206 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



differences in the electrical characteristics of the 

 muscle fiber membrane in parallel with the junc- 

 tionally responding region. 



In the mammalian muscle fiber under normal 

 conditions, the endplate potential does not form a 

 conspicuous step on the rising phase of the spike as it 

 does in the amphibian (4). The explanation for this 

 lies in the fact that the threshold depolarization for 

 initiating an action potential is here much lower 

 (about 10 mv compared with 40 mv in the frog), and 

 at this level the transition between endplate potential 

 and spike does not involve an appreciable change in 

 rate of rise of potential. In the curarized preparation 

 the response differs from that seen in the frog in hav- 

 ing a shorter decaying phase and in becoming at- 

 tenuated more rapidly with increasing distance from 

 the junction. These differences are attributable 

 entirely to a higher conductance of the muscle fiber 

 membrane with a consequent reduction in the electric 

 time and space constants. 



The tonic muscle fibers of the frog, supplied by the 

 small diameter motor nerve fibers, display differences 

 in their electrical response from the twitch fibers of 

 the same animal which are again mainly attributable 

 to the electrical properties of the fiber membrane, 

 though the disposition of the nerve endings also plays 

 an important part (10, 11, 54). The tonic muscle 

 fiber is unable to develop an action potential, due 

 apparently to the absence of the mechanism by which 

 the sodium permeability of the membrane is increased 

 by depolarization. The entire time course of the 

 junctional response can therefore be observed under 

 all conditions without the complication of a super- 

 imposed spike. In addition the amplitude of the junc- 

 tional response can be varied by stimulation of differ- 

 ent nerve fibers. Owing to a wide and relatively 

 uniform distribution of their endings along the muscle 

 fiber, the potential wave does not show a marked 

 attenuation with distance, and at no position does it 

 have the initially rapid and later relatively slow 

 decline of the endplate potential recorded at the 

 junction of a twitch fiber. Another distinctive feature 

 of the response in these fibers is that the membrane 

 potential goes through a phase of hyperpolarization 

 after recovering from the depolarization. A similar 

 phase of hyperpolarization is found to follow a wave 

 of depolarization elicited by a current pulse applied 

 directly to the muscle fiber, from which it is inferred 

 that this feature is not due to .some peculiar charac- 

 teristic of the transmission process but depends rather 

 on the electrical behavior of the membrane. 



.\CT1VITY OF THE NERVE TERMIN.^LS 



In the preceding .section, the local electrical changes 

 brought about in the normal and in the curarized 

 muscle fiber by the arrival of an impulse in the pre- 

 junctional nerve terminals have been described. In 

 this section the behavior of the terminals will be con- 

 sidered under various conditions, in .so far as this 

 throws light on their specialized properties. Almost 

 all the information to be presented is derived from 

 recording potential changes in the muscle fiber. 

 According to the chemical theory of transmission, 

 activity in the nerve terminals causes a release of 

 acetylcholine which then reacts with the muscle to 

 produce an alteration in it. Hence, when recording 

 from the muscle, an indication of activity at the 

 terminals is obtained, provided that allowance is 

 made for possible effects in the later stages of the 

 transmission process. An example of such an effect is 

 the reduction of the responsiveness of the muscle 

 fiber by curare through its competition with acetyl- 

 choline. 



When the membrane potential is recorded in the 

 junctional region of a muscle fiber, a sequence of 

 small transient changes of potential (as shown in fig. 

 5) is observed even in the absence of a nerve impulse 

 (3, 41, 62). Although their peak amplitude is only of 

 the order of 0.5 mv, these potential changes have 



2raV 



50 msec 



FIG. 5. Spontaneously occurring miniature endplate poten- 

 tials recorded at the junctional region of a muscle liber of a 

 frog. The location of the recording position was confirmed by 

 the form of the response elicited by a nerve impulse. [From 

 Fatt & Katz (39).] 



