The extraocular muscles (extrinsic ocular muscles), are the seven
extrinsic muscles of the
human eye
The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm.
...
.
Six of the extraocular muscles, the four recti muscles, and the
superior
Superior may refer to:
*Superior (hierarchy), something which is higher in a hierarchical structure of any kind
Places
*Superior (proposed U.S. state), an unsuccessful proposal for the Upper Peninsula of Michigan to form a separate state
*Lake ...
and
inferior oblique muscle
The inferior oblique muscle or obliquus oculi inferior is a thin, narrow muscle placed near the anterior margin of the floor of the orbit. The inferior oblique is one of the extraocular muscles, and is attached to the maxillary bone (origin) and ...
s, control
movement of the eye and the other muscle, the
levator palpebrae superioris, controls
eyelid elevation
The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Vert ...
. The actions of the six muscles responsible for eye movement depend on the position of the eye at the time of
muscle contraction
Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...
.
Structure
![1412_Extraocular_Muscles](https://upload.wikimedia.org/wikipedia/commons/3/3b/1412_Extraocular_Muscles.jpg)
Since only a small part of the eye called the
fovea
Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure.
Human anatomy
*Fovea centralis of the retina
* Fovea buccalis or Dimple
* Fovea of the femoral head
* Trochlear fovea of the fr ...
provides sharp vision, the eye must move to follow a target.
Eye movements must be precise and fast. This is seen in scenarios like reading, where the reader must shift gaze constantly. Although under voluntary control, most eye movement is accomplished without conscious effort. Precisely how the integration between voluntary and involuntary control of the eye occurs is a subject of continuing research.
["eye, human."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009] It is known, however, that the
vestibulo-ocular reflex plays an important role in the involuntary movement of the eye.
The
levator palpebrae superioris is responsible for raising the upper
eyelid, and this can be a voluntary or involuntary action. The other six extraocular muscles are involved in movements of the eye; these are the four recti (straight) muscles, and two oblique muscles.
Recti muscles
The four recti muscles are named according to their relative positions of attachment – the
superior rectus muscle
The superior rectus muscle is a muscle in the orbit. It is one of the extraocular muscles. It is innervated by the superior division of the oculomotor nerve (III). In the primary position (looking straight ahead), its primary function is elevatio ...
,
lateral rectus muscle,
medial rectus muscle
The medial rectus muscle is a muscle in the orbit near the eye. It is one of the extraocular muscles. It originates from the common tendinous ring, and inserts into the anteromedial surface of the eye. It is supplied by the inferior division of t ...
, and
inferior rectus muscle
The inferior rectus muscle is a muscle in the orbit near the eye. It is one of the four recti muscles in the group of extraocular muscles. It originates from the common tendinous ring, and inserts into the anteroinferior surface of the eye. It de ...
. The recti muscles are all of almost equal length of around 40 mm but the lengths of their associated tendons differ.
Oblique muscles
The two oblique muscles are the
inferior oblique muscle
The inferior oblique muscle or obliquus oculi inferior is a thin, narrow muscle placed near the anterior margin of the floor of the orbit. The inferior oblique is one of the extraocular muscles, and is attached to the maxillary bone (origin) and ...
, and the
superior oblique muscle.
Pulley system
The movements of the extraocular muscles take place under the influence of a system of extraocular muscle pulleys, soft tissue
pulley
A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt, or transfer of power between the shaft and cable or belt. In the case of a pulley supported by a frame or shell that ...
s in the orbit. The extraocular muscle pulley system is fundamental to the movement of the eye muscles, in particular also to ensure conformity to
Listing's law Listing's law, named after German mathematician Johann Benedict Listing (1808–1882), describes the three-dimensional orientation of the eye and its axes of rotation. Listing's law has been shown to hold when the head is stationary and upright and ...
. Certain diseases of the pulleys (heterotopy, instability, and hindrance of the pulleys) cause particular patterns of incomitant
strabismus. Defective pulley functions can be improved by surgical interventions.
Origins and insertions
Four of the extraocular muscles have their origin in the back of the orbit in a fibrous ring called the
common tendinous ring
The common tendinous ring, also known as the annulus of Zinn, or annular tendon, is a ring of fibrous tissue surrounding the optic nerve at its entrance at the apex of the orbit. It is the common origin of the four recti muscles of the group of ex ...
: the four recti muscles. The four recti muscles attach directly to the front half of the eye (anterior to the eye's equator), and are named after their straight paths.
Note that medial and lateral are relative terms. Medial indicates near the midline, and lateral describes a position away from the midline. Thus, the medial rectus is the muscle closest to the nose. The superior and inferior recti do not pull straight back on the eye, because both muscles also pull slightly medially. This posterior medial angle causes the eye to roll with contraction of either the superior rectus muscle or the inferior rectus muscle. The extent of rolling in the recti is less than the oblique, and opposite from it.
The ''superior oblique'' muscle originates at the back of the orbit (a little closer to the medial rectus, though medial to it), getting rounder as it
courses forward to a rigid, cartilaginous pulley, called the
trochlea
Trochlea (Latin for pulley) is a term in anatomy. It refers to a grooved structure reminiscent of a pulley's wheel.
Related to joints
Most commonly, trochleae bear the articular surface of saddle joint, saddle and other joints:
* Trochlea of hume ...
, on the upper, nasal wall of the orbit. The muscle becomes tendinous about 10mm before it passes through the pulley, turning sharply across the orbit, and inserts on the lateral, posterior part of the globe. Thus, the superior oblique travels posteriorly for the last part of its path, going over the top of the eye. Due to its unique path, the superior oblique, when activated, pulls the eye downward and laterally.
The last muscle is the ''inferior oblique'', which originates at the lower front of the nasal orbital wall, and passes under the LR to insert on the lateral, posterior part of the globe. Thus, the inferior oblique pulls the eye upward and laterally.
Blood supply
The extraocular muscles are supplied mainly by branches of the
ophthalmic artery. This is done either directly or indirectly, as in the lateral rectus muscle, via the
lacrimal artery, a main branch of the ophthalmic artery. Additional branches of the ophthalmic artery include the
ciliary arteries
The ciliary arteries are divisible into three groups, the long posterior, short posterior, and the anterior.
* The short posterior ciliary arteries from six to twelve in number, arise from the ophthalmic artery as it crosses the optic nerve.
* The ...
, which branch into the
anterior ciliary arteries
The anterior ciliary arteries are seven small arteries in each eye-socket that supply the conjunctiva, sclera and the recti muscles. They are derived from the muscular branches of the ophthalmic artery.
Course
The anterior ciliary arteries are ...
. Each rectus muscle receives blood from two anterior ciliary arteries, except for the lateral rectus muscle, which receives blood from only one. The exact number and arrangement of these ciliary arteries may vary. Branches of the
infraorbital artery
The infraorbital artery is an artery in the head that branches off the maxillary artery, emerging through the infraorbital foramen, just under the orbit of the eye.
Course
The infraorbital artery appears, from its direction, to be the continuatio ...
supply the inferior rectus and inferior oblique muscles.
Nerve supply
The nuclei or bodies of these nerves are found in the brain stem. The nuclei of the abducens and oculomotor nerves are connected. This is important in coordinating the motion of the lateral rectus in one eye and the medial action on the other. In one eye, in two antagonistic muscles, like the lateral and medial recti, contraction of one leads to inhibition of the other. Muscles show small degrees of activity even when resting, keeping the muscles taut. This "
tonic" activity is brought on by discharges of the motor nerve to the muscle.
Development
The extraocular muscles develop along with
Tenon's capsule
Tenon's capsule (), also known as the Tenon capsule, fascial sheath of the eyeball () or the fascia bulbi, is a thin membrane which envelops the eyeball from the optic nerve to the corneal limbus, separating it from the orbital fat and forming a so ...
(part of the ligaments) and the fatty tissue of the
eye socket (orbit). There are three centers of growth that are important in the development of the eye, and each is associated with a nerve. Hence the subsequent nerve supply (innervation) of the eye muscles is from three
cranial nerves. The development of the extraocular muscles is dependent on the normal development of the eye socket, while the formation of the ligament is fully independent.
Function
Eye movement
The
oculomotor nerve
The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of ...
(III),
trochlear nerve (IV) and
abducens nerve
The abducens nerve or abducent nerve, also known as the sixth cranial nerve, cranial nerve VI, or simply CN VI, is a cranial nerve in humans and various other animals that controls the movement of the lateral rectus muscle, one of the extraocula ...
(VI) coordinate
eye movement. The oculomotor nerve controls all muscles of the eye except for the
superior oblique muscle controlled by the trochlear nerve (IV), and the
lateral rectus muscle controlled by the abducens nerve (VI). This means the ability of the eye to look down and inwards is controlled by the trochlear nerve (IV), the ability to look outwards is controlled by the abducens nerve (VI), and all other movements are controlled by the oculomotor nerve (III)
Movement coordination
Intermediate directions are controlled by simultaneous actions of multiple muscles. When one shifts the gaze horizontally, one eye will move laterally (toward the side) and the other will move medially (toward the midline). This may be neurally coordinated by the central nervous system, to make the eyes move together and almost involuntarily. This is a key factor in the study of strabismus, namely, the inability of the eyes to be directed to one point.
There are two main kinds of movement: conjugate movement (the eyes move in the same direction) and disjunctive (opposite directions). The former is typical when shifting gaze right or left, the latter is convergence of the two eyes on a near object. Disjunction can be performed voluntarily, but is usually triggered by the nearness of the target object. A "see-saw" movement, namely, one eye looking up and the other down, is possible, but not voluntarily; this effect is brought on by putting a prism in front of one eye, so the relevant image is apparently displaced. To avoid double vision from non-corresponding points, the eye with the prism must move up or down, following the image passing through the prism. Likewise conjugate torsion (rolling) on the anteroposterior axis (from the front to the back) can occur naturally, such as when one tips one's head to one shoulder; the torsion, in the opposite direction, keeps the image vertical.
The muscles show little inertia - a shutdown of one muscle is not due to checking of the antagonist, so the motion is not ballistic.
Compensatory movements
The
vestibulo-ocular reflex is a
reflex
In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus.
Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...
that stabilizes gaze when the head is moved. The reflex involves compensatory eye movements driven by inhibitory and excitatory signals.
Table
Below is a table of each extraocular muscle and their innervation, origins and insertions, and the primary actions of the muscles (the secondary and tertiary actions are also included, where applicable).
Clinical significance
![Lawrence 1960 14](https://upload.wikimedia.org/wikipedia/commons/7/77/Lawrence_1960_14.13.png)
Damage to the
cranial nerves may affect the movement of the eye. Damage may result in double vision (
diplopia) because the movements of the eyes are not synchronized. Abnormalities of visual movement may also be seen on examination, such as jittering (
nystagmus).
Damage to the oculomotor nerve (III) can cause double vision and inability to coordinate the movements of both eyes (
strabismus), also eyelid drooping (
ptosis) and pupil dilation (
mydriasis
Mydriasis is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of certain types of drugs.
Normally, as ...
).
Lesions may also lead to inability to open the eye due to paralysis of the levator palpebrae muscle. Individuals suffering from a lesion to the oculomotor nerve may compensate by tilting their heads to alleviate symptoms due to paralysis of one or more of the eye muscles it controls.
Damage to the trochlear nerve (IV) can also cause double vision with the eye adducted and elevated.
The result will be an eye which can not move downwards properly (especially downwards when in an inward position). This is due to impairment in the superior oblique muscle.
Damage to the abducens nerve (VI) can also result in double vision.
This is due to impairment in the lateral rectus muscle, supplied by the abducens nerve.
Amblyopia
Amblyopia, also called lazy eye, is a disorder of sight in which the brain fails to fully process input from one eye and over time favors the other eye. It results in decreased vision in an eye that typically appears normal in other aspects. Amb ...
also known as ''lazy eye'' is a condition of
diminshed sight in one eye.
Ophthalmoparesis
Ophthalmoparesis refers to weakness (-paresis) or paralysis (-plegia) of one or more extraocular muscles which are responsible for eye movements. It is a physical finding in certain neurologic, ophthalmologic, and endocrine disease.
Internal o ...
is weakness or paralysis of one or more extraocular muscles.
Examination
The initial clinical examination of the extraoccular eye muscles is done by examining the movement of the globe of the eye through the ''six cardinal eye movements''. When the eye is turned out (temporally) and horizontally, the function of the lateral rectus muscle is tested. When the eye is turned in (nasally) and horizontally, the function of the medial rectus muscle is being tested. When turning the eye down and in, the inferior rectus is contracting. When turning it up and in the superior rectus is contracting. Paradoxically, turning the eye up and out uses the inferior oblique muscle, and turning it down and out uses the superior oblique. All of these six movements can be tested by drawing a large "H" in the air with a finger or other object in front of a patient's face and having them follow the tip of the finger or object with their eyes without moving their head. Having them focus on the object as it is moved in toward their face in the midline will test ''convergence'', or the eyes' ability to turn inward simultaneously to focus on a near object.
To evaluate for weakness or imbalance of the muscles, a penlight is shone directly on the corneas. Expected normal results of the corneal light reflex is when the penlight's reflection is located in the centre of both corneas, equally.
Gallery
Image:Gray776.png, Nerves of the orbit. Seen from above.
Image:Gray785.png, Figure showing the mode of innervation of the Recti medialis and lateralis of the eye.
Image:Gray890.png, Dissection showing origins of right ocular muscles, and nerves entering by the superior orbital fissure.
Image:Eye orbit anterior.jpg, View of the orbit from the front, with nerves and extraocular muscles.
Image:Extraocular muscles.jpg, Extraocular muscles
Image:Eyemuscles.png
Image:Extraocular muscle actions and innervation.png, Schematic demonstrating the actions and cranial nerve innervation (in subscript) of extraocular muscles.
See also
*
ALS#Late stages
*
Hering's law of equal innervation Hering's law of equal innervation is used to explain the conjugacy of saccadic eye movement in stereoptic animals. The law proposes that conjugacy of saccades is due to innate connections in which the eye muscles responsible for each eye's moveme ...
*
Park's three-step test
*
Sherrington's law of reciprocal innervation
References
Further reading
*
External links
* - "Extraocular Muscles, Actions"
*
Animations of extraocular cranial nerve and muscle function and damage (University of Liverpool)
{{DEFAULTSORT:Extraocular Muscles
Muscles of the head and neck
Human eye anatomy