Despite the spiritual aura surrounding the term, Descartes attached to it actual and, for his time, completely scientific significance based on ideas from mechanics, kinematics, and hydraulics.
He described the impulses conducted along peripheral nerves by the term “animal spirits,” which he borrowed from the ancient physicians. As examples, Descartes cited blinking in response to the sudden appearance of an object before one’s eyes and withdrawal of a limb after the sudden application of a painful stimulus. The entire process of nervous activity, characterized by automatism and involuntariness, consists in stimulation of the sensory apparatus and conduction of the apparatus’ impulses along peripheral nerves to the brain and from the brain to the muscles. Descartes’s teaching on the reflex principle of nervous activity was based on the mechanism of involuntary movements. The ancient physicians, for example, Galen in the second century, divided human motor actions into voluntary actions, which require the participation of consciousness in their execution, and involuntary actions, which are performed without the participation of consciousness. The concept of reflexes was first conceived by the French philosopher Descartes. The reflex activity of the nervous system assures the organism’s functional integrity and controls the organism’s interaction with the external environment, that is, its behavior. The biological significance of reflexes consists in the regulation of the work of organs and their functional interactions to maintain the stability of the organism’s internal environment (homeostasis) while preserving its integrity and ability to adapt to the external environment. The structural mechanism of a reflex is the reflex arc, which includes receptors, a sensory (afferent) nerve that conducts excitation from receptors to the brain or spinal cord, a nerve center located in the brain and spinal cord, and an efferent nerve, which conducts excitation from the brain or spinal cord to effector organs, that is, muscles, glands, and internal organs. The term “reflex,” adopted from the physical sciences, emphasizes the fact that nervous activity is “reflected,” that is, it is a response to influences from the external or internal environment. Other muscle groups show similar reflexes.Ī response of an organism mediated by the central nervous system after stimulation of receptors by internal or external environmental agents it is manifested by the occurrence of or change in the functional activity of individual organs or the body as a whole. It involves the patellar (kneecap) tendon and a group of upper leg muscles. A familiar reflex is the knee-jerk or stretch reflex. The remainder of the reflex response is governed by the specific synaptic connections that lead to the effector neurons. The sensory side of the reflex arc conveys specificity as to which reflex will be activated. The central neurons which are often interposed between the sensory and motor neurons are called interneurons.
Such a sequence begins with sensory neurons and ends with effector cells such as skeletal muscles, smooth muscles, and glands, which are controlled by motor neurons. The neurons involved in most reflexes are connected by specific synapses to form functional units in the nervous system. However, most reflexes require activity in a large sequence of neurons. For example, ciliated protozoa, which are single cells and have no neurons, nevertheless exhibit apparently reflexive behaviors.
The simplest known reflexes require only one neuron or, in the strictest sense, none. In higher animals, such as primates, where learned behavior dominates, reflexes nevertheless persist as an important component of total behavior. Along with other, more complex stimulus-bound responses such as fixed action patterns, they constitute much of the behavioral repertoire of invertebrates. Reflexes are exhibited by virtually all animals from protozoa to primates. ReflexĪ simple, unlearned, yet specific behavioral response to a specific stimulus. The Columbia Electronic Encyclopedia™ Copyright © 2013, Columbia University Press.
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