The scapulohumeral reflex is an intrinsic reflex of the scapulohumeral musculature. A blow to the medial edge of the scapula causes reflex adduction and external rotation of the arm. Changes in reflex movement refer to central or peripheral nerve lesions.
What is the scapulohumeral reflex?
The scapulohumeral reflex is an intrinsic reflex of the scapulohumeral muscles. The scapulohumeral muscles connect the humerus to the scapula. The muscle group consists of a total of seven muscles of the shoulder and shoulder girdle muscles. In addition to the hooked arm muscle (Musculus coracobrachialis), the deltoid muscle (Musculus deltoideus), and the lower limb muscle (Musculus infraspinatus), the scapulohumeral group includes the lower shoulder blade muscle (Musculus subscapularis), the upper limb muscle (Musculus supraspinatus), the small round muscle (Musculus teres minor), and the large round muscle (Musculus teres major). The scapulohumeral reflex is a monosynaptic intrinsic reflex of this muscle group. The reflex corresponds to a stretch reflex whose afferent and efferent pathways are located in the same organ. A blow to the medial scapular border triggers the automatic movement. With mediation of spinal cord segments C4 to C6, adduction and external rotation of the arm in the shoulder joint occurs via the axillary nerve and suprascapular nerve. Both the shoulder girdle and shoulder muscles of the scapulahumeral muscle group are involved in the reflex movement.
Function and task
Each reflex involves a reflex arc. These arcs are composed of what is called an affector and an effector. The effector is the sensitive pathway of the reflex arc. It registers the stimuli that trigger the reflex movement. The effector is a motor pathway that implements the movement. In muscle intrinsic reflexes, the two pathways are located in the same organ. The triggering stimulus is thus detected at the same body site where the movement is executed at the end of the reflex arc. Affector and effector of the scapulohumeral reflex are the axillary nerve and the suprascapular nerve. The axillary nerve is a mixed nerve that originates at the brachial plexus to the posterior fasciculus. The axillary nerve is connected by fibers to the C5 and C6 spinal cord segments. It runs together with the arteria circumflexa humeri posterior and the vena circumflexa humeri posterior close to the joint capsule at the collum surgicum of the humerus. Across the lateral axis gap, it is bordered on the side by the humerus through the long head of the triceps, crossing the teres major muscle to reach the deltoid muscle. Before the nerve crosses the axillary gap, it delivers the cutaneus brachii lateralis superior sensory nerve through the fascia into the subcutaneous tissue of the lateral shoulder. Motorically, the nerve innervates the shoulder muscles deltoid and teres minor muscles. The axillary nerve sensitively innervates the skin of the lateral shoulder region. The mixed nerve suprascapularis is also important for the scapulohumeral reflex. It arises at the brachial plexus from the superior truncus and is connected by fibers to the C4, C5, and C6 spinal cord segments. From the junction of the cervical nerve, it runs laterally under the trapezius muscle and the omohyoideus muscle. Through incisura scapulae of the scapula, it enters the supraspinous fossa. Here it crosses the ligamentum transversum scapulae superius and continues under the supraspinatus muscle. To this muscle it gives off several branches and from there reaches the lateral border of the spina scapulae. The nerve innervates the infraspinous fossa muscle, the supraspinatus muscle, the deltoid muscle and the teres minor muscle motorically. Its sensitive branches run around the shoulder joint and lie in the skin of the lateral shoulder region. During the scapulohumeral reflex, the afferent nerve senses stretch at the contractile midpoint of the muscle spindle fibers. An action potential is thus generated in the afferent Ia fibers, which travels via the spinal nerve to the posterior horn of the spinal cord. There, the signal is monosynaptically transmitted to the α-motoneurons, which initiate the skeletal muscle fibers of the scapulohumeral muscle group. Negative feedback maintains a constant muscle length during the stretch reflex.
Diseases and disorders
The scapulohumeral reflex has medical relevance as a symptom of lesions in the peripheral and central nervous systems.During the reflex examination, the neurologist checks the integrity of the nerve guidance pathways and, if necessary, determines the localization of a neurological lesion. Such a lesion is present, for example, in the case of paralysis of the scapulohumeral muscles, as may be present due to damage to the C4, C5 and C6 spinal cord segments or to the axillary and suprascapular nerves. As a monosynaptic intrinsic reflex, the scapulohumeral reflex has only a short latency and cannot be fatigued. Therefore, if it can no longer be triggered or can only be triggered with a long delay, there must be a nerve injury that preferably affects the peripheral nervous system. Polyneuropathies, for example, can bring the reflex to a standstill. Such disorders affect the peripheral nervous system and may present as a result of malnutrition, infection, poisoning, or reclined nerve injury. On the other hand, paralysis of the axiliar or suprascapular nerve may be traumatic or caused by neuritis. On the other hand, when lesions are present in spinal cord segments C4 to C6, the scapulohumeral reflex is usually exaggerated. When there is damage to the 1st motoneuron in the pyramidal system, intrinsic reflexes such as the scapulohumeral reflex occur in muscles in which they are not actually observed. An exaggerated scapulohumeral reflex is therefore interpreted as a pyramidal tract sign and may present in the context of diseases such as ALS or multiple sclerosis, both of which affect the central nervous system.
