Impulse 

 from here 



■>^. 



Sensation 

 , received 

 here 



%, 



/ 



Impulse 



received here 



(receptors) 



? 



Action 



produced here 



(effectors) 







FUNCTIONS OF NERVE CELLS 



A living body senses outside events at the ends of "receiving" nerves, or receptors. 

 Nerve impulses are transmitted by nerves toward central organs. The living body 

 also produces "effects" upon the outside world, through special organs, such as 

 muscles of the hands, called effectors 



spinal cord, and in special clumps called ganglia. These masses of cell 

 bodies make up the "gray matter" of the nervous system; the strands of 

 fibers make up the "white matter". 



There are also special neurons in the gray portions of the brain that are 

 related to knowing, feeling, imagining, and the voluntary control of muscles 

 (see illustration above). 



The protoplasm of one nerve cell and the protoplasm of the next are 

 connected through the branching ends of the axons and the dendrites. If 

 the endings of a sensory, or afferent, neuron are stimulated, the disturbance 

 passes through the cell body and the axon to the terminals, which are in 

 contact with the dendrite of an associative cell. From this cell the impulse 

 is transmitted to the dendrites of an efferent cell and on through the axon 

 of this one to the terminals in some effector — for example, a muscle or a 

 gland (see illustration, p. 275), 



The Main Nerve Axis^ Among the vertebrate animals, as among other 

 bilaterally symmetrical animals, such as insects and segmented worms, a 



^See Nos. 1 and 2, p. 298. 

 276 



