[The physiology of the lid motor system]

The physiology and anatomy of the lid motor system have been widely studied from experimental and clinical points of view during last years. These studies, mainly carried out in animals, have helped to understand its functioning in normal and pathologic situations. Despite the apparent simplicity of this motor system, lids are able of doing many types of movements, depending on each particular situation. Besides participating in reflex and spontaneous blinks, lids take part actively in motor responses of major complexity and that requires a better precision, for instance, during emotional responses as laughing or crying in human beings. Lid movements also accompanies the eyes in such a manner not to cover the pupil while they maintain a fixed position in the orbit or during reflex and voluntary saccades.

This paper deals with the neural circuits underlying lid motor system and how the activity in them translates, through the corresponding muscles, in a coherent response despite the different situations imposed by the different stimuli. In this way, it is described each element and how they contribute in the whole we observe as lid movements, in its repertoire.

The facial motor system has been quite preserved through phylogeny, so it can be said that human beings possess a mechanism for lubricating and protecting the eye that, except for certain differences, share with the rest of the terrestrial living beings.