72 The Conduction of Impulses by Nerves /4 : 2 



2. A Brief Glance at Electricity 



Physicists consider all matter to be made up of neutral atoms, which, in 

 turn, are made up of positively and negatively charged particles. 

 Although large chunks of matter are electrically neutral, on a subatomic 

 scale, many particles have a net charge. In a liquid or a crystal, there 

 are often ions or groups of atoms which are likewise charged. For 

 instance, NaCl splits into Na + and CI - ions in a water solution. In an 

 NaCl crystal, the sites are occupied by Na + and Cl~ ions. Even water 

 has measurable H + and OH" concentrations. Thus, on an atomic or 

 molecular scale, charges frequently do not balance out even though a 

 volume containing many molecules is approximately electrically neutral. 



Likewise, when a metal is placed in a liquid, or when two dissimilar 

 metals are placed in contact, the two faces of the surfaces of discontinuity 

 become charged. The "dry" cell and storage battery are examples of 

 two metals in a liquid. Unlike charges are separated at the metal- 

 liquid interfaces. If two dissimilar metals are used, the charge separa- 

 tion will be unequal; charges will flow when these two metals are 

 connected by an external conductor. The thermocouple is an example of 

 a practical use of the charge separation at the junction of two metals. 



If charge is not allowed to flow after equilibrium has been established, 

 the actual charge separation is very small in each of the cases above ; 

 the net charge separation is negligible compared to a coulomb. This 

 leads one to suspect that although matter is approximately neutral, in 

 no case do the charges balance out to the last electron. Biological cells 

 and parts of cells are not exceptions. The net charge on any cell 

 measured in coulombs is infinitesimal, but measured in the units of the 

 charge on an electron e, it is appreciable. The neurons are distinguished 

 from most other cells in that they are specialized to transmit changes in 

 their surface potential rapidly. (Muscle fibers are similar to neurons in 

 this respect.) 



The flow of electrical charge is known as electrical current. Currents are 

 measured in units called amperes. Early investigators of bioelectrical 

 phenomena regarded the current as the fundamental event in the con- 

 duction of impulses by neurons. Hence, they referred to these as 

 action currents. 



Considerable experimental evidence, however, supports the electrical 

 potential changes as being uniquely a property of the neuron membrane. 

 In any case, the potential is the parameter actually measured in most 

 experiments. The electrical potential difference between two points is 

 defined in elementary physics as the energy received by a unit charge 

 when it is carried between these two points. Thus, it is a potential 

 energy per unit charge. Electrical potential is usually measured in volts. 



