CHAPTER IV 



Initiation of impulses at receptors 



J. A. B. GRAY I Department oj Physiology, University College, London, England 



CHAPTER CONTENTS 



General Properties 



Type of Energy Required to Excite Receptors 



Adaptation 



Receptive Fields 



Information 

 Repetitive Responses and Tonic Receptors 



Stimulus-Frequency Relations 



The Effect of a Reduction of Excitation 



The Nature of Repetitive Firing 



Modification of Afferent Discharges by Current 

 Excitation of Impulses by Controlled Pulses and Phasic Re- 

 ceptors 



Quantitative Aspects of Excitation 



On and Off Responses 



Summation 

 Receptor Potentials and Other Generator Potentials 



Generator Potentials in Complex Organs 



Receptor Potentials Generated in Nerve Terminals 



Relation of Receptor Potentials to Impulse Initiation 



Quantitative Relations between Stimulus and Receptor 

 Potential 



Absolute Magnitude of the Receptor Potential 



Summation of Receptor Potentials 



Depression 

 Site of Impulse Initiation 



Effect of Procaine and Sodium Lack on Receptor Potentials 

 Transmission of Energy to the Receptor Elements 

 Effects of 'Transmitter' Substances 



Action of Acetylcholine 



Action of Blocking Agents and Anticholinesterases 



Effects of Sympathetic Stimulation and Epinephrine 



Other Substances 

 Minute Structure of Receptors 

 Hypotheses Concerning the Mechanisms of Receptors 



IN THE INTACT ORGANISM impulses are set up in pri- 

 mary afferent fibers as a result of activity in those 

 receptors with which the fibers are associated. These 

 receptors may consist solely of specialized termina- 

 tions of the afferent nerve fibers, or the nerve endings 



may be associated with other cells which play a 

 significant role in the initiation of impulses. In either 

 instance, the role of the receptor is to record the 

 state of, or changes in, the physical or chemical en- 

 vironment by the initiation of impulses which are 

 then conducted in the primary afferent fibers to the 

 central nervous system. A primary afferent fiber may 

 be connected with a single receptor or with many; 

 but even when it is supplied by numerous receptors, 

 a single afferent fiber remains a single channel into 

 the central nervous system and must be considered as 

 such. When dealing with the activity in such a fiber 

 it is necessary to consider the fiber and all its periph- 

 eral connections as a whole, that is as a sen.sory unit. 

 It is the purpose of this section to consider something 

 of the general behas'ior of sensory units and of the 

 mechanisms by which indi\idual receptors initiate 

 impulses in the primary afferent neurons. 



GENERAL PROPERTIES 



A few words should first be said concerning classi- 

 fication. Sensory units can be described by reference 

 to the properties of the specific stimulus, the nature 

 of the activity and the site and distribution of the re- 

 ceptive field. All these factors, together with the con- 

 duction velocity of the fiber, are measurable quanti- 

 ties and a precise description of a sensory unit is thus 

 possible. It seems better in this context to avoid terms 

 such as warmth, pain or red; these terms describe 

 sensations which depend on the activity of the whole 

 nervous system, not just on the properties of one 

 sensory unit. 



Type of Energy Required to Excite Receptors 



In most animal organisms there are receptors that 

 respond to the following forms of energy: mechanical, 



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