Friday, June 19, 2020

Sternocleidomastoid Muscle

ORIGIN

1. The sternal head is tendinous and arises from the superolateral part of the front of the manubrium sterni.
2. The
clavicular head is musculotendinous and arises from the medial one-third of the superior surface of the clavicle.

It passes deep to the sternal head, and the two heads blend below the middle of the neck. Between the two heads, there is a small triangular depression of the lesser supraclavicular fossa, overlying the internal jugular vein.

INSERTION

It is inserted:
1. By a thick tendon into the lateral surface of mastoid process, from its tip to superior border. 

2 By a thin aponeurosis into the lateral half of the superior nuchal line of the occipital bone.



Nerve Supply

1. The spinal accessory nerve provides the motor supply. It passes through the muscle. 

2. Branches from the ventral rami of C2 are proprioceptive

Actions

1. When one muscle contracts:
a. It turns the chin to the opposite side.
b. It can also tilt the head towards the shoulder of same side.
2 When both muscles contract together:
a. They draw the head forwards, as in eating and in lifting the head from a pillow.
b. With the longus colli, they flex the neck against resistance.
c. It also helps in forced inspiration.

Clinical Anatomy:

Torticollis is a deformity in which the head is bent to one side and the chin points to the other side.
This is a result of spasm or contracture of the muscles supplied by the spinal accessory nerve/ these being the stemocleidomastoid and trapezius,
Although there are many varieties of torticollis depending on the causes, the common types are: a. Rheumatic torticollis due to exposure to cold
or draught.
b. Reflex torticollis due to inflamed or suppurating cervical lymph nodes which irritate the spinal accessory nerve.
c. Congenital torticollis due to birth iniury.
Wry neck: Shortening of the muscle fibres due to intravascular clotting of veins within the muscle. It usually occurs during difficult delivery of the baby. 


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