24. The pectoral region and axilla Flashcards
Where is the pectoral region located?
located on the anterior chest wall.
Describe the Pectoralis Major: • where does it originate • where does it insert • what is its action • what is it innervated by
• It is triangular in shape and the origin is composed of a sternal head and a clavicular head.
• The clavicular head originates from the anterior surface of the medial clavicle
• The sternocostal head originates from the anterior surface of the sternum, the upper six costal cartilages and the aponeurosis of the external oblique muscle
• Pectoralis Major inserts into the intertubercular sulcus of the humerus. The fibres from the clavicular head overlap those of the sternal head and insert inferior to them in the intertubercular sulcus
Actions:
If the arm is abducted at the shoulder joint, contraction of the sternocostal head pulls the humeral insertion towards the fixed sternocostal origin, and therefore adducts the arm.
if the upper limb starts in the anatomical position,
contraction of the clavicular head will flex the arm at the shoulder joint.
If instead, the humerus is already externally (laterally) rotated at the shoulder joint, contraction of pectoralis major will internally (medially) rotate it
• Innervation: Medial pectoral nerve (C8-T1 from the medial cord of the brachial plexus) and lateral pectoral nerve (C5-7 from the lateral cord)
Describe the Pectoralis Minor: • where does it originate • where does it insert • what is its action • what is it innervated by
- lies deep to pectoralis major. Both muscles form part of the anterior wall of the axilla
- originates from the 3rd-5th ribs
- inserts into the coracoid process of the scapula
- Action: Pectoralis minor stabilises the scapula by drawing it anteroinferiorly against the thoracic wall.
- Innervation: Medial pectoral nerve (C8-T1 from the medial cord of the brachial plexus).
Describe the Serratus Anterior: • where does it originate • where does it insert • what is its action • what is it innervated by
• located more laterally on the chest wall and forms the medial border of the axilla
• It originates as separate ‘slips’ from the lateral aspects of ribs 1-8
• The fibres pass deep to the scapula and insert into the
costal (rib-facing) surface of its medial border (the border nearest the vertebral column).
• Action:
- Serratus anterior protracts the scapula.
- rotates the medial border of the scapula anteroinferiorly so that the glenoid cavity (site of articulation with the humerus at the shoulder joint) rotates upwards. This enables the upper limb to be abducted above 90° at the shoulder.
- also holds the medial border of the scapula against the ribcage.
• Innervation: Long thoracic nerve (C5-7, directly from anterior rami of spinal nerves i.e. from the ‘roots’).
What is Winging of the Scapula?
One of the actions of the serratus anterior is to ‘hold’ the scapula against the ribcage. If the long thoracic nerve is damaged (and the serratus anterior is therefore paralysed), ‘winging of the scapula’ occurs, so called because of its winged appearance. The medial border of the scapula is no longer held against the chest wall so protrudes posteriorly.
When can winging of the scapula clearly be seen?
• can be seen clearly when the patient is asked to
place the palm of their hand on a wall and push; the
force is transmitted back along their upper limb to the
scapula which ‘lifts off’ the underlying ribs.
• Alternatively, the patient can be asked to hold their unaffected shoulder with the hand of the affected limb
and pull the unaffected shoulder forwards; this places traction on the scapula of the affected limb and elevates its medial border.
What are the causes of winging of the scapula?
• The most common cause of dysfunction of the long
thoracic nerve is trauma. The long thoracic nerve is vulnerable to surgical trauma during a mastectomy
with axillary clearance (excision of the breast and axillary lymph nodes for cancer) as it passes superficial to the serratus anterior muscle in the medial wall of the axilla and can be ‘stripped’ along with the axillary nodes and fat.
• Also, a blunt trauma to the neck or shoulder causing sudden depression of the shoulder girdle, or wearing a heavy backpack, can both cause traction injuries to the long thoracic nerve.
Which muscles assist respiration in conditions of high metabolic demand?
In conditions of high metabolic demand (e.g. when hyperventilating to compensate for severe metabolic acidosis) or in respiratory disease (e.g. severe asthma), pectoralis major and minor and serratus anterior can all act as accessory muscles of respiration by elevating the ribs and therefore increasing the volume of the thoracic cavity
Describe the Coracobrachialis: • where does it originate • where does it insert • what is its action • what is it innervated by
• lies deep to the biceps brachii in the arm. It is included
here because it will be an important landmark when you dissect the brachial plexus.
• Coracobrachialis originates from the coracoid process of the scapula.
• It passes through the axilla and inserts onto the medial side of the humeral shaft, at the same level as the deltoid tubercle.
• Action: Coracobrachialis flexes the arm at the shoulder and is also a weak adductor of the arm.
• Innervation: Musculocutaneous nerve (C5-7 from the lateral cord of the brachial plexus)
Describe the Subclavius: • where does it originate • where does it insert • what is its action • what is it innervated by
- small muscle, which is located directly underneath the clavicle, running horizontally. It affords some minor protection to the underlying neurovascular structures (e.g. in cases of clavicular fracture or other trauma).
- Subclavius originates from the junction of the 1st rib and its costal cartilage
- inserts onto the inferior surface of the middle third of the clavicle.
- Action: Subclavius anchors and depresses the clavicle.
- Innervation: Nerve to subclavius
Describe the Deltoid: • where does it originate • where does it insert • what is its action • what is it innervated by
• The deltoid muscle is a triangular muscle, shaped like the Greek letter delta: Δ. It can be divided functionally into anterior, middle and posterior parts.
• Deltoid originates from the anterior border and upper surface of the lateral third of the clavicle, and from the acromion and spine of the scapula.
• It inserts into the deltoid tuberosity on the lateral surface of the humerus
• Actions:
Anterior fibres – flex and medially rotate the arm at the shoulder.
Middle fibres – abduct the arm from 15-90° (Deltoid takes over from the supraspinatus, which abducts the arm for the first 15° from the anatomical position).
Posterior fibres – extend and laterally rotate the arm at the shoulder.
• Innervation: Axillary nerve (C5,6 from the posterior cord of the brachial plexus)
What is the axilla?
The axilla is a pyramidal space at the junction of the upper limb and the thorax. It provides a passageway by which neurovascular and muscular structures can enter and leave the upper limb.
What are the borders of the axilla
The axillary borders consist of four walls, an apex and a base
• Apex (axillary inlet): lateral border of the first rib, superior border of scapula, and the posterior border of the clavicle.
• Lateral wall: intertubercular sulcus of the humerus.
• Medial wall: serratus anterior, ribs, intercostal muscles.
• Anterior wall: pectoralis major, pectoralis minor and subclavius.
• Posterior wall: subscapularis, teres major and latissimus dorsi.
• Base: axillary fascia (thick layer of fascia spanning between the inferior borders of pectoralis major and latissimus dorsi) and skin
What does the size and shape of the axilla vary with?
the size and shape of the axilla varies with the degree of
arm abduction. When the arm is fully abducted, the axillary fascia becomes taught and the axillary contents are compressed against the proximal humerus
What are the contents of the axilla?
• Axillary artery (and branches) – the main artery supplying the upper limb.
• Axillary vein (and tributaries) – the main vein draining the upper limb
• Cords of the brachial plexus
• Branches of the brachial plexus (long thoracic, median, ulnar, musculocutaneous, radial etc.)
• Intercostobrachial nerve (T2) – cutaneous nerve supplying the upper medial arm and part of the floor of the axilla
• Axillary lymph nodes: these filter lymphatic fluid that has drained from the upper limb and pectoral region.
• Biceps brachii (short head) and coracobrachialis: these
muscles originate from the coracoid process of the scapula and their tendons pass through the axilla on route to the arm
What are the 3 main routes by which structures leave the axilla?
• Inferiorly and laterally, into the upper limb.
• Via the quadrangular space to the posterior arm and shoulder area. This is an intermuscular space through
which the axillary nerve and posterior circumflex humeral artery (a branch of the axillary artery) pass to encircle the surgical neck of the humerus.
• Via the clavipectoral triangle (or deltopectoral triangle), which is an opening in the anterior wall of the axilla bounded by the pectoralis major, deltoid, and clavicle. The cephalic vein enters the axilla via the clavipectoral triangle, whilst the medial and lateral pectoral nerves leave.
What are the 6 groups that the axillary lymph nodes are divided into?
APICAL
A Anterior (pectoral) group P Posterior (subscapular) group C Central group I Infraclavicular (deltopectoral) group A Apical group L Lateral group
Describe where the Anterior (pectoral) group is and where it receives lymph vessels from
Lying along the lower border of the pectoralis minor behind the pectoralis major, these nodes receive lymph vessels from the lateral quadrants of the breast and the anterolateral abdominal wall above the level of the umbilicus.
Describe where the Posterior (subscapular) group is and where it receives lymph vessels from
Lying in front of the subscapularis muscle, these nodes receive superficial lymph vessels from the back, down as far as the level of the iliac crests
