A reflex is an involuntary, automatic response of a body part or organ to a stimulus. Within this, a foreign reflex describes a specific type of reflex and is also called a polysynaptic reflex.
What is a foreign reflex?
Many extraneous reflexes serve a protective purpose. For example, the swallowing reflex enables fluid and food intake while protecting the airway and lungs. Unlike the intrinsic reflex, in the extrinsic reflex the receptor and effector are not located in the same organ. This means that the perception of a stimulus and the body’s action in response to it occur in different organs. A receptor is a cell or cell association that can translate chemical or physical stimuli into a neuronal form. The sensory cells in the eye or ear and the sensitive cells of the skin are examples of receptors. An effector is an association of cells that can receive neuronal signals and produce a response in response. The organ belonging to the effector is also called the organ of success.
Function and task
The path that the stimulus takes from the receptor to the effector is also called the reflex arc. The stimulus is first registered by the receptor. There, it triggers an excitation in the nerve cells. This excitation is transmitted to the central nervous system (CNS) via so-called afferent nerve fibers. Afferents are nerve fibers that lead from the periphery, for example from the extremities, to the CNS. Most reflexes travel from their receptor to the spinal cord via afferent fibers. The spinal cord is part of the central nervous system and runs in the vertebral canal of the spine. In the spinal cord, the stimulus is then transferred from afferent sensory nerve pathways to a motor nerve pathway. The motor nerve pathway in the anterior horn of the spinal cord is an efferent, meaning that the nerve pathway travels from the spinal cord to the periphery to the organ of success. There, the stimulus then triggers a corresponding response. Reflexes enable humans to react quickly to various living conditions in their fairly constant environment. Reflexes are automatic, schematic, and stereotyped, allowing for very little reaction time. Innate reflexes facilitate survival. They have been tested by previous generations and thus provide an increase in adaptation and survivability from birth. Many extraneous reflexes serve to protect. For example, the eyelid closure reflex protects the eye from foreign objects, and the swallowing reflex enables fluid and food intake while protecting the airway and lungs. In addition to the eyelid closure and swallowing reflexes, physiological reflexes include the abdominal skin reflex and the cremasteric reflex. In the abdominal skin reflex, the muscles of the abdominal wall contract when the abdomen is brushed from the flank side toward the navel. The cremasteric reflex is the lifting of the testis by brushing the inner side of the thigh. The pupillary reflex is also a physiological external reflex. It causes the pupils to adapt to different light conditions. Both pupils always constrict or dilate, even if only one of the pupils is illuminated. The gag reflex occurs when liquid or other foreign objects enter the trachea. Spoiled or very bitter foods can also trigger the gag reflex. Thus, just like the swallowing reflex, this reflex is a protective reflex. In the infant, the sucking reflex and the sole-of-foot reflex are also part of the physiological reflex repertoire. However, the plantar reflex, also known as the Babinski reflex, is pathologic in the adult.
Diseases and disorders
Pathologic extraneous reflexes are involuntary body responses that do not occur in healthy individuals. They usually provide evidence of diseases of the central nervous system. In the Babinski reflex, the outer edge of the foot is brushed. In healthy infants and in diseases of the central nervous system, toe spreading is observed. The big toe pulls in the direction of the dorsum of the foot. If this toe spreading occurs, it is called a positive Babinski reflex. A positive Babinski reflex provides an indication of damage to the pyramidal tract. The fibers of the so-called motoneurons run in the pyramidal tract. They supply the muscles of the body. The Chaddock reflex also belongs to the pyramidal tract signs, i.e. to the pathological extraneous reflexes that indicate damage to the pyramidal tract.Similar to the Babinski reflex, pressure on a point on the foot triggers toe spreading. The Gordon reflex is also a pyramidal pathway sign. Here, spreading of the toes and tightening of the big toe are triggered by pressure on the calf muscles. Other pathological extraneous reflexes caused by pyramidal tract damage include the ankylosing spondylitis Mendelian reflex, the Oppenheim reflex, and the Rossolimo reflex. A well-known disease in which pathological foreign reflexes occur is multiple sclerosis. Here, the meylin sheaths of the nerves are damaged by autoimmune processes. In addition to pathological external reflexes, absent or weakened physiological external reflexes also provide clues to possible diseases. A missing or weakened abdominal skin reflex is a sign of multiple sclerosis, just like the Babinski or Oppenheim reflex. If the cremasteric reflex is absent when the inner thigh is stroked, this indicates testicular torsion or damage in the area of the L1 and L2 spinal cord segments. In turn, an absent anal reflex indicates damage to spinal cord segments S3-S5. The eyelid closure reflex is absent in the case of damage to the afferent or efferent nerve fibers in the area of the eye, as well as in the case of paralysis of the facial nerve. Disorders of the eyelid closure reflex may indicate damage to the optic nerve as well as disorders of the motor fibers in the area of the eye. If the optic nerve is damaged, the pupillary reflex fails when illuminating the affected eye, but when illuminating the healthy eye, the pupillary reflex can be triggered in both eyes. On the other hand, if the motor part of the eye is damaged, the pupillary reflex can no longer be triggered in the affected eye even when the healthy eye is illuminated.
