Introduction 
cell body 
dendrites 
axon (less than 1/20 inch to 
more than three feet in length) 
terminal branches of axon 
Figure 21. A schematic diagram of a typi- 
cal neuron. The arrows indicate the di- 
rection in which nerve signals are con- 
veyed. The largest axons in the human 
brain and spinal cord extend for about 
three feet and have a diameter of less 
than 1/1, 000th of an inch. Many axons 
are covered by an insulating layer 
known as the myelin sheath. The myelin 
sheath is interrupted at intervals known 
as nodes of Ranvier. 
From Alberts, B., Bray, D., Lewis, J., 
Raff, M., Roberts, K., and Watson, J.D. 
1989. Molecular Biology of the Cell, 2nd 
edition. New York: Garland, p. 1061. 
1 
Figure 22. A few of the many types of neu- 
rons in the vertebrate nervous system. 
(S. Ramon y Cajal, Histologic du Systeme 
Nerveux de I'Homme et des Vertebres. 
Paris: Maloine, 1901- 191 1; reprinted, 
Madrid: C.S.I.C., 1972.) 
From Alberts, B., Bray, D., Lewis, J., 
Raff, M., Roberts, K., and Watson, J.D. 
1989. Molecular Biology of the Cell, 2nd 
edition. New York: Garland, p. 1061. 
Ivi 
