. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 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 ma
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 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 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
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