Reflex

A reflex ^[4] is defined as any behavior of the organism that occurs in response to a particular stimulus without the intervention of consciousness. Most often, this reactionary behavior is motor (muscular) in nature, but it can also be of a different nature, such as glandular.

Roles of reflexes :

Reflexes are essential behaviors for the organism, given that they are, in most cases, extremely rapid and stereotyped. They allow us to adapt to various situations, particularly those where the physical integrity of the organism is threatened. This is notably the case when one immediately and involuntarily withdraws their hand even before realizing it has been burned ^[82].

Nature of the reflexes :

Reflexes can be innate or acquired through various life experiences ^[1]. The reflexes we possess are numerous and varied, but they all obey the same principle: few synaptic relays, which guarantees the speed of the reaction.

A reflex requires a sensory receptor that captures the signal and an afferent fiber that carries the signal to the CNS, often the spinal cord or the brainstem. From there, a motor impulse arises and travels along a motor neuron to stimulate an effector organ (often a muscle), which then reacts. This sequence of events is called the reflex arc ^{[1, 54]}.

The reflex arc :

There is no voluntary control over the reflex arc. That said, there are sometimes fibers that ascend to the cerebral cortex to keep us informed of what has occurred.

When a physician strikes the patellar ligament ^[57] (during a physical examination), this percussion causes an elongation of the quadriceps muscle. The neuromuscular spindles ^{[3, 38, 109]} located within it stretch and send a signal to the spinal cord via proprioceptive nerve fibers (Type A-alpha). These fibers terminate in the anterior horn of the spinal cord ^[1] on the dendrites of the motor neurons that contract the quadriceps.

The proprioceptive fibers also activate interneurons that inhibit the motor neurons of the antagonist muscle ^{[130, 133]}. This results in a knee extension reaction.

Reflexes in medicine :

Reflexes are widely utilized in clinical practice; they often provide information about the nature and site of a nervous system lesion. Thus, if the patellar reflex is absent, it indicates a peripheral failure. If, on the other hand, it is exaggerated and brisk, it is concluded that there is a loss of central control over the reflex arc, indicating a central lesion ^[67].