EXCITATORY AND INHIBITORY PROCESSES 



303 



Inhibition of Sensory Discharges 



While a slowly adapting receptor is stretched, rhythmic discharges occur 

 which may be recorded from the cell soma by means of intracellular micro- 

 electrodes. If during stretch the inhibitory nerves are stimulated at a certain 

 frequency, sensory discharges stop immediately and this effect remains for 

 the duration of inhibitory stimulation (see Fig. 13). In most cases a small 

 hyperpolarization occurs in an apparent effort to bring the membrane 

 potential back to its resting level. In other instances inhibition is accomphshed 

 although inhibitory stimulation results in a small depolarization which, if 

 maintained below the firing level, accomplishes essentially similar results. 

 During inhibitory stimulation even strong stretch is incapable of making the 

 receptor fire. The effectiveness of the inhibitory impulses in stopping sensory 

 activity depends on the relationship between afferent discharge rate and 

 efferent inhibitory frequency. An inhibitory impulse arriving at a dendrite just 

 after the cell soma has conducted will delay the subsequent discharge only a 

 little but if it arrives when the prepotential has already developed and the 

 cell is ready to fire it will cause an appreciable delay because the prepotential 

 will have then to develop anew. 



Fig. 13. Intracellular recording from a slowly adapting receptor cell. The regular 

 train of afferent discharges (11/sec) set up by maintained, stretch is interrupted 

 by stimulation of the inhibitory axon between arrows, at 34/sec. Small deflections 

 are inhibitory potentials. (From Kuffler and Eyzaguirre, 7. Gen. Pliysiol.39 : 155- 



184, 1955.) 



Membrane Potential Changes during Inhibition 



The potential changes produced by inhibitory stimulation on the receptor 

 cell membrane have been observed both in rapidly and in slowly adapting 

 cells. In general, when the receptor cell membrane potential decreases during 

 progressive depolarization by stretch, inhibitory stimulation produces a 

 repolarization and the magnitude of this repolarization is dependent on the 

 magnitude of the membrane potential level obtained during stretch. This has 

 been clearly seen in rapidly adapting neurons since once adaptation occurs 

 no sensory discharges appear which could obfiterate the smaller inhibitory 



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