The eye muscles serve the motor function of the eyeballs, accommodation of the lenses, and adaptation of the pupils. The 6 external eye muscles are able to move the two eyeballs in unison and synchronously or to focus on a gaze target. The inner eye muscles are responsible for focusing on near or far vision and for the adaptation of the pupils, the adjustment to the strength of the light incidence (comparable to the selection of the aperture on a camera).
What are eye muscles?
The external eye muscles provide the necessary eye movement in the three possible directions of rotation: Nodding (up and down), lateral rotation (right and left), and tilting (torsion). While the two directions of rotation, pitching and lateral rotation, can be controlled voluntarily, torsion is physically very limited. It is activated almost exclusively via involuntary stimuli by the vestibular system (organ of balance). The eyeballs are normally rotated in the same direction and synchronously. However, voluntary movements in the opposite direction are also possible to a limited extent, for example internal strabismus. Since the external eye muscles are skeletal muscles, the eyes can be moved voluntarily. However, there is also an involuntary eye movement in all directions, which works almost without distortion and is controlled by the vestibular system in the middle ear in order to avoid losing the last image from the eye as much as possible during fast head movements or accelerations. This is comparable to the images taken by a gyro-stabilized camera. The inner (smooth) eye muscles, which are subject to the autonomic nervous system, accommodate the eye lens from near vision to distance vision and vice versa. Two tiny inner eye muscles provide adaptation of the pupil to the appropriate light conditions.
Anatomy and structure
The external eye muscles include 4 straight and 2 oblique eye muscles, each acting in pairs as antagonists. Except for the superior oblique eye muscle, all external eye muscles originate at the tip of the bony orbit. From there they run funnel-like to the eyeball (bulbus oculi), where they are attached to the sclera of the eyeball. The eyelid elevator also originates in the same place and runs in the upper orbit to the eyelid. The eyelid elevator is not only activated voluntarily, but it is also connected to the superior straight muscle. The latter supports it as an agonist, which means that the eyelid automatically moves upward when the eye rolls upward and vice versa. The external eye muscles consist of striated skeletal muscles subject to will and are innervated by three cranial nerves. The inner eye muscles consist of the paired ciliary muscles, which, when tensed, flatten the lens and cause a higher focal length. From the two antagonistic muscles that cause adaptation of the pupil in response to incident light intensity. The internal eye muscles are parasympathetically stimulated and therefore cannot be voluntarily controlled.
Tasks and function
The external eye muscles serve primarily to rotate the eyes synchronously and in parallel in the two directions up-down and right-left. To enable spatial vision, the external eye muscles align the eyes so that the object we want to look at is respectively imaged in the fovea centralis of both eyes, the point of sharpest vision on the retina. This means that the central visual axes of both eyes always intersect at the height of the object. At close distances, this is equivalent to internal strabismus, whereas the visual axes of the eyes are aligned virtually parallel for objects at long distances. If we turn the eyes in any direction, willingly or unwillingly, the muscles report the movement to the visual center in the brain so that the brain interprets the image shift on the retina as the eyes’ own movement and not as movement of the object or the entire environment. Another task is to perform a so-called microsaccade one to three times per second. In this process, the eyes are jerkily rotated by less than 30 arcminutes, which occurs autonomously and completely unnoticed. The microsaccades cause the image on the retina to shift by about 40 photoreceptors. This prevents the photoreceptors (cones and rods) from being damaged by uniform exposure for too long.The internal eye muscles have the task of autonomously accommodating the lens to changing distances and independently controlling the incidence of light by adapting the pupil.
Diseases
Dysfunction of one or more nerves that provide motor control to the external or internal eye muscles is called ophthalmoplegia. There are then signs of paralysis (paresis) in the affected eye muscles. A distinction is made between internal and external ophthalmoplegia. If the outer and inner eye muscles are affected equally, this is called total ophtalmoplegia. If only the outer eye muscles are affected, the exact automatic alignment of the eyes is disturbed, which can manifest itself in strabismus and the production of double images or similar symptoms. If the inner eye muscles are affected, this may be manifested, for example, by a wide, fixed pupil and/or the inability to adjust the eyes to a certain distance, i.e., focus is lost. Nerve damage can be caused, for example, by neurotoxins, by tumors, or by aneurysms. If certain areas in the visual center of the brain are disturbed, there will be disturbances in the alignment of the eyes to gaze targets or eye tremor (nystagmus), which, however, may be normal for a few seconds when stopping sustained body turns (pirouette). If the transmission of stimuli from the nerve to the eye muscles is disturbed, there may be myasthenia gravis, an autoimmune disease that manifests itself in symptoms of muscle weakness in the eye muscles. Another autoimmune disease is Graves’ disease, a disease usually coupled with thyroid dysfunction. The disease is symptomatic by bulging eyes, which is caused by changes in the tissue behind the eyeball.
