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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



postsynaptic neuronal discharge under ordinary 

 conditions. It is well known that the dendritic shafts of 

 the large pyramidal neurons in the fifth and sixth 

 layers of the sensory cortex do not give off branches in 

 the fourth layer where the terminals of the thalamo- 

 cortical fibers and the aggregations of Golgi type II 

 cells arc located. The only contact between the large 

 pyramidal cells and the afferent elements is made 

 through the comparatively few paradendritic synapses 

 which are not sufficient to bring about a postsvnaptic 

 discharge. It is most unlikely that the afferent fibers 

 from the thalamus and Golgi type II cells in layer IV 

 can ever directly acti\ate the large pyramidal cells. 

 The discharge of the large pyramidal cells on arrival 

 of an afferent volley of impulses must be achieved 

 through the action of pericorpuscular synapses sup- 

 plied by small and medium pyramidal cells. 



Apical Dendrites and Electrical Signs 

 of the Evoked Potential 



From the point of view set forth above, the surface- 

 positivity of the primary response of the evoked corti- 

 cal potential can be reasonably explained. When the 

 pyramidal cells are indirectly activated by afferent 

 impulses through chains of internuncial neurons 

 including Golgi type II cells, star pyramids and small 

 and medium pyramidal cells which make pericor- 

 pu.scular synapsis with the large pyramids, the post- 

 synaptic impulses initiated at the cell body will serve 

 as a sink and the apical dendrites as a source of the 

 current flow. The record of such electrical change 

 taken from the cortical surface will be a positive wave. 

 As soon as the impulses arrive at the apical dendritic 

 ple.xus at the cortical surface, the electrical sign of the 

 potential will be reversed. The surface-negative wave 

 following the positive deflection of the primary 

 response may be accounted for, at least in part, on 

 this basis. As pointed out previously (14) this interpre- 

 tation does not exclude the possibility that other 

 cortical elements participate in the elaboration of the 

 surface negative deflection. In fact, the neurons 

 situated in the upper layers of the cortex must also 

 be involved. 



The conduction velocity of impulses passing along 

 dendrites is less than 2 m per sec, which is many 

 times slower than that of impulses passing along axons. 

 There is also a decremental reduction in velocity as 

 the impulses are propagated from the proximal part 

 to the terminal regions of the dendrites (21). The 

 long duration of the surface-positive wave of the 

 evoked potential gives every indication of being a 



manifestation of dendritic activity. It is more than 

 probable that the process underlying the surface- 

 positive wave and the following negativity lie mainly 

 in the apical dendrites of different groups of cortical 

 pyramids. The deeply situated basal dendrites of the 

 pyramidal cells are mostly oriented toward the sub- 

 cortical white matter or more or less horizontallv. In 

 other words, they are arranged in a direction roughly 

 opposite to that taken by the apical dendrites. Thus, 

 during the discharge of the pyramidal cells the po- 

 tential changes originating from the basal dendrites 

 must be greatly neutralized by the divergently 

 propagating potentials along the overwhelmingly 

 dominant apical dendrites, if the potentials are 

 recorded from a lead on the cortical surface. 



Micrnelectrode Findings Concerning the Mechanism 

 of Impulse Initialiiin in Single .Neurons 



Without going into a detailed discussion of the 

 fundamental mechanism by which the propagated 

 impulse is initiated hw electrical or synaptic excitation, 

 it may be relevant to mention here a few experimental 

 facts which seem to characterize the synaptically 

 evoked potentials as contrasted with the antidromi- 

 cally produced potentials. As revealed by intracellular 

 microelectrode recordings, the discharge of a neuron 

 elicited by synaptic excitation of the cell body or 

 dendrites is characteristically different from that 

 elicited by stimulation of its axon. The synaptically 

 produced discharge as well as the spontaneous firing 

 of a neuron is usually preceded by a slowly rising 

 positive deflection upon which the sharp spike rides 

 when the discharge threshold is reached. On sub- 

 liminal stimulation only the small deflection will be 

 present. Such preliminary potentials have been 

 observed in spinal motoneurons (11), in the thalamus 

 (60) and in Betz cells of the motor cortex (59). It has 

 sometimes been called the * synaptic potential". Since 

 it may be present without necessarily involving 

 synaptic transfer of impulses and since the term also 

 describes the potentials recorded by other means, it 

 has been suggested to adopt the noncommittal term 

 ' prepotential' in its place (68). 



The fact that the prepotential is present mostly in 

 the evoked or spontaneously occurring responses 

 which involve the activity of cell body and dendrites 

 but not in the antidromic responses seems to suggest 

 that activity of the dendrites plays an important role 

 in initiation of the spike discharge. In this respect 

 Eyzaguirre & Kuffler's study on single neurons of the 

 lobster and cravfish has thrown much light on the 



