662 PRINCIPLES OF GENERAL PHYSIOLOGY 



SUMMARY 



In the use of instruments for recording the changes in the electrical state 

 of tissues, the important point to be kept in mind is that the moving parts shall 

 be able to follow rapid alterations in current or potential, and this without 

 overshooting the correct position. They must either be aperiodic, but without 

 more damping than just necessary, or their own vibration period must be shorter 

 than that of any change to be measured. 



The interest of electrical changes is not only as giving insight into the 

 processes going on in the cells, but also as a means of investigation of the time 

 relations and other properties of these processes. In the case of nerves, there 

 is frequently no other method available for detecting the existence of the passage 

 of impulses. 



Although the ultimate source of differences of potential in cells must be due 

 to the separation of electrically charged ions, it is found that the different rates 

 of movement of ions free to diffuse is inadequate, while the presence of metallic 

 electrodes similar to those of the usual form of concentration battery is out of the 

 question. 



On the other hand, the existence of a cell membrane permeable to one only 

 of the oppositely charged ions of a binary electrolyte is capable of accounting 

 satisfactorily for all the phenomena met with. 



It is shown that the electromotive force of a concentration battery of this 

 particular kind follows the same formula as that with metallic electrodes. It 

 may, indeed, be regarded as a model of the process in, the case of metallic 

 electrodes. The difference is that the membrane cell, or electrode, is indifferent 

 to the chemical nature of the ions, being concerned only with the sign of the 

 charge, while the metallic electrode only takes account of ions of the same 

 chemical nature as itself. The reason for this difference in behaviour is that the 

 membrane allows free interchange between diffusible ions of the same sign 

 between the interior and exterior, so long as the potential difference is 

 unchanged thereby. 



The contact surface between phases is, similarly, the site of a potential 

 difference, if one of the ions is soluble in both phases, the other in one only. 



There is reason to believe that the electrical change in nerve and muscle is 

 inseparable from the state of excitation, but that the state of contraction of muscle 

 may be absent, although the electrical phenomena remain. 



Description is given in the text of the way in which the " demarcation current " 

 and the " negative variation " in nerve and muscle are explained on the basis of 

 membrane potential. 



The phenomena in smooth muscle are discussed in the text. 



In the ventricle of the heart, when led off directly, there is a simple diphasic 

 change, indicating the progression of a wave of negativity from base to apex. 



When the electro-cardiogram is obtained from the unexposed heart it has 

 hree phases, the third one indicating negativity of the base. This may be due 

 either to the excitatory state lasting longer at the base than at the apex, or be 

 due to the course of the wave not being so simple as a progression from base to 

 apex merely. The former hypothesis is more in accordance with facts. But 

 it must be remembered that transmission is by means of Purkinje tissue, which 

 conducts faster than ordinary heart muscle, so that contraction may be practically 

 simultaneous in all parts of the ventricle. By shortening, artificially, the dura- 

 tion of the excitatory state at the apex, or lengthening that at the base, the 

 triphasic curve can be obtained from a diphasic one. 



There is no reason to suppose that there is any difference between the 

 components of the electro-cardiogram other than that due to the different times 

 of arrival and duration of the excitatory state at the various parts of the heart 

 muscle. * 



