UPPER LIMB MUSCLES (2/6) - SHOULDER REGION

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Shoulder muscles are categorized as intrinsic and extrinsic. This is based on their attachment points. The scapula, clavicle, and humerus are the bones of the shoulder. The intrinsic muscles originate from the scapula and/or clavicle and attach to the humerus. Together with the coracobrachialis, which is an upper arm muscle, the intrinsic muscles compose the scapulohumoral group. Meanwhile, the extrinsic ones originate from the torso, and attach to the clavicle, scapula, or humerus.
Let’s discuss the intrinsic muscles first. There are 6 – the deltoid, teres major, and 4 rotator cuff muscles - the teres minor, subscapularis, supraspinatus, and infraspinatus.
The deltoid has 3 heads – anterior, intermediate or middle, and posterior, which converge at their attachment point to the humerus at the deltoid tuberosity. These 3 heads have different origins and different actions. The anterior fibers arise from the anterior border and upper surface of the lateral third of the clavicle. The intermediate fibers originate from the lateral border of the acromion. Finally, the posterior fibers originate from the lower lip of the posterior border of the spine of the scapula. The anterior fibers assist the pectoralis major, causing flexion and medial rotation. The intermediate fibers take over arm abduction after the supraspinatus abducts the first 15 degrees. Finally, the posterior fibers cause extension and lateral rotation.
When all 3 heads of the deltoid contract simultaneously, the deltoid is the prime mover of arm abduction along the frontal plane.
Teres major is on the inferior border of the quadrangular space, which is found between teres minor and major, the long head of triceps brachii, the humerus, and the subscapularis. It originates from the posterior surface of the inferior angle of the scapula and attaches to the medial lip of the intertubular groove of the humerus. Teres major adducts and extends the humerus, and medially rotates the humerus. It also helps stabilise the humeral head in the glenoid cavity.
Now we come to the 4 rotator cuff muscles, which all originate from the scapula and attach to the humerus. In addition to their individual actions, these muscles together pull the humeral head into the glenoid fossa, which is in the scapula. This provides stability to the glenohumeral joint.
Teres minor originates from the middle half of the lateral border of the scapula and attaches to the inferior facet of the greater tubercle of the humerus. It laterally rotates the arm.
Subscapularis originates from the subscapular fossa on the costal surface of the scapula and attaches to the lesser tubercle of the humerus. It medially rotates the arm.
Supraspinatus originates from the supraspinous fossa of the scapula and attaches to the superior facet of the greater tubercle of the humerus. It abducts the arm up to 15 degrees and then helps the deltoid in abduction from 15-90 degrees.
Infraspinatus originates from the infraspinous fossa of the scapula and attaches to the middle facet of the greater tubercle of the humerus. It is the main external rotator of the shoulder. It also adducts the inferior angle of the scapula. Note that this muscle works with the deltoid and teres minor.
Now for the extrinsic shoulder muscles. These muscles originate from the torso and attach to the scapula, clavicle, or humerus. The extrinsic shoulder muscles can be organized into 2 layers – deep and superficial.
The deep muscles include the rhomboids and the levator scapulae. The rhomboid minor is above the rhomboid major. Rhomboid minor originates from the spinous processes of C7-T1, major from the spinous processes of T2-T5 vertebrae. Both rhomboids attach to the medial border of the scapula and serve to retract and rotate the scapula.
The levator scapulae originate from the transverse processes of the C1-C4 vertebrae and attach to the medial border of the scapula. They elevate the scapulae.
The superficial muscles include the trapezius and latissimus dorsi. The trapezius lies overtop of the rhomboids and the levator scapulae. The trapezius originates from the skull, the nuchal ligament, and the spinous processes of C7-T12. The fibers attach to the clavicle, acromion, and the scapula spine. The trapezius has 3 heads – upper, middle, and lower. The upper fibers elevate the scapula. The middle fibers retract the scapula, and the lower fibers depress the scapula.
The latissimus dorsi originates from the spinous processes of T7-T12, the iliac crest, the thoracolumbar fascia, and the inferior three ribs. The latissimus dorsi’s fibers converge into a tendon attaching to the intertubular sulcus of the humerus. The latissimus dorsi causes extension of the humerus, medial rotation of the humerus, and it causes adduction of the humerus, together with teres major and pectoralis major.

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AMAZINGGGG! First year medico's life saver! It has 3D animations, points of attachments and actions!! Like...how cool is that?! Thank you!🎉

sakthipriyakurmapu

This is a very effective video to me and I get to remember it easily too which is incredible. I can't wait for another video to be uploaded. And thank you for your hard work. Thank you.

dreamies

Please please continue making such videos. I always find the topic of muscle origin, insertion and action very difficult to visualise but u made it easier. Thank u so much.

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