The reflex arc is the shortest neuronal connection between receptors and target organs and initiates a body reflex. Input occurs via the afferent limb of the arc, whereas output occurs via the efferent limb. Changes in reflex arches can be diagnosed by electrophysiologic examination.
What is the reflex arc?
In general, the term reflex arc refers to the shortest connection between specific receptors and effectors that passes over the neurons of a given excitatory circuit. Reflex arc refers to the sequence of neuronal processes that trigger a body reflex. In general, the term refers to the shortest connection between specific receptors and effectors that runs across the neurons of a given excitatory circuit. Each reflex arc involves the neuronal influx of information from the central nervous system. This inflow is also called the afferent limb and is used to input information. At least one central neuron forms the basis of the reflex arc. In addition, the reflex arc always contains an efferent structure in which information is conducted away from the central nervous system to the periphery. This structure is also called the efferent limb of the reflex arc. The last part of the reflex arc is the effector, which is the organ that performs the reflex. The effector is thus the end point of the goal-directed, neuronal process. In the simplest and fastest form, the efferent and afferent legs are connected by a single synapse in the anterior horn of the spinal cord. In this case, we speak of monosynaptic reflexes. To be distinguished from this are polysynaptic reflex arcs, in which several central neurons are connected in series.
Function and task
When humans stumble, they often catch themselves before they fall, such as by changing the position of their legs. If he chokes, he does not suffocate because a cough reflex is triggered. If an object flies towards him, he automatically pulls his arms in front of his face, and if something approaches his eye, his eyelid closes involuntarily. Reflexes like these are rapid and involuntary movements in response to a specific stimulus. Most reflexes are designed to protect the organism from harm, such as the eyelid closure reflex. All reflexes consist of an interaction of the sensory organs, nerves and muscles. In this way, stimulus-adequate reactions can be delivered to certain stimuli. While some reflexes are innate, others are acquired based on experience. For all of them, the reflex arc plays a major role, because only through this system is the timely reflex response to a particular stimulus assured. Protective reflexes in particular depend on this rapid response, otherwise they would no longer serve any purpose. In addition to a receptor, each reflex arc contains an afferent limb for input of information, central neurons, an efferent limb for reflex output, and an effector that carries out the output information. Afferent limbs consist of afferent receptor nerve fibers, such as those represented by the class I neurons on the spindles of muscles. Axons or motor neurons make up the efferent limb. To some extent, postganglionic fibers are also involved in the efferent limb. Effectors can be either organs such as the heart or specific muscles and glands. The afferent limb originates from sensory organs and their receptors in all monosynaptic reflexes. Muscle spindles may also be involved as sensory receptors on the afferent limb. The afferent impulse is ever transmitted to the spinal cord. If transmission to the brain were required, the reflex response would take too much time. Projection into the spinal cord occurs via the sensory neurons. The pyramidal tracts of the spinal cord are involved in monosynaptic reflexes with inhibitory or facilitatory influences. As the afferent limb opens into the spinal canal, the efferent limb of the reflex arc opens into muscles, organs, or glands. The efferent impulses are transmitted from the spinal cord along motor nerve pathways located in the motor anterior horn. Motoaxons thus conduct the efferent output to the target organ. These axons belong to the Aα-fibers and have a correspondingly high conduction velocity. In intrinsic reflexes, the receptor and effector are located in the same organ. In the case of extrinsic reflexes, on the other hand, they are located in different organs.
Diseases and ailments
Reflex examination is one of the standard neurological examinations. This reflex examination is primarily used to detect pathologic reflexes, as they may occur in the context of various diseases. Pathological reflexes are mainly the Babinski reflex, the Chaddock reflex and the Gordon reflex, but also the Mendel-Bechterew reflex, the Oppenheim reflex and the Rossolimo reflex. Pathological reflexes belong to the so-called pyramidal tract signs and thus give an indication of damage to the pyramidal tracts in the spinal cord. In such damage, the course of the reflex arc is disturbed, since all monosynaptic reflexes run through this center. Pyramidal tract signs can occur in the context of various diseases. For example, in the autoimmune disease multiple sclerosis (MS), inflammation can cause lesions to develop in the pyramidal pathways, triggering pyramidal pathway signs. In MS, the presence of pyramidal tract signs shortly after the onset of the disease is considered a bad sign and thus negatively affects prognosis. Usually, pathological reflexes are associated with central paralysis, such as hemiplegia, whose origin is in the central nervous system. Central and peripheral reflex arc patterns can be traced during electrophysiological examination. Pathological changes in reflex arcs can also be diagnosed in this way. Such changes can narrow down the localization of brain lesions and occur, for example, in the context of stroke.
