Nervous Systems 



779 



pears only at the cathode (on the make), is reduced or absent if the nerve 

 is refractory, and is graded in size. Its increase is non-hnear, the local re- 

 sponse increasing out of proportion to the stimulus strength. Local responses 

 have been seen in thin-sheathed fibers of Crustacea^"'' and of Cephalo- 

 poda,^^ and in myelinated fibers of the frog-'*'* (Fig. 290). Lorente de No^^*^ 

 classes this graded response as part of the electrotonic potential. 



(3) When the stimulus is of threshold magnitude the local graded poten- 

 tial reaches a critical height and a propagated nerve impulse (spike) arises 

 out of it. The critical height of the local potential in crustacean fibers is 

 about 20 per cent of the spike height.^'^'^ Under conditions of membrane 

 instability, as in decalcified giant nerve fibers of the squid, electrical oscilla- 

 tions of variable size appear, and out of the largest of these conducted im- 

 pulses arise^'^' ^^ (Fig. 291). The spike is all-or-none, rises rapidly to its 

 crest, and then declines at a decreasing rate. The spike duration varies with 



Fig. 293. A, Changes produced in the after-potentials of a phrenic nerve (A fibers) 

 by short tetanic stimulations. The spike potentials are not visible so the records begin 

 with the negative after-potential and continue into positive after-potential. The top 

 record shov.'s the after-potential response evoked by a single stimulus, for comparison 

 with the curves below at the designated frequencies of stimulation. Time line=60 cycles. 

 B, C, Negative after-potentials in response to single and tetanic stimulation in veratrinized 

 nerve. Time 60 cycles. From Gasser and Grundfest.^^" 



conduction velocity and is roughly 1.5 msec, for fast frog and crustacean 

 fibers. It vi'as formerly believed that the action potential spike was merely 

 a reduction in the resting potential of the nerve fiber. However, in giant 

 nerve fibers of the squid it has been possible to insert an electrode inside 

 the fiber and thus record the maximum potential developed across the mem- 

 brane. In such fibers with resting potentials of about 55 mv. where the in- 

 side is negative, the outside may become negative, sometimes by an addi- 

 tional 50 or 60 mv.^^'^' ^^^ (Fig. 292). Thus the spike is more than a mere 

 reduction of resting potential; it is also an active reversal in potential. 



(4) Following the spike and delaying its downward deflection is a nega- 

 tive after-potential (Fig. 293, B, C). The negative after-potential repre- 

 sents a process of negativity which starts during the spike and reaches its 

 peak much later (4-6 msecs. in frog A fibers, which have a spike crest time 



