INITIATION OF IMPULSES AT RECEPTORS 



127 



instance, the applied force will cause a relatively large 

 increase in the frequency of the discharge, an increase 

 which will then decline until the correct frequency 

 for the new steady state has been reached. A similar 

 process occurs if there is a sudden decrease in, again 

 for instance, the applied force. In this instance the 

 frequency falls abruptly to a value below that ex- 

 pected for the new steady state and then increases 

 with time. Thus, if a muscle spindle is discharging 

 rhythmically and the muscle in which it lies is 

 stretched for a time and then suddenly returned to 

 its resting length, the frequency of the discharge from 

 the spindle falls well below its resting value, possibly 

 to zero; after a time the resting rhythm re-establishes 

 itself (75). Similar changes can be observed in other 

 types of unit, for example in temperature sensitive 

 units (46), and the pressure sensiti\e units of the cat's 

 carotid sinus (60). It should be noted that the beha- 

 vior of such units contrasts with that of phasic units 

 which are considered in another section below. 



Nature of Repetitive Firing 



Ideas on the mechanisms by which firing takes 

 place started with the proposals of Adrian (i). Essen- 

 tially these were that special nonaccommodating 

 regions of nerve exist at sensory nerve endings and 

 that repetitive activity is initiated in these regions; the 

 frequency of the discharge depends on the refractory 

 period which may be longer here than in other parts 

 of the nerve. Broadly speaking, work on the nature of 

 repetitive firing by sensory receptors has followed two 

 lines. The first has attacked the problem of nerve 

 accommodation and the other, the mechanism that 

 determines the interval between impulses. 



Many investigations have been carried out on the 

 rate of accommodation of nerve and these have shown 

 that accommodation need not be rapid and that in 

 crustacean (49), amphibian (26) and mammalian 

 (32, 89) nerve it is in fact possible to obtain main- 

 tained repetitive firing during the passage of a con- 

 stant current. Further it has been found that most 

 experimental procedures tend to increase the rate of 

 accommodation (81); it is possible that the common 

 eflfect of all these procedures is to lower the membrane 

 potential, a reduction of which is known to increase 

 the rate of accommodation (94). These findings led 

 to the view that the mechanism of repetitive firing 

 from sensory receptors could be explained on the 

 known properties of nerve fibers. This view was 

 elaborated in particular by certain Scandinavian 



workers (9, 30) who suggested that the receptor 

 develops a 'generator potential' which causes current 

 to flow in the nerve fiber so acting like a constant 

 current stimulus in setting up a train of impulses. 

 This idea has remained the basis of most subsequent 

 work on the subject. 



The concept that the inter\als between the im- 

 pulses of a train are dependent on the rate of recovery 

 after an impulse is faced with the difficulty that 

 rhythmic discharges of very low frequency, a few 

 impulses per second, can be obser\ed. These intervals 

 are much longer than the total duration of the re- 

 covery process as known in nerve. Investigations on 

 the repetitive firing of crustacean nerve during the 

 passage of a constant current have introduced another 

 idea C49)> that the intervals between impulses are 

 determined by the response time. That is to say the 

 intervals are determined in the same manner as the 

 latency from the l:)eginning of a current stimulus to 

 the initiation of the first impulse. 



The passage of a constant current through a crusta- 

 cean axon sets up a repetitive discharge as shown in 

 figure 3. Several points can be seen in this figure; the 

 frequency of discharge is related to the current 

 strength; the interval between the beginning of the 

 current and the first impulse is always closely related 

 to the intervals between the other impulses; these 

 intervals are all dependent on the development of 

 the local response, an impulse being initiated when- 

 ever this local response reaches the critical potential; 

 the critical potential at which the impulses are set up 

 is the same with all but the greatest strengths of cur- 

 rent and all but the highest frequencies of impulses. 

 Apart from this direct evidence that it is the time 

 course of the development of the local response that 

 sets the interval between impulses, the recovery time 

 of these axons is such that it cannot explain the fre- 

 quencies observed. These crustacean axons have long 

 response times and can therefore give regular low 

 frequency discharges. 



The events taking place in certain stretch receptors 

 in Crustacea are very similar (27). A microelectrode 

 in the cell body of one of these primary sensory neu- 

 rons is able to detect a receptor potential generated in 

 the terminals and, superimposed on it, a discharge of 

 nerve impulses. The receptor potential will be con- 

 sidered in a later section. Here it is sufficient to point 

 out that after an impulse the membrane potential 

 builds up again in a manner very similar to that 

 shown in figure 3, and the next impulse is set up 

 when this potential reaches the critical value. The 



