Reflex arc

The shortest connection of a neural excitation circuit is called a reflex arc. A shortest connection is limited as to the interconnection between two different neurons. This takes place at the level of the spinal cord via a synapse in the ventral horn ( Fig. 1 - Section 1: anterior horn. ). One speaks in this simplest form of a reflex also more accurate from a simple monosynaptic reflex arc.

Proprioception and control loop

The term reflex arc and the physiological neuron theory related is based on the concept of technical control loop and to the corresponding input / output systems. In contrast to the purely physiological concept of the reflex is stressed with the notion of the reflex arc, the biological principle of organization in topological terms. The exact knowledge of topographical constraints enables accurate topical diagnosis. It is facilitated by the entire monosynaptic reflex arc is in the same spinal cord level, the exact knowledge therefore is also clinically important. The examination of reflexes belongs to the standard neurological examination.

The problem of a topical diagnosis and the associated localization theory sounds also with the concept of proprioception or with the principle of self- control or self-regulation. Even if only a small part of the pulses of proprioception to consciousness, and thus reaches the cortex, so the concept of the reflex arc can not be understood solely in terms of a simple mechanical automatism. This would be an oversimplification, that is the essence of the living organism not fair. A single nerve cell receives stimuli from not only one or two neurons, but by numerous, even up to thousands of neurons. This is the next chapter for the motor anterior horn cell in the spinal cord, cf. Elements of the reflex arc. So, for example, receives the reflex arc promoting or inhibiting central influences of the pyramidal tracts, which are activated in the reflex arc at the peripheral motor neuron. They act physiologically in the proprioceptive reflex inhibitory to a reflex response in damage to the pyramidal tract system, however, promoting, see pyramidal tract signs.

From the variety of the actual reflex arcs is here mainly the monosynaptic proprioceptive reflex shown.

Elements of the reflex arc

According -called principle of a control loop is distinguished:

• (. Fig.2 symbol w ) on the side of the entry of the afferent limb of the arc ( blue in Figure 1. ); Origin = sensor or sensor in the art = receptor (muscle spindle) in biology; Forwarding of the stimulus by unipolar nerve cell in the spinal ganglion
• (. Fig.2 symbol y ) on the side of the output of the efferent limb of the arc ( in red in Figure 1. ); Forwarding of responses by motor anterior horn cell; Target = actuator or effector in the art = effector ( physiology ) or effector in physiology (muscle or gland )

Afferents have for simple ( monosynaptic ) reflexes originate in sense organs or other sensitive or sensory receptors in muscles (receptor: muscle spindles), tendon or in the skin ( touch). The forwarding of the afferent impulses to the spinal cord via sensory nerve cells (usually Aα - fibers by Erlanger and Gasser Ia-Fasern/Ib and II fibers by Lloyd / Hunt). It is in terms of the neural cell type to pseudounipolar neurons whose cell bodies in the dorsal root ganglion (Fig. 1 - Section 13:. Spinal ganglion ) is situated. This is located within the spinal canal, but not part of the central, but to the peripheral nervous system, see the definition of the polysynaptic reflex.

Efferent fibers have their destination in muscle or gland. The forwarding of the efferent impulses from the spinal cord via motor nerves ( motor neurons ), whose cell bodies in the gray matter of the spinal cord in motor anterior horn (Fig. 1 - Section 1:. Anterior horn ) is. That the effector (muscle ) leading Motoaxon heard with respect to the conduction velocity of the Aα fibers ( short α - motoneuron ). The muscle spindles are supplied by motor about different types of A? Fibers (short: γ - motoneurons ).

Physiological components

The reflex response of an organism within the meaning of the reflex arc consists entirely of the following functional single facts:

Other terms

Occasionally, one also speaks of a reflex circuit, but also reflexive relationships more complex nature and can be meant by nested loops. The time from exposure to the stimulus to motor response is referred to as reflex time.

Mono- and polysynaptic reflexes

About monosynaptic reflex arcs ( reflexes ) concluded upcoming reflex reactions are the fastest running organismic reflexes, as they run only a single synapse in the spinal cord. As spinal cord reflexes they can not be deliberately influenced in their course, when they come once in motion; a conscious influence can only be achieved indirectly via a stimulus control. This monosynaptic reflexes are also called reflexes, because the receptor and effector are in the same organ.

Examples of reflexes:

• Patellar = PSR
• Achilles tendon reflex = ASR

The polysynaptic reflexes more central neurons are connected in series. Receptor and effector are usually spatially separated, so they are also referred to as a foreign reflexes.

Examples of foreign reflexes:

• During coughing, the stimulus caused by foreign bodies in the neck ( receptor) is recorded. The stimulus response is performed by the respiratory muscles ( effector ).
• When the corneal cornea of ​​the eye (receptor ) is touched, the stimulus response is made by the Lidmuskulatur ( effector ).

Reflex arc as a model

The reflex arc as a physiological model for understanding the operation of the entire nervous system of far-reaching significance. This already applies to the ( in the opening credits ) definition given conceptual understanding of the reflex arc as a special case of the far more complex operation of neuronal excitation circuits. The vegetative and the animal nervous system each operate according to this principle, but by their very nature subunits only elements within the entire nervous system dar. Based on the function of neuronal networks the activities of the higher centers of the CNS can be understood fundamental, such as the distinction between association cortex and sensorimotor cortex. Projection centers can be understood, for example, in analogy to the spinal centers as part of a "psychic reflex arc " ( Jaspers ), see also the theory of spinal irritation of Wilhelm Griesinger. The function of the association cortex can be understood in analogy to the polysynaptic reflex.

This model character thus also extends to the understanding of mental conditions including mental illnesses and disorders. The latter can z.T. be understood as a failure of homeostasis or as a failure of unconscious self- regulation. Even the modern concept of a mental disorder has such parallels of technical terms from the control equipment such as the disturbance. Biological disturbances in accordance with the discussion of the changing situations of the environment. In this way, as information can be obtained through physical control mechanisms in mental illness. This point of view concerns mainly questions of psychophysical correlation.

The description of the conditioned reflex by the Russian scientist Ivan Petrovich Pavlov (1849-1936) presents the history of science is already a first step in this direction; This phenomena of perception and learning were included in the theory of reflex. Using the example of Pavlov's show also socio-political ( materialistic ) behind his reflective teaching, which were sometimes referred to by its opponents as " reflex mythology ". Pavlov tried with his preference for the physiology to banish all psychological terms from his vocabulary and his staff. For his work on the activities of digestive glands, he received the Nobel Prize in 1904. Even modern representations of this historical phenomenon, which has also in the West German past for movement, showing different cybernetic interpretation issues. The introduction of cybernetic terminology can lead to an enrichment of the description of mental disorders, which is not possible in the language of classical physics. Terms of control engineering such as "in- service - company " illustrate integrative functions of the CNS ( inter alia also comparable with social influence factors and the appropriate "in- service acquisition " ) and comparable with certain technical solutions modular structure the nervous system. Also in the neurosciences, there is the concept of modules. The more complex these tasks, the sooner they can be regarded as a mental functions, which often then happens in practice when certain reactions a person can not explain.

Figure 4 psychophysical correlation as a control loop on the animal level

The classic reflex theory Pavlov, as it is also the basis of behaviorism, is based on the experimental observation in the lab → Experimental Psychology. John B. Watson (1878-1958) laid in 1913 established the mission statement of this new science, which has later been proven as a science of behavior or behavior analysis. First, the externally observable physiological processes were investigated, see blue area in Figure 3 ( the "outside world "). Afferent or rezeptorisch external physical influences were acting as a stimulus ( stimulus engl. ) named, emanating from the biological system efferent or effector responses to stimulation as reactions ( engl. response or reaction) on the biological system ( " red zone " ). In this case, the stimulus came to a decisive, almost causal and superficial meaning. However, this mainly determined by the → Machine paradigm ideas required a revision, since the biological system plays its own effectiveness and role in these processes. The so-called S / R operation ( engl. stimulus / response model) in biological organisms is therefore of purely physical processes to vary according to the cause-and- effect mechanism, although it suggests such an interpretation. For its own laws (so-called "primary active " ) processes of biological systems and the polysynaptic connections with reflex processes are adhered to, particularly the influence of the vegetative and animal nervous system and its responsiveness. In addition to the innate reflex processes not only learned influences must be taken into account (conditioning), and the general development and maturation of the investigated biological system plays a role, see the blue area in Figure 3 (" inner world " - vegetative NS) and in Figure 4 ( " psychic system " - animalistic NS) further split structure of this inner world into a somatic and psychic realm. The " inner world " of the organism to Figure 3 corresponds to both the " somatic system " and the " mental health system " in Figure 4, see also the only preliminary Hilfskonstuktion the black box. This "box" was therefore " black box " named because of the complexity of the inner life should be excluded for methodological reasons. As an example of the effectiveness of the internal world factors ( the negative epitome of the "black box" ) here would be the Babinski reflex to call. This is bound in its occurrence to the function of mature central railways. Normally when mature adults it is missing, the child he is under 2 years old physiologically.

An early criticism of the behaviorist reflex arc theory can be found in the neurologist and shape theorist Kurt Goldstein. Through his work with brain-injured soldiers during the First World War, he comes among other things, to the conclusion that there are no isolated stimulus-response processes in the organism, but that the organism always reacts as a whole.

Kurt Goldstein influenced his work significantly the development of Gestalt therapy. Fritz Perls and Laura Perls, the founder of Gestalt therapy, are directly related to Goldstein.

The model character of the reflex arc and its far-reaching clarification based on bionic examples still leaves many questions unanswered. If when making everyday about comparing the similarity between the central nervous system and telephone exchange is illuminated by the analogy between polysynaptischem reflex arc and telephonic communication, at least about the issue of communication in plants is a unberücksichtigtes and entirely different phenomenon. Plants have known about no nervous system. But they have faced many similar biological functions as well as animals and humans. The reflex arc can thus serve only partly as an overarching model in biology. Very different conditions here apply to the plant kingdom about.