The Ocular Motor Nuclei and Nerves Flashcards
Name all six of the rotary extra-ocular muscle and list which cranial nerve they are innervated by.
The way to remember this is every rotary EOM is innervated by the third (Oculomotor) nerve (from the upper midbrain) except:
LR6SO4.
The Lateral Rectus is innervated by the sixth (Abducent) cranial nerve (from the mid-caudal pons).
The superior Oblique is innervated by the Fourth (Trochlear) Nerve (from the lower midbrain).
True or False- All oculomotor nerves pass through the cavernus sinus
True, in order to get through the superior orital fissure ( i.e. the hole in the orbit for nerves to pass through) to the midbrain, they have to pass through the cavernous sinus.
[The pathway of these nerves is drawn in purple - to get from the eyes to the midbrain they pass through the black cicrcular thing which is part of the cavernous sinus]
Each Motor Nucleus gives rise to a Nerve, the axons of which follow which three paths?
- a short intra-cerebral course, before exiting the brainstem
- a long intra-cranial course, to reach the superior orbital fissure
- a long intra-orbital course, to reach & innervate their target muscle(s)
Where is the Oculomotor Nuclei located?
•It is located at the midline in the upper midbrain, at the level of the superior colliculus. (Pointed out in the picture).
Why is the Oculomotor Nuclei refered to as an Oculomotor Nuclear Complex?
As it is a collection of seven sub-seperate nuclei
What are the 7 sub-seperate nuclei of the Oculo Motor Nuclear Complex and what do they innervate?
The seven sub-nuclei are refered to as motor neuron pools.
- There are 4 somatic motor neuron pools these each innervate a rotary EOM (i.e. the MR,SR,IR,IO).
[Remember the Oculomotor complex is to do with the oculomotor nerve (3rd nerve)]
[Are called somatic as they supply skeletal muscle which EOMS are]
- 1 midline somatic motor neuron pool (called the Caudal Central Nucleus) which innervates the LPS (Levator Palpebrae Superioris)
- 2 autonomic motor neuron pools (Edinger-Westphal & Anteromedian nuclei) - autonomic thus parasympathetic system- these innervate different neurons in the Ciliary Ganglion, which supply the sphincter pupillae & ciliary muscles, respectively, in the eye
What are distinguishing features of the OculoMotor Nerves (3rd)?
- Largest of the 3 Nerves that supply EOMs: ~25,000 axons
- Has both Somatic & Parasympathetic axons:
–majority are voluntary motor axons (to the MR SR IR IO & LPS) which occupy the centre of the nerve
–the rest are pre-ganglionic autonomic axons (to the Ciliary Ganglion) which are superficial in the nerve
•Most of these axons are uncrossed, but those supplying the SR project contralaterally: their axons cross in the upper midbrain just as they exit the nucleus
How does the intra-cranial third nerve exit the midbrain?
- Into sub-arachnoid space at inter-penduncular fossa
- It Passes between the posterior cerebral artery (PCA) & superior cerebellar artery (SCA), then infero-lateral to the posterior communicating artery (PCoA)
- It Travels forward in sub-arachnoid space
- To enter dural sheath forming the lateral wall of the Cavernous Sinus and..
- Enters orbit via superior orbital fissure (SOF)
Where is the Trochlear Nuclei located?
•It is located at the midline in lower midbrain, at the level of the inferior colliculus
What are the 2 distinguishing features of the Trochlear Nerve?
(1) emerges from the dorsal (posterior) surface of the brainstem (i.e. it goes the long way around)
(2) carries axons that cross completely inside the brain
- So, the Nuclei supply the opposite SO muscle
- But the Nerve outside the brain supplies SO muscle on the same side (geddit?)
- Also, smallest diameter (only 3000-4000 axons), but longest intra-cranial course, due to (1) above
Describe the path of the intra-cerebral and intra-cranial 4th Nerve courses
Axons of Trochlear motor neurons
- they come out from the trochlear nucleus andRun laterally & dorsally around the edge of the Peri-Aqueductal Grey (PAG)
- Cross the midline (decussate) in the Superior Medullary Velum (SMV), a thin sheet of tissue forming part of the roof of the 4th Ventricle
- Exit the brainstem dorsally just behind the inferior colliculi to enter the sub-arachnoid space
- Run forward near the cerebral peduncles, passing close to superior cerebellar artery (SCA)
- Then enter the dural sheath forming lateral wall of the Cavernous Sinus, below 3rd nerve
- To enter the orbit via the Superior Orbital Fissure
[This path can be followed in the picture]
Where is the Abducens Nuclei located?
•It is Located close to midline of Middle Pons, just below the ‘facial colliculi’
What is the pyramidal tract also known as?
The cortico-spinal tract
Describe the Intra-Cerebral & Intra-Cranial
6th Nerve Courses
Axons (~7,000) of Abducens motor neurons:
- Run ventrally through the basal pons, passing between fibres of the Pyramidal (also known as cortico-spinal) Tract
- Exit close to midline at pons-medulla junction, close to the basilar & anterior inferior cerebellar arteries
- Ascend vertically in the sub-arachnoid space to pierce the dura overlying the Clivus bone
- Turn sharply over the Apex of Petrous Temporal bone, under the petro-sphenoidal ligament through ‘Dorello’s canal’ to enter the Cavernous Sinus
- Run through middle of sinus, just lateral to the Internal Carotid Artery (ICA)
- Enter the orbit via the Superior Orbital Fissure
How do the different nerves, 3rd, 4th, and sixth, pass through the cavernous sinus?
The third and fourth nerve pass through the lateral wall of the cavernous sinus (which is made up of dura mater).
The sixth nerve passes straight through the cavernous sinus next to the internal carotid artery.
What are the infra-orbital (in the orbit) nerve courses?
The third and sixth nerve pass through the common anular tendon (of zinn - where EOMS meet).
The third nerve actually splits into 2 branches - a superior division which supplies the SR and LPS (because they are muscles towards the top of the eye), and a larger inferior divison which supplies the IR,MR & IO and carries parasympathetic fibres to the ganglion [Can see this on diagram].
The fourth, trochlear, nerve runs medially and dorsally to the common anular tendon so it can get to the SO muscle.
[•Fibres from the EW & AM Nuclei synapse on separate Ganglion Cells, whose post-ganglionic fibres run together in the (6-10) short ciliary nerves into the eye
•Then between the sclera & choroid (supra-choroid) to innervate the iris sphincter or ciliary muscles, respectively]
