802 



Comparative Animal Physiology 



covery resembles the supernormal period in cardiac mxuscle (Ch. 15). Ex- 

 planations have been suggested as follows: 



(a) It is difficult to account for this delayed facilitation on the basis of 

 persistence of any chemical transmitter unless it is assumed that a second 

 excitatory substance is liberated. 



(I?) In nerve fibers the excitability returns during the refractory period 

 and is supernormal during the negative after-potential. Similar enhanced ex- 

 citability can be produced by cathodal polarization, catelectrotonus. In some 

 nerve centers the late facilitation is at a time of delayed negativity. A rela- 

 tion between reflex facilitation and slow negative potentials in the spinal 



(A) 



— > 



(C) 



Fig. 302. (A) Diagram of delay path circuit in oculomotor nucleus. F, Floor neurone; 

 I, intemeurone; M, motoneurone; O.N., oculomotor nucleus; COL, coUiculus; MED., 

 medulla. Modified from Lx)rente de N6.^ (B) Schema of a delay path (multiple chain) 

 circuit with three successive intemeurones. (C) Schema of a closed reverberating chain 

 circuit of internuncial neurones. From Lorente de N6.^ 



cord is indicated.^^'' In the superior cervical sympathetic ganglion the gan- 

 glionic response consists of one or more spikes, lasting 6-8 msec, followed by 

 a slow negative wave persisting 50-100 msec.^-^' -''^ When two preganglionic 

 volleys enter the ganglion at such an interval that the second enters during 

 the slow negative wave due to the first, a spike results which is facilitated 

 and has shorter latency than when the second stimulus is applied alone. The 

 time course of this facilitation follows the slow negative wave according to 

 Eccles,^-'' but not according to Rosenblueth and Simeone. '''''' 



(c) A third explanation for delayed facilitation is by-pass conduction in 

 intemeurones. In the oculomotor nucleus^''-' -''^ e.g., neurones are ar- 



