Experimental physiology / by Sir Edward Sharpey-Schafer.

  • Sharpey-Schäfer, E. A. (Edward Albert), Sir, 1850-1935.
Date:
1927
Catalogue details

Licence: In copyright

Credit: Experimental physiology / by Sir Edward Sharpey-Schafer. Source: Wellcome Collection.

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    of the variation in amount of current; but if quick, the muscle-nerve responds to each movement of the bridge. The rheonome may also be made with a copper bridge, the ends of which dip into solution of sulphate of copper. Excitation by induced current; Determination of excitability of a nerve.—An induction shock is a stimulus, and the break induction shock a far stronger stimulus than the make. Get the minimal effect of each by sliding the secondary coil to the necessary distance from the primary, and make a note as to the respective positions of the secondary coil. This gives a measure of the excitability of the nerve. Its excitability to tetanisation (use the Neef’s hammer with and without the Helmholtz wire) is determined in the same way. Fig. 31.—Stimulation of nerve by condenser discharge. Reaction of nerve and muscle to galvanic and faradic excitation.— If the above experiment is tried (a) with a nerve-muscle preparation and (6) with nerveless muscle (the nerves within the muscle are paralysed by curari), it will be found that the nerve will react to a weaker stimulus than the muscle when induction shocks are employed ; but if the make and break of a galvanic circuit be used as the stimulus the muscle will respond to the weaker stimulus. Plain muscle shows this even more strikingly. Chronaxy.—A galvanic current must, in order to produce excita- tion, not only be of sufficient strength, but must also be allowed to act for a sufficient time on the preparation which is to be stimulated. The minimum duration of the action of such a stimulating current is termed the chronaxy (time value) of the tissue. Different kinds of nerve and muscle vary greatly in their chronaxy : that of ordinary skeletal muscle and myelinated nerve being, for example, much shorter than that of involuntary muscle and non-myelinated nerve.