Musculus Teres Minor: Structure, Function & Diseases

The teres minor muscle is a skeletal muscle that belongs to the shoulder musculature. It forms part of the rotator cuff, which holds the upper arm bone (humerus) to the shoulder. Damage to the teres minor muscle or its nerve can affect the stability of the cuff and increase the likelihood of shoulder dislocation (luxation).

What is the teres minor muscle?

The teres minor muscle is a striated skeletal muscle that is subject to voluntary control in humans. It extends between the edge of the scapula and the humerus and is part of the rotator cuff (muscle-tendon cap), which attaches the humerus to the shoulder and stabilizes the joint. The shoulder joint needs this additional support because it has only a relatively shallow socket from which the condyle can easily pop out. Dislocations are therefore particularly common at this joint. Together with the teres major muscle, the teres minor muscle also encloses the axial gap. It is also known as the small round muscle and is part of the shoulder musculature. Above it lies the deltoid muscle, which extends as a triangle between the clavicle, scapula and humerus.

Anatomy and structure

The teres minor muscle originates from the scapula, where the margo lateralis scapulae is the outer edge of the bone. On the upper arm, the muscle attaches to the humerus. There, a larger protrusion is located laterally in the bone, which anatomy calls the tuberculum majus humeri. This is also where the infraspinatus and supraspinatus muscles attach, which, like the teres minor muscle, belong to the rotator cuff. The fourth muscle in this unit is the subscapularis muscle; however, this muscle does not attach to the humerus tuberculum majus, but to the tuberculum minus, a smaller projection of the humerus. Between the two bony prominences runs a pit in which the tendon of the biceps brachii muscle finds support. The teres major or great round muscle is also important for shoulder stability. Like the teres minor muscle, it is also responsible for a number of arm movements. The teres minor muscle receives the command to contract via the axillary nerve, which also innervates the teres major muscle as well as the deltoid muscle.

Function and Tasks

The teres minor muscle is a striated muscle and consists of a large number of muscle fibers, which in turn are grouped into bundles. A muscle fiber represents a muscle cell, but unlike other cells, it contains multiple nuclei because the classic unit with one nucleus in a membrane-enveloped cell does not exist in muscle tissue. Instead, the fine structure within the muscle fiber forms myofibrils that run longitudinally through the fiber. Their transverse sections (sarcomeres) are characterized by an alternation of actin/tropomyosin filaments and myosin filaments. Z-discs demarcate the sarcomeres from each other. When the muscle contracts, the fine filaments of the striated muscle push into each other; the myosin filaments have heads with which they can dock to the complementary filament. When they subsequently fold over, they pull the filaments together, thereby shortening the length of the muscle fiber. This process is made possible by calcium ions that originate from the sarcoplasmic reticulum. The sarcoplasmic reticulum is a tubular system that surrounds the myofibrils in the muscle fiber. When an electrical nerve signal (action potential) reaches the muscle, it first crosses a synapse and triggers the so-called endplate potential in the muscle: an electrical charge shift in the muscle cell. This endplate potential propagates through the sarcolemma, the T-tubules and finally the sarcoplasmic reticulum. Nerve cells whose job is to control muscles are called motoneurons. They stimulate not only a single muscle fiber, but several at the same time. The ratio varies from muscle to muscle: fine movements require a lower ratio than coarse ones; for example, on the biceps, one motoneuron excites about 700 muscle fibers. Contractions of the teres minor muscle participate in various arm movements. The muscle is active when a person pulls the previously splayed arm back toward the trunk (adduction) and when they rotate it outward (external rotation).In addition, the teres minor muscle participates in retroversion; this movement extends the arm backward away from the body.

Diseases

Complaints related to the teres minor muscle often occur as part of problems with the rotator cuff. Rotator cuff tears are tears of the tendons that attach the muscles of the cuff to the bone. Basically, both a single tendon and several tendons can tear at the same time. The tendon of the supraspinatus muscle is particularly frequently affected. Functional impairment of the teres minor muscle can also result from lesions to the axillary nerve, which supplies the muscle with neuronal signals. One possible cause of damage to the axillary nerve is a fracture of the humerus at the collum surgicum. This site breaks particularly easily and can also damage the axillary nerve in the process. Lesion of the nerve is also possible during bone healing: to repair the fracture, the body forms new bone tissue that establishes a callus over the fracture site. In addition, dislocation can damage the axillary nerve if it is overextended during dislocation of the joint. In either case, impairment of the axillary nerve causes the nerve pathway to fail to provide motor signals to the teres minor muscle and other muscles as usual.