ALL OR NOTHING 



185 



The neuron may be likened to this electrical model (Fig. 33 A). 

 C is then the nerve cell, F the process or nerve fibre, the key the 

 receptor mechanism and M the effector mechanism. A second 

 circuit of the nature of a telephone relay could be added to the 

 first, S being an electro-magnet which closes the second circuit 

 when the key of the first is depressed (Fig. 34). 



S may be termed the synapse joining an effector and receptor 

 neuron. The student will notice that there is no material con- 

 tinuity between the two neurons and that no energy passes from 

 one to the other. 



4. " All or nothing." It is obvious in the electrical model that 

 connection is either made or not made. The energy available 



JIEACTING OR. EFFECTOR. UNIT 



RECEIVING OR. RECEPTOR UNIT 



S \ 



N, 



SYNAPSE Ci 



B 



ELECTROMAGNET 



FIG. 34. A. Diagram showing a rereirinu (N,) and a reacting Xeuron (A 7 ..), each 

 with dend rites at its extremities, and their connection to one another through a 

 Synapsis (.S'). 



B. Electrical Model to illustrate the functional continuity of two neurons. 

 See text. 



from Cj is- a fixed quantity independent of the energy used to 

 close the circuit, and similarly the energy in the system of which 

 C 2 is the cell is independent of the energy used in the electro- 

 magnet S. 



It is true also for the nervous system that the maximum motor 

 effect is produced, if any effect is produced at all. It is a case of 

 " all or nothing." Nerves are, however, made up of several more 

 or less parallel neurons, and some of these may be stimulated 

 and some may not. As each nerve fibre activates one muscle 

 fibre and as each muscle fibre responds to the activation by a 

 maximal contraction, the result is that the extent of contraction 

 depends on the number of muscle fibres which shorten and which, 

 in turn, depends on the number of nerve fibres implicated. 



5. Temperature coefficient of the nervous impulse. When a 

 length of nerve is cooled its power to conduct an impulse is 



