PHYSIOLOGY OF NERVE TISSUE. 83 



Stimuli of Nerves. Nerves do not possess the power of spon- 

 taneously generating and propagating nerve impulses ; they can be 

 aroused to activity only by the action of an extraneural stimulus. In 

 the living condition the stimuli capable of throwing the nerve into 

 an active condition act for the most part on either the central or 

 peripheral end of the nerve. In the case of motor nerves the stimulus 

 to the excitation, originating in some molecular disturbance in the 

 nerve-cells, acts upon the nerve-fibers in connection with them. In 

 the case of sensor or afferent nerves the stimuli act upon the peculiar 

 end organs with which the sensor nerves are in connection, which 

 in turn excite the nerve-fibers. Experimentally, it can be demonstated 

 that nerves can be excited by a sufficiently powerful stimulus applied 

 in any part of their extent. 



Nerves respond to stimulation according to their habitual func- 

 tion ; thus, stimulation of a sensor nerve, if sufficiently strong, 

 results in the sensation of pain ; of the optic nerve, in the sensation 

 of light ; of a motor nerve, in contraction of the muscle to which it 

 is distributed; of a secretor nerve, in the activity of the related 

 gland, etc. It is, therefore, evident that peculiarity of nerve func- 

 tion depends neither upon any special construction or activity of 

 the nerve itself, nor upon the nature of the. stimulus, but entirely 

 upon the peculiarities of its central and peripheral end organs. 



Nerve stimuli may be divided into 



1. General stimuli, comprising those agents which are capable of 

 exciting a nerve in any part of its course. 



2. Special stimuli, comprising those agents which act upon nerves 

 only through the intermediation of the end organs. 



General stimuli: 



1. Mechanical: as from a blow, pressure, tension, puncture, etc. 



2. Thermal : heating a nerve at first increases and then decreases its 

 excitability. 



3. Chemic : sensor nerves respond somewhat less promptly than 

 motor nerves to this form of irritation. 



4. Electric : either the constant or interrupted current. 



5. The normal physiologic stimulus : 



(a) Centrifugal or efferent, if proceeding from the center toward 



the periphery. 

 (&) Centripetal or afferent, if in the reverse direction. 



