The biceps tendon reflex is an innate and monosynaptic intrinsic reflex that belongs to the stretch reflexes. Reflectively, the biceps muscle contracts after a blow to the biceps tendon, thereby flexing the forearm at the elbow joint. The biceps tendon reflex may be altered in peripheral and central nerve damage.
What is the biceps tendon reflex?
The biceps tendon reflex is classified among the innate reflexes and corresponds to a stretch reflex to protect associated structures. The biceps brachii muscle is a two-headed humeral muscle with two joints. The associated tendon is the biceps tendon. The reflex contraction of the biceps muscle following a blow to the biceps tendon is called the biceps tendon reflex. Motor reflexes of the human body are either foreign or intrinsic reflexes. The biceps tendon reflex is an intrinsic reflex. It thus has its afferent and efferent pathways in the same organ. It is triggered directly at the site of reflex response, so to speak, and is monosynaptic. Reflex contraction of the biceps brachii muscle causes the forearm to flex at the elbow joint. The effector and receptor for this reflex are located in the musculocutaneous nerve. The nerve mediates the reflex response via motoneurons in spinal cord segments C5 and C6. The biceps tendon reflex is classified as an innate reflex and corresponds to a stretch reflex to protect associated structures.
Function and task
The biceps brachii muscle, which has two limbs, passes over the shoulder joint and the joint of the elbow. The muscle is a flexor muscle and flexes the forearm at the elbow via contraction. The origin of the long muscle part is the supraglenoid tuberosity at the scapula. The short muscle head originates at the coracoid process. The tendinous insertion is the radial tuberosity of the radius and fascia on the forearm. The original tendon of the longer head passes through the humeral sulcus intertubercularis and the joint capsule in the shoulder joint to the supraglenoid tuberosity. There it is surrounded by the vagina synovialis intertubercularis. The musculocutaneous nerve arises from the brachial plexus of spinal cord segments C5 to C6 and C7. This nerve innervates the biceps muscle, thus tethering it to the nervous system. The musculocutaneous nerve is a mixed nerve that innervates its supply area both sensitively and motorically. Motorically, the nerve innervates the upper arm muscles Musculus coracobrachialis, Musculus brachialis and Musculus biceps brachii. Sensitively, it innervates the joint capsule in the elbow joint and some skin sections of the radial side on the forearm. This mixed innervation allows the nerve to serve as both effector and receptor in the biceps tendon reflex. The stretch receptors of the sensitive sections register the stretch that the biceps tendon and muscle spindle undergo in one stroke. This stretch information is reported to the spinal cord, where they receive motor reflex responses. The motor portions of the musculocutaneous nerve relay this information to the biceps muscle, initiating the reflex contraction. Interconnection via the spinal cord ensures a rapid reflex response. The sensitive afferents of the biceps tendon reflex are located at the contractile center of the biceps muscle spindle fibers. An action potential is generated in these fibers during stretch, which is transmitted to α-motoneurons via a single synapse in the anterior horn of the spinal cord. The motoneurons cause contraction of skeletal muscle fibers in the biceps. Negative feedback maintains a fixed muscle length during reflex movement regardless of any interference. Because the reflex response is intended to protect the muscle, a high conduction velocity is imperative for the success of the reflex movement. The conduction velocity of α-motoneurons is approximately 80 to 120 ms-1.
Diseases and disorders
The physician examines the biceps tendon reflex as part of the reflex examination or neurologic diagnosis. The reflex can be elicited while the patient is sitting or lying down. The slightly bent forearm of the patient is stabilized by the physician. With the reflex hammer he strikes lightly on the biceps tendon in the elbow. He performs this procedure on both sides and observes the reflex response by comparing the sides. If the biceps tendon reflex behaves abnormally on one or both sides, various nerve damage is a possible cause. The reflex is either diminished or exaggerated.For example, if the biceps muscle does not contract or shows diminished response after striking the tendon, peripheral nerve injury is likely the cause. Nerve injuries in the peripheral nervous system can be caused by accidental trauma. Nerve disease may also be to blame for the decreased reflex response of the humeral muscle. For example, a conceivable disease would be polyneuropathy, which is often triggered by malnutrition, a poisoning condition, or an infectious disease. If the biceps tendon reflex is not absent but pathologically increased, then a lesion of the pyramidal tracts in the spinal cord is probably responsible for the altered reflex behavior. The pyramidal tracts connect the central motoneurons and control voluntary and reflex motor activity. When this area is damaged, so-called pyramidal tract signs appear. To confirm the suspected diagnosis of pyramidal damage, the physician tests the patient not only for the biceps tendon reflex, but also for pathological reflex movements from the Babinski group. If these are present, he assumes damage to the central motor neurons. Such damage can occur in the context of diseases such as multiple sclerosis or ALS. In MS, the patient’s own immune system causes inflammatory lesions in the central nervous system. ALS, on the other hand, is a degenerative disease that specifically degrades the motor nervous system. A slightly increased biceps tendon reflex does not necessarily have pathologic disease value, but may also be related to a physiologically vivid reflex response in the patient.
