Week 1 - Revision: Muscles part 1 and 2

Question 1

What muscles comprise the Muscle Cone?

Answer 1

• Superior Rectus
• Lateral Rectus
• Medial rectus
• Inferior rectus
• Superior oblique
• Inferior oblique

Question 2

Where do all the muscles originate?

Answer 2

• Common Tendon of Zinn
- Comes through the superior orbital fissure

Question 3

What is the spiral of Tillaux?

Answer 3

• Rectus muscle passes throgh the tendons capsule and insert into sclera
• Muscles insert into at different distances from the cornea
• The insertion pattern is a spiral with the medial rectus closest to the cornea (5.5 mm) and the superior rectus the furthest away from the cornea (7.4 mm).

Question 4

What are the distances for the spiral if Tillaux?

Answer 4

• Medial - 5.5mm
• Inferior - 6.7mm
• Lateral rectus - 6.9mm
• superior rectus - 7.7mm
Measured from the Limbus to insertion:
S - 8mm
L - 7mm
I - 6mm
M - 5mm

Question 5

What is Tenon’s capsule?

Answer 5

• Is the bulk of the orbital fascial system
•Forms the envelope within which the eyeball moves
• Fuses posteriorly with optic nerve sheath, anteriorly with the intermuscular septum
• Posterior portion is thin and flexible
Posterior to the equator, it is thick and tough, suspending the globe to the periorbital tissues
ALL MUSCLES attach to Tenons capsule; with the obliques penetrating through it

Question 6

Describe the medial rectus:

Answer 6

• Originates on both the upper and lower limb of the common tendous ring and the optic nerve sheath.
• Inserts vertically 5.5 mm from the cornea. The horizontal plane of eye bisects the insertion.
• Fascial expansion from muscle sheath forms the medial check ligament and attach to medial wall of orbit.

Question 7

What nerves and innervations does the medial rectus have?

Answer 7

• Innervation is via cranial nerve III, the oculomotor nerve, and the specific branch runs along the inside of the muscle cone, on the lateral surface.
• The superior oblique, ophthalmic artery and nasociliary nerve all lie above the medial rectus.

Question 8

What is the Origin, Insertion, direction, innervation, bloody supply and action of medial rectus?

Answer 8

× Origin: annulus of zinn
* Insertion: medially, in hori meri, 5.5 mm from limbus
* Direction: 90°
* Innervation: lower CN III
* Blood supply: Inf. Mus. Branch Of Oph. A.
× Action: adduction- inwards to middle

Question 9

Where is the origination and insertion of the lateral rectus?

Answer 9

• Originates on both the upper and lower limb of the common tendous ring, AND a process of the greater wing of the sphenoid bone.
• Inserts parallel to medial rectus 6.9 mm from the cornea. (Tendon 9.2 mm wide, 8.8 long).
• Fascial expansion from muscle sheath forms the lateral check ligament and attach to lateral wall of orbit at Whitnalls tubercle.

Question 10

What is the innervation, bloody supply and innervations of the lateral rectus muscle?

Answer 10

• Innervated by the abducens nerve, Cranial n VI which enters the muscle on the medial surface.
• The lacrimal artery and nerve run along the superior border.
• The abducens n., ophthalmic artery and ciliary ganglion lie medial to the lateral rectus and between it and the optic nerve.

Question 11

What is the Origin, insertion, innervation, bloody supply and action of the lateral rectus?

Answer 11

• Origin: annulus of zinn
• Insertion: laterally, in horizontal meridian, 6.9mm from limbus
• Innervation: CN VI
• Blood supply: Inf. Mus. Branch Of Oph. A.
• Action: abduction or outwards

Question 12

What is the origin and insertions of the superior rectus?

Answer 12

• Originate on superior limb of the tendonous ring, and optic nerve sheath.
• Muscle passes forward underneath the levator, but the two sheaths are connected resulting in coordinated movements.
• Insertion 7.4 mm from limbus, and obliquely.
• The angle from the origin to the insertion is 23° beyond the sagital axis.

Question 13

How do the nerves and blood supply innervate the superior rectus?

Answer 13

• Frontal nerve runs above the s. rectus & levat.
• The nasociliary nerve and ophthalmic artery run below.
• The tendon for insertion of the superior oblique muscle runs below the anterior part of the superior rectus.
• Innervations via superior division of CN III, from the inferior surface; additional branches make their way to the levator.

Question 14

What is the action of the superior rectus?

Answer 14

• Primary action is elevation . But since the insertion on the globe is lateral as well as superior, contraction will produce rotation about the vertical axis toward midline
• Thus secondary action is adduction
Finally, because the insertion is oblique, contraction produces torsion nasally Intorsion.

Question 15

What is the Origin, insertion, direction, innervation and action of the superior rectus?

Answer 15

• Origin: annulus of zinn
• Insertion: superiorly, in ver. Mer. 7.7 mm from limbus
• Direction:23°
• Innervation: upper CN III
• Action: Elevation , Intorsion, Adduction

Question 16

What are the insertions and origins of the inferior rectus?

Answer 16

• Originates on lower limb of common tendonous ring.
• Inserts 6.7 mm from limbus, insertion is an arc
• It is parallel to superior rectus, making a 23° angle beyond the sagittal axis.
• Innervated by inferior division of CN III which runs above it (within the muscle cone).
• Below is the floor of the orbit and inf. oblique

Question 17

What is the Origin, Insertion, direction, innervation and action of the Inferior rectus?

Answer 17

• Origin: annulus of zinn
• Insertion: inferiorly, in ver. Mer. 6.5 mm from limbus
• Direction:23°
• Innervation: lower CN I
• Action: Depression. Extorsion, Adduction,

Question 18

Where is the inferior rectus bound to the lower eyelid?

Answer 18

• The inferior rectus muscle is distinctly bound to the lower eyelid by the fascial extension from its sheath

Question 19

What is the superior oblique, where is its origin and what is it innervated by?

Answer 19

• Anatomical origin: lesser wing of the sphenoid bone. The physiological origin is the trochlea, a cartilagenous “U” on the superior medial wall of the orbit.
• Longest thinnest EOM, the muscle ends before the trochlea, tendon is 2.5 cm, smooth movement through trochlea.
• Innervation by CN IV, the trochlear nerve posterior in the orbit.

Question 20

What is the action of Sup. Oblique?

Answer 20

• Primary action: depression and intorsion
• Due to insertions laterally and posterior:
- ABDUCTION
- DEPRESSION

Question 21

What is the origin, insertion, innervation and action of superior oblique?

Answer 21

• Origin: superior of annulus of zinn (func. At trochlea
• Insertion: post. to equator in suprotemp.
• Innervation: CN IV, 4 , Trochlear
• Action: Intorsion, Dep, Abd,

Question 22

What is the Origination, insertion and innervation of the inferior oblique?

Answer 22

• Originates on the maxillary bone inferior to the nasolacrimal fossa. The ONLY EOM originating in the anterior orbit.
• Inserts on the posterior. lateral aspect of globe mostly inferior, below the ant.-post.
horizontal plane.
• Innervation from inferior division of CN III inserts on the upper surface (within muscle cone.)

Question 23

What is the action of Inf. Oblique?

Answer 23

• Primary is extorsion
• 2° is due to posterior, lateral, inferior insertion being pulled around, underneath globe and toward the anterior inferior insertion medially.
• Rotation about the Z axis will be nasal to temporal (abduction).
• Rotation about the X axis will be elevation

Question 24

What is the origin, insertion, length, direction, innervation and action of the inferior oblique?

Answer 24

• Origin: behind of lacrimal fossa
• Insertion: post. to equator in macular area.
• Direction:51°
• Innervation: lower CN III
• Action: Ext, Elev, Abd,

Question 25

What is the origin, insertion, innervation and action of the Levator Palpebrae superiors?

Answer 25

• Origin: above of annulus of zinn
• Insertion: above and anterior surface of tarsus.
• Innervation: upper CN III
• Action: eyelid elevation

Question 26

What are surgical considerations of surgery on the rectus muscles?

Answer 26

• Blood supply to extra ocular muscles provides almost all of temporal half of anterior segment
- Therefore simultaneous surgery on all 3 rectus muscles, can induce anterior segment ischaemia

Question 27

What is the range of action of the globe?

Answer 27

• Can be moved 50° in each direction from primary position
• But under normal circumstances eyes move 15-20° from primary position before head movement

Question 28

What are the primary and secondary muscle action definitions?

Answer 28

• Primary action : major effect of a muscle, when muscle contracts while eye primary position
• Secondary and tertiary actions: additional effects on position of eye in primary
• Remember RADSIN - Recti Adduct, Superiors Intort

Question 29

What are the innervations of all 3 cranial nerves?

Answer 29

• Cranial nerve VI (abducens) innervates the lateral rectus muscle
• Cranial nerve IV (trochlear) innervates the superior oblique muscle
• cranial nerve III has an upper and a lower division: innervates the levator palpebrae, superior rectus, medial rectus, inferior rectus, and inferior oblique muscles.

Question 30

What are Striate muscles?

Answer 30

• Skeletal muscles consist of 100,00s of muscles cells (muscle fibres)
• A myofibril consists if two types of protein filaments called thick and thin filaments
- hundreds of myofibrils in each muscle fibre
- Myofibrils line up with each other such that Z-lines of each sarcomere in one myofibril lines up with Z lines of sarcomeres in adjacent myofibrils

Question 31

What is the Ratchet Model?

Answer 31

• Myosin head binds to actin filament.
• The ratchet motion moves the two filament about 12 nm with respect to each other.
• It takes only 5 ms
• Because of the large number of z-line segments or contractile units along a fiber, a fast motion is attained.

Question 32

What is the difference in action between thick and thin muscles?

Answer 32

Thick Fibers vs Thin Fibers
• Striated vs striated
• Single innervation vs multiply innervated
(1 nerve 1 branch vs many branches 1 nerve)
• Motor end plate vs En Grappe end plate
• Fast and slow twitch fibers vs slow sustained (tonic)
• All or none contraction vs graded contractions