NEURONAL INTEGRATIVE MECHANISMS 11 



ture and in which physiologically initiated movements or impulse bursts 

 can be nearly completely accounted for in the neurograms. 



In the simplest case there is a cell which has a fixed frequency of firing 

 and this cell is simply turned on and off by input from the periphery or from 

 higher centers. This has been found in the control of (neurogenic) sound 

 production in a cicada (Hagiwara and Watanabe, 1956) and in the control 

 of electric organ discharge in Torpedo (Albe-Fessard and Szabo, 1954.) 

 In both the pacemaker cell is an interneuron, not a motoneuron, and the 

 fixed frequency is high — 200 per second in the former, 100 in the latter. 

 The frequency or intensity of stimuli to sensory nerves does nothing in 

 the cicada but determine how long the pacemaker will buzz and how 

 promptly it will start. The system is like an oscillator controlled by a switch 

 which can be only on or off but which can be turned on with various speeds 

 due to the finite distance the switch must be moved before it changes its 

 state. In Torpedo it does not yet seem clear whether the 100 per second 

 frequency is independent of the input. In both cases the frequency-deter- 

 mining interneuron is penultimate — it controls the motoneuron directly, 

 one motoneuron on each side in the cicada, about 70,000 on each side or 

 100 for each interneuron in the electric lobe of Torpedo. There is one other 

 step in the cicada. Whereas the electric-lobe motoneurons follow the in- 

 terneurons 1 :1 after the first impulse of a series, the cicada motoneurons 

 follow every other interneuron impulse, therefore firing the muscles at 

 100 per second. Moreover, the two sides are always 180 degrees out of 

 phase, so that there must be some reciprocal inhibition of the two sides. 



The only other preparation which I will discuss here is the lobster heart 

 ganglion (Fig. 4), which is somewhat more complicated. This is largely 

 based on the work of my former associate. Dr. Donald Maynard, but some 

 aspects have been extended by Dr. Hagiwara and myself (Maynard, 1953 

 a,b,c, 1956a,b,c; Bullock, Cohen, and Maynard, 1954; Hagiwara and 

 Bullock, 1955) . Here a pattern is repeated at regular intervals, correspond- 

 ing to each heart beat ; and normally the heart beat, or as we shall call it the 

 burst, is paced by the activity of certain of the four small posterior cells. 

 Here, as in the system we have just examined, each of the follower cells 

 responds to the pacemaker, or to some other cell triggered in turn by the 

 pacer, with a train of impulses whose frequency is not the same as that of 

 any other cell but is peculiar to the cell. But this frequency is not fixed. 

 It starts high or quickly rises to a maximum and then falls along a curve 

 characteristic for the cell over some hundreds of heart beats. This fre- 

 quency/time curve could conceivably be determined entirely by the prop- 

 erties of the given cell since the cell can respond to a single incoming im- 

 pulse by a repetitive discharge, as we have seen happen in intracellular 

 records. A single large, slowly decaying synaptic potential can, by the 



