172 DONALD M. MAYNARD 



variation (Fig. 22). It would seem, therefore, that even in such a Hmited 

 system as the ganglion where the only known synaptic connections between 

 intrinsic neurons are excitatory, the integrating neuron gains striking freedom 

 with respect to its patterned activity. Such a unit may do much more in 

 response to a given stimulus than merely increase or decrease its discharge 

 frequency. 



One problem in the analysis of a neural system is the selection of para- 

 meters significant in its normal function. In the cardiac ganglion, this diffi- 

 culty is partially answered. The heart can function as an efficient pump only 

 with co-ordinated contractions which develop intracardiac pressures greater 

 than those in the arteries. These contractions in turn require a co-ordinated 

 burst of nervous activity from the motor neurons. The sequence, amplitude, 

 and duration of contraction of the individual muscle bundles within the 

 heart presumably depend upon the sequence, frequency, and number of 

 discharges arising in particular motor neurons in a given burst. In terms of 

 heart contraction, therefore, the temporal pattern and synchronization of 

 impulse discharge in motor units becomes more significant than such para- 

 meters as "average discharge frequency" or non-propagated membrane 

 potential shifts. 



Two aspects of the inhibitor junctions seem to favor the retention of some 

 such synchronized and patterned activity during the otherwise disruptive 

 effects of partial inhibition. First, a differential sensitivity to inhibition exists 

 in that spontaneous discharges tend to be blocked more easily in the followers 

 than in the pacemakers. Such a difference insures bursts as long as neurons 

 remain active, and the range of inhibition giving a graded heartbeat becomes 

 determined by the hmits of activity of the followers rather than a failure in 

 co-ordination. Second, inhibitor action is reported to facilitate synaptic 

 transmission in some of the followers. If this phenomenon is substantiated, 

 and occurs in the normal ganglion, it must extend the normal limits of 

 follower activity during inhibition and permit eff'ective heartbeats over a 

 wider range of inhibition than otherwise possible. 



Under the above conditions, it is not surprising to find burst co-ordination 

 retained at the expense of regularity. Indeed, the reported patterning effects 

 follow directly from the characteristics of the e.p.s.p. and i.p.s.p. described 

 for single units in earlier sections. It is clear, however, that knowledge of 

 single unit characteristics alone could not exclude the very real possibility 

 of indiscriminate reduction in excitability during inhibition and the conse- 

 quent disruption of the synchronized burst with release of spontaneous, 

 independent motor neuron discharge (see Fig. 25). The latter prediction 

 would require further knowledge of relative sensitivities of the interacting 

 units of a system where, as Fig. 22 shows, a very small excitability change in 

 one unit may lead to much greater and opposite alterations in total system 

 output. 



