804 Comparative Animal Physiology 



agent may be excitatory for some, and inhibitory for other receptor cells. In- 

 hibitory mediators occur in the heart and in smooth muscle, but there is 

 no evidence for inhibitory agents in nerve centers. 



An electrical hypothesis of direct inhibition makes use of the short (2-3 

 mm.) "Golgi" interneurones which are found in many parts of the central 

 nervous system, particularly in the spinal cord (Fig. 303).^^^ Such a neurone 

 might be excited by an inhibitory axon to produce a graded synaptic poten- 

 tial which would be conducted electronically over the short neurone and 

 on entering the motoneurone produce there regions of decreased excitability 

 or anelectrotonus. These would compete with excitatory synaptic potentials 

 set up on the motoneurone and prevent the initiation of a propagated im- 

 pulse. 



Another postulate is that inhibitory endings are so distributed that excita- 

 tory fibers are unable to depolarize a sufficient area for excitation. 



The second type of inhibition has a much longer latency. Sherrington ob- 

 tained maximum inhibition of flexor reflexes in cats if the inhibitory volley 

 preceded the excitatory volley by 30-80 msec. This inhibitory process builds 

 up and declines more slowly than the excitatory process . 



The delayed inhibition may result from slow positive potentials in nerve 

 centers. It was stated above (p. 780) that, during the positive after-poten- 

 tial, nerve fibers are depressed in excitability. This is similar to lowered excit- 

 ability in anodal polarization, anelectrotonus. Slow positive waves have been 

 recorded from numerous centers, but their interpretation is difficult because 

 they vary with electrode placement and the origin of slow potentials in nerve 

 centers is not well known. Correspondence between diminished excitabilit)' 

 and positivity in the anterior horn of the spinal cord has been indicated.^**"^ 

 In sympathetic ganglia a long-lasting positive wave is maximal at 100-200 

 msec.125, 257 yj^g excitabihty of the ganglion for preganglionic volleys is 

 diminished in a time course corresponding well with the slow positive poten- 

 tial.^-^ Such correspondence did not appear when the ganglion was condi- 

 tioned by tetanic preganglionic stimulation.^''^ In the vertebrate retina a cor- 

 relation has been suggested between positivity of the retinal potential (neg- 

 ativity of the ganglion cells of the inverted retina) and the setting up of 

 optic nerve impulses, also between the reverse state (positivity of the gan- 

 glion cells) and cessation of optic nerve discharge (inhibition).^''* What- 

 ever the temporal relation between inhibition and positive after-potentials in 

 nerve centers, any prolonged positivity will certainly lower the excitability of 

 synapses within its electric field. 



The neurone circuits of parts of central nervous systems are very com- 

 plex. It is possible to construct circuits which are reasonable anatomically and 

 which are properly timed to account for inhibition. ^^^ Impulses entering a 

 center over one path and acting through interneurones may be cancelled by 

 impulses from another pathway which arrive at such a time earlier as to 

 leave the interneurones inexcitable. 



The time course of inhibition has not been examined in any non-mam- 

 malian center. There are many examples, however, of the inhibitory action 

 of one center on another. Integrative centers maintain a balance between 

 inhibition and excitation of motor centers. Examples of these will be con- 

 sidered below. 



