By the abdominal skin reflex, neurologists mean an extraneous reflex that causes the abdominal muscles to contract when the abdominal skin is brushed. The polysynaptic reflex is wired through the spinal cord, and its absence may indicate pyramidal damage. Multiple sclerosis is a possible cause of such a lesion.
What is the abdominal skin reflex?
Neurologists refer to the abdominal skin reflex as an extraneous reflex that causes the abdominal muscles to contract when the abdominal skin is brushed. Humans are equipped with a variety of reflexes. They are all triggered by certain stimuli. Which perceptual organ is associated with a reflex depends on the type of reflex. Reflexes are divided into two main groups: intrinsic reflexes and extrinsic reflexes. Unlike intrinsic reflexes, the affectors and effectors in extrinsic reflexes are not located in the same organ. Effectors and afferents include the descending and ascending nerve fibers. Effectors carry action potentials toward a specific stimulus, that is, toward the central nervous system, while effectors carry stimuli away from the central nervous system. The abdominal skin reflex is one of about ten extraneous reflexes in humans. The reflex is triggered by brushing the abdominal wall and is an important part of the neurological examination. The site of stimulation of the abdominal skin reflex is the skin. Its target organ is the abdominal wall muscles. Neurology distinguishes the abdominal skin reflex into an upper, middle and lower reflex. Neurological reflex examination checks the abdominal skin reflex at all three levels.
Function and task
Like all motor reflexes, the abdominal skin reflex is connected to the pyramidal system in the spinal column. This interconnection ensures a particularly rapid reflex response, as is critical for the function of all reflexes. The thoracic nerve pathways Th8 to Th12, which are interconnected in the pyramidal system of the spinal cord, are involved in the reflex movement of the abdominal skin reflex as triggering pathways. As a foreign reflex, the abdominal skin reflex is a polysynaptic reflex whose reflex arc occurs across multiple synapses. The sensory cutaneous nerves are involved in the reflex as triggering nerves. The motor nerves of the abdominal muscles are the organ of success and initiate the contraction movement of the muscles. The smooth muscles of the abdominal wall consist of the rectus abdominis muscle, the transversus abdominis muscle and the obliquus externus abdominis muscle. These three muscles belong to the skeletal musculature and are innervated by motor nerves, which in the case of the abdominal skin reflex receive a potential from the spinal cord and contract in response. During the neurological reflex examination, the patient places himself in the supine position. The physician swipes a needle briskly from the lateral abdominal skin toward the center. This stroking motion occurs from both sides and is performed at three different heights of the abdominal skin. Usually, the reflex is tested at the level of the costal arch as well as at the level of the umbilicus and above the groin. If the reflex is obtained, the abdominal muscles contract at all three levels. Like all other reflexes, the abdominal skin reflex occurs completely involuntarily and has primarily protective functions. Contraction of the abdominal wall muscles hardens the abdominal wall and thus protects the internal organs. Like all external reflexes, the abdominal skin reflex belongs to the exhaustive reflexes. Thus, the reflex movement cannot be triggered any number of times in succession. Age is also related to the reflex. In terms of age physiology, the abdominal skin reflex can therefore no longer be triggered in some patients of advanced age. In the course of reflex examination, the physician must consider these parameters in order to correctly interpret an absent abdominal skin reflex.
Diseases and complaints
On a flaccid abdominal wall, the abdominal skin reflex cannot be triggered at all or only with difficulty. The same is true on an obese abdominal wall. If abdominal skin reflexes cannot be triggered on a physiologically normal abdominal wall, this is interpreted as a so-called pyramidal tract sign. Pyramidal tract signs refer to damage to the pyramidal tract, which connects the first and second motoneurons in the spinal cord. Other pyramidal tract signs are the reflexes of the so-called Babinski group.These are pathological reflex movements that cannot be triggered in healthy people and refer to damage to the motoneurons themselves. The abdominal skin reflex is often absent unilaterally or bilaterally in multiple sclerosis (MS) patients. Immunologically induced inflammation demyelinates the central nervous system in this disease, thus depriving the nerves of conduction. In an acute MS flare-up, sensitivity disorders of the abdominal skin can also be the cause of the absent reflex. If the receptors of the abdominal skin do not register any excitation, they cannot transmit the triggering information to the central nervous system and the motor reflex response is accordingly absent. Thus, an extinguished abdominal skin reflex is not necessarily associated with a pyramidal lesion, but may also be due to a peripheral lesion, such as may occur in the setting of a polyneuropathy. Polyneuropathies can be caused by malnutrition, nerve damage in the peripheral nervous system, or infection and poisoning. If spinal cord damage is indeed causally related to an absent abdominal skin reflex, these lesions may also have been caused by a spinal infarction or a degenerative disease of the central nervous system, such as ALS, and are often accompanied by spastic paralysis. In most cases, an increased abdominal skin reflex has no pathologic value and may be physiologically related to a generally brisk reflex response, for example.
