280 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Proprioceptors (e.g., muscle spindles) receive stimuli arising within the 

 body, such as pain and variations in the tension of a muscle or tendon. Re- 

 ceptors are changed chemically or physically, often in both ways, by stimu- 

 lation. These stimuli set up impulses, actually changes going through the 

 nerve cells with which the receptors are in contact. Thus the receptors bring to 

 the nerve cells the raw materials with which they work. Many receptors, 

 such as those of the muscle sense of position, are associated with nerves that 

 end in the spinal cord and the cerebellum where their activity is below the 



scnsorij 

 cqII 



m.TjscLc 

 cell 



cell 



nerve 

 rtet 



muscle 

 cell 



Fig. 16.1. Diagrams of simple associations of receptor and eflfector cells, those 

 that receive the stimulus and those that act in response to it. Left, A receptor cell 

 (sensory) in direct contact with an effector; right, a more complex type in which 

 conducting cells (nerve net) act as middle men. Such arrangements occur in 

 simpler animals, e.g., sea anemones. (Courtesy, Parker: Elementary Nervous 

 System. Philadelphia, J. B. Lippincott and Co., 1919.) 



region of consciousness. We do not decide to turn over in our sleep. Other 

 receptors such as sight and hearing are intimately connected with the higher 

 centers of the brain. Sense organs are parts of the nervous system but are 

 more conveniently discussed in another chapter (Chap. 17). 



Conduction. A pinch at one end of a fresh frog muscle immediately starts 

 waves of contraction moving toward the opposite end. When a cell is injured 

 by a microdissection needle a "death wave" begins at the point of injury 

 and quickly overspreads the cell. This is the conduction that is characteristic 

 of all protoplasm. In paramecia and some other protozoans conductile 

 fibrils connect the basal bodies of the cilia with each other and with a center of 

 coordination near the mouth (Fig. 16.2). Conduction reaches its highest de- 

 velopment and speed in the nerve impulse. 



Nerve Cell 



Characteristics. Nerve cells or neurons are the basic units of structure in 

 the nervous system. Each neuron has threadhke extensions called fibers. 



