INVERTEBRATE PHYSIOLOGY 



B 



J 



Fig. 3. A. Intracellular potential of the soma of a large neuron of the cardiac 

 ganglion of a lobster. The activity results from stimulation applied to the ganglion 

 a few mm. away. The first deflection is an antidromically conducted spike. The large 

 deflection is interpreted as a synaptic potential resulting from arrival of an impulse 

 in a presynaptic fiber ; it is all or none by this form of presynaptic stimulation. The 

 record shows two spikes arising from the synaptic potential, one at its peak and one 

 on the falling phase ; other records show from one to five, the number apparently de- 

 termined by the condition of the postsynaptic element, not by a difference in the 

 number of presynaptic elements. Time marks, 10 msec. ; vertical calibration, 10 mv. 



B. Intracellular potential of the soma of a large neuron of the cardiac ganglion 

 of a lobster. The activity is spontaneous in the ganglion and in the form of bursts, 

 one for each heart beat. The large deflection is regarded as a synaptic potential result- 

 ing from arrival of presynaptic impulses, and the smaller slower deflections are 

 interpreted similarly. The sharp spikes (7-9 per burst) are the impulses in the axon 

 leaving this soma ; they arise from or near the crest of a synaptic potential. Note 

 that, as in A and C, they do not have a fixed voltage threshold but a threshold which 

 depends on other factors, e.g., time, even in naturally occurring repetitive firing. The 

 spikes between bursts are preceded by a slow depolarization or generator potential ; 

 these are regarded as spontaneous activity of the cell we are in. The record illustrates 

 the complexity of the interaction of integrative properties in a simple ganglion. The 

 record is 3.6 seconds long. 



C. Intracellular potentials of a median giant fiber of an earthworm. Two micro- 

 electrodes are inside the fiber, 11.3 mm. apart, the one farther from the stimulating 

 electrodes is the lowest beam, the nearer one is the middle beam. The top beam is an 

 extracellular monitor still farther down the fiber. Two shocks are given, 2 msec, apart. 

 The first elicits action potentials of 98 and 110 mV. on resting potentials of 72 and 

 74 mV. The second elicits smaller direct spikes plus complex small potentials in the 

 nearer penetration only (arrow), regarded as synaptic potentials from small fiber 

 bombardment eventually leading to a spike which propagates. (There is also a lateral 

 giant spike on the external leads, in the middle of the sweep, but this has no reflection 

 in the internal median giant leads.) Like the preceding, the record shows the inter- 

 action of prepotentials and a complex recovery of excitability in determining firing. 

 Time in msec. (The experiment and permission to use the record are due to the kind- 

 ness of C. Y. Kao.) 



