The teres major muscle is one of the skeletal muscles that humans can control voluntarily and forms part of the rotator cuff. It extends from the lower edge of the scapula to the upper arm and participates in the movements of the arm.
What is the teres major muscle?
On the back is located the teres major muscle, whose name means as much as “large round muscle“. Its origin is at the lower edge of the scapula (at the angulus inferior scapulae) and it attaches to the humerus. The insertion of the teres major muscle is located on the crista tuberculi minoris on the front side of the bone, which is also the insertion point of the latissimus dorsi muscle. The teres major muscle belongs to the skeletal musculature and consists of striated fibers, the pattern of which is based on the structure of the muscle. Within the skeletal musculature, the teres major muscle can be classified as part of the shoulder muscles. According to some definitions, it is part of the rotator cuff, while others consider it merely a secondary stabilizer of the cuff. Conscious control of the teres major muscle originates in motor areas of the brain and proceeds via sympathetic nerve fibers.
Anatomy and structure
Nerve pathways supplying the teres major muscle pass through the spinal nerves of the neck. The commands for contraction and relaxation originate mainly from the subscapular nerve, which also supplies neural stimuli to the subscapular muscle. Less frequently, the teres major muscle also receives nerve signals from the thoracodorsal nerve, which belongs to the brachial plexus and also controls the large back muscle (latissimus dorsi muscle), or from the axillary nerve, which belongs to the same nerve plexus and is mainly responsible for the deltoid and teres minor muscles. The tendon of the teres major muscle is 5 cm long and connects to the humerus via a bursa synovialis, which reduces friction. The muscle is surrounded by a sheath of connective tissue; its internal structure consists of bundles of muscle fibers, each of which combines several muscle fibers. The muscle fibers make up the muscle cells, but they are not demarcated from one another like other cells in the body. Instead, they form a continuous tissue with many cell nuclei. Running longitudinally through the muscle fibers are myofibrils, whose sections (sarcomeres) are composed of actin/tropomyosin and myosin filaments.
Function and tasks
At the junction of a motor nerve fiber and muscle is the motor end plate. The electrical action potential from the axon of the nerve cell triggers the release of neurotransmitters here, as in an interneuronal synapse. These neurotransmitters (often acetylcholine) evoke the endplate potential at the muscle, which propagates across the cell membrane of the muscle cell, channels in the cell (T-tubules) and the guidance system of the sarcoplasmic reticulum. The sarcoplasmic reticulum releases calcium ions in response to the electrical potential, whereupon the filament-like myofilaments push into each other, shortening the muscle. When the nerve no longer excites the muscle, the electrical endplate potential also falls away, the calcium ions remain in the sarcoplasmic reticulum, and the muscle relaxes again. The function of the teres major muscle is to move the arm in certain directions; in doing so, it participates in internal rotation, which turns the arm inward, and retroversion, which pulls it backward. The great round muscle is also active in movements of the upper arm toward the body (adduction). The latissimus dorsi muscle is also involved in these movements. In addition, the teres major muscle, together with the pectoralis major muscle and the latissimus dorsi muscle, stabilizes the shoulder.
Diseases
As part of the rotator cuff, the teres major muscle can be affected by medical conditions that affect this anatomic structure. In rotator cuff rupture, the tendon that connects the muscle to the bone tears. The rupture triggers pain and hinders the mobility of the arm-shoulder part. As one of the stabilizing muscles of the cuff, the teres major muscle is of great importance in rehabilitation after rotator cuff injuries, as it can relieve the damaged structure. This case often occurs in impingement syndrome, when the tendon of the shoulder muscle is pinched.In myofascial pain syndrome, the muscle hardens as a result of a persistent state of tension. The tension at the teres major muscle can affect the mobility of the shoulder muscle and thus the mobility of the arm. The hypertonus also causes pain, especially during movements and pressure on the affected area. Such a trigger point is caused, for example, by overloading and fine tears in the muscle. Movement pain can also be due to other causes, for example, bursitis. Depending on the severity, it manifests itself in weaker or stronger pain, and the area may be locally heated or swollen. Fluid may also collect in the tissue. Occasionally, control of the teres major muscle does not originate from the subscapular nerve but from the axillary nerve. This nerve runs along the axilla near the humerus – a site that is highly susceptible to fracture. If the humerus bone is fractured, the surrounding tissue can suffer damage and the axillary nerve may also be affected. Lesions to the nerve are also possible when the shoulder dislocates (luxates). Regardless of the cause, injuries to motor nerve fibers controlling the teres major muscle can limit the muscle’s ability to move. This is also true for the other two nerves (subscapular nerve and thoracodorsal nerve).