The reflex arc and reflex action The fundamental and primary purpose of the neurones is to furnish a line of communication between the sense organs or receptors and the effectors or reacting structures. The central nervous system acts as a mediator and director of such communications. In man and the higher animals, this is accomplished in each instance by no less than two neurones to the voluntary organs and three neurones to the involuntary structures. Part of these are afferent and the rest efferent. At the distal end of the afferent neurone is to be found the receptor; in the spinal cord is the center or communication between the two neurones (synapse); and at the distal end of the efferent neurone is the effector, for instance, the muscle. The simplest structural basis for the communication between the environmental changes and stimuli which set up a disturbance in the receptor and the response of the organism in its adjustment to that change, as muscle contraction, is known as a reflex arc. The resulting response from such a mechanism is known as reflex action. A third neurone, situated wholly within the central nervous system, is usually intercalated between the afferent and efferent neurones. This then becomes a connector neurone. In normal coördinated reflex response, these reflex-arc elements most probably act in groups. The receptors are more or less specialized receiving organs situated peripherally. They are specialized to the extent that their stimulation threshold is greatly lowered for one particular type of stimulus, as the eye is most sensitive to light, the ear to air waves, while similar thresholds for other stimuli have been greatly increased. Reflex action may be defined as an action which takes place independently of volition but in response to peripheral stimulation. The reactions of the involuntary organs naturally, at all times, come under this caption. Examples of reflex action in the voluntary system, however, are familiar to us all. The time-worn example is the jerking of the hand away when it accidentally touches something hot. Such responses as sneezing, coughing, winking of the eye, and the knee jerk although, with the possible exception of the winking reflex, they involve more than one muscle and efferent nerve fiber, are typical of purposeful reflexes. Such reflexes are exact and well-timed and are, therefore, called coördinated reflexes. When the reflex involves the response of a single effector it is termed a simple reflex. Again, reflexes may become convulsive (convulsive reflexes). In this case the response spreads to many, if not all, parts of the body. Under these conditions, the normal resistance (probably synaptic) which retains the nerve impulse within certain definite bounds or in certain nervous channels, is diminished or overcome. The result is that the excitation spreads from neurone to neurone throughout the nervous system, simultaneously bringing into action very remote and usually unrelated parts of the body. Certain reflex acts are said to be inborn since the structural mechanism upon which they depend is present and functional at birth. Since these are not dependent for their existence on the circumstances in which the individual happens to be placed, they have been termed by Pavlov unconditioned reflexes. Biologically they are of great importance since they contribute directly to the well-bring and survival of the individual. They are common to all members of a given species. In the higher animals, while the major part of behavior has as its basis the unconditioned reflexes, other wellestablished responses, of a perfectly definite character, fall quite logically into another and somewhat different category. These have been acquired. The salient feature of these is that, although their motor response is typical of some one of the unconditioned reflexes, the sensory or afferent stimulation is evoked by some other than that usually associated with the observed response. These reflexes are thus built upon the structural and functional integrity of the unconditioned reflexes or some other acquired reflex. They have, therefore, been called conditioned reflexes by Pavlov. It is probably upon this type of reflex that the process of learning depends. Without the cerebrum, the individual would be reduced to an automaton, a slave to environmental changes (stimuli) through the medium of his unconditioned-reflex mechanisms. The simplest reflex arc and reflex act is confined to a single segment of the spinal cord with its pair of nerve roots and to the same side only. By means of connector neurones which cross to the opposite side of the cord, the reflex may be extended to the efferent neurones of the other side of the body. Through other connector fibers the reflex may extend to other levels or segments of the cord or brain stem, either on the same or opposite side. In this way parts quite remote from the point of the stimulus may be brought into action reflexly. Connector fibers in the central nervous system run in the white columns or tracts, the corresponding nerve cells lie in the gray matter while their terminal end brushes come into synaptic relation with nerve cells situated at other levels of the cord and stem. In man and the higher animals which possess a cerebrum and are capable of volitional responses, other mechanisms are brought into action. Volitional responses can be instigated or stopped at will. In these, the impulse which initiated the activity originated, not in a peripheral receptor as in the unconditioned responses, but in the uppermost part of the central nervous system-the cerebrum. Connections exist between these segmental reflex arcs and the higher brain centers. The afferent connections are completed by at least two neurones in addition to the peripheral afferent neurone. The second neurone takes origin from the gray matter of the cord at the point of termination of the first, the third terminates in the sensory cortex of the opposite side. Complete crossing or decussating occurs in the medulla oblongata. Through association neurones, connections between the sensory and motor areas of the cortex are formed. The circuit is completed through the motor efferent fibers leading from the motor cortex to the medulla, where most of these fibers cross or decussate to the opposite side of the cord, where they pass to make synaptic connections with the common efferent path (neurone) of the reflex are to the muscle. In addition there is a two-neurone pathway from the cerebrum to the cerebellum and thence to the muscles. The response may even be initiated volitionally (within the cortex) without an immediate sensation from the periphery. Thus, it is obvious that volitional and conditioned responses are built upon the structural foundation of the inherited unconditioned-reflex mechanisms.		Home Classes of levers Varieties of muscle Smooth muscle The blood supply to muscles The neurone Cardiac muscle On the nomenclature of skeletal muscles The nerve supply to muscles Structure of a skeletal muscle Central nervous system The reflex arc and reflex action Physiologic properties of muscle Mechanical Changes The effects of temperature on muscular contraction Muscle glycogen, its source and fate The reaction of muscles Oxygen consumption of muscle Determination of the oxygen consumption of muscle The viscosity factor in muscle and muscle efficiency Significance of the recovery heat Production of Electricity