Reflex arc

“ The first link of each reflex chain is a neurone which starts in a receptor organ, e. g., a sense-organ. A receptive field, e. g., an area of skin, is always analyzable into receptive points, and the initial nerve-path in every reflex arc starts from a receptive point or points. A single receptive point may play reflexly upon quite a number of different effector organs. It may be connected through its reflex path with many muscles and glands in various parts. ”

“ The same motor neurone may however be harnessed into two or more such teams, as is seen from the fact that the same muscle may participate in different reflex movements; and for a similar reason we believe that the same sensory neurone may be utilized in more than one reflex arc. ”

“ The central organs or adjusters include the proximal ends of these elements, all the association neurones, and the proximal ends of the efferent neurones. The effectors are not neurones at all, but muscle fibers, gland cells or other types of cells under the control of nerves. Thus the ordinary reflex may be said to involve in sequence the activity of a receptor, adjustor and an effector, to use modern terminology, and these three elements are recognizable in every complete reflex arc.
Our own reflexes are sometimes associated with consciousness and sometimes not. ”

“ The external stimulus acts upon the organ of sense, and the impression is transmitted along the centripetal way to the nerve centre—the corresponding motor impulse is elaborated, and is transmitted along the centrifugal path to the organ of motion, provoking a movement. Although the arc represents diagrammatically the mechanism of reflex spinal [Pg 223] actions, it may still be considered as a fundamental key explaining the phenomena of the more complex nervous mechanisms. Man, with the peripheral sensory system, gathers various stimuli from his environment. ”

“ The more complex habits, as we shall presently see more fully, are, from the same neural point of view, nothing but concatenated discharges in the nerve-centers, due to the presence there of systems of reflex arcs, so organized as to wake each other up successively—the impression produced by one muscular contraction serving as a stimulus to provoke the next, until a final impression inhibits the process and closes the chain. ”

“ Results. - Intensity of stimulation, fatigue and freshness, spinal induction, functional species of reflex, are all, therefore, physiological factors influencing the result of the interaction of reflex-arcs at a common path. It is noticeable that they all resolve themselves ultimately into intensity of reaction. Thus, intensity of stimulus means as a rule intensity of reaction. Those species of reflex which are habitually prepotent in interaction with others are those which are habitually intense; those specially impotent in competition are those habitually feeble in intensity, e.g. ”

“ Reflex arcs from widely different parts conjoin and pour their influence harmoniously into the same muscle. The motor neurones of a muscle of the knee are the terminus ad quern of reflex arcs arising in receptors not only of its own foot, but from the crossed fore foot and pinna and tail, also undoubtedly from the otic labyrinth, olfactory organs and eyes. ”

“ I see an example of it in the way in which many of them account for our thoughts and resolutions: they call them reflex action. The discovery of the nature of automatic motion was one of the most important discoveries of the last age. An action goes along a nerve to the centre of a ganglion, and comes out in motion by another nerve: thus, if a frog's foot is pricked, it is immediately drawn in. Of much the same kind is the reflex action of the sensori-motor system. ”

“ Several peripheral pathways for afferent impulses may be joined to an efferent pathway so as to complete reflex arcs and the efferent cell be under the controlling influence of some upper neuron coming down from the central axis with the power either to permit or to inhibit the reflex acts which would otherwise take place as a result of peripheral stimuli. ”

“ If we fancy the farther end of the string to be divided into several strands, each of which is attached to a different object, and which may be, separately or together, affected by a pull on the nearer end of the string, we shall form a still more definite notion of reflex action, for an impression made upon the same sensory nerve may produce various results, according to the strength of the impression and the efferent nerve-channel along which it is thrown back by the nerve-center. ”

“ It also indicates how such nervous relations may be complicated by other connections of [Pg 41] the nerve-cells involved in the reflex action. It will be seen that they make many upward connections with the brain, in consequence of which consciousness may be involved. Ordinarily one is more or less conscious of reflex action, though the will is not involved; in fact, a willed or voluntary action is usually considered the reverse of a reflex or involuntary action. But for a reflex action the brain is not essential. ”

“ Numerous other reflex centers are described by physiologists, but enough has been said to emphasize the great importance of the spinal cord as an independent nerve center, besides its function as a conductor of nervous impulses to and from the brain.
280. The Brain as a Reflex Center. The brain, as we have just stated, is the seat of consciousness and intelligence. It is also the seat of many reflex, automatic, and coordinating centers. These give rise to certain reflex actions which are as entirely independent of consciousness as are those of the spinal cord. ”

“ Diagram of the nerve path of a simple reflex action.
Reflex Actions; their Meaning.—If through disease or for other reasons the cerebrum does not function, no will power is [Pg 354] exerted, nor are intelligent acts performed. All acts performed in such a state are known as reflex actions. The involuntary brushing of a fly from the face, or the attempt to move away from the source of annoyance when tickled with a feather, are examples of reflexes. In a reflex act, a person does not think before acting. The nervous impulse comes from the outside to cells that are not in the cerebrum. ”

“ We can only discriminate one thrill at a time; but the neural excitations408 simultaneously pressing towards a discharge are many; and the conditions which determine now this, and now the other excitation to predominate by its differential pressure, are far beyond any mechanical estimate. I mention this because the advocates of the Reflex Theory maintain that the neural processes are the same whether a sensation be produced or not; and that since the same actions follow the external stimulation whether sensation be produced or not, this proves the actions to be purely mechanical. ”

“ Such reflexes involve not only the excitation of certain movements appropriate to the stimulus but the inhibition of antagonistic muscles and reflex movements. Further in the integration of the spinal system, reflexes are compounded, one bringing to the support of another allied accessory reflexes so that various coöperativecoöperative movements are executed. A constellation of reflexes leads to quite complex spinal mechanisms responsive to groups of stimuli acting concurrently and resulting in behavior which is purposive and adaptive to the situation. ”

“ The Reflex Theory asserts that when a sensory nerve is stimulated, the excitation of the centre may either subdivide into two waves, one of which passes directly to the brain and there awakens sensation, the other passes over to the motor-roots and causes muscular contractions ”

“ Broadly understood, this consists of the chain of consequences that begins with the reception of a stimulus on the surface of the animal and, leading through the central nervous organs, ends in the excitation of a reaction by some such organ as a muscle. The term reflex is made to apply nowadays to nervous operations involving conscious states as well as to those that are carried out unconsciously. In its greatest simplicity the conventional reflex involves at least two nervous cells or neurones and some form of reacting organ ”

“ When a stimulus falls upon the appropriate sensory surface, a wave of molecular movement is sent up the attached sensory nerve to a nerve-centre, which thereupon issues another wave of molecular movement down a motor nerve to the group of muscles over whose action it presides; and when the muscles receive this wave of nervous influence they contract. This kind of response to stimuli is purely mechanical, or non-mental, and is ordinarily termed reflex action. The whole of the spinal cord and lower part of the brain are made up of nerve-centres of reflex action ”

“ The jerking away of the hand on accidentally touching a hot stove, the winking of the eyes on sudden exposure to danger, and the quick movements from slight electrical shocks are familiar examples. The explanation of reflex action is that external stimuli start impulses in neurons terminating at the surface of the body and these, in turn, excite impulses in neurons which pass from the spinal cord or brain to the muscles (Fig. 138) . Since there is an apparent turning back of the impulses by the cord or brain, the resulting movements are termed reflex.105 ”

“ Although there is still a good deal of uncertainty respecting the exact composition of the feelings of movement, it may be taken as fairly proved that they include an active element or "feeling of innervation," which is correlated with the central excitation of motor fibers, and a passive element or tactual sensation which is connected with a reflex excitation of sensory fibers, consequent on certain differences in the tensions and mutual pressure of various parts of the skin which result from the movement. ”

“ In the brain we are sure that we do not know how to localize functions; in the spinal cord, we think we do know something; but there are so many anomalies, and seeming contradictions, and sources of fallacy, that beyond the facts of crossed paralysis of sensation, and the conducting agency of the gray substance, I am afraid we retain no cardinal principles discovered since the development of the reflex function took its place by Sir Charles Bell's great discovery. ”

“ DIAGRAM ILLUSTRATING A REFLEX An object O suddenly appears in front of the eye. Its image, formed on the retina R, a sense organ, starts impulses along the fibers of the sensory nerve cell s, which in turn stimulate the motor nerve cells m. These in turn stimulate the appropriate muscles of the eyelid, compelling a wink. (After Hough and Sedgwick, “The Human Mechanism.”)
A nervous activity which consists of the passage of a disturbance from a sense organ to a muscle is called a reflex. We have many examples in ourselves; if we inadvertently touch something hot, the hand is jerked away. ”

“ When an excitation increases, as we have seen, the number of muscular groups set in motion, resistance to transmission increases in the same proportion. “The sensation of pain presupposes a reflex movement and an arrest of nervous conduction in the grey substance of the spinal marrow.” It is this process of inhibition, in varying degrees, which is felt by the consciousness as pain.
2. Vascular reflexes.—Peripheral excitation also has reflex effects on the vaso-motor system; it produces contraction of the spinal arteries, of the carotids, and of the cerebral 85arteries. ”

“ Before we can leave the subject of nervous activity, there are some special kinds of reflexes to be spoken of, and there are also some additional things to say about the working of the emotions. When we speak of special reflex actions we have in mind some of the most useful of our own reflexes. They are not different from the simple reflexes that are described in Chapter X, except that they involve a larger number of sense organs and many muscles, and are correspondingly more complicated. ”

“ A represents a nervous centre and B the point of stimulus and AB the line of transmission. AC represents the reflex act. The whole series of phenomena occurring between B and C is simply one of mechanical causation, according to our supposition, and inasmuch as volition in the mature subject passes along the same lines of muscular egress, anteceded by some stimulus known either as an impression or a motive, or both, the character of that event seems to be the same as the action from which it is presumably developed. ”

“ In other words, the more intense part of each reflex obtains possession of the final common paths at the expense of the less intense portion of the reflex. But if the intensity of the stimuli applied to the right and left feet be not closely enough balanced, the crossed extension of the reflex excited by the stronger stimulus is found to exclude even the homonymous flexion that the weaker stimulus should and would otherwise evoke from the leg to which it is applied. ”

“ It is namely a well-known fact that when a nerve-current has to pass through the spinal cord to excite a muscle by reflex action, the time is longer than if it passes directly down the motor nerve: the cells of the cord take a certain time to discharge. Similarly, when a stimulus is applied directly to the cortex the muscle contracts two or three hundredths of a second later than it does when the place on the cortex is cut away and the electrodes are applied to the white fibres below. ”

“ It is now established that the paths of reflex action, traversed by external stimuli reappearing as motions, are continuous; that is, that the afferent nerves terminate in ganglia whence originate the efferent nerves by which the muscles are set in motion. This statement is subject, however, to the qualification that no nerve seems to be strictly continuous, but to consist in a series of elements through which motion has continuity. The in-carrying and out-carrying tracts of nerves are woven into a complex central network, so that stimulus may traverse the whole system. ”