TRANSIENT BIOELECTRICS IN NERVE 275 



logued as coming from normal or diseased tissue. Patterns taken on an indi- 

 vidual vary with the emotional state. A creative man is said to have pat- 

 terns quite different from one who lacks new ideas. However, the fine struc- 

 ture of these waves is not well understood. Recorded spikes are only about 

 150 nv high. Characteristic spikes of different shapes and frequencies have 

 been named alpha, theta, delta, etc. These are depicted in Figure 10-6. 



Location of tumors, via predominance of the delta waves (see Table 10-2), 

 has been particularly successful, with 73 to 90 per cent accuracy claimed. 

 Bagchi has reported 84 per cent in 333 tries. Other abnormalities, such as 

 epilepsy, have been studied by this technique. 



TABLE 10-2. Classification of Electroencephalograph Waves. 



Names of 

 Waves 



Frequency (cps) Association 



delta 0.5 to 3.5 "disease, degeneration, 



death; defence"* 

 theta 6 to 7 



alpha 8 to 13 a scanning mechanism? 



beta 14 to 30 alertness; active response 



♦Walter, W. Gray, "The Living Brain," Penguin Books, Baltimore, 1960, p. 81. 



While the all-encompassing phenomenological techniques of EEG have 

 been making useful contributions to life, studies of individual neurons, via 

 microelectrodes in the cortex, and studies of the properties of synapses and 

 ganglia in the spinal cord have demonstrated interesting phenomena such 

 as: inhibition of transmission across nerve endings (strong signals passed 

 through one nerve ending reduce the effectiveness of one close by); post- 

 tetanic potentiation (faster and more energetic transmission through a par- 

 ticular nerve path following a rapid succession of pulses through that path); 

 and the promotion of epileptic-like seizures and peculiar mental images in 

 man by electrical stimulation of particular spots in the cortex via micro- 

 electrodes. 



Transfer of an impulse across a synapse (Figure 10-7) is currently thought 

 to be by means of "chemical" transfer rather than by "electrical," for two 

 reasons: the observed salt concentration changes associated with a single im- 

 pulse are very small; and there is fairly good evidence that acetylcholine 

 (ACh) accumulates in the gap during transmission across the gap. A theory 

 is that ACh is contained in the many little vesicles in the pre-synaptic end- 

 ing: that, during "activity," ACh is expelled through the membrane and 

 diffuses to the post-synaptic membrane and locally depolarizes it. The de- 

 tails of this mechanism are still unknown. 



