The radius periosteal reflex is an intrinsic reflex of the human body. Normally, a blow to the arm triggers a slight flexion of the forearm; if the reflex is absent, this may indicate a neurological or muscular disorder.
What is the radial periosteal reflex?
The radius periosteal reflex is an intrinsic reflex of the human body. Normally, a blow to the arm triggers a slight flexion of the forearm. The radial periosteal reflex or brachioradialis reflex is an intrinsic reflex of the arm. Medicine calls a reflex an intrinsic reflex when stimulation and reaction occur in the same organ. The radius periosteal reflex is triggered by a blow to the radius. The radius is a bone of the forearm, which is also called the radius. The radius is a so-called tubular bone: The medullary cavity of the bone forms a uniform tube inside, which contains the bone marrow. Together with the ulna (ulna), the radius forms the skeleton of the forearm. To trigger the radius periosteal reflex, the forearm must be bent in relation to the upper arm. It must be rotated neither outward nor inward. A position that is rotated outward is referred to in medicine as suspension, while the position of the forearm that is rotated inward is referred to as pronation. A blow to the radius triggers the radius periosteal reflex and consequently causes pronation of the hand and forearm.
Function and task
Underlying the radius periosteal reflex is a simple neurological circuit. Receptors on the forearm register impact against the bone: The mechanical stimulation causes the sensory neuron to fire, i.e., to produce a signal. This happens by changing the electrical charge of the neuron. Changes in the cell membrane shift the ionic ratio between the inside and the outside of the cell, and the neuron depolarizes. As a summed electrical potential, the neuron transmits the stimulation through its axon. At the end of the neuron is an interface between the first and a second cell. The natural sciences refer to this interface as the synaptic cleft. The signal travels across the synaptic cleft by first being translated into a chemical form: The electrical voltage of the first nerve cell causes the release of neurotransmitters. Neurotransmitters are messenger substances that enter the synaptic cleft and reach the second nerve cell at its other end. There, the neurotransmitters bind to specialized receptors, to which they fit like a key in a lock. The occupied receptors now also trigger a change in the electrical charge in the second nerve cell by opening ion channels in the cell membrane: The second neuron depolarizes and the information about the stimulation is successfully transferred to the second neuron. In the radius periosteal reflex, this connection is monosynaptic: only a single synapse is involved in the transmission of the signal from the receptor to the spinal cord. Biology also refers to this conduction of signals as afferent, from the Latin word for “to carry” (“affere”). On the opposite pathway, the efferent (“carrying out”) nerve pathway, the motoneuron then sends the signal for muscle contraction. This signal is directed to the brachioradialis muscle. The humeral radius muscle is a skeletal muscle located in the upper arm and directed toward the thumb-facing side. Contraction of the humeral radius muscle shortens the associated tendon and flexes the forearm. Neurology refers to the nerve pathways responsible for the radial periosteal reflex as numbers C4 and C6. In addition, the radial nerve also participates in the neuronal transmission of information. Like all intrinsic reflexes, the radial periosteal reflex occurs without brain involvement; therefore, humans cannot consciously control, suppress, or willfully trigger it.
Diseases and complaints
When checking the radial periosteal reflex, physicians compare whether the response is the same on both sides. This comparison minimizes misdiagnosis because it allows the examination to account for interindividual differences in responsiveness. The absence of the radial periosteal reflex may indicate radialis palsy. This is a paralysis of the arm that affects the wrist and finger extensors.This is caused by damage to the nerves of the upper arm, in particular the radial nerve. Radialis palsy manifests itself in a characteristic finger posture: The relatively high muscle tension slightly flexes the muscles of the fingers and wrist, giving the impression that the affected person wants to point at something or hold out the hand for a kiss. For this reason, the vernacular also calls this posture kissing or drop hand. The cause of radialis paralysis is often a fracture of the upper arm or other severe mechanical impact. It can also occur, for example, when the affected person lies immobile on his or her side for a long time, as is the case during anesthesia or in bedridden patients. In this case, the body weight presses on the radial nerve for a long time, possibly causing damage. Treatment of radial nerve palsy depends on the causes in each individual case; the damaged nerve may be able to be reconstructed or regenerate itself with adequate rest. The absence of the radial periosteal reflex also potentially represents the symptom of a myotonic disease. This is a group of various muscle disorders for which prolonged muscle tension and delayed relaxation are typical. This group includes various syndromes of muscle wasting. Myotonic dystrophy type 1, for example, is a genetic disease characterized by muscle weakness, cardiac arrhythmia and hormonal abnormalities. In it and similar diseases, the main focus is on treating the symptoms.
