CHAPTER IX. 



THE FUNCTIONAL INTERRELATION OF RECEPTORS, NEURONS, AND 



EFFECTORS. 



The human body is a mechanism for producing response to stimula- 

 tion. Response is always the modification of the activity of muscles or 

 glands, or of both. Other activities in the body (such as the activities of 

 blood corpuscles) are subsidiary to these responses, modifying their char- 

 acter, temporal course, or extent. 



The responses of a muscle or gland are of two kinds : increase in activ- 

 ity, or decrease in activity. In the muscle the increase in activity is mani- 

 fested in contraction; the decrease, .in relaxation. In the gland, the 

 activity which is subject to increase and decrease is secretion, that is, the 

 formation, or separation, of some substance or substance (saliva, mucus, 

 sweat, adrenalin, etc.) to the elaboration of which the gland is especially 

 adapted. 



Striped muscle contracts normally only when irritated by a nerve cur- 

 rent (excitatory or acceleratory current). Relaxation supervenes on con- 

 traction when the exciting current ceases, but is also facilitated by nerve 

 currents. These latter currents are called inhibitory. Whether there are 

 two specifically different classes of efferent neurons, one class having ac- 

 celeratory effects, and the other inhibitory effects ; or whether two branches 

 of the same axon may have contrary effects (on two different groups of 

 muscle fibers), has not been made clear. 



In distinction from the production of positive contractions, acceleratory 

 nerve impulses to muscles may have a tonic effect (may give tone to the 

 muscles). Tone in a muscle is a condition of preparedness for contrac- 

 tion, and may be called the normal condition of muscle. If the efferent 

 nerves supplying a muscle are completely severed, the muscle becomes 

 flabby, and much greater stimulus (e. g., electricity applied directly to 

 the muscle or to the cut end of the nerve) is required to produce contrac- 

 tion than is required in the case of muscle having tone. 



The unstriped and cardiac muscle also receives tone through nerve cur- 

 rents. In the normal body, therefore, there is a constant flow of accel- 

 eratory current through the efferent nerve fibers to the general muscu- 

 lature, keeping it in condition for action. 



