The infraspinatus muscle extends between the scapula, glenohumeral joint capsule, and greater humerus. It is part of the striated (skeletal) musculature and is important for external rotation, abduction, and adduction of the arm. As part of the rotator cuff, it can be damaged if the cuff ruptures.
What is the infraspinatus muscle?
A person normally has 656 different muscles that control voluntary movements, reflexes, and other motor activities. The muscles responsible for voluntary movements are called skeletal muscles or striated muscles. Their name comes from the striated pattern of the muscle fibers, which can be seen under the microscope and is caused by filaments that are pushed into each other. These filaments are thread-like structures of myosin and actin/tropomyosin. The latter are attached to the Z-disks that mark the transverse segments (sarcomeres) within a myofibril. Many myofibrils are grouped into a muscle fiber and surrounded by a layer of connective tissue. Several muscle fibers in turn form a muscle fiber bundle that makes up the muscle as a whole unit. One of the skeletal muscles is the infraspinatus muscle. It is located on the back in humans and lies in the lateral shoulder region, where it participates in external rotation, abduction as well as adduction of the arm.
Anatomy and structure
The origin of the infraspinatus muscle is at the scapula. There, the organ is attached to the infraspinous fossa, which is a pit in the scapula, for movement. From there, the infraspinatus muscle extends over the lateral shoulder area to the upper arm, where it attaches to the greater tuberosity. The greater tuberosity is the large humeral hump located at the end of the upper arm bone (humerus) near the shoulder. It points posteriorly and laterally; at right angles from the greater tuberosity is the lesser tuberosity or small humerus. The infraspinatus muscle is part of the rotator cuff, which in addition to it includes the teres minor muscle, the subscapularis muscle, and the supraspinatus muscle. The suprascapular nerve is responsible for the nervous supply of the infraspinatus muscle. Other fibers of the nerve pathway also innervate the supraspinatus muscle, which is another shoulder muscle and is also responsible for external rotation and abduction of the arm. The suprascapular nerve also has sensitive fibers that transmit sensations from the shoulder joint capsule to the nervous system.
Function and tasks
Effectively, the infraspinatus muscle primarily causes external rotation of the arm. In addition, it participates in movements of the arm toward the limb axis (adduction) and away from the body (abduction). The suprascapular nerve tells the muscle whether to tense or relax. Electrical impulses travel along the axons of the nerve cells, which originate as projections from the cell body (soma). A sheath of myelin surrounds the axons and electrically insulates them from the surrounding tissue. This connection of axon and myelin sheath is what biology calls a nerve fiber. A nerve carries a large number of such fibers; the extensions of the suprascapular nerve terminate at the infraspinatus muscle, among others, where they form a motor end plate. At this station, the electrical action potential briefly transforms into a biochemical signal: the electrical stimulation causes bubbles (vesicles) in the motor end plate to release the neurotransmitters they contain. These neurotransmitters cross the short distance to the muscle fiber, where they encounter receptors in the membrane. A specific neurotransmitter – in this case acetylcholine – matches its associated receptor like a key in a lock, thereby opening ion channels in the muscle’s membrane. The incoming ions trigger the endplate potential in the muscle; the information now spreads out again in electrical form, using the sarcolemma and the T-tubules – microscopic structures in the muscle cell. In the sarcoplasmic reticulum, its path continues so that the signal spreads throughout the muscle fiber. Calcium ions stored in the sarcoplasmic reticulum then leave the membrane system. They bind to the fine fibrils within the muscle fiber and cause them to push into each other, shortening the muscle and initiating the intended movement of the arm.
Diseases
As part of the rotator cuff, the infraspinatus muscle plays a role in injuries that affect this overall structure. For example, a common injury to the rotator cuff is a rupture. It is characterized by the tearing of tendons and/or muscles and is particularly common after a fall on the arm. Additionally, the shoulder joint may dislocate; the rupture causes the rotator cuff to be unable to adequately support the joint, facilitating dislocation of the joint. Dislocation can also occur simultaneously during injury. In addition to acute rupture, degenerative rotator cuff rupture is possible: it occurs when the joint becomes progressively closed with age, and degenerative joint disease can exacerbate or accelerate this process. Depending on the type of injury and individual conditions, various options can be considered to treat a rotator cuff rupture. Within the framework of conservative therapy, medications on the one hand and physiotherapeutic measures on the other hand can be applied. Surgical interventions, on the other hand, aim to surgically repair the damage to the rotator cuff. Which measures are necessary and possible in individual cases depends on numerous influencing factors. As with all medical interventions, the decision is therefore up to the treating physician. In addition to direct injuries to the infraspinatus muscle, damage to the suprascapular nerve can also lead to functional impairment of the muscle, since this nerve is responsible for controlling the infraspinatus muscle.
