60 



GENERA/. CONCEPTS 



Mviscle 



Figure 3.15. Diagrams of an efferent neuron (A) and an afferent neuron (B). The 

 arrows indicate the direction of the normal nerve impulse. (Millard, King and Showers: 

 Human Anatomy and Physiology.) 



certain invertebrates the oxygen-carrying pigment is not localized in 

 cells, but is dissolved in the plasma and colors it red or blue. Platelets 

 are small fragments broken off from cells in the bone marrow; they play 

 a role in the clotting of blood (p. 541). 



Nervous Tissues. Cells specialized for the reception of stimuli and 

 the transmission of impulses are called neurons. A neuron typically has 

 an enlarged cell body, containing the nucleus, and two or more cyto- 

 plasmic processes, the nerve fibers, along which the nerve impulse travels 

 to the next neuron (Fig. 3.15). Nerve fibers vary in width from a few 

 microns to 30 or 40 microns and in length from a millimeter or two to 

 several feet. The neurons are connected end to end so that impulses 

 may be transmitted all through the body. Two types of nerve fibers are 

 distinguished: axons, which transmit impulses away from the cell body, 

 and dendrites, which transmit them to the cell body. The junction be- 

 tween the axon of one neuron and the dendrite of the next neuron in 

 the chain is called a synapse. At the synapse the axon and dendrite do 



