The Shoulder

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The shoulder complex consists of three bones: the clavicle, the scapula, and the humerus. The complex is connected to the axioskeleton via the ster num and rests on the thorax. Thus the shape of the thorax affects the function of the shoulder.

The shoulder complex is composed of four joints: sternoclavicular, acromioclavicular, scapu-lothoracic, and glenohumeral.

The movements of the shoulder can be described as elevation and depression about the anterior-posterior axis, protraction and retraction about the superior-inferior axis, and upward and downward rotation about the medial-lateral axis.

The movements of the shoulder are controlled by three groups of muscles: the axioscapular plus axioclavicular muscles (Table 12-10), whose origins are on the trunk and whose insertions are on

TABLE 12-10 Axioscapular and Axioclavicular Muscles*

Muscles

Origin (Proximal

Insertion (Distal

Innervation

Action

Attachment)

Attachment)

Trapezius

Medial third of

Lateral third of

Spinal root of

Superior fibers: elevate

superior nuchal line,

clavicle, acromion,

accessory nerve,

Middle fibers: retract

external occipital

and spine of

cranial nerve

Inferior fibers: depress

protuberance,

scapula

XI, and cervical

Superior and inferior

ligamentum nuchae,

nerves C3 and

fibers act together

spinous processes of

C4

in superior rotation

C7-T12, lumbar and

sacral processes

Levator scapulae

Posterior tubercles of

Superior part of

Dorsal scapular

Elevate scapula and

C1-C4 vertebrae

medial border of

nerve (C5) and

tilt glenoid cavity

scapula

cervical nerves

inferiorly by

C3 and C4

rotating scapula

Rhomboid minor

Rhomboid minor:

Medial border of

Dorsal scapular

Retract and rotate

and rhomboid

ligamentum nuchae

scapula from level

nerve (C4 and

scapula to depress

major

and spinous processes

of spine to inferior

C5)

glenoid cavity; fix

of C7 and T1

angle

scapula to thoracic

Rhomboid major:

wall

spinous processes of

T2-T5

Sternocleidomastoid

Lateral surface of

Sternal head: attached

Spinal root of

Acting alone: tilts head

mastoid process

to anterior surface

accessory

to ipsilateral side,

and lateral half of

of manubrium

nerve (XI) and

laterally flexes neck

superior nuchal line

Clavicular head:

branches of

Acting bilaterally:

of occipital bone

superior surface

cervical nerve

flexes neck

of medial third of

(C2 and C3)

clavicle

Pectoralis minor

Third to fifth ribs near

Medial border and

Medial pectoral

Stabilizes scapula by

their costal cartilages

superior surface of

nerve (C8 and

drawing it inferiorly

coracoid process of

T1)

and anteriorly against

scapula

thoracic wall

the scapula; scapulohumeral muscles (Table 12-11), whose origins are on the scapula and whose insertions are into the humerus; and axiohumeral muscles (Table 12-12), whose origins are on the trunk and whose insertions are on the humerus.

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Essentials of Human Physiology

Essentials of Human Physiology

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