The sternocleidomastoid (SCM) is a prominent, paired muscle located on each side of the neck. It has two heads of origin: the sternal head attaches to the manubrium of the sternum, and the clavicular head attaches to the medial third of the clavicle. Both heads insert at the mastoid process of the temporal bone behind the ear and the superior nuchal line of the occipital bone.
Functionally, the sternocleidomastoid muscle plays a key role in head and neck movements. When one side contracts, it rotates the head to the opposite side and tilts it laterally. When both sides contract together, they flex the neck forward. It also assists in elevating the sternum during deep inhalation, aiding respiration.
The SCM is a superficial muscle, easily palpable and visible when turning or tilting the head, and serves as an important anatomical landmark in the neck region. It is innervated primarily by the accessory nerve (cranial nerve XI) with contributions from cervical spinal nerves.
The trapezius is a large, superficial, triangular-shaped muscle located in the upper back and neck region. It connects the shoulder girdle to the axial skeleton and is divided into three parts based on fiber orientation: the upper (descending), middle (transverse), and lower (ascending) fibers.
Functionally, the trapezius is crucial for movements of the shoulder girdle and upper limb. It helps in elevating, retracting, and rotating the scapula, as well as stabilising the shoulder during arm movements. Additionally, it assists in extending and rotating the neck.
This muscle plays an important role in maintaining posture and enabling shoulder and arm mobility. It is innervated by the accessory nerve (cranial nerve XI) and cervical spinal nerves.