Pectoral girdle

Question 1

Pectoralis major:
* Origin
* Insertion
* Nerve supply
* Action

Answer 1

• Origin - clavicular head - medial half of the anterior surface of the clavicle. Sternocostal head - lateral half of the anterior surface of the manubrium and body of sternum, the upper 6 costal cartilages and the aponeurosis of the external oblique muscle over the upper attachment of rectus abdominis.
• Insertion - clavicular head - anterior lamina of the tendon into the lateral lip of the intertubercular (bicipital) sulcus of the humerus.
  Sternocostal head - manubrial part - anterior lamina of the tendon into the lateral lip of intertubercular sulcus behind (deep to) clavicular fibres. Sternocostal part - the lower sternocostal and abdominal fibres course upwards and laterally to be inserted progressively higher into the posterior lamina of tendon, producing the rounded. appearance of the anterior axillary fold.
• Nerve supply - lateral and medial pectoral nerves from the brachial plexus (C8-T1).
• Action - flexion, adduction and IR of humerus. With the upper limb fixed in abduction, the muscle is a useful accessory muscle of inspiration, drawing the ribs upwards towards the humerus.

The sternocostal fibres are the chief adductors.
The clavicular head assists in flexion at the shoulder joint.

Question 2

Pectoralis minor:
* Origin
* Insertion
* Nerve supply
* Action

Answer 2

• Origin - small triangular muscle arises from the 3rd, 4th and 5th ribs under cover of pectoralis major.
• Insertion - medial border and upper surface of the coracoid process of the scapula.
• Nerve supply - lateral and medial pectoral nerves (C6-8).
• Action - stabilisation, depression, protraction/abduction, IR and downward rotation of scapula.

Question 3

Subclavius:
* Origin
* Insertion
* Nerve supply
* Action

Answer 3

• Origin - costochondral junction of 1st rib.
• Insertion - subclavian groove of the inferior surface of middle third of clavicle.
• Nerve supply - nerve to subclavius from the upper trunk of the brachial plexus (C5, 6).
• Action - stabilisation of the clavicle at the sternoclavicular joint during movements of the shoulder and arm (pectoral girdle).

Question 4

Trapezius:
* Origin
* Insertion
* Nerve supply
* Action

Answer 4

• Origin - skull, nuchal ligament and spinous processes of C7-T12.
• Insertion - clavicle (upper fibres), acromion (middle fibres) and scapula spine (lower fibres).
• Nerve supply - spinal part of the accessory nerve - motor. Branches from the cervical plexus (C3 and C4) - pain and proprioception.
• Action - all fibres help to retract/adduct the scapula.
  Upper fibres - elevation of scapula, extension and rotation of head and neck.
  Middle fibres - elevation, upward rotation and retraction of scapula.
  Lower fibres - depression, upward rotation and retraction of scapula. Lower and upper fibres together - rotates scapula upwards.

Question 5

Latissimus dorsi:
* Origin
* Insertion
* Nerve supply
* Action

Answer 5

• Origin - the lower 6 thoracic vertebral spinous processes anterior to the attachment of the trapezius on the spinous processes.
  Through the thoracolumbar fascia, it attaches to the lumbar and sacral spinous processes (T6-S5) and the supraspinous ligament.
  Other attachments include posterior iliac crest, lower 3 to 4 ribs where it interdigitates with the external oblique muscle and inferior angle of scapula.
• Insertion - As the muscle fibres extend toward the axilla, the fibres wind around the anterior aspect of the teres major muscle to insert as a flat tendon on the floor of intertubercular (bicipital) groove of humerus.
• Nerve supply - thoracodorsal nerve (C6-8) from the posterior cord of the brachial plexus.
• Action - shoulder adduction, IR, extension. Its costal fibres of origin can assist in deep inspiration, elevating the lower 4 ribs towards the fiixed humerus.

Question 6

Rhomboid major/minor:
* Origin
* Insertion
* Nerve supply
* Action

Answer 6

• Origin - major - arises from 4 vertebral spines (T2-5) and the intervening supraspinous ligaments.
  Minor - narrow ribbon of muscle parallel with the above, arising from 2 vertebral spines (C7-T1).
• Insertion - major - medial border of scapula between the root of the spine and inferior angle. Minor - medial border of the scapula at the root of the spine.
• Nerve supply - dorsal scapular nerve from brachial plexus (C5)
• Action - The rhomboids draw the vertebral border of the scapula medially and upwards. With trapezius they contract in squaring the shoulders i.e. scapula retraction.

Question 7

Levator scapulae:
* Origin
* Insertion
* Nerve supply
* Action

Answer 7

• Origin - transverse process of atlas (C1) and axis (C2) and from the posterior tubercles of C3 and C4.
• Insertion - medial border of the scapula from the superior angle to the spine.
• Nerve supply - from the cervical plexus (C3, 4, anterior rami), reinforced by dorsal scapular nerve (C5).
• Action - with the upper part of trapezius, it can elevate the scapula and laterally flex the neck.

Question 8

Serratus anterior:
* Origin
* Insertion
* Nerve supply
* Action

Answer 8

• Origin - it arises by a series of digitations from the upper 8 ribs.
  The 1st digitation arises from the 1st and 2nd ribs.
  All the other digitations arise from their corresponding ribs and the lower 4 interdigitate with external oblique.
• Insertion - costal (inner) surface of scapula.
  The 1st and 2nd at the superior angle, the 3rd and 4th as a thin sheet to the length of the vertebral border, and the lowest 4 at the inferior angle.
• Nerve supply - long thoracic nerve (C5-7) of brachial plexus.
• Action - scapula protraction, upward rotation of scapulothoracic joint. A further highly important action is that of the lower 4 digitations which powerfully assist trapezius in rotating the scapular laterally and upwards in raising the arm above the shoulder.

Question 9

Describe the sternoclavicular joint.

Answer 9

• Synovial joint between the bulbous medial end of the clavicle, the superolateral part of the manubrium of the sternum and the adjoining first costal cartilage.
• The joint is separated into two cavities by an intervening disc of fibrocartilage, which is attached at its periphery to the capsule of the joint.
• Although synovial, it is atypical as the bony surfaces are covered by fibrocartilage, not the usual hyaline variety.
• The sternal end of the clavicle projects above the upper margin of the manubrium so that only about the lower half of the clavicular articular surface lies opposite the sternal articular facet.
• The capsule invests the articular surfaces like a sleeve. The articular disc is attached to the capsule. The disc is also firmly attached to the medial end of the clavicle above and behind, and to the first costal cartilage below.
• The capsule is thickened in front and behind as the anterior and posterior sternoclavicular ligaments.
• The interclavicular ligament joins the upper borders of the sternal ends of the two clavicles and is attached to the suprasternal (jugular) notch of the manubrium.
• The costoclavicular ligament binds the clavicle to the first costal cartilage and the adjacent end of the first rib, just lateral to the joint. It is in two laminae. The fibres of the anterior lamina run upwards and laterally, and those of the posterior lamina upwards and medially (these are the same directions as those of the external and internal intercostal muscles). The ligament is very strong and is the major stabilising factor of the sternoclavicular joint.
• The stability of the joint is maintained by the ligaments especially the costoclavicular ligament. It takes all strain off the joint transmitting stress from clavicle to first costal cartilage.

Question 10

Describe the acromioclavicular joint.

Answer 10

• This is a synovial joint between the flat overhanging lateral end of the clavicle and the underlying medial border of the acromion.
• The articulating surfaces are covered (like those of the sternoclavicular joint) by fibrocartilage (so it is an atypical synovial joint).
• A sleeve-like capsule surrounds the articular surfaces; it is not strong, but on top there is a thickening of fibres which constitutes the acromioclavicular ligament. * An incomplete disc of fibrocartilage hangs down into the upper part of the joint cavity.
• The coracoclavicular ligament, extremely strong, is the principal factor in providing stability to the joint.
• It consists of two parts, conoid and trapezoid.
• The conoid ligament, an inverted cone, extends upwards from the knuckle of the coracoid process to a wider attachment around the conoid tubercle, on the undersurface of the clavicle.
• The trapezoid ligament is attached to the ridge of the same name on the upper surface of the coracoid process and extends laterally, in an almost horizontal plane, to the trapezoid ridge on the undersurface of the clavicle. * The two ligaments are connected to each other posteriorly, forming an angle that is open anteriorly.