CHAPTER XV 



Sensory mechanisms — introduction 



LORD E. D. ADRIAN | Trinity College, Cambridge, England 



THE ESSENTIAL ELEMENTS of the sense oigaiis are the 

 receptor cells which respond to physical and chem- 

 ical disturbance and transmit information about it 

 to the central nervous system. Naturally in these days 

 they are fascinating material for the cell physiologist. 

 The electron microscope gives him new data about 

 their structure, and there are new biophysical and 

 biochemical techniques for investigating their re- 

 actions. If all goes well, our understanding of the 

 changes which take place in the receptors will soon 

 have reached the molecular level. 



The sense organs also provide ample material for 

 the electrophysiologist who deals with them as con- 

 stituent elements of the nervous system. The technique 

 of recording nervous activity has reached great pre- 

 cision and the flow of information can be studied in 

 the cell units and pathways of the central nervous 

 system as well as in the peripheral nerves. In the ani- 

 mal kingdom there is still a vast range of receptor 

 apparatus awaiting investigation and even in the 

 vertebrate there is still a good deal of exploration to 

 be done, particularly about the receptors which signal 

 internal rather than external events. 



Another line of research leads beyond the receptors 

 and their afferent connections, for the physiology of 

 the sense organs must include the study of their func- 

 tion as well as of the properties which make them 

 react to the stimulus. Some of them, pain receptors 

 for instance, may be no more than warning devices 

 which signal whenever their environment sets them 

 in action, but many are used actively to explore the 

 environment and such use involves movement directed 

 by the central nervous system. We look with our eyes, 

 feel with our fingers and sniff to identify a smell. 

 Aclivit}- directed by the central nervous s\stem may 

 also be needed to protect the sense organ when the 



stimulus is too strong. We may have to constrict our 

 pupils and shade our eyes, or cover our ears or hold 

 our ijreath. Since the receptors will give most informa- 

 tion when the stimulus falls within a particular range 

 of intensity, we have to study the different adjust- 

 ments \vhich keep it within that range. 



The analysis of this kind of central control has been 

 carried out most fully for the receptors which signal 

 muscular contraction. The muscle spindle is a sense 

 organ excellently adapted for investigations of this 

 kind, for in it the signaling and adjusting mechanisms 

 are coupled together in a single structure and its 

 function is to guide the relatively simple operations 

 involved in posture and limb movement. Recent in- 

 formation on the efferent innervation of the spindle 

 has given us a much clearer picture in which it ap- 

 pears as an active participant in the feed-back mech- 

 anism which ensures smooth movement against a 

 continuous postural background. 



The action involved in adjusting the stimulus to 

 the sense organ can vary greatly in .scale and com- 

 plexity, from a simple reflex contraction to an elab- 

 orate sequence of skilled movement, as when the 

 microscopist places the slide in position, focuses first 

 with the coarse adjustment and then with the fine 

 and makes appropriate use of his ocular muscles, ex- 

 ternal or internal. In such operations the adjustment 

 is carried out by muscles in the organ or elsewhere. 

 But in addition we may have to consider a more 

 direct central adjustment which does not operate 

 throuQjh the muscular link but by efferent nerve fibers 

 leading directly to the receptors or to some part of 

 the pathway from them to the central nervous system. 

 At present we know that there are efferent fibers to 

 the retina and the olfactory bulb. There are indica- 

 tions of a control of this kind in the cochlea also and. 



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