons on the CONTRALATERAL side of the brain stem
what is the Edinger=Westphal nucleus?
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
lesions of the oculomotor nerve cause:
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
what is the functional component of the facial nerve?
SVE, GVE, SVA, GSA
what does the motor division of the facial nerve innervate? what is its functional component?
muscles of facial expression platysma stylohyoid stapedius posterior belly of digastric
SVE
what does the sensory/parasympathetic division of the facial nerve innervate? what is its functional component?
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)
what is the nervus intermedius?
contains the sensory and parasympathetic fibers of the facial nerve
part of the vagal system
what is the course of the facial nerve?
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
what happens if the facial nerve is lesioned at the stylomastoid foramen?
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
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?
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.)
