Sterno-Cleido-Mastoid

By the spinal accessory nerve which is formed by the spinal roots (upper 5 cervical ventral roots) they ascend through the foremen magnus, to exit through the jugular foremen, after joining the cranial roots. 

Two heads (a) Sternal or medial from the anterior surface of the manubrium sterni, (b) Clavicular or lateral from the upper surface of the medial third of the clavicle. 

The two heads merge together and the muscle end over the mastoid process. Before the muscle end, it is pierced by the spinal accessory nerve. 

The patient is in supine position with the head in neutral position. 

Four-fingers breadth cephalad to the muscle origin (at the level of the thyroid cartilage) (Adam s apple). Because of the muscle s mobility and the skin looseness, it is practical to pinch the muscle between the index and thumb to anchor it firmly and to allow a safer electrode insertion. 

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