i6o 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



FIG. 12. Different effects on spikes and p.s.p.'s of cat motoneurons produced with different amounts 

 of membrane polarization. Tlie membrane potential was changed by passing an appropriate cur- 

 rent through the recording microclectrode. A Wheatstone bridge arrangement balanced out the 

 artifacts caused by this current, but as a consequence absolute levels of the membrane potential 

 could not be measured. Upper set (^A to G): Two traces are simultaneously recorded, the upper indi- 

 cating the amount of current flow through the electrode, the lower showing the recorded potentials. 

 .-1 to C, decreasing amounts of depolarizing current; D, no applied current; E to G, increasing 

 amounts of hyperpolarizing current. The records are aligned so that the peaks of the spikes coincide 

 (upper broken line). The parallel lower broken line passes through the point at which the spike 

 begins. When the strong depolarizing current was applied in .4, it quickly evoked a direct spike. 

 A subsequent orthodromic volley evoked a p.s.p. which reached the critical firing level but found 

 the electrically excitable membrane still refractory. Hence, an orthodromically evoked spike was 

 absent. At the end of this and subsequent records is a 50 mv calibrating pulse. B, C, the depolariza- 

 tions from the applied current were smaller. They did not elicit a spike; but summing with the de- 

 polarization of the p.s.p. evoked a spike earlier than the orthodromic volley alone did (D). Hyper- 

 polarization of the membrane operated in the opposite direction, hindering the orthodromically 

 evoked spike which appeared markedly late on the p.s.p. in F, and was absent in 6', although the 

 p.s.p. in the hyperpolarized neuron was larger (compare the p.s.p.'s in A and G). A small deflection 

 which follows the artifact of the stimulus to the nerve and precedes the p.s.p. by neaily i msec, is 

 probably elcctrotonic pick-up of activity from the presynaptic impulses. Note that it is too small to 

 evoke the spike. Lower set QA' to F'). In this experiment the spikes were elicited by antidromic in- 

 vasion from the motor axons. A' to C, decreasing amounts of membrane depolarization; D\ no 

 applied current; E' and F', currents applied so as to produce increasing membrane hyperpolariza- 

 tion. The antidromic spike (Z)') shows an inflection which probably represents a response first in 

 the axon hillock portion, succeeded by involvement of the rest of the cell. Depolarization of the 

 cell body facilitates its invasion by the antidromic spike and minimizes the inflection on the rising 

 phase. It is almost absent when the cell is strongly depolarized (.-l')- Hyperpolarization hinders the 

 invasion of the cell body (F') and when it is strong (F') prevents the response of the soma. The 

 smaller, early component is then seen in isolation as pick-up at the cell body of the response in the 

 axon hillock and nerve fiber. Timing pulses at i msec, intervals are injected into the records. [From 

 Frank & Fuortes (79).] 



in the same cell also permits blockade of spikes by 

 synaptic depolarization induced by drugs that excite 

 the synaptic membrane (fig. 13). This blockade is 

 frequently useful clinically but it in often misnamed 

 as'curarization" (cf. 96). Blockade by o'-tubocurarine 

 and other similarly acting agents operates through an 



entirely different mechanism as will be described 

 below. 



c) ELECTROCHEMICAL GRADATION AND REVERSAL OF 



POSTSYNAPTIC POTENTIALS. Although Synaptic trans- 

 ducer action is not responsive to electrical stimuli, 



