PROPERTIES OF THE NERVE CELL. 139 



which was formerly supposed to average about 10 per second, 

 but is now estimated as varying between certain wide limits, 

 perhaps from 40 to 100 per second (p. 46). For any particular 

 group of these motor cells the evidence indicates that it has a prac- 

 tically constant rate whatever may be the intensity of the stimulus 

 and, indeed, when artificial stimulation is used and the rate is 

 varied, the evidence that we have so far appears to show that 

 the nerve cells do not discharge in a one to one correspondence 

 with the rate of stimulation, as is the case, within limits, for 

 muscle and nerve fibers. On the contrary, under such circum- 

 stances the discharge from the nerve cells takes place in a rhythm 

 characteristic of the cells and independent of that of the stimula- 

 tion.* From this point of view we must look upon these nerve 

 cells as possessing fundamentally a rhythmic activity, as in the 

 case of the heart. There is no doubt, however, that some at least 

 of the motor cells of the spinal cord can be stimulated by a single 

 stimulus so as to respond with a single discharge instead of a rhyth- 

 mical series of discharges. As will be described below, the knee- 

 kick is a simple muscular contraction, not a tetanus, which is 

 aroused by reflex stimulation of the corresponding motor cells in 

 the spinal cord. 



The Refractory Period of the Nerve Cell. It will be recalled 

 that the nerve fiber exhibits what is called a refractory period for a 

 brief interval (0.002 to 0.006 sec.) after it is stimulated. During 

 this period it is not irritable to a second stimulus. The same 

 phenomenon is exhibited to a marked degree by the heart muscle 

 and likewise by many nerve cells. In the motor nerve cell which 

 shows the property of discharging a series of impulses with rhythmic 

 regularity it may be supposed that the refractory period is marked, 

 and indeed is connected probably with the rhythmic character of 

 the cell's activity. But in this as in other properties it is certain 

 that there are great differences in the many varieties of nerve cells 

 found in the central nervous system. While those that act rhyth- 

 mically have probably a relatively long refractory period, others 

 may exhibit a period of unirritability but little longer than that 

 shown by the nerve fibers. In the case of the reflex motor centers 

 in the lumbar spinal cord of the frog it is stated (Langendorff) that 

 a second stimulus falling at an interval of 0.04 sec. after the first 

 is effective. The refractory period of these cells is less, therefore, 

 than this interval. 



* Horsley and Schafer, "Journal of Physiology," 7, 96, 1886. 



