The abdominal wall reflex is an intrinsic reflex of the human body that leads to involuntary contraction of the abdominal muscles. The function of the abdominal wall reflex is to protect the abdominal muscle from passive overstretching, thereby preventing damage to it. Its absence may indicate damage to the pyramidal tract, for example, as a result of a stroke.
What is the abdominal wall reflex?
The abdominal wall reflex is an intrinsic reflex of the human body that leads to involuntary contraction of the abdominal muscles. The abdominal wall reflex represents an intrinsic reflex of the human body. Intrinsic reflexes are characterized by the fact that stimulation and reaction occur in the same organ. In neurophysiology, which deals with the mechanisms of the human nervous system, this refers to a reflex that is neuronally connected in a special way: extensions of the nerve cell, the dendrites, receive the stimulating signal. These specialized dendrites of sensory neurons are called afferent dendrites, derived from the Latin ‘affere’, meaning ‘to feed’ or ‘to carry’. In an intrinsic reflex, a so-called Ia afferent transmits this nerve signal to a motoneuron via a single switching point. Accordingly, two nerve cells and one interface (synapse) are involved in the information transmission of the triggering stimulus. In neuroscience, the term motoneuron refers to efferent nerve cells, derived from the Latin term ‘efferre’, ‘to carry out’. Unlike afferent neurons, which feed information to the nervous system, efferent nerves carry commands from the nervous system to the muscles. This efferent signal causes the corresponding muscle to contract, resulting in the abdominal wall reflex in this discussed case. As with all intrinsic reflexes, the abdominal wall reflex occurs without the involvement of the central nervous system. Therefore, the human body cannot consciously trigger or suppress the movement.
Function and task
Passive stretching of the abdominal muscle triggers the abdominal wall reflex. Special sensory nerve cells located in the muscle detect the stretch. Mechanical stimulation of the dendrites (afferents) causes a change in the electrical potential of the neuron. This, in turn, causes depolarization of the cell and makes it produce a chemical signal: It releases certain messenger substances, called neurotransmitters, into the synaptic cleft located between the sensory neuron and the next neuron. The following neuron detects the chemical stimulation with the help of the receptors on its dendrites. In this way, the nerve signal is transmitted from cell to cell. Various mechanical stimuli are capable of triggering the abdominal wall reflex on one or both sides. A blow to the costal arch can elicit the abdominal wall reflex, as can a blow to the iliac crest. Also, if the abdominal muscle above the pubic bone is stretched too much passively, the human body automatically triggers the abdominal wall reflex. One of the functions of the reflex is to protect the muscle from overstretching. Muscle fibers are elastic tissue that behaves flexibly and is stretchable to a certain degree. However, this stretchability is limited. In a slow, gradual overstretch, initially only individual fine fibers of the muscle tear. The effects of such slight overstretching are unpleasant for the person affected, but manifest themselves only in the form of the familiar muscle soreness and have no far-reaching health consequences. However, an increasing pull on the muscle fibers can tear entire muscle fibers and even the muscle as a whole. The abdominal wall reflex also has a second important protective function: It protects the organs of the abdominal cavity from possible damage caused by blows or falls. The contraction causes the abdominal muscles to stiffen, thereby forming a natural shield to shield the organs from mechanical stimuli. However, this protection only extends to a certain extent. For example, in the case of traffic accidents or intentionally inflicted violence, the protection provided by the abdominal wall reflex is not sufficient. One consequence is often injuries to the internal organs in the abdominal cavity. During a medical examination, doctors usually place two fingers of one hand on the abdominal muscle and briefly tap the back of the hand with the other hand or reflex hammer.This allows them to determine whether the abdominal wall reflex is triggered as expected. Its absence may point to a neurologic or other condition.
Diseases and medical conditions
An absent abdominal wall reflex may indicate a variety of diseases. However, it does not form the only symptom of one, and medical professionals cannot always clearly attribute its absence to a single cause. If stimulation of the abdominal muscle does not result in the abdominal wall reflex, this may indicate damage to the pyramidal tract. The pyramidal tract is a part of the pyramidal nervous system, which in its entirety controls the movements of the human body. The pyramidal tract begins at the medulla oblongata, which is a part of the brain, and continues down the spinal cord where most of the nerve fibers cross. The motoneurons involved in the abdominal reflex are also located there. Neuroscience refers to them by the code Th6-L1. For this reason, damage to the pyramidal pathway affects, among other things, the abdominal wall reflex. In particular, if the abdominal muscle responds to stimulation with the abdominal wall reflex on only one side, damage to the pyramidal tract is obvious. A stroke (apoplexy) potentially evokes this lesion. A stroke is damage to the brain that often affects other parts of the nervous system and is caused by inadequate blood supply to the brain. Therefore, the absence of the abdominal wall reflex represents a serious symptom that requires further clinical diagnosis by a health care professional.
