NEURON PHYSIOLOGY INTRODUCTION 



65 



the potential of which was virtually controlled by the 

 (C'i„)/(Cli) ratio and then later to restore the normal 

 ionic composition of the fiber, as revealed by a normal 

 behavior of the membrane potential to variations in 

 (K„). 



In conclusion it may he stated that, though detailed 

 modifications and developments of the ionic hypoth- 

 esis are recjuired in order to explain such phenomena 

 as the falling phases of the action potentials of medul- 

 lated nerve fibers and cardiac muscle fibers and the 

 effect thereon of repolarizing currents, in essentials 

 the ionic membrane hypothesis has survived the most 

 severe tests and remains as the only conceptual frame- 

 work for our discussion of the electrical events that 

 are so essentially concerned in all activities of the 

 neuron. It will therefore be pertinent to consider now 

 the mode of operation of synapses in the light of the 

 ionic hypothesis. 



TRANSMISSION BETWEEN NEURONS 



The synapse is a device for the transmission of 

 information from one neuron to another. Excitatory 

 synaptic action is effective only in so far as it leads to 

 the discharge of an impulse by the postsynaptic 

 neuron, for only under such conditions does this 

 neuron in turn exert effective action on other neurons. 

 It may be provisionally concluded from the available 

 experimental evidence that any neuron, other than a 

 primary sensory neuron, requires excitatory synaptic 

 action in order to generate an impulse. In the absence 

 of an afferent input even the most complex assem- 

 blages of neurons remain silent, as may be seen in the 

 isolated cortical slabs of Burns (10). 



On the other hand, inhibitory synaptic action 

 attempts to suppress the discharge of impulses and is 

 effective in so far as it diminishes or shortens the dis- 

 charge produced by any given synaptic excitation. 

 Inhibition can be thought of as exercising a sculptur- 

 ing role on what would otherwise be the massive 

 incoordinate activity of a convulsing nervous system, 

 thus reducing it to the organized responses character- 

 istic of normal nervous activity. However, just as 

 with the excitatory synapses, inhibitory synap.ses 

 require activation by presynaptic impulses. Hence, an 

 investigation of the transmis.sion between neurons 

 can be reduced to a study of the mode of operation of 

 excitatory and inhibitory synapses. It will emerge 

 that the ionic hypothesis of the nerve membrane 



provides the basis for our atteinpls to understand both 

 these types of synaptic activity. 



Excitatory Synaptic Action 



Excitatory synaptic action on neurons is exhibited 

 in its simplest form by the monosynaptic action which 

 afferent impulses from the annulospiral endings of 

 muscle spindles exert on motoneurons. When recorded 

 by an intracellular electrode, the monosynaptic 

 action by a single volley generates a depolarizing 

 potential, the excitatory postsynaptic potential 

 (EPSP), that runs virtually the same time course 

 regardless of volley size (fig. 4.4 to C). This observa- 



FiG. 4. A to C. EPSP's obtained in a biceps-semitendinosus 

 motoneuron with afferent voIley.s of different size. Inset records 

 at the left of the main records show the afferent \olley recorded 

 near the entry of the dorsal nerve roots into the spinal cord. 

 They are taken with negativity downward and at a constant 

 amplification for which no scale is given. Records of EPSP are 

 taken at an amplification that decreases in steps from A lo C as 

 the response increases. Separate vertical scales are given for 

 each record of EPSP. All records are formed by superposition of 

 about 40 faint traces. D to G. Intracellularly-recorded po- 

 tentials of a gastrocnemius motoneuron (resting membrane 

 potential, —70 mv) evoked by monosynaptic activation that 

 was progiessively increased from D to G. The lower traces are 

 the electrically differentiated records, the double-headed arrows 

 indicating the onsets of the IS spikes in E to G. HtoK. Intra- 

 cellular records evoked by monosynaptic activation that was 

 applied at 12.0 msec, after the onset of a depolarizing pulse 

 whose strength is indicated in m^ia. A pulse of 20 m^ua was just 

 below threshold for generating a spike. H shows control EPSP 

 in the absence of a depolarizing pulse. Lower traces give 

 electrically differentiated records. Note that the spikes are 

 truncated. [From Coombs el at. (14).] 



