TRANSIENT BIOELECTRICS IN NERVE 273 



(4) Chemical: foreign chemicals applied, or changes in concentration of 

 natural chemicals — taste buds, dehydrated tissue cells, etc. 



(5) Gravitational: continuous attraction to earth, occasionally varied by 

 superposition of various accelerations — balance-detectors in middle 

 ear, for example. (These are essentially of type (2).) 



Deserving special mention as a trigger is trie "pacemaker" of the heart, 

 which in man repetitively stimulates the pump to compress and relax once 

 about every 1.3 sec 24 hr a day for life. Recordings from microelectrodes 

 inserted into pacemaker cells show that they are self-contained oscillators. 

 Very recently D. Noble has shown 19 that if certain limiting conditions are 

 imposed on the cable-and-changing-permeability theory described above, 

 the theory can describe the condition of oscillating permeability and oscil- 

 lating potential of the membrane of the pacemaker cell. 



When and if the pacemaker fails, it has been shown to be possible to 

 stimulate the heart artificially. With small transistor circuitry and small 

 zinc-mercuric oxide batteries, it has been demonstrated recently that an 

 artificial pacemaker can be buried, by surgery, in the abdominal cavity 

 under the skin and stimulate a weak heart regularly for at least a year be- 

 fore the battery has to be changed (again by surgery). This device has 

 brought a normal life to many people. 



Recent advances in microelectrode preparation have permitted glass tubes 

 to be drawn down to an outer diameter of 0.0005 cm, filled with electrolyte, 

 and the ends inserted carefully right into the individual muscle cells in the 

 animal's beating heart. Thus the electrical measurements on cells working in 

 situ are now being made. Great care has to be taken that the electrical meas- 

 urements are not affected by the huge electrical resistance of these micrelec- 

 trodes (try Problem 10-5). For steady potentials an electrometer with a high 

 impedance is usually used; but for rapidly-varying potentials, such an in- 

 strument is too slow to follow the potential changes without inducing dis- 

 tortion. There this problem of measurement presently rests. Once it is 

 solved, although the cross-correlation of electrical and chemical information 

 may still not be possible in these small cells because of the size of the object 

 under study, pharmacological problems should receive much attention with 

 this technique. Indeed the neuromuscular junction is already being so ex- 

 plored. 



Studies on the Central Nervous System 



By contrast with the normally resting peripheral nerve tissue, which is 

 activated upon demand, the brain is a mass of spontaneously pulsating neu- 

 ral networks, seemingly continuously energized and active. It is usually as- 

 sumed that the basic processes are electrochemical, like those just outlined 

 as being proper to nerve conduction. However, biophysical knowledge of 



