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HANDBOOK OF PHVSIOLOGV 



NEUROPHVSIOLOCi' I 



cession. Thus motor nerve cells in the vertebrate 

 spinal cord discharge impulses repetitively over a 

 wide rang;e of frequency depending; on the state of 

 the cell. Similarly, sensory nerve fibers carry a series 

 of impulses toward the spinal cord in response to 

 sensory stimuli delivered to their endings. There is 

 at present a large amount of data concerning the 

 pattern of impulse discharge obtained by the method 

 of recording single fiber responses originated by 

 Adrian (3, 4). 



In many excitable tissues, application of a long 

 constant current generates a train of action potentials, 

 as shown by Arvanitaki (8), Fessard (33), Erlanger & 

 Blair (29), Katz (72) and others. The records fur- 

 nished in figure 35 show repetitive firing of action 

 potentials in the .squid giant a.xon induced by con- 

 stant outward membrane currents of four different 

 intensities. The stimulating pulses are sent into the 

 a.xon through a long intracellular metal wire elec- 

 trode, and the responses are recorded with another 

 intracellular electrode. It is difficult to maintain 

 repetitive firing indefinitely under these experimental 

 conditions. It is to be observed that each action po- 

 tential is preceded by a slowly rising phase of the 

 membrane potential. This slowly rising phase has 

 been demonstrated at the sites of naturally induced 

 repetitive responses in the automatically beating 

 cardiac muscle [cf. VVeidmann (143)]. 



The site at which impulses are initiated repetitively 

 is called a 'pacemaker'. At present, it is not clear how- 

 sensory nerve endings or the motor nerve cells become 

 pacemakers. However, there is one thing that can be 

 inferred from the mechanism of the nervous conduc- 

 tion in the peripheral nerve fiber. As has been dis- 

 cussed on previous pages, nervous conduction is ef- 

 fected through excitation of each segjment (or node 



of RanvierJ l)y the electric current generated b\ the 

 adjacent active segment. From this one can infer that 

 a sensory stimulus or a natural stimulus for the motor 

 nerve cell has to be transformed eventually into an 

 electric stimulus in order that it initiates a propagated 

 impulse. (If the size and shape of the electric current 

 generated by a sensory stimulus are similar to those 

 of the ordinary action current, the statement just 

 made has no meaning; however, it is generally ac- 

 cepted that the first electrical sign of the response to 

 a sensory stimulus is variable in size and very differ- 

 ent from the ordinary all-or-none response.) Since a 

 constant current applied to a peripheral nerve fiber 

 can gi\'e rise to a repetitive firing of impulses, it is 

 generally believed that natural pacemakers resemble 

 in some respect an artificial one induced i)y applica- 

 tion of a constant current (fig. 35). 



The mechanism of repetitive firing proposed by 

 Adrian (3, 4) to interpret the injury and sensory dis- 

 charges of impulses is as follows. An electric stimulus 

 of a constant intensity sets up the first action potential 

 in accordance with the law of electric excitation. 

 Then, the nerve fiber falls into the refractory period 

 which makes the stimulus totally ineffective. As the 

 fiber recovers from this refractoriness, the stimulus 

 becomes effective again and the second action po- 

 tential is set up. The second response leaves behind 

 it another refractory period. The nerve fiber thus 

 exhibits a kind of oscillatory phenomenon similar to 

 that in a neon lamp connected to a battery, a con- 

 denser and a resistor. 



It is simple to express Adrian's concept in terms of 

 the membrane potential and the threshold depolariza- 

 tion. At the beginning of the refractory period, the 

 critical membrane potential is close to the level of 

 the shoulder of the action potential (see fig. 20). 



FIG. 35. Repetitive firing of action potentials in a squid giant axon. The relative intensities of the 

 stimulating currents used are indicated by the broken lines. Both stimulating and recording elec- 

 trodes were long intracellular metal wires. [From S. Hagiwara e! al., unpublished.] 



