The reflex with the simplest structure in medicine is called the intrinsic reflex. This means that the reflex occurs in the very place where it was triggered. An example of this is the patellar tendon reflex in the area of the kneecap, which is caused by a light blow on the same.
What is an intrinsic reflex?
The reflex with the simplest structure in medicine is called an intrinsic reflex. An example of this is the patellar tendon reflex in the area of the kneecap. In the intrinsic reflex, the response is elicited exactly where the stimulus occurred. The term originates from neurophysiology and usually refers primarily to muscle reflexes. If a muscle is stretched by a stimulus, this leads to a twitching of the irritated muscle via synapses and circuits in the spinal cord. The strength of the reaction depends on the reflex arc and the state of the motoneuron in the spinal cord. However, the intrinsic reflex is not immutable. It can be influenced by exercises, increased attention or appropriate tension of the muscle. In addition, in some cases it can be absent altogether. In contrast to the external reflex, the intrinsic reflex cannot be habituated. In medicine, this is the term used to describe habituation to a stimulus. Thus, in the case of intrinsic reflexes, the reactions are not weakened by repetition or fail to occur altogether. It is not fatigable. In addition, it has a short latency period. Due to the less complex branching, the reaction is therefore immediate and without major delay. In the case of the external reflex, on the other hand, the nerves are first switched in the spinal cord to other organs or muscles, which then react to the reflex. An example of this is the examination of the pupillary reflex when brain damage is suspected. Incidence of light on one eye usually causes constriction of both pupils.
Function and task
The list of intrinsic reflexes is long. Some are used by physicians to test the function of various spinal cord nerves. Intrinsic reflexes perform various tasks, and they are generally protective mechanisms. For example, the stretch reflexes protect people from falling, as is the case with the PSR (patellar tendon reflex). If the knee buckles, the lower leg is reflexively moved to advance, which can prevent a fall by stabilizing balance. Thus, it is the performance of a brisk routine task. Therefore, the short latency is necessary. The intrinsic reflexes thus fulfill a vital function. The so-called reflex arc consists of a receptor – the sensory organ -, the nerve fiber involved, the central nervous system as well as the motor fiber and the effector. The effector is the muscle or organ that performs the response to the stimulus. In the case of the intrinsic reflex, the receptor and effector are identical according to the above explanation. Triggering stimuli can be, for example, tension, acceleration, pressure, sound, temperature, light or chemical substances. In the following, some of the intrinsic reflexes are taken up for illustration purposes in order to clarify the function and reaction. In the Achilles tendon reflex, for example, a light blow to the taut Achilles tendon triggers a reaction from the ankle. The abdominal wall reflex responds to the placement of fingers on the abdominal muscles and a blow to the backs of the fingers. The resulting tightening of the abdominal muscles, for example, is a mechanism that protects against damage to internal organs from dull blows to the abdomen. Blows to the chin, in turn, trigger the automatic closing of the mouth to prevent damage to the mouth. In addition, however, there are reflexes that occur only when damage to the central motor system has occurred. These include, for example, the toe flexor reflex, which occurs when the toe berries are struck briefly and forcefully. If the toes are flexed, this indicates injury.
Diseases and ailments
Accordingly, damage to nerves or the spinal cord can affect intrinsic reflexes. For example, damage to the first motor neuron leads to a strong increase in reflexes. This includes the toe flexor reflex. However, if damage occurs within the reflex arc, the reflex fails completely. This can be the case due to mechanical damage or neuritis. Thus, a detailed neurological examination can determine whether individual nerve pathways are damaged or whether a nerve dysfunction is present.After strokes, there can be an increase in the intrinsic reflexes of muscles. Physicians refer to this type of abnormal behavior as spasticity. External reflexes in neighboring muscle groups are also possible. In addition to the toe flexor reflex, the so-called adductor reflex (ADR) also provides information about diseases. Here, if a blow is applied to the tendons of the inner side of the femur at the knee joint, the leg reacts with adduction in the hip. If the reflex is increased, both legs react to the unilateral stimulus as is the case with the reaction of both eyes to light conditions. Responsible for the ADR are the L2, L3 and L4 nerves and the obturator nerve. This is a nerve of the lumbar plexus. In the case of a herniated disc, there may also be a failure of the Achilles tendon reflex if there is compression of the roots. The orbicularis oris reflex causes trunk-like protrusion of the lips when the mouth muscles are tapped. This intrinsic reflex is a sign of muscle hyperexcitability and may occur, for example, in tetany. Not every absence of an intrinsic reflex is due to a disease. For example, there are some reflexes that are close to swelling and therefore cannot always be triggered. When in doubt, a medical professional will refer to other tests for a definite diagnosis of disease.
