#17 Flashcards
(21 cards)
The corticobulbar system is like the corticospinal system in most respects, except
it innervates cranial nerve motor nuclei in the brainstem instead of ventral horn cells in the
spinal cord
Corticobulbar tract
target
function
two important caveats
Targets: pontine and medullary reticular formations (majority) and cranial nerve motor nuclei (lower motor neurons)
* Function: voluntary control of…
o jaw movements (motor V nucleus)
o mm. of facial expression (VII nucleus)
o swallowing and voice modulation (nucleus ambiguus)
o tongue mobility (XII)
* Two important caveats:
o Caveat #1: Most CN motor nuclei receive bilateral corticobulbar inputs.
Therefore, unilateral corticobulbar lesions usually produce no clinical effect.
There are two exceptions:
§ Facial nucleus (VII): Neurons that innervate the muscles of the lower quadrant of the face receive mainly crossed input from the opposite motor cortex.
§ Hypoglossal nucleus (XII): Neurons that innervate the tongue receive mainly crossed input from the opposite motor cortex.
o Caveat #2: Motor nuclei for CNs III, IV, an VI (eye movements) do not
receive direct corticobulbar inputs, but are activated instead by brainstem pattern generators controlled by superior colliculus.
§ Even so, descending cortical fibers (from frontal and parietal eye fields, not M1) activating these pattern generators travel down through internal capsule (corticopontine system).
Corticobulbar tract
target
function
two important caveats
Targets: pontine and medullary reticular formations (majority) and cranial nerve motor nuclei (lower motor neurons)
* Function: voluntary control of…
o jaw movements (motor V nucleus)
o mm. of facial expression (VII nucleus)
o swallowing and voice modulation (nucleus ambiguus)
o tongue mobility (XII)
* Two important caveats:
o Caveat #1: Most CN motor nuclei receive bilateral corticobulbar inputs.
Therefore, unilateral corticobulbar lesions usually produce no clinical effect.
There are two exceptions:
§ Facial nucleus (VII): Neurons that innervate the muscles of the lower quadrant of the face receive mainly crossed input from the opposite motor cortex.
§ Hypoglossal nucleus (XII): Neurons that innervate the tongue receive mainly crossed input from the opposite motor cortex.
o Caveat #2: Motor nuclei for CNs III, IV, an VI (eye movements) do not
receive direct corticobulbar inputs, but are activated instead by brainstem pattern generators controlled by superior colliculus.
§ Even so, descending cortical fibers (from frontal and parietal eye fields, not M1) activating these pattern generators travel down through internal capsule (corticopontine system).
Q. Where do crossed corticobulbar fibers decussate?
A. At or near the level of the nuclei they innervate
: When you ask a patient with hypoglossal nucleus or CN XII damage to
protrude the tongue
it will point to the side of the lesion (due to intact genioglossus
Patients
with XII lesions have
dysphagia (trouble swallowing) and dysarthria (trouble with
articulation).
Fascia bulbi (Tenon’s capsule)
Most of the eye is enclosed in a sheath (fascia bulbi or Tenon’s capsule) that is
perforated by the extraocular muscles, and is reflected backward on each as a tubular sheath. The muscles insert into the sclera of the eye. This complex allows the eye to move up/down, side to side, and rotate
All extraocular muscles originate from the
common tendinous ring at the back of the eye
except inferior oblique which is attached to the antero-medial floor of the orbit.
The abducens nucleus contains motor neurons innervating the
e lateral rectus muscle
Lesions to the abducens nerve result in
palsy of the ipsilateral lateral
rectus muscle. The eye on the affected side drifts medially
(unopposed MR) and fails to abduct past the midline on horizontal conjugate gaze to that side
At this point, note that lesions to the abducens nucleus will produce
the same defect, but with additional signs because the nucleus also
sends important projections to the contralateral oculomotor nucleus
via the MLF
The trochlear nucleus and nerve innervate one muscle:
superior oblique
CN IV role in binocular vision
The action of superior oblique is critical for animals, including humans, who have
binocular vision. As objects become closer, the eyes converge to keep retinal images in register but these objects appear lower in the visual field as they get closer. Superior oblique allows depression of the eye when the eyes are adducted.
Lesions of the trochlear nerve result in
paralysis of the ipsilateral superior oblique muscle. The patient complains of diplopia (double vision) that is maximal when the patient is looking down and medially (for example, when looking while walking down stairs). To compensate for the relative extorsion of the eye due to this lesion, the patient tends to tilt the head towards the non-lesioned side.
CN IV
lesions to nucleus
lesions to nerve
- Lesions to nucleus: contralateral signs (extraordinarily rare)
- Lesions to nerve: ipsilateral signs; far more common due to the long peripheral
course of the nerve…
The large oculomotor complex contains motor neurons innervating
The large oculomotor complex contains motor neurons innervating
: Unlike the trochlear nerve (which runs dorsolaterally), oculomotor nerve fibers
pass
ventrally through the tegmentum
Third nerve palsy – “down and out”
Lesions of the III nucleus or nerve results in “down and out” syndrome:
* Abducted eye (loss of MR)
* Depressed eye (loss of both elevators: SR and IO)
* Complete ptosis (loss of LPS)
* Ipsilateral mydriasis (dilated pupil) – loss of constrictor
pupillae
* Loss of direct and consensual pupillary light reflexes in the ipsilateral eye
Conjugate gaze
ability of the eyes to work in unison, moving both eyes in
the same direction at the same time
Mediating conjugate movements requires integration of activity in the
III, IV, and VI nuclei by the medial longitudinal fasciculus (MLF).
Lateral movements of the eyes depend on interactions between the
abducens nucleus in
the pons and the oculomotor nucleus in the midbrain. The abducens nucleus is the
primary driver of horizontal conjugate gaze.
As well as motorneurons innervating the lateral rectus (via the abducens nerve), the
abducens nucleus contains a separate population of interneurons that send their axons through the MLF to the
contralateral oculomotor nucleus, specifically to the III
subnucleus innervating medial rectus (illustration bottom left).
