HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 1 



Other muscle groups that participate in an organized 

 movement (cf. 140). These effects frequently are 

 fully reciprocal, excitation in either path being asso- 

 ciated with inhibition in the other, and thus they 

 involve processes of reciprocal inhibition as well as 

 primary excitation. 



A mechanism, discovered by Lloyd (cf. 138) and 

 termed direct inhibition, was believed to be mediated 

 monosynaptically, the collaterals of the same afferent 

 nerve fiber at one motoneuron evoking excitation, 

 tho.se to another producing inhibition. This mecha- 

 nism would imply that the same transmitter evokes 

 depolarizing p.s.p.'s in one neuron and hyperpolariz- 

 ing p.s.p.'s in another cell. From the point of view of 

 theoretical considerations this means of achieving 

 reciprocal actions is perfectly feasible, as is the possi- 

 bilitv that different transmitters are released at the 

 different terminals of the same primary afferent nerve 

 fiber. In the lobster cardiac ganglion (33, 186) one 

 presynaptic nerve causes excitation in one neuron 

 and inhibition in another. 



The reality of monosynaptic direct inhibition of 

 this type in the cat spinal cord is at present in dis- 

 pute. Lloyd and his colleagues (cf. 194) maintain 

 that a monosynaptic pathway exists while Eccles 

 and his associates (cf. 60) consider that 'direct' in- 

 hibition is a disynaptic event. Whether the particu- 

 lar reflex pathways under discussion are mono- 

 synaptic or disynaptic is probably a matter of the 

 specific structures involved and perhaps of the 

 methodological details.* In principle, direct inhibi- 

 tion by monosynaptic reciprocal innervation can 

 occur. Since it is theoretically feasible it seems un- 

 likely that among the many types of connections 

 elaborated in the nervous system one possible and 

 rather simple variety has been omitted. 



ROLE OF ELEMENT.-^RY SYN.\PTIC PROPERTIES 

 IN INTEGRATIVE ACTIVITY 



Spatial Interrelations of Synaptic and 

 Conductile Aiembrane 



Since p.s.p.'s are 'standing' nonpropagated po- 

 tentials, their effect upon the electrically excitable 



" Drugs such as pentobarbital, for example, can alter pro- 

 foundly the pathways that produce pyramidal tract activity 

 through thalamocortical relays. This is disclosed by changes in 

 latency of several msec, when 4 to 10 mg per kg of pentobarbital 

 are administered (99, loi, 161 and unpublished work). 



membrane of the same cell depends upon the spatial 

 arrangement of these differently excitable structures. 

 Assuming as a first approximation that the elec- 

 trically excitable membrane everywhere in a cell is 

 triggered to discharge a spike by the same level of 

 critical depolarization, and that the depolarizing 

 p.s.p.'s are everywhere equal in amplitude, the in- 

 tensity of excitation of the former by the latter will 

 depend upon the distance between the synaptic 

 focus and the nearest conductile membrane. The 

 more closely the two electrogenic membrane sites 

 approximate each other the more intense will be the 

 excitation for triggering a spike. The apical dendrites 

 of the cerebral cortex are not electrically excitable 

 (107) and the p.s.p.'s of the axodendritic synapses 

 generated at some distance from electrically ex- 

 citable membrane therefore would not be expected 

 to be as effective as the axosomatic p.s.p.'s generated 

 in close contiguity with electrically excitable mem- 

 brane. Thus, the apical dendrites of cortical neurons, 

 although they generate intense synaptic activity 

 (165) are not as effective in triggering spikes as are 

 the depolarizing synaptic loci at the soma (27, loi). 

 Spatial considerations may also be applied to the 

 effects of hyperpolarizing p.s.p.'s. The latter would 

 be most intensely inhibitory if they are interposed 

 between sites of excitatory p.s.p.'s and electrically 

 excitable membrane. The depolarizing p.s.p., in 

 that case, would be diminished not only by elec- 

 trotonic averaging between the opposed electro- 

 genic actions. The interposed hyperpolarizing site 

 would receive more outward current flow than rest- 

 ing membrane since its resistance would be lower, 

 and the potential gradient steeper. Consequently 

 this bypass would result in less current flow from 

 the depolarizing synaptic site to the electrically 

 excitable but as yet inactive membrane. Thus, the 

 loci at which depolarizing and hyperpolarizing 

 p.s.p.'s are generated, both relati\e to each other 

 and to electrically excitable membrane, must play 

 an important role in determining the effectiveness 

 of transmission from a given afferent volley. The 

 simplifying assumption that all s\naptic sites are 

 electrogenically equivalent is probably not justified 

 (see below). It is also likely that the conductile 

 membrane in different parts of a cell varies with 

 respect to its electrical threshold (79) or that it is 

 differently electrogenic (69), and these factors may 

 reinforce the transmissional inhomogeneity of dif- 

 ferent synaptic sites. 



