The Efferent (Motor) System










CHAPTER 16 The Efferent (Motor) System


The efferent ocular motor pathway includes the cortical centers, intermediate gaze centers, cranial nerve nuclei, cranial nerves, and extraocular muscles (EOMs) which produce eye movements.


Extraocular Muscles


Ocular muscles may be extrinsic or intrinsic (Flowchart 16.1).




Flowchart 16.1 Muscles of eyeball.


The extraocular muscles (EOMs) and their central nervous control subserve the motility and coordination to the eyes.


Anatomy of EOM


Origin


There are six EOMs. Five out of six extraocular muscles (except inferior oblique) originate at the orbital apex (Fig. 16.1). These EOMs are:




Fig. 16.1 Figure depicting origin of extraocular muscles at orbital apex (right superior fissure and annulus of Zinn). Abbreviations: CN, cranial nerve; IR, inferior rectus; LPS, levator palpabrae superioris; LR, lateral rectus; MR, medial rectus; O, inferior oblique; SO, superior oblique; SR, superior rectus.




Superior oblique (SO) arises near the apex of the orbit, superomedial to the optic foramen.


Inferior oblique (IO) arises anteriorly from the inferonasal angle of the orbit, just behind the orbital rim and lateral to the lacrimal fossa.



SR and MR muscles are closely attached to the dural sheath of the optic nerve. That is why the pain occurs during elevation and adduction in patients with retrobulbar neuritis.


Insertion


Recti muscles are inserted into sclera via tendons anterior to the equator of globe (Fig. 16.2).




Fig. 16.2 Figure depicting insertion points of recti muscles.


Table 16.1 shows that the MR tendon inserts closest to the limbus. Insertions get further away from limbus and make a spiral pattern (spiral of Tillaux).


























Table 16.1 Insertion of recti muscles


Muscle


Insertion


MR


5.5 mm from nasal limbus


IR


6.6 mm from inferior limbus


LR


6.9 mm from temporal limbus


SR


7.7 mm from superior limbus


Abbreviations: MR, medial rectus; IR, inferior rectus; LR, lateral rectus; SR, superior rectus.


Oblique muscles are inserted into sclera, posterior to the equator of globe (Fig. 16.3).




Fig. 16.3 Figure depicting insertion points of recti muscles.


The SO tendon is inserted into the posterior upper temporal quadrant of the globe in a fan-shaped manner under the SR muscle. The insertion extends near the superotemporal vortex vein.


The IO muscle has almost no tendon at its insertion and inserts into the posterior lower temporal quadrant of the globe, close to the macula and inferotemporal vortex vein.


Nerve supply (Mnemonic-LR6 (SO4)3)


The third, fourth, and sixth cranial nerves supply the EOMs in the following manner:


LR is supplied by abducens (6th) nerve.


SO is supplied by trochlear (4th) nerve.


SR is supplied by superior division of oculomotor (3rd) nerve.


MR, IR, and IO are supplied by the inferior division of the oculomotor (3rd) nerve.


Course of EOM (from Origin to Insertion)


Visual axis: It is the line passing from the object of fixation to the fovea through the nodal point of the eye.


Orbital axis: Medial orbital walls are approximately parallel to each other. The lateral and medial walls of the orbit are at an angle of 45° with each other. The orbital axis therefore forms an angle of 22.5° (≈23°) with both the lateral and medial walls (Fig. 16.4a).




Fig. 16.4 (a) Orbital axis. (b) Primary position of eye.


Primary position of eye: The primary position is defined as the position when the eye and head are both directed straight ahead. In the primary position, the visual axis forms an angle of 23° with the orbital axis (Fig. 16.4b).


Horizontal Recti


Horizontal recti include MR and LR which arises from a common tendinous ring (the annulus of Zinn) and inserted into the sclera by a tendon. These rectus muscles move the eyes about the vertical axis (z-axis of Fick) (Fig. 16.5).




Fig. 16.5 MR and LR muscles. Abbreviations: LR, lateral rectus; MR, medial rectus.


MR muscle: It courses anteriorly along the medial wall of the orbit.


As MR muscle runs along the medial orbital wall, it can sustain damage during ethmoid sinus surgery.


MR is the only rectus muscle that does not have a facial attachment to an oblique muscle. Thus, MR muscle is at a greatest risk of slippage during strabismus surgery.


LR muscle: It courses anteriorly along the lateral orbital wall.


Vertical Recti


Vertical recti (SR and IR muscles) run in line with the orbital axis, so they form an angle of 23° with the visual axis in primary positions (Fig. 16.6).




Fig. 16.6 Relation of vertical recti and oblique muscles in relation to visual axis in primary position. Abbreviations: IO, inferior oblique; IR, inferior rectus; SO, superior oblique; SR, superior rectus.


Obliques


Obliques (SO and IO muscles) form an angle of 51° with the visual axis in primary position.


Actions of EOM


EOMs rotate the eye around the center of rotation situated about 13 mm behind the cornea. It lies, therefore, behind the nodal point of the eye on the visual axis. Ocular movements take place around three axes (termed as axes of Fick) passing through the center of rotation. These three axes are:


Horizontal axis (x-axis of Fick): Movements around the horizontal axis are elevation or upward rotation and depression or downward rotation.


Vertical axis (z-axis of Fick): Movements around the vertical axis are adduction or medial rotation and abduction or lateral rotation.


Anteroposterior axis (y-axis of Fick): Movements around the anteroposterior axis are intorsion or incycloduction and extorsion or excycloduction (Fig. 16.7).




Fig. 16.7 Ocular movements around three axes.


The actions of EOMs depend on the position of the globe at the time of muscle contraction and may be defined as primary or secondary. Primary action is the major effect of the EOM when the eye is in the primary position, while secondary actions are the additional actions which depend on the position of the eye.


These actions can be easily understood by the course of the respective EOMs and their relationship with the visual axis.


When visual axis lies in the muscle plane, the direction of movement is only upward and downward.


If the muscle plane (line of muscle pull) is at a greater angle to the visual axis, the EOM produces a much greater degree of torsion. In the primary position of the eyeball, obliques are at a greater angle (51°) with the visual axis than the vertical recti (23°). So, obliques produce a much greater degree of torsion than the vertical recti, that is, the primary action of obliques is torsion. The SO muscle is an intortor, while the IO muscle is an extortor (Table 16.2, Fig. 16.8).








































Table 16.2 Actions of EOM


Muscle


Primary action


Subsidiary actions of EOM


MR


Adduction



LR


Abduction



SR


Elevation


Intorsion and adduction


IR


Depression


Extorsion and adduction


SO


Intorsion


Depression and abduction


IO


Extorsion


Elevation and abduction


Abbreviations: EOM, extraocular muscle; IO, inferior oblique; IR, inferior rectus; LR, lateral rectus; MR, medial rectus; SO, superior oblique; SR, superior rectus.




Fig. 16.8 Actions of EOM. Abbreviations: EOM, extraocular muscle; IR, inferior rectus; LR, lateral rectus; MR, medial rectus; IO, inferior oblique; SO, superior oblique; SR, superior rectus.


The torsional movements are defined in terms of the direction of movement of the upper pole of cornea. When the upper pole of cornea rotates nasally, it is termed intorsion. When the upper pole of cornea rotates temporally, it is termed extorsion. Superiors (SO and SR) are intortors, while inferiors (IO and IR) are extortors.


Horizontal recti: As horizontal recti run along the orbital walls; therefore, the sole action of the MR muscle in the primary position is adduction and that of the LR muscle is abduction.


Vertical recti: As these muscles form an angle of 23° with the visual axis in the primary position, therefore:


If globe is abducted 23°, visual axis and muscle axis align; SR acts only as an elevator and IR acts only as a depressor in this position.


If the eye is adducted 67°, the angle between the visual and orbital axes would be 90°; SR acts only as an intortor and IR acts only as an extortor in this position (Fig. 16.9).




Fig. 16.9 (a) SR acts only as an elevator and IR acts only as a depressor in this position. (b) SR acts only as an intortor and IR acts only as an extortor in this position. Abbreviations: IR, inferior rectus; SR, superior rectus.

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Nov 20, 2022 | Posted by in OPHTHALMOLOGY | Comments Off on The Efferent (Motor) System

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