Question

What are the 6 extrinsic muscles of the eye and their functions?

Answer 1

The basic function of an animal’s or a higher organism's eye is to receive and process visual detail and various other photo response functions which are independent of vision. These functions are aided by the 6 extrinsic muscles of the eye. These muscles are also called extraocular muscles and the movement of the eye specifically depends on them.

Complete answer:
Four of the extrinsic eye muscles are called the rectus muscles and the rest two of them are called the superior oblique and the inferior oblique. The movements of these extraocular muscles are assisted by extraocular muscle pulleys (soft tissue pulleys).
The rectus muscles have their origin in a fibrous ring called the annulus of Zinn. They are attached directly to the front of the eye and have names depending on their paths. The four rectus muscles are: superior rectus, inferior rectus, lateral rectus and medial rectus. The ring surrounds the upper, medial and lower margin of the optic foramen and encircles the optic nerve. The ring also has a tendinous bridge over the lower and medial part of the superior orbital fissure. The tendon of Zinn gives rise to the inferior part of rectus internus, the lower head of rectus lateralis. These three form the lower part of the fibrous ring. The rectus superior, the rest of rectus medialis and the upper head of rectus lateralis form the upper part of the fibrous ring. The upper part is also called by the term ‘superior tendon of Lockwood’. Each of these aforementioned muscles are inserted by a tendinous expansion into the sclera from the margin of the cornea. The basic difference between these muscles are that the rectus medialis is the broadest, rectus lateralis is the longest and the rectus superior is the thinnest and the narrowest.
The superior oblique muscle stems from the back of the orbit from the sphenoid bone. It eventually extends into the trochlea, a rigid cartilaginous pulley on the upper, nasal wall of the orbit. This muscle becomes tendinous about 10mm before the trochlea. In the last part of the superior oblique’s pathway, it travels posteriorly over the top of the eye. The primary function of these muscles is intorsion, i.e., internal rotation, the secondary function is depression and the tertiary function is abduction, i.e., lateral rotation.
The inferior oblique muscle originates from the lower front of the nasal orbital wall and inserts on the lateral, posterior part of the globe. Therefore, these muscles pull the eye upward and laterally. It is also known as obliquus oculi inferior. It performs the following functions: extorsion, elevation and abduction of the eye. Among these the primary action is extortion, i.e., external rotation. The secondary action is elevation and the tertiary action is abduction, i.e., moving eyes upward and outwards.

Note:
The extraocular muscles are innervated by three cranial nerves. Any kind of damage to these cranial nerves result in paralysis of the respective muscles altering the resting gaze of the affected eye. Oculomotor nerve (CN III) is the innervation of superior rectus, inferior rectus, medial rectus and inferior oblique. Trochlear nerve (CN IV) is the innervation of superior oblique while the Abducens nerve (CN VI) is the innervation of lateral rectus.