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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



depolarizing and hyperpolarizing potentials without 

 spikes (184). 



Some invertebrate muscle filjers possess dual 

 synaptic activity (72, 73) and it has been suggested 

 (96) that vertebrate smooth muscle may also belong 

 to this category. If the fibers are not electrically 

 excitable, the contractions caused by their depolariz- 

 ing p.s.p.'s would be local, as in frog "slow' muscle 

 fibers (34, 35, 132). The hyperpolarizing p.s.p.'s 

 would serve the function of diminishing or regulating 

 the degree of the mechanical response by decreasing 

 the depolarizations of the 'excitatory' p.s.p.'s. 



By far the most prominent class are the cells in 

 which spikes as well as the two kinds of p.s.p.'s are 

 generated. Most, and perhaps all, neurons of the 

 vertebrate central nervous system probably belong 

 to this group (cf. 59, 60, 158, 159, 161 -167). The 

 hyperpolarizing and depolarizing p.s.p.'s appear to 

 have nearly identical durations and the superposi- 

 tion of the two p.s.p.'s may decrease membrane 

 depolarization sufficiently to eliminate spike produc- 

 tion by an orthodromic excitatory pathway. This 



interaction of depolarizing and hyperpolarizing 

 p.s.p.'s adds to the variety and flexibility of integra- 

 tive activity within the central nervous system. The 

 effects are achieved not only by relatively simple 

 algebraic summation of the potentials but also by 

 the interplay of more subtle factors which will be 

 described in the next section of this chapter. 



Fast and Slaw Respunses of Iinrrtebratt' Muscles 



The muscle fillers of some insects and Crustacea 

 (cf. 116, 117) are known to be electrically excitable, 

 but they also respond diff^erently to stimulation of 

 different excitatory nerves (fig. 21). Their 'fast' in- 

 nervation, which may be constituted of one or several 

 nerve fibers, produces large depolarizing p.s.p.'s 

 upon which is superimposed a spike-like response, 

 often showing a small overshoot. Stimulation of the 

 'slow' nerve fiber leads to a small depolarizing p.s.p. 

 Upon this there may develop various gradations of 

 the electrically excitable response. The mechanical 

 acti\ities are also different. The fast nerve fiber 



mV 



FIG. 21. Different responses produced in insect muscle fibers on stimulating their fast and slow 

 innervation. Intracellular recording from e.xtensor tibiae of the mesothoracic leg of Schistocerca gri'- 

 garia. A: The responses of six different muscle fibers, first to stimulation of the fast nerve fiber and 

 then the slow. In all but one muscle fiber, the fast response developed an overshoot. .^K notch on the 

 response of fiber ii indicates the level on the p.s.p. out of which the spike-like activity developed. 

 In fiber ;, as in about 50 per cent of the muscle fibers, no response resulted on stimulating the slow 

 nerve fiber. Various grades of activity are shown in the other examples. In three of these (iii, iv, r) 

 the p.s.p. was large enough to evoke some local response of the electrically excitable membrane. B: 

 Three examples of facilitation of the p.s.p.'s by repetitive stimulation of the slow nerve at about 

 30 per sec. The augmented p.s.p.'s evoked larger pulsatile local responses, and in one case (i.v) an 

 overshoot was obtained. Time and amplitude calibration in (0 apply to all records. [From Hoyle 

 (1 16).] 



