Human Anatomy Lesson 21
Shoulder, Posterior Scapular Region, Axilla
In this lesson, we will cover the shoulder joint and surrounding muscles, posterior scapular region (including rotator cuff muscles), and axilla. The shoulder is a very interesting joint from an evolutionary standpoint, since the head of the humerus is not tightly constrained to the joint capsule as the head of the femur is in the acetabulum of the pelvis. For this reason, it is relatively easy to "separate" your shoulder or otherwise injure your rotator cuff muscles, whereas it is harder (and more disastrous) to do the same at the hip joint. On the other hand, the "looseness" of the shoulder allows a really incredible range of motion at the shoulder joint which is a remnant of our ape ancestry, which allowed us to climb and swing from our upper limb.
Learning Objectives - By the end of this lesson, you should be able to ...
- Describe movements at the shoulder joint
- List and describe the bones and joints in the shoulder region
- List and describe the origins, insertions, innervations, and actions of the muscles in the shoulder joint and posterior scapular region
- List and describe the courses of nerves and vessels in the posterior scapular region, and understand how they enter and/or exit the axilla
- Define the borders of the axilla
- Briefly describe the contents of the axilla, including muscles, nerves, arteries, and veins
- Understand the brachial plexus
Movements of the Scapula and Glenohumeral Joint
- AB-duction/AD-duction: movements of the arm at the shoulder joint in the coronal plane. After about 90 degrees, the scapula itself rotates.
- Flexion/Extension: movement of the arm in the sagittal plane anterior to the body (flexion) and posterior to the body (extension).
- Medial Rotation/Lateral Rotation: a small amount of medial and lateral rotation of the humerus at the shoulder joint is possible.
- Retraction/Protraction: the scapula can be retracted (pulled posteriorly, as in upright rows) and protracted (sliding anteriorly and laterally along the rib cage).
- Circumduction: this movement is a combination of all of the above movements, as when you windmill your arms up and down around your head (think Pete Townsend from The Who).
Bones and Joints
- Clavicle: this "S"-shaped bone articulates with the manubrium of the sternum and the acromion process of the scapula.
- Scapula: the medial border of the scapula is straight and the lateral border is slanted superolaterally, so that the scapula overall has a triangular shape. The anterior border of the scapula rides up against the rib cage, so it is smooth, and the posterior surface has a scapular spine along the superior portion. A suprascapular notch in the superior border forms a very important exit pathway for the suprascapular nerve (see below). The glenoid cavity is on the lateral side of the scapula and is the point of articulation with the humerus. A supraglenoid tubercle and infraglenoid tubercle provide important attachment areas for the long head of the biceps brachii muscle and the long head of the triceps brachii muscle, respectively. As noted above, the acromion process articulates with the clavicle, and a coracoid process provides attachment area for two muscles running into the arm - coracobrachialis and the short head of biceps brachii.
- Humerus: the head of the humerus is round, and a laterally-positioned greater tubercle and anteriorly-positioned lesser tubercle provide attachment areas for muscles. The intertubercular sulcus in between the two tubercles forms the lateral wall of the axilla, as well as a groove through which tendons run from the shoulder into the arm.
- Sternoclavicular, Acromioclavicular, and Coracoclavicular joints: these three joints are between the sternum and clavicle and different parts of the scapula and clavicle, respectively, and anchor the upper limb to the thorax and scapula. The upper limb is not attached (via bony attachment, at least) to the rib cage.
- Glenohumeral Joint: this is the shoulder joint and, like most of the other joints we've discussed in this course, it is a synovial joint with surrounding fibrocartilage.
There are two sets of muscles surrounding the shoulder joint. The first set attaches the scapula to the trunk, humerus, and vertebral column, and includes the trapezius, deltoid, levator scapulae, rhomboid major and minor. The second set is composed of posterior scapular muscles, including supraspinatus, infraspinatus, teres major and minor, and the long head of triceps brachi.
- Trapezius: runs from cranium, nuchal ligament, and C7-T12 to insert onto the scapular spine, acromion process, and lateral 1/3 of the clavicle. Receives motor innervation from the accessory nerve (CN XI). Elevates the scapula and rotates it during AB-duction of arm above horizontal; also retracts and depresses the scapula.
- Deltoid: runs from inferior edge of the scapular spine, lateral margin of the acromion process, and the lateral 1/3 of the clavicle to the deltoid tuberosity of humerus. Innervated by axillary nerve. AB-ducts the arm after 15 degrees; in addition, assists in flexion and extension of the arm.
- Levator scapulae: runs from transverse processes of C1 to C4 vertebrae to the medial part of the scapula. Innervated by anterior rami of C3-C4 spinal nerves and C5 fibers from the dorsal scapular nerve. Elevates the scapula.
- Rhomboid major and minor: run from nuchal ligament down to T5 vertebrae to medial border of scapula. Elevates and retracts the scapula. Innervated by the dorsal scapular nerve. The levator scapulae, rhomboid minor, and rhomboid major attach to the medial border of the scapula all the way from its superior angle down to its inferior border, and connect the scapula to the vertebral column.
- Supraspinatus: runs from the medial 2/3 of the suprapinous fossa to the greater tubercle of the humerus. Innervated by C5 and C6 fibers of the suprascapular nerve. AB-ducts the arm up to 15 degrees.
- Infraspinatus: runs from medial 2/3 of the infraspinous fossa to the greater tubercle of the humerus. Innervated by C5 and C6 fibers of the suprascapular nerve. Laterally rotates the arm at the shoulder joint.
- Teres minor: runs from posterior surface of scapula near its lateral border to the greater tubercle on the posterior surface of the humerus. Innervated by C5 and C6 fibers of the axillary nerve. Along with infraspinatus, it laterally rotates the arm at the shoulder joint. The supraspinatus, infraspinatus, and teres minor muscles together, along with subscapularis (a muscle on the anterior surface of the scapula), are known as the rotator cuff. These muscles are also called the S.I.T.S. muscles (after the first letter of each muscle).
- Teres major: runs from posterior surface of the inferior angle of the scapula to the intertubercular sulcus on the anterior surface of the humerus. Innervated by the inferior (lower) subscapular nerve. Medially rotates and extends the arm at the shoulder joint. Note that teres minor, but NOT teres major, is a rotator cuff muscle.
- Long head of triceps brachii: runs from the infraglenoid tubercle on the scapula to the olecranon process of the ulna. Innervated by C6, C7, and C8 fibers from the radial nerve. Extends the forearm at the elbow joint, and aids in AD-ducting and extending the arm at the shoulder joint.
Nerves and Vessels in the Posterior Scapular Region
Nerves and vessels in the posterior scapular region pass in the (1) suprascapular foramen, which passes the suprascapular nerve (but not the suprascapular artery, which passes above the suprascapular foramen) from the neck into the posterior scapular region; (2) the quadrangular space, which is bordered by teres minor, long head of triceps brachii, teres major, and the humerus, and passes the axillary nerve and posterior circumflex humeral artery; (3) the triangular space, bordered by triceps brachii, teres major, and teres minor, and passes the circumflex scapular artery and vein; and (4) the triangular interval, bordered by teres major, triceps brachii, and the humerus, and passes the radial nerve and profunda brachii artery.
- Suprascapular nerve: originates from the superior trunk of the brachial plexus, passes through the suprascapular foramen, and curves around the scapular spine by running through the greater scapular notch. It innervates the suprapinatus and infraspinatus muscles.
- Suprascapular artery: runs from the thyrocervical trunk (a branch of the subclavian artery) and runs just above the suprascapular foramen to supply blood to the supraspinatus and infraspinatus muscles.
- Axillary nerve: runs from the posterior cord of the brachial plexus through the quadrangular space to wind around the surgical neck of the humerus.
- Posterior circumflex humeral artery: runs from the 3rd of the axillary artery through the quadrangular space to wind around the surgical neck of the humerus with the axillary nerve. Supplies adjacent muscles and the shoulder joint.
- Circumflex scapular artery: runs from the subscapular artery (a branch of the 3rd part of the axillary artery) through the triangular space and through the origin of the teres minor muscle to supply adjacent muscles.
Borders of the Axilla
The axilla is the gateway between the neck and upper limb. It is a pyramidal space with four sides, an inlet, and a floor, and is bordered by the clavicle, scapula, upper thoracic wall, humerus and surrounding muscles.
- Inlet: formed by the lateral margin of the 1st rib, posterior surface of the clavicle, and superior margin of the scapula as far laterally as the coracoid process. The brachial plexus and axillary artery and vein pass over the 1st rib in the axillary inlet.
- Anterior wall: formed by the deep surfaces of the pectoralis major muscle, the underlying pectoralis minor and subclavius muscles, and the clavipectoral fascia.
- Medial wall: formed by the upper thoracic wall and serratus anterior muscle (which runs from the upper 8-9 ribs to the medial border of the scapula, and is innervated by the long thoracic nerve).
- Lateral wall: formed by the intertubercular sulcus of the humerus.
- Posterior wall: formed by the the costal surface of the scapula and surrounding muscles, including subscapularis, teres major, latissimus dorsi, and the long head of triceps brachii.
- Floor: formed by fascia and skin.
Contents of the Axilla
- Muscles: the proximal parts of the biceps brachii and coracobrachialis muscles run through the axilla. The biceps brachii has a long head that originates on the supraglenoid tubercle of scapula and a short head that originates on the coracoid process. It runs from these origins to the radial tuberosity. Innervated by C5 and C6 fibers of the musculocutaneous nerve. Flexes the forearm at the elbow joint and supinates the forearm; in addition, assists in flexion of the arm at the shoulder joint. The coracobrachialis muscle runs from the coracoid process to the medial side of the humerus at midshaft. Innervated by C5, C6, and C7 fibers of the musculocutaneous nerve. Flexes and AD-ducts the arm at the shoulder joint.
- Axillary artery and its branches: the axillary artery is the continuation of the subclavian artery in the axilla, where it changes name after passing over the lateral margin of rib 1. The axillary artery can be divided into 3 parts by the pectoralis minor muscle (see, I told you this muscle would come up again!). The 1st part of the axillary artery is proximal to pectoralis minor, and it gives off one branch, the superior thoracic artery, which supplies upper regions of the medial and anterior axillary walls. The 2nd part lies deep to pectoralis minor, and it gives off two branches, the thoracoacromial artery and the lateral thoracic artery. The thoracoacromial artery supplies the anterior axillary wall via pectoral, deltoid, clavicular, and acromial branches. The lateral thoracic artery runs along the inferior margin of the pectoralis minor muscle and supplies the breast. The 3rd part of the axillary artery is distal to pectoralis minor, and it gives off three branches: the subscapular artery, anterior circumflex humeral artery, and posterior circumflex humeral artery. The subscapular artery runs along the subscapularis muscle a short distance before splitting into the circumflex scapular artery, which runs through the triangular space to supply the posterior scapular region, and the thoracodorsal artery, which supplies the posterior and medial walls of the scapula. The axillary artery continues as the brachial artery in the arm.
- Axillary vein: continuation of the basilic vein at the lower margin of teres major. It passes through the axilla medial and anterior to the axillary artery, and continues as the subclavian vein after crossing the lateral border of the first rib. The cephalic vein, which is a vein that drains the lateral and posterior parts of the hand, forearm, and arm and runs between the deltoid muscle and pectoralis major in the axillary region, drains into the axillary vein deep to the clavicular head of pectoralis major.
- Brachial plexus: formed by anterior rami of C5 through C8 and most of the anterior ramus of T1. Passes over the first rib to enter the axilla.
The brachial plexus can be organized into roots (anterior rami of spinal nerves), trunks, divisions, cords, and terminal nerves.
- Roots: the anterior rami of C5, C6, C7, C8, and part of T1 spinal nerves make up the brachial plexus. In addition, the dorsal scapular nerve, which innervates rhomboid major and rhomboid minor, comes off the C5 root, and the long thoracic nerve, which innervates serratus anterior, comes directly off of C5, C6, and C7 roots.
- Trunks: the roots of C5 and C6 come together to make the superior trunk, C7 continues as the middle trunk, and C8 and T1 come together as the inferior trunk. In addition, the suprascapular nerve and nerve to subclavius branch off of the superior trunk.
- Divisions: each of the trunks splits into an anterior division and posterior division. The posterior divisions of the three trunks form the posterior cord and ultimately the radial nerve, the anterior divisions of the superior and medial trunks make up the lateral cord, and the anterior division of the inferior trunk makes up the medial cord.
- Cords: so, to recap, the lateral cord is composed of the anterior divisions of the superior and middle trunks; the posterior cord is composed of the posterior divisions from all three trunks; and the medial cord is composed of the anterior division of the inferior trunk. In addition, the lateral pectoral nerve, which provides motor innervation to pectoralis major, branches from the lateral cord; the medial pectoral nerve, which provides motor innervation to pectoralis major and pectoralis minor, branches from the medial cord; and the axillary nerve (which provides motor innervation to deltoid and teres minor and sensory innervation to skin on the upper lateral part of the arm), thoracodorsal nerve (which provides motor innervation to latissimus dorsi), inferior (lower) subscapular nerve (provides motor innervation to teres major), and superior (upper) subscapular nerve (along with inferior subscapular nerve, provides motor innervation to subscapularis) branch from the posterior cord.
- Terminal nerves: the lateral cord splits to form the musculocutaneous nerve and 1/2 of the median nerve. Likewise, the medial cord splits to form the ulnar nerve and the other 1/2 of the median nerve. The radial nerve is the continuation of the posterior cord. Because of the structure of the brachial plexus, the musculocutaneous nerve is composed of C5 to C7 fibers; the median nerve is composed of C6 to T1 fibers (but can also have C5 fibers); the radial nerve is composed of C5 to T1 fibers; the axillary nerve is composed of C5 and C6 fibers), and the ulnar nerve is composed of C8 and T1 fibers.
Including this lesson, the final 7 lessons cover the upper and lower limbs.
- Human Anatomy Lesson 22
This lesson covers the muscles, nerves, and blood vessels that are in the arm.
- Human Anatomy Lesson 23
Lesson #23 covers the forearm, wrist, and hand
- Human Anatomy Lesson 24
Lesson #24 covers the gluteal region
- Human Anatomy Lesson 25
Lesson #25 covers the thigh
- Human Anatomy Lesson 26
Lesson #26 covers the leg and ankle
© 2014 Robert McCarthy