Reponse in the Living and Non-Living/Chapter 1
RESPONSE
IN THE
LIVING AND NON-LIVING
CHAPTER I
THE MECHANICAL RESPONSE OF LIVING SUBSTANCES
Mechanical response —Different kinds of stimuli—Myograph—Characteristics of response-curve: period, amplitude, form—Modification of response-cumes.
ONE of the most striking effects of external disturbance on certain types of living substance is a visible change of form. Thus, a piece of muscle When pinched contracts. The external disturbance Which produced this change is called the stimulus. The body which is thus capable of responding is said to be irritable or excitable. A stimulus thus produces a state of excitability which may sometimes be expressed by change of form.
Mechanical response to different kinds of stimuli.— This reaction under stimulus is seen even in the lowest organisms; in some of the amoeboid rhizopods, for instance. These lumpy protoplasmic bodies, usually elongated while creeping, if mechanically jarred, contract into a spherical form. If, instead of mechanical disturbance, we apply salt solution, they again contract, in the same way as before. Similar effects are produced by sudden illumination, or by rise of temperature, orby electric shock. A living substance may thus be put into an excitatory state by either mechanical, chemical, thermal, electrical, or light stimulus. Not only does the point stimulated show the effect of stimulus, but that effect may sometimes be conducted even to a con- siderable distance. This power of conducting stimulus, though common to all living substances, is present in very different degrees. While in some forms of animal tissue irritation spreads, at a very slow rate, only to points in close neighbourhood, in other forms, as for example in nerves, conduction is very rapid and reaches far.
The visible mode of response by change of form may perhaps be best studied in a piece of muscle. When this is pinched, or an electrical shock is sent through it, it becomes shorter and broader. A responsive twitch is thus produced. The excitatory state then dis- appears, and the muscle is seen to relax into its normal form
Mechanical lever recorder.-In the case of contraction of muscle, the effect is very quick, the twitch takes place in too short a time for detailed observation by ordinary means. A myographic apparatus is therefore used, by means of which the changes in the muscle are self-recorded. Ts we obtain a history of its change and recovery from the change. The muscle is connected to one end of a writing lever. When the muscle contracts, the tracing point is pulled up in one direction, say to the right. The extent of this pull depends on the amount of contraction. A band of paper or a revolving drum-surface moves at a uniform speed at right angles to the direction of motion of the writing lever. When the muscle recovers from the stimulus, it relaxes into its original form, and the writing point traces the recovery as it moves now to the left, regaining its first position. A curve is thus described, the rising portion of which is due to contraction, and
the falling portion to relaxation or recovery. The ordinate of the n issa the ti response, and the abscissa the time , iMacsaaaue Davies (fig. 1). RECORDER Characteristics of the response- ““)one is securely held at one curve: (1) Period, (2) Amplitude, °%% the other end being con- Under the action of stimu- lus the contracting muscle over the travelling recording ° cle recovers from contrac- may these response-curves be dis- tion, the tracingpointretumns tinguished from each other. Ag on P the record of muscle 5 - curve. regards the period, there is an activity of the muscle. For instance, in tortoise it may be as high as a second, whereas in the wing-muscles of ‘It is probable that a continuous graduated scale might, as suggested by Hermann, be drawn up in the animal curve represents the intensity of The muscle M with the attached nected with the writing lever. (3) Form.—Just as a wave of sound pulls the lever and moves (2) amplitude, and (3) form, SO surface P. When the mus- to its original position. See enormous variation, corresponding to the functional many insects it is as small as z4, part of a second. kingdom, from the excessively rapid contraction of B32 insects to those of tortoises and hibernating dormice.'1[1] Differences in form and amplitude of curve are well illustrated by various muscles of the tortoise. The curve for the muscle of the neck, used for rapid withdrawal of the head on approach of danger, is quite different from that of the pectoral muscle of the same animal, used for its sluggish movements.
Again, progressive changes in the same muscle are well seen in the modifications of form which consecutive muscle-curves gradually undergo. In a dying muscle, for example, the amplitude of succeeding curves is continuously diminished, and the curves themselves are elongated. Numerous illustrations will be seen later, of the effect, in changing the form of the curve, of the increased excitation or depression produced by various agencies.
Thus these response records give us a means of studying the effect of stimulus, and the modification of response, under varying external conditions, advantage being taken of the mechanical contraction produced in the tissue by the stimulus. But there are other kinds of tissue where the excitation produced by stimulus is not exhibited in a visible form. In order to study these we have to use an altogether independent method, the method of electric response.
- ↑ 1 Biedermann, Electro-physiology, p. 59.