Extraocular Muscles
The extraocular muscles are the six
muscles that control the movements of the eyes. These six muscles can be group
into 3 separate pairs of muscles and can be divided into 2 different types of
muscles as shown in table below.
Muscles Type
|
Muscles Pairs
|
Individual Muscles
|
Rectus Muscles
|
Vertical Rectus
Muscles
|
Superior Rectus
Muscles
|
Inferior Rectus
Muscles
|
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Horizontal
Rectus Muscles
|
Medial Rectus
Muscles
|
|
Lateral Rectus
Muscles
|
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Oblique Muscles
|
Oblique Muscles
|
Superior Oblique
Muscles
|
Inferior Oblique
Muscles
|
The medial and lateral recti contract reciprocally, thus enables the eyeball from side to side. The superior and inferior recti contract reciprocally to move eyeball upward and downward. While the oblique muscles operate mainly to rotate the eyeballs for keeping the visual in the upright position.
The distribution of the muscles controlling the two eyes is symmetrical as shown in figure below. Each of the these three pairs of muscles is reciprocally innervated so that one muscles of the pairs relaxes while the other contract. These three pairs of the muscles is said to be antagonistic muscles as the contraction one of the muscles will leads to the relaxation of the other muscles pairs.
By making the above figure as reference, we can take a few examples. For the upward fixation, the recti muscles involve are superior and inferior recti. In order to move the eyeballs upward, the superior rectus muscle need to contract while the inferior rectus muscle become relaxes. Apart of recti muscles, the involvement of oblique muscles to rotate the eyeballs in order to keep the visual field still maintain in the upright position. In this case, the inferior oblique muscle need to contract while the superior oblique muscle become relaxes. This type of contraction and relaxation of the muscles in known as antagonistic as mentioned above.
Same goes to downward fixation, the
inferior rectus muscle need to contract while the superior rectus muscle become
relaxes. Apart of recti muscles, the superior oblique muscle need to contract
while the inferior oblique muscle become relaxes in order to maintain the
visual field in the upright position.
However, for the rightward fixation
or leftward fixation, the contraction & relaxation of the recti muscles are
reciprocal between the right eye & left eye. In case of rightward fixation,
the right eyeball lateral rectus muscle contract while the medial rectus muscle
relax. On the other hand, the left eyeball medial rectus muscle contract while
the lateral rectus muscle relax. While for the leftward fixation, the left
eyeball lateral rectus muscle contract while the medial rectus muscle relax. On
the other hand, the right eyeball medial rectus muscle contract while the
lateral rectus muscle relax.
The most important movement of the
eyes are those that cause the eyes to “fix” on a discrete portion of the field
of vision. The fixation mechanism is primarily controlled by the secondary
visual areas of the occipital cortex. Visual fixation can lock the target on
the central area of the retina. When the object images from the centre of the
fovea to the fovea edge, the reflex response through the negative feedback
mechanism will regulate the three pairs of ocular muscles to move the object
image back to the centre of the fovea. Thus, as the image move upwards, the
eyeball will move downwards; when the image moves to the left, the eyeballs
will move to the right. Both action as described above will finally move the
image back to the centre of fovea. Therefore, the locked object will be always
kept, by means of the eye movements, on the centre of the fovea in order to
obtain the clearest image.
Eye Muscles Anatomy
1 = Annulus tendineus communis
2 = Superior rectus muscle
3 = Inferior rectus muscle
4 = Medial rectus muscle
5 = Lateral rectus muscle
6 = Superior oblique muscle
7 = Trochlea of superior oblique
8 = Inferior oblique muscle
9 = Levator palpebrae superioris muscle
10 = Eyelid
11 = Eyeball
12 = Optic nerve
Eye Muscles Pictures
List of Muscles with Their Respective Functions
Muscle
|
Origin
|
Insertion
|
Primary function
|
Secondary function
|
Tertiary function
|
Inserted into the sclera distance
|
|
Annulus
of Zinn from tendinous ring
|
eye
(anterior, superior surface)
|
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Annulus
of Zinn from tendinous ring
|
eye
(anterior, inferior surface)
|
Adduction[1]
|
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Annulus
of Zinn from tendinous ring
|
eye
(anterior, lateral surface)
|
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Annulus
of Zinn from tendinous ring
|
eye
(anterior, medial surface)
|
Adduction
|
|||||
Superior and Medial to Annulus
of Zinn via the Trochlea of superior oblique which
forms a 'pulley system'.
|
eye
(posterior, superior, lateral surface)
|
Intorsion
|
Depression
|
Abduction[1]
|
|||
eye
(posterior, inferior, lateral surface)
|
Extorsion
|
Elevation
|
Abduction[1]
|
Reference
1. Book
a) K
& H Student Module Experiment 3 Electro – Oculargram(EOG) Measurement.
2. Website
a) http://en.wikipedia.org/wiki/Extraocular_muscles
b) http://emedicine.medscape.com/article/1189799-overview#aw2aab6b2
