Sternocleidomastoid muscle

Source: Wikipedia, the free encyclopedia.
Sternocleidomastoid
spinal accessory nerve
sensory: cervical plexus
Proprioceptive: ventral rami of C2-3
ActionsUnilaterally: contralateral cervical rotation, ipsilateral cervical flexion Bilaterally: cervical flexion, elevation of sternum and assists in forced inhalation.
Identifiers
Latinmusculus sternocleidomastoideus
TA98A04.2.01.008
TA22156
FMA13407
Anatomical terms of muscle

The sternocleidomastoid muscle is one of the largest and most superficial cervical muscles. The primary actions of the muscle are rotation of the head to the opposite side and flexion of the neck. The sternocleidomastoid is innervated by the accessory nerve.

Etymology and location

It is given the name sternocleidomastoid because it originates at the

skull.[3]

Structure

The sternocleidomastoid muscle originates from two locations: the

skull by a thin aponeurosis.[3][4]
The sternocleidomastoid is thick and narrow at its center, and broader and thinner at either end.

The sternal head is a round

manubrium sterni
. It travels superiorly, laterally, and posteriorly.

The clavicular head is composed of fleshy and

aponeurotic fibers, arises from the upper, frontal surface of the medial third of the clavicle
; it is directed almost vertically upward.

Sternocleidomastoid Labelled

The two heads are separated from one another at their origins by a triangular interval (lesser supraclavicular fossa) but gradually blend, below the middle of the neck, into a thick, rounded muscle which is inserted, by a strong tendon, into the lateral surface of the

superior nuchal line of the occipital bone
.

Nerve supply

The sternocleidomastoid is innervated by

ventral primary rami of C2 and C3.[5]

Variation

The clavicular origin of the sternocleidomastoid varies greatly: in some cases the clavicular head may be as narrow as the sternal; in others it may be as much as 7.5 millimetres (0.30 in) in breadth.

When the clavicular origin is broad, it is occasionally subdivided into several slips, separated by narrow intervals. More rarely, the adjoining margins of the sternocleidomastoid and

posterior triangle
.

The supraclavicularis muscle arises from the manubrium behind the sternocleidomastoid and passes behind the sternocleidomastoid to the upper surface of the clavicle.

Function

The function of this muscle is to rotate the head to the opposite side or obliquely rotate the head.[3] It also flexes the neck.[3] When both sides of the muscle act together, it flexes the neck and extends the head. When one side acts alone, it causes the head to rotate to the opposite side and flexes laterally to the same side (ipsilaterally).

It also acts as an accessory muscle of respiration, along with the scalene muscles of the neck.

Contraction

The signaling process to contract or relax the sternocleidomastoid begins in Cranial Nerve XI, the

motor endplates on the muscle fibers located at the clavicle. Acetylcholine (ACH) is released from vesicles and is sent over the synaptic cleft to receptors on the postsynaptic bulb. The ACH causes the resting potential to increase above -55mV, thus initiating an action potential which travels along the muscle fiber. Along the muscle fibers are t-tubule openings which facilitate the spread of the action potential into the muscle fibers. The t-tubule meets with the sarcoplasmic reticulum at locations throughout the muscle fiber, at these locations the sarcoplasmic reticulum releases calcium ions that results in the movement of troponin and tropomyosin on thin filaments. The movement of troponin and tropomyosin is key in facilitating the myosin head to move along the thin filament, resulting in a contraction of the sternocleidomastoid muscle.[7]

Anatomical landmark

The sternocleidomastoid muscle with nearby structures labeled, such as the triangles of the neck.

The sternocleidomastoid is within the

landmark of the neck, as it divides the neck into anterior and posterior cervical triangles (in front and behind the muscle, respectively) which helps define the location of structures, such as the lymph nodes for the head and neck.[8]

Many important structures relate to the sternocleidomastoid, including the common carotid artery, accessory nerve, and brachial plexus.

Clinical significance

Examination of the sternocleidomastoid muscle forms part of the examination of the cranial nerves. It can be felt on each side of the neck when a person moves their head to the opposite side.[8]

The triangle formed by the clavicle and the sternal and clavicular heads of the sternocleidomastoid muscle is used as a landmark in identifying the correct location for central venous catheterization.[medical citation needed]

Contraction of the muscle gives rise to a condition called torticollis or wry neck, and this can have a number of causes. Torticollis gives the appearance of a tilted head on the side involved. Treatment involves physiotherapy exercises to stretch the involved muscle and strengthen the muscle on the opposite side of the neck. Congenital torticollis can have an unknown cause or result from birth trauma that gives rise to a mass or tumor that can be palpated within the muscle.

See also

Wikimedia Commons has media related to Sternocleidomastoid muscles.

References

Public domain This article incorporates text in the public domain from page 390 of the 20th edition of Gray's Anatomy (1918)

  1. ^ "Sternocleidomastoid". Merriam-Webster.com Dictionary. Retrieved 2016-01-21.
  2. ^ "Sternocleidomastoid". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 2020-03-22.
  3. ^
    ISBN 978-0-323-02752-6
    , retrieved 2020-11-06
  4. ISBN 978-1-4377-0740-3
    , retrieved 2020-11-20
  5. ^
    ISBN 978-0-12-802653-3
    , retrieved 2020-11-06
  6. ISBN 978-1-4160-2971-7
    , retrieved 2020-11-06
  7. PMID 21250228
    . NBK387.
  8. ^
    ISBN 978-1-4377-2419-6
    .
This article uses anatomical terminology.
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