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Note: Every general physician must learn to detect ocular malalignment. First learn that the lateral rectus muscle has only one action, abduction of the eye, and the medial rectus has only one action, adduction. All other extraocular muscles (EOMs) have multiple actions, depending on the position of the eye. If the time allotted to study the neurologic examination (NE) is short, photocopy Fig. 4-18, which summarizes these actions, paste it in your handbook for reference, and skip to Section II, J and K. If you wish to understand the actions of the EOM, work through the text, which calls for you to make two simple models enabling you to reason out the actions and recall them as needed.
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I find that many students automatically reject model-making as a waste of time. Let me say this: You will never really understand the ocular rotations unless you experience these actions by seeing them happen and feeling them with your own fingers. It might interest your skeptics to know that Leonardo da Vinci (1452–1519), who dissected many human bodies to learn about the actions of muscles, devised the method of tugging on tapes attached to the insertion of muscles to teach himself how the muscles worked. To experience the actions of the muscles, get these materials to make the two models:
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To make an axial rotation model get:
A small ball of clay, a piece of a kitchen sponge, or even a wad of gum
Three toothpicks or, better, thin round applicator sticks
To make an eyeball rotation model get:
A soft, sponge rubber ball, 2 to 3 in. in diameter, that you can stick pins into
Several straight pins, preferably with large heads
A piece of leather (from the tongue of an old shoe) or plastic
Scissors
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A. Model 1: Axial rotation of the eyeball around three axes
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Each eye has to aim its visual axis at any point within the perimeter of movement. To achieve infinitely variable movement within that perimeter, the eyeball rotates axially around three axes: a vertical axis, a lateral axis, and an anteroposterior axis (A-P; Fig. 4-7).
To visualize the axial rotation of the eyes, reproduce Model 1 as shown in Fig. 4-7B and label the axes. Rotate each of the three sticks around the vertical, lateral, and A-P axes. Actually hold the stick between your thumb and forefinger and spin the model. Then and only then, as you see it and your fingers feel it, will you understand that ocular rotation is axial rotation, not eccentric rotation. In Fig. 4-8 check the eye which shows the correct, axial rotation.
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B. Model 2: Eyeball rotation model to demonstrate the pull of the extraocular muscles
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On your rubber ball, draw a pupil and an iris. Then place dots for the points of emergence of the three axes and label them (Fig. 4-9).
Cut two strips of leather. Mark them MR and LR for the medial and lateral rectus muscles, respectively. Consider this model as the right eyeball.
Draw an arrow along the strips to represent the vector or line of pull of the contracting muscle (see the arrow in Figs. 4-9 and 4-11).
Stick a pin through the anterior end of each strip and into the ball. Stick the pin anterior to, but exactly in line with, the lateral axis. If you insert the pin posterior to the lateral axis, the muscles would retract, not rotate, the eyeball (Fig. 4-9).
Although in vivo the muscular actions differ somewhat from a mechanical model, the model provides valuable insights with regard to these actions.
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C. Types of eye movements and nomenclature
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The eyeball can rotate laterally or medially around its vertical axis, upward or downward around its horizontal axis, and torsionally (rotate in or out) around its A-P axis.
Ductions are monocular rotations when the opposite eye is covered, e.g., adduction.
Versions are binocular parallel rotations to the sides, up, or down.
Vergences are binocular non-parallel rotations, e.g., convergence or divergence.
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D. Action of the medial and lateral rectus muscles
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The simplest way to move the eyes around two axes is by four muscles (Fig. 4-10). Label the axes in Fig. 4-10 and learn the names of the four rectus muscles. Especially note their insertion anteriorly on the eyeball.
While holding the rubber ball with your index finger on the top of the vertical axis and your thumb on the bottom (pincer's grasp), pull straight back on the lateral rectus strip with your other hand. Alternately pull the medial or lateral recti strips and observe the exact axial rotation of the ball around the vertical axis. Study Fig. 4-11 and label the axes.
Only from your model will you fully appreciate this fact: You have to pull exactly straight back on the strips that represent the medial and lateral recti, along the vector arrow shown in Fig. 4-11, or the eye will wobble around rather than demonstrating precise axial rotation around the vertical axis.
Because the medial and lateral recti pull exactly straight "on center," they have one and only one action: to adduct or abduct the eye. The actions of the medial and lateral recti rotate the eye around the _____________ axis. Because the other ocular rotatory muscles pull off center in relation to the ocular axes, they display more than one effective action.
vertical
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E. Action of the superior rectus muscle
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Attach another leather strip, label it SR for superior rectus, and draw an arrow to represent its vector or line of pull. Figure 4-12B shows that contraction of the superior rectus would rotate the eye upward around the ____________ axis. Reproduce this action by holding the rubber ball with its lateral axis between your thumb and second finger. This is the primary action of the muscle.
lateral
The angulation, or "off center" pull, of the superior and inferior recti causes a difference in the strength of the primary action and permits secondary and tertiary actions, depending on the position of the eye. To understand how the actions of some ocular muscles change as the eyes rotate, you must know the origin and insertion of the muscles in relation to the axes of the eyeball.
The recti all originate from the annulus of Zinn, a cuff that encircles the optic foramen. In Fig. 4-10 (top view of the right eye), note that from their origin the recti angle □ laterally/□ medially.
☑ laterally
Figure 4-12B shows that, with the eyes in the primary position, the superior rectus runs somewhat □ medial to/□ lateral to the vertical axis.
☑ medial to
We have already seen that the superior rectus has the primary action to rotate the eye __________________ around the lateral axis.
upward
To analyze the other actions of the superior rectus, angle the strip along the normal line of pull of the muscle, as in Fig. 4-12B. Now hold the eyeball by its vertical axis and notice that the ball rotates medially when you pull back on the strip.
Because its line of pull runs slightly medial to the vertical axis, the superior rectus has a secondary action to rotate the eye medially around the vertical axis. The muscle whose sole action is medial rotation of the eye is the __________________ rectus. This action is termed _____ duction.
medial; adduction
To better visualize the secondary and tertiary actions of the superior rectus, imagine the eye in a position of extreme adduction, as in Figs. 4-12A and 4-13. Hold the ball by the A-P axis and pull on the superior rectus strip after the ball has been turned medially, as in Fig. 4-13. You will find that it rotates inward.
Now you have observed that the superior rectus can elevate the eye, adduct it, and tilt the vertical axis inward. Inward tilting of the vertical axis is called intorsion, as shown in Fig. 4-14.
In Fig. 4-14, the right eye has rotated around the A-P axis, tilting the top of the vertical axis in. This action is called □ intorsion/□ extorsion.
☑ intorsion
In the left eye of Fig. 4-14, the top of the vertical axis tilts out. Therefore, it is called _____torsion.
extorsion
The torsions involve rotation of the eye around the _____ axis.
A-P
When the eye is abducted, the point of insertion of the tendon of the superior rectus shifts laterally in relation to the vertical axis, as shown in Fig. 4-12C. The vector (arrow) now pulls directly over the vertical axis.
Would the superior rectus act to adduct or intort when the eye is abducted? □ Yes/□ No.
See next frame.
With the eye abducted, the superior rectus pulls directly over the vertical axis. It dissipates none of its strength in adduction or intorsion. Therefore, the superior rectus elevates the eye most strongly when the eye is □ adducted/□ straight ahead/□ abducted.
☑ abducted
In what position of the eye would the superior rectus have the weakest action of elevation? ____________
Adduction
In summary, the primary action of the superior rectus is ____________ of the eye. This action is strongest when the lateral rectus has ____________ the eye.
elevation; abducted
The secondary and tertiary actions of the superior rectus are ____________ and ____________ of the eye.
adduction; intorsion
What direction would you ask the Pt to look to test the strongest elevating action of the right superior rectus?
____________________________________________________
Up and to the right
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F. Action of the inferior rectus muscle
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The inferior rectus muscle has the same direction of origin and insertion as the superior rectus (Fig. 4-10). The primary action of the inferior rectus is to □ depress/□ elevate the eye.
☑ depress
Its secondary action would be to ____________ the eye.
adduct
To analyze the tertiary inferior rectus action, pin another strip to the rubber ball to represent the inferior rectus. Hold the ball by the A-P axis and consider the inferior rectus action with the eye adducted. Then, when the inferior rectus contracts, the top of the vertical axis should tilt □ internally/□ externally.
☑ externally
External tilting of the vertical axis is called _________________. Internal tilting is called _________________.
extorsion; intorsion
What is the only eye position in which the superior and inferior recti could rotate the eye in the same direction? ______________________________
Adduction
The superior and inferior recti are ineffective adductors until the medial rectus muscle, the most critical muscle for adduction, has already begun to act.
To test the strongest action of the right inferior rectus as a depressor, in what direction would you ask the Pt to look?
_____________________________________________________.
Down and to the right (down and out)
In summary, list the primary and two supplementary actions of the inferior rectus muscle.
_________________________
Depress; adduct; extort
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G. Action of the superior oblique muscle
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The superior oblique originates from the lesser wing of the sphenoid bone, just above the annulus of Zinn. Its tendon runs through a trochlea (pulley) attached to the rim of the bony orbit (Fig. 4-15A). When the tendon runs to the eye, it inserts posteriorly to allow the superior oblique to have an effective pull when contracting. In so attaching, the tendon runs somewhat medial to the vertical axis, like the superior and inferior recti. Cut another strip, label it SO (superior oblique), and draw an arrow to represent its vector along the line of the tendon.
The vector diagram of Fig. 4-15B resolves arrow R-B into effective components.
Vector R-A □ depresses/□ elevates the eye around the ____________ axis.
☑ depresses; lateral
Vector R-C □ abducts/□ adducts the eye around the ____________ axis and □ intorts/□ extorts the eye around the ____________ axis.
☑ abducts; vertical; □ intorts; A-P
Therefore, vector R-B acts to ____________ the eye, ____________ the eye, and ____________ the eye.
depress; abduct; intort
To recapitulate: Contraction of the superior oblique, when the eye starts in the primary position, causes:
A primary action to □ depress/□ elevate the eye.
☑ depress
A secondary action to □ adduct/□ abduct/□ elevate the eye.
☑ abduct
A tertiary action to □ intort/□ extort the eye.
☑ intort
With the eye in the primary position, the line of pull of the superior oblique tendon runs medial to the vertical axis (arrow in Fig. 4-15A). Complete Fig. 4-16 to show the relation of the vertical axis of the eye to the line of pull of the superior oblique tendon when the eye is adducted.
When the eye is adducted, the tendon of the superior oblique pulls directly over the vertical axis. Therefore, none of the depressor action of the muscle is dissipated in the other actions, which are to ____________ and to ____________ the eye.
abduct; intort
In what position of the eye does the superior oblique show the strongest primary action of downward rotation of the eye? ____________
Adduction
Hence, the clinical test for the strongest action of the superior oblique is to ask the Pt to look ________________________________________.
in and down
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H. Action of the inferior oblique muscle
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The inferior oblique muscle, in contrast to the other ocular muscles, originates from the medial inferior rim of the bony orbit. No other ocular muscle originates anteriorly. In inserts on the posterior part of the eyeball. To attain sufficient resting length, it wraps further around the eye than the other muscles.
The inferior oblique passes posteriorly, somewhat medial to the vertical axis. Its obliquity and alignment with the vertical axis matches the superior oblique (Figs. 4-15A and 4-15C).
The primary action is exactly antagonistic to the superior oblique. The inferior oblique acts to _____________________ the eye.
elevate
The secondary action is to _____________________ the eye, acting in harmony with the superior oblique.
abduct
The tertiary action is to _____ the eye, antagonistic to the superior oblique.
extort
The one direction in which the superior and inferior obliques rotate the eyeball the same way is _____________________.
abduction
The obliques can abduct strongly only after the lateral rectus has already started to rotate the eye laterally. After lateral rectus paralysis, the obliques cannot initiate abduction and the eye cannot be abducted. We run squarely into a problem. Intorsion, the so-called tertiary action of the superior oblique, is clinically one of its most important actions. Therefore, in classifying intorsion as a tertiary action, we do not dismiss it as a negligible action.
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I. A vector diagram of ocular muscle action
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In the blanks of Fig. 4-17, place the initials of the muscles represented by the vector arrow numbers.
The vector diagram combined with a knowledge of the origin and insertion enable you to remember forever the actions of the ocular muscles. We can state that, when the eye is in the primary position, the pull or vector of all the ocular muscles is medial to the vertical axis except for the _____________________________ muscle.
lateral rectus
Because four of the ocular muscles pull "off center," i.e., medial to the vertical axis, they display their particular secondary and tertiary actions. The four muscles that pull off center when the eye is in the primary position are the _________________________________________________
_________________________________________________
__________________________________.
superior and inferior obliques and the superior and inferior recti
The two muscles that always pull "on center" and therefore have only primary actions are the _____________________.
medial and lateral recti
Cover the answers in Table 4-1 and recite them. Use drawings or the ball model, when necessary, to deduce the answers.
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J. Mnemonic summary of the actions of the ocular rotatory muscles
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The only action of the medial and lateral recti is to rotate the eyeballs medially and laterally, respectively.
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The superior and inferior recti rotate the eyeball superiorly and inferiorly, respectively, as their primary actions when the eyes are turned outward. Hence, all recti primarily rotate the eyeball in the direction of the adjective component of their names.
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With the eyes turned in the, superior and inferior obliques rotate the eyeballs inferiorly and superiorly, respectively, as their primary actions, an action opposite to the adjective component of their names.
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The superior oblique intorts the eye, except when the eye looks in.
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The inferior oblique extorts the eye, except when the eye looks in.
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K. Yoking of ocular muscles
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Note: If you did not do Sections II, A to J, use Table 4-1 and Fig. 4-18 for the following frames:
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The ocular muscles of the two eyes collaborate with each other to keep the eyes aligned. If a Pt has diplopia when looking to the left, you would suspect weakness of the abductor of the left eye or the adductor of the right eye.
The strongest abductor of the left eye is the muscle whose only function is abduction, the _____________________ muscle.
lateral rectus
The strongest adductor of the right eye is the muscle whose only function is adduction, the _____________________ muscle.
medial rectus
When muscles of the two eyes act in unison for conjugate gaze, we say that they are yoked.
Suppose a Pt complains of diplopia only when looking up and to the left.
When the left eye is abducted, the strongest elevator is the ___________________ muscle.
superior rectus
When the right eye is adducted, the elevating power of its superior rectus is diverted to the secondary action of the muscle, ___________________, and to the tertiary action of the muscle, ___________________.
adduction; intorsion
As the superior rectus loses its elevator strength during adduction, the muscle that converts its action solely to elevation is the ___________________.
inferior oblique
During adduction of an eye, the elevator action of the superior rectus □ decreases/□ increases, and the elevator action of the inferior oblique simultaneously □ decreases/□ increases.
☑ decreases; ☑ increases
Thus for upward gaze to the left, the muscle that elevates the left eye, the ___________________ muscle, is yoked to a muscle of the right eye. The yoked muscle of the right eye that replaces the vanishing elevator action of the right superior rectus as the eye adducts is the ___________________ muscle.
superior rectus; inferior oblique
Which muscle has the strongest depressant action when the eye is adducted? ___________________
Superior oblique
The muscle with the strongest depressor action with the eye abducted is the ___________________.
inferior rectus
A Pt looking to the right has diplopia when he looks down. Which of the yoked muscles should you suspect of weakness: the ___________________ muscle of the right eye or the ___________________ muscle of the left eye.
inferior rectus; superior oblique
Hering's law (Ewald Hering, 1834–1918) of equal stimulation of the yoke muscles. The law states that the muscles yoked for conjugate eye movements receive equal stimulation by the nervous system. Thus, if the right lateral rectus is stimulated to rotate the right eye to the right, the left medial rectus receives equal stimulation. This principle is called Hering's law. State Hering's law in your own words. _____________________________________________________
During conjugate eye movements, the yoke muscles (the muscles of the two eyes that rotate the eyes in the same direction) receive equal stimulation.
A summary of the yoke muscles. If you studied pages 130–139, complete Fig. 4-18 by writing in the initials of the muscles most important for the movement indicated by the arrows. If you did not read those pages, simply copy in the correct initials for reference.
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L. Oppositional action of pairs of ocular muscles of one eye and the tonic innervation of the extraocular muscles
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The medial and lateral recti of one eye exemplify a general law of the EOMs: The muscles of one eye act in agonist and antagonist pairs. For each direction of eye movement, one or more muscles act in exactly the opposite direction.
An electromyographic needle inserted into the EOMs records a continuous play of nerve impulses called tonic innervation that maintains light tension in the muscles with the eyes still and in the primary position. Other skeletal muscles are electrically silent when the part they move is at rest. During conjugate movement of the eyes, Sherrington's law of reciprocal inhibition holds: The muscle or muscles in one eye that cause the rotation are actively innervated, whereas the antagonists are inhibited. After the eye movements stop, tonic innervation resumes.
Because each agonist–antagonist pair of EOMs receives a tonic, equal play of nerve impulses, the pull of one muscle balances the pull of the other, like opposing rubber bands under slight tension. Thus, the position of the eyes is always positively determined.
After paralysis of one EOM, the eye deviates in the direction of pull of the intact, oppositional muscle, which continues to receive its tonic innervation. After paralysis of the lateral rectus muscle (VI nerve palsy), tonic innervation of the medial rectus causes the eye to rotate □ inward/□ outward.
☑ inward
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M. Review of actions of the extraocular muscles
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List the six ocular rotatory muscles and their origins and insertions in relation to the axes of ocular rotation.
The muscles (or tendons in the case of the superior obliques) all insert distal to the way they approach the globe (Fig. 4-17).
With the eyes in the primary position, the vector of only one muscle, the lateral rectus, pulls lateral to the vertical axis (Fig. 4-17).
Only one muscle, the inferior oblique, originates anteriorly, and only one muscle, the superior oblique, runs through a trochlea (pulley).
Reason out the position of the eye when only one muscle is paralyzed. The eye turns away from the pull of the paralytic muscle because the intact muscles act unopposed.
Reason out the position of the eye when only one nerve is intact.
Distinguish between the possible movements of the muscle according to the mechanics of origin and insertion and the strongest movements of the muscle when the eye is rotated into the optimum position.