17 The Pectoral Girdle and Arm
Learning Objectives:
- Recognize the different categories of muscles with respect to movement.
- Describe the different types of muscle contractions
- Identify, describe, and palpate the joints of the pectoral girdle.
- Identify the rotator cuff muscles and describe their action, origin, insertion, and innervation.
- Identify and describe the posterior muscles acting on the shoulder girdle.
- Identify and describe the anterior muscles acting on the pectoral girdle.
Terms to Know
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Joints of the Pectoral Girdle
Muscles Acting on the Elbow
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Muscles Acting on the Pectoral Girdle
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Introduction
In this lab you will explore the joints and muscles of the pectoral girdle. As a study tip, you may find it helpful to write a description of each muscle/structure as you identify them. When you write it in your own words, it may spark your memory as you look for that structure the next time you study.
Lab Activities
Activity 1: Joints of the Pectoral Girdle and Shoulder
Examine the three joints of the pectoral girdle and shoulder and the structures listed with them.
- Observe the sternoclavicular joint. This is between the sternum and the clavicle. The sternoclavicular ligament helps to stabilize this joint. Palpate your own sternoclavicular joint.
- Observe the acromioclavicular joint. Palpate this joint on yourself, where the clavicle meets the acromion at the “point of your shoulder.” Observe the acromioclavicular, coracoacromial, and coracoclavicular ligaments. Notice how the supraspinatus runs underneath this ligament as it moves towards the greater tubercle.
- Observe the glenohumeral joint. Notice how smooth the articular cartilage on the head of the humerus is. Also, note how thick the articular capsule is. Lastly, observe the ring of tissue just surrounding the glenoid fossa. This is the glenoid labrum.
- Notice the scapulothoracic joint. This is not a true articulation. It is considered a functional joint between the scapula and the thoracic wall.
Activity 2: Muscles of the Pectoral Girdle and Shoulder
Explore the muscles of the pectoral girdle and arm listed below. You will be asked to identify the origins, insertions, actions, and/or innervations of these muscles listed in the muscle charts. However, you may not need to know certain specifics for each muscle. Follow what is on the posted muscle charts for the specifics of what you need to know for each muscle. It can be helpful for you to have the muscle charts with you while you are examining these structures. Looking at the orientation of muscle fibers and the origin/insertions can help you understand the actions of these muscles.
- Examine the trapezius. This muscle has fibers running superiorly, horizontally, and inferiorly from the vertebral column to insert onto the scapular spine, acromion, and clavicle.
- Observe the rhomboid major, rhomboid minor, and levator scapulae. The rhomboid major is larger and inferior to the rhomboid minor. The rhomboid minor is thinner and located between the rhomboid major and the levator scapulae. The levator scapulae is the most superior of these muscles. These muscles run from the vertebral column to the medial border or superior angle of the scapula.
- Observe the subclavius just inferior to the clavicle.
- Observe the supraspinatus muscle in the supraspinous fossa, just superior to the scapular spine. Inferior to the scapular spine, observe the infraspinatus in the infraspinous fossa. Just inferior to the infraspinatus, observe the teres minor. The teres minor, infraspinatus, and supraspinatus are all part of the rotator cuff group and are the three rotator cuff muscles located on the posterior aspect of the scapula. They all insert onto the greater tubercle of the humerus.
- Inferior to teres minor, observe the larger teres major. Also, observe the larger latissimus dorsi just inferior to teres major. The latissimus dorsi fans out over the back region, with fibers merging near its insertion at the intertubercular groove of the humerus. Now notice how the latissimus dorsi and teres major both run to insert on the anterior aspect of the humerus. This is important to appreciate so that you can understand how these muscles contribute to internal rotation of the glenohumeral joint.
- Observe the deltoid on the lateral aspect of the shoulder. This muscle wraps around the shoulder superiorly and comes together at a common insertion point on the humerus (the deltoid tuberosity). Notice how the orientation of the fibers changes as it wraps around the shoulder from anterior to posterior. This can help you to understand why the functions of the posterior, middle, and anterior fibers of the deltoid differ from each other.
- On the anterior aspect of the scapula, observe the subscapularis muscle. This is the fourth rotator cuff muscle, and it sits in the subscapular fossa. It inserts at the lesser tubercle of the humerus.
- Observe the muscle on the anterior aspect of the scapula running from the thoracic wall to the medial border of the scapula. This is the serratus anterior. Think about how its structure contributes to its function of stabilizing the scapula.
- Observe pectoralis major and pectoralis minor. Pectoralis major is larger and superficial to pectoralis minor. Pectorals major has an insertion on the humerus, while pectoralis minor inserts onto the coracoid process of the scapula. As a result, pectoralis major acts on the glenohumeral and scapulothoracic joints, while pectoralis minor only acts on the scapulothoracic joint.
- Observe the coracobrachialis. This muscle originates on the coracoid process of the scapula and inserts on the humerus. This muscle is normally pierced by a nerve, the musculocutaneous nerve, in the middle portion of the muscle belly.
- Now observe the biceps brachii. This muscle has two heads. The long head originates on the supraglenoid tubercle of the scapula. The short head originates from the coracoid process of the scapula.
- Just lateral to the biceps brachii, observe the brachialis. This muscle is the primary flexor of the elbow.
- On the posterior aspect of the arm, observe the triceps brachii. The long head is superficial and in the middle, and this is the only portion of the triceps brachii that crosses the glenohumeral joint. Also, observe the medial and lateral heads.
Activity 3: Radiology of the Pectoral Girdle and Shoulder
Open the Radiology of the Pectoral Girdle PowerPoint on canvas. Explore the structures listed above in the radiological images.
Wrap up and clinical connection
The clinical connection for today involves a couple of videos discussing the rotator cuff. Injuries to the rotator cuff are among the most common musculoskeletal conditions in people over 40 years old. An orthopedic surgeon reviews the rotator cuff muscles and treatments for rotator cuff injuries in the first video. The second video provides an in-depth review of the shoulder’s anatomy and thoroughly discusses rotator cuff tears.