FUNCTIONS OF THE NERVOUS SYSTEM 299 



Excessive stimulation of any of these nerves results in 

 pain. 



435. Efferent Nerves carry impulses of many kinds be- 

 sides those of motion. Five classes of these nerves in the 

 human system have been described: 



1. Motor nerves, distributed to the voluntary and in- 

 voluntary muscles, and carrying impulses resulting in 

 movement. 



2. Accelerator nerves, which increase the rate of rhyth- 

 mical action, as in the heart. 



3. Inhibitory nerves, which retard or wholly check 

 rhythmical action, as in certain nerve fibers of the heart. 



4. Secretory nerves, which convey from the central sys- 

 tem to the glands impulses resulting in the secretion of 

 their special products. 



5. Trophic (from a Greek word meaning " nursing ") 

 nerves, which control the nutrition of the parts to which 

 they go. 



Still other nerves exist which can be classed neither as 

 afferent nor efferent. They are those which connect dif- 

 ferent parts of the brain and cord with one another, and 

 in general form lines of communication between nerve 

 centers. They have been called intracentral nerves. 



436. The Nervous Discharge. When a nervous impulse 

 reaches a nerve cell, a remarkable change takes place. 

 Something similar to an explosion occurs, and a new, 

 different, and more powerful current issues from the cell. 

 An afferent impulse arrives at a nerve center; that change 

 which is called the nervous discharge takes place; energy 

 is set free which is conducted by efferent fibers to the 

 terminal plate in a muscle fiber, it may be. Here again 

 a second nervous discharge results, and a still larger 

 amount of force is liberated. 



