Unconditioned Reflexes

or species reflexes, relatively constant, stereotypical, innate reactions of an organism to the influence of the external and internal environment, effected through the mediation of the central nervous system and requiring no special conditions for their occurrence. The term “unconditioned reflex” was introduced by I. P. Pavlov to designate reflexes that occur unconditionally under the action of appropriate stimuli on the sensory nerve endings (receptors). Examples of unconditioned reflexes include secretion of saliva when food enters the mouth and jerking away the hand when the finger is pricked. The biological role of unconditioned reflexes, which serve as the foundation for the rest of the nervous activity of the organism, is adaptation of the behavior of an animal of a given species to the usual, constant conditions of its environment. The dynamic interaction of unconditioned reflexes with acquired ones that develop in the course of an organism’s life, the so-called conditioned reflexes, ensures adaptation of the organism to changes in the internal and external environments. The anatomic bases of unconditioned reflexes are various divisions of the spinal cord and brain, including its cerebral hemispheres. Precise correspondence of unconditioned reflexes to the qualitative and quantitative characteristics of the stimulus has made it possible to interpret the problem of their biological expediency in a materialist manner, in view of the striking adaptation of the function to the character of the stimulus. An important role in the mechanism of unconditioned reflexes belongs to so-called reverse afferentation, that is, the entry into the organism of information about results and about the degree of success of the completed act. The distinction between unconditioned and conditioned reflexes is relative. According to Pavlov, unconditioned reflexes arise as conditioned reflexes and later (when the same life conditions are maintained in a number of generations) are strengthened in the course of evolution and transformed into innate reflexes.

Notions about the extreme stability and immutability of unconditioned reflexes have not been confirmed. It has turned out that unconditioned reflexes exhibit noticeable shifts due to many factors (interaction with other reflexes, hormonal-humoral influences, and the functional state of the organism and of its central nervous system). Unconditioned reflexes “in pure form” may occur once or several times after the birth of the animal, and then, in quite a short time, be “overgrown” by conditioned reflexes and other unconditioned reflexes. All this hampers classification of unconditioned reflexes. At the base of some classifications of unconditioned reflexes (A. D. Slonim) lies the principle of equilibration of the organism with the external environment and maintenance of the constancy of its internal environment (homeostasis); other classifications take into account the ecological and biological characteristics of unconditioned reflexes (N. A. Rozhanskii). The classification accepted in Pavlov’s laboratory takes into account the acting stimulus and the biological meaning of the response. In accordance with this classification, the following unconditioned reflexes are distinguished: alimentary, defensive, sexual, and orientational-exploratory. Pavlov classified instincts and other manifestations of complex acts of innate activity in animals and man that have a cyclic or behavioral character in a special category of complex unconditioned reflexes. Similar types of innate activity have been intensively studied in recent decades by the science of behavior—ethology.