Inferior tarsal muscle

The inferior tarsal muscle is a thin sheet of smooth muscle located within the lower eyelid and is considered homologous to the superior tarsal muscle (Müllers muscle) of the upper eyelid. Unlike the superior tarsal muscle, however, its attachments to the tarsal plate are less distinct and more variable.

Attachments of the muscle

1. Origin

The inferior tarsus muscle extends from the fascial sheath of the inferior rectus muscle and adjacent orbital connective tissue

2. Insertion

The muscle inserts into the inferior border of the inferior tarsal plate. Microscopic studies have shown that the inferior tarsal muscle does not attach directly to the inferior tarsal plate. Instead, its smooth muscle fibres are enclosed within and associated with the capsulopalpebral fascia, through which they gain an indirect attachment to the inferior tarsus. The surrounding fibrous fascia gradually tapers and blends with the inferior border of the tarsal plate.

Innervation

It consists of involuntary smooth muscle fibres supplied by postganglionic sympathetic fibres from the superior cervical ganglion.

Function

The inferior tarsal muscle receives sympathetic innervation from postganglionic fibres arising from the superior cervical ganglion and contributes modestly to maintenance of lower eyelid position and the width of the palpebral fissure.

Clinical correlate

Clinically, interruption of sympathetic innervation, as occurs in Horner syndrome, may cause slight elevation of the lower eyelid, contributing to narrowing of the palpebral fissure.