Explain about the physiology of eyelid movements.
Physiology of Eyelid Movements:
It is quoted that the eyelids move more than any other part of the body. The major movement of the eyelids include opening and closing of the eye. Other movements that need to be understood are blinking, winking, peering and forceful closure. The opening movements of the upper and lower lids are determined by the contraction of yoke muscles known as the levator palpebrae superioris (LPS) (supplied by the 3rd nerve) and the smooth muscle fibres which arise from the under surface of the LPS (sympathetic nerve). Opening movements are bilaterally symmetrical in direction and amplitude. The upper eyelid can be raised by 15 mm by the LPS muscle arid an additional elevation of 3-5 mm by the frontalis muscle.
The closing action of the lids occurs through the movement of the orbicularis oculi muscle and is supplied by the 7th cranial nerve. The muscle contains pretarsal, preseptal and orbital fibres. The pretarsal fibres function in spontaneous blinking and tactile corneal reflex. The preseptal fibres are responsible for voluntary blinking and sustained activity. Lastly, the orbital fibres are used in forceful closure of the eyelids. The exposed zone between the upper and lower eyelids is known as the palpebral fissure. In adults, it is 27-30 mm long and 8-11 mm wide.
The upper lid moves vertically downwards while the lower lid moves medially. Their rate of movement is similar but the lower lid movement begins 10-20 milliseconds earlier than the upper lid. In this mechanism Shesington's law of reciprocal innervation is obeyed.