NEURON PHYSIOLOGY — INTRODUCTION 



69 



of the primary afferent volley and hence depresses its 

 excitatory action (8, 45, 55). This effect has been 

 attributed to the dorsal root reflex and the dorsal root 

 potential set up by the powerful conditioning volley 

 (8) and probably is of little significance with more 

 physiological types of afferent input. Apart from this 

 effect it has been shown that inhibitory actions on 

 motoneurons are explained satisfactorily by the tran- 

 sient increase which is produced in their membrane 

 potentials and which has been designated the inhibi- 

 tory postsynaptic potential, IPSP (6, 16, 18). A com- 

 parable synaptic inhibitory action has been observed 

 with crustacean stretch receptor cells (60), and has 

 also been recorded on the neurons of Clarke's column 

 by Curtis, Eccles & Lundberg (19a). 



As shown in figure 75 to H, a single volley in the 

 afferent fibers from annulospiral endings in quadriceps 

 muscle evokes a hyperpolarizing response, the inhibi- 

 tory postsynaptic potential (IPSP) in a motoneuron 

 of the antagonist muscle (biceps-semitendinosus). The 

 IPSP is observed to be increased in a series of stages 

 as the afferent volley is increased in size, but it is not 

 altered in time course, showing that a simple spatial 

 summation occurs when several inhibitory synapses on 

 the same neuron are simultaneously activated. With 

 the maximum spatial summation in figure jE the 

 membrane potential was increased from —60 to 

 -63.5 mv. 



In order to produce the observed hyperpolariza- 

 tion, current must be flowing inward across the moto- 

 neuronal membrane in general, and there must be a 

 corresponding outward current in the region of the 

 activated inhibitory synapses (fig. 8A, inset). As with 

 the excitatory synaptic action in figure 5^, the time 

 course of the current that produces the IPSP may be 

 determined if the time constant of the membrane is 

 known. The broken line in figure 8.4 plots the time 

 course so determined and shows that the high intensity 

 phase has virtually the same time course as with 

 excitatory synaptic action, though there is much less 



r-r-i~r-rT-n 



msec 



Fig. 7. A to H. Lower records give intracellular responses of 

 a biceps-semitendinosus motoneuron to a quadriceps volley of 

 progressively increasing size, as is shown by the upper records 

 which are recorded from the si.xth lumbar dorsal root by a 

 surface electrode (downward deflections indicating negativity). 

 All records are formed by tfie superposition of about 40 faint 

 traces. 



B I ELEMENT ^ ORDINARY 



i-l 



ELEMENT 



I 70 mV I 90 mV | TO mV 



I T . T 



INSIDE CELL 



FIG. 8. A. Continuous line plots the mean time course of the IPSP set up in a biceps-semitendinosus 

 motoneuron by a single quadriceps la volley. The measured time constant for the membrane was 

 2.8 msec. The broken line gives the time course of the inhibitory subsynaptic current that would 

 produce the IP.SP, the calculation being similar to that used in deriving figure ^A. Inset shows lines 

 of postsynaptic current flow in relationship to an inhibitory synaptic knob. B. Diagrammatic 

 representation of the electrical properties of an ordinary element on the neuronal membrane and of 

 an inhibitory element with K+ and Cl~ ion components in parallel. Further description in the 

 text. 



