352 THE NERVE SYSTEM 



of any excitation coming through afferent (sensor) 

 nerves from the periphery. If the cell activity is 

 continuous it causes an even and regular control over 

 the processes of cell nourishment, development, etc. 

 (trophic); muscle and vascular activity, which is 

 spoken of as tonus. 



Reflex Actions. They are defined as the reception 

 of nerve impulses transmitted to the nerve cells in 

 the spinal cord and higher centres in the brain, by 

 afferent (sensor) nerves; and the response of the nerve 

 cells in the efferent centres to this stimulation, which 

 result in nerve impulses being excited in the nerve 

 cells, and conveyed by the efferent (motor) nerves, 

 which pass from these cells to the structures to be 

 innervated, and cause them to act, as, to muscles, 

 causing contraction; to gland cells, secretion; to 

 bloodvessels, increasing or decreasing their caliber; 

 and to organs, increasing or decreasing their activity. 



For any reflex act to be mechanically possible there 

 must be present the following structures: 



1. A surface to receive the stimuli: skin, mucous 

 membrane, sense organ. 



2. An afferent nerve fiber and cell, to convey the 

 nerve-impulse arising as a result of the stimulus 

 exciting the sense organ, etc., into activity. 



3. An emissive cell, from which arises an (4) efferent 

 nerve, distributed to a responsive organ, as muscle, 

 gland, bloodvessel, etc. // 



In a more practical way the most sim|lfe reflex 

 action can be explained as follows: If a muscle is 

 stimulated by a strong current of electricity or pinched 

 by an instrument, there is developed in the terminals 

 of the afferent nerve a nerve impulse which is conveyed 

 to the nerve cells in the posterior horn of the spinal 

 cord, the dendrites of these cells transmit the impulse 

 to the dendrites of the nerve cells in the anterior 

 horn, where by means of a molecular jpllturbance 



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