CHAPTER 29 



Sense Organs 

 and Nervous Coordination 



If an organism is to be successful and survive in the complex world in 

 which it lives, the activities of all of its organs must be integrated so 

 that the organism will function and will make appropriate responses 

 to its external and internal environment. In the higher animals, in- 

 tegration is accomplished by special receptors, or sense organs, which 

 detect changes in the environment, and by the nervous system, which 

 conveys the impulses initiated by the sense organs to appropriate ef- 

 fectors (muscles, glands), whose activity brings about the appropriate 

 response. Many vertebrate and invertebrate effectors are regulated in part 

 by hormones that are secreted by endocrine glands and transported in the 

 blood stream. It will be shown in the next chapter that endocrine integra- 

 tion tends to be general rather than specific in its action; that is to say, 

 one hormone may affect more than one organ. Endocrine integration is 

 generally slower but longer lasting than nervous integration; it is espe- 

 cially effective in controlling continuing processes such as metabolism and 

 growth. In a few instances, e.g., in the control of pancreatic secretion, 

 endocrine integration is specific and rapid, but most of the specific and 

 rapid adjustments are achieved by the sense organs and the nervous 

 system. Nervous integration is highly specific; the neurons carry im- 

 pulses from specific receptors to the spinal cord or brain, from which 

 impulses go out through other neurons to specific effectors. It is rapid 

 because the nerve impulse can travel very fast— as fast as 140 meters per 

 second in the larger, myelinated mammalian neurons— and a second 

 impulse can follow after a brief recovery period that lasts at most only 

 several milliseconds. 



It will be recalled from Chapter 5 that our ability to perceive 

 different kinds of stimuli (touch, light, sound, etc.) is a function of 

 the specificity of the receptors, which are attuned to specific stimuli, 

 and of their specific connections within the nervous system. The nerve 

 impulse that is initiated is not specific and is fundamentally the same 

 regardless of where it comes from. Awareness of the sensation depends 

 on the precise part of the brain the impulse reaches. This can be dem- 

 onstrated by by-passing the receptor and stimulating its neurons di- 

 rectly. The subject then feels the same sort of sensation as if the 

 receptors themselves had been stimulated. People who have had ampu- 



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