EXCITATORY AND INHIBITORY PROCESSES 299 



phase of the first impulse the membrane potential transiently returns toward 

 its "relaxed" equilibrium state. During that period, therefore, the generator 

 effect is decreased and the axon impulse may not produce the adequate 

 depolarization which normally constitutes the discharge threshold for anti- 

 dromic excitation of the cell soma. 



Effect of Antidromic Impulses on Discharge Frequency 



If an antidromic impulse arrives at the cell soma when an orthodromic 

 discharge is just about to fire it will have little effect on the timing of the 

 subsequent response and the rhythm of discharge is not appreciably disrupted. 

 If, however, the antidromic Impulse arrives during the after-positivity of the 

 orthodromic discharge it delays the next orthodromic impulse and at the 

 same time the prepotential rise after the antidromic impulse is slowed. It is 

 assumed that the antidromic impulse, by having spread into the dendrites, 

 delays re-excitation by interfering with the generator action. The depressant 

 action of antidromic stimuli, as measured by the reduction of afferent 

 activity, is best seen during low-frequency discharge under weak stretch. 

 Thus, a train of antidromic impulses may stop a sensory discharge for a 

 second or more while its effect on high-frequency rhythmic activity may be 

 relatively small. The different results obtained during weak and strong 

 stretch may be expected if the antidromic impulses either depress the generator 

 itself or attenuate the electrotonic spread from dendrites to the cell soma. 

 At near threshold stretch a diminution of the barely adequate generator 

 potential would therefore be more readily detected. 



Grouped Discharges 



A number of slowly adapting cells are capable of producing spontaneous 

 and grouped discharges when they are completely relaxed or slightly stretched. 

 These discharges are short high-frequency bursts which are followed by a 

 pause. Besides occurring "spontaneously" an orthodromic train from the 

 receptor cell may be elicited by a single antidromic impulse. These discharges 

 have been recorded by means of an intracellular microelectrode in the cell 

 soma and by placing recording electrodes on the sensory axon. Therefore, all 

 the discharges going in as well as those leaving the soma-dendrite region 

 could be monitored and correlated. It is of some interest that changing the 

 tension apphed to the receptor muscle influences the number of impulses 

 within each high-frequency burst. Applied stretch is capable of abohshing 

 this type of response. In a naturally occurring impulse burst, recorded intra- 

 cellularly, the first orthodromic volley shows part of the usual pre-potential 

 which is absent in subsequent smaller components. When the action potentials 

 are recorded from the axon several conducted impulses are detected. An 

 essentially similar picture is obtained when this burst is produced by anti- 



