SENSE ORGANS AND NERVOUS COORDINATION 585 



together. Conduction deafness of these types can be corrected by a 

 hearing aid that amplifies vibrations enough to be transmitted directly 

 through the skull bones to the cochlea. More rarely the acoustic 

 nerve or the cochlea may be damaged. Deafness of this sort cannot be 

 corrected. If only a part of the cochlea is injured, one may become deaf 

 only to sounds of certain frequencies. The continuing, loud, high- 

 pitched noises to which boilermakers are subjected sometimes destroy a 

 part of the cochlea, and they become deaf to sounds of this frequency. 

 Observations of this type, and similar experiments performed on vari- 

 ous mammals, established the fact that sounds of different frequency 

 are detected by different parts of the cochlea. 



245. Organization of the Nervous System 



Neurons and the Nerve Impulse. The nervous system provides 

 for the coordination and integration of the body's many activities by 

 conducting impulses from the receptors to the appropriate effectors. 

 It is composed of nerve cells or neurons, which conduct the impulses, 

 and of supporting cells known as neuroglia. We previously considered 

 the morphology and many aspects of the physiology of these cells, but 

 it is appropriate at this time to examine the nature of the nerve im- 

 pulse more thoroughly. 



The biochemical processes that are responsible for a nerve im- 

 pulse are not completely imderstood, but the impulse itself is a wave 

 of "depolarization" that spreads along the plasma membrane of the 

 neuron (Fig. 29.7). An electric potential exists across the membrane 

 of a resting netnon, for there are more positively charged ions on the 

 outside of the membrane than on the inside; sodium ions (Na+) in 

 particular are abundant on the outside. The membrane is said to be 

 polarized. Stimulating the neuron at any point increases the perme- 

 ability of its plasma membrane. Ions that were held apart now can 

 and do move from one side to the other. The outside of the mem- 

 brane at the point of stimulation loses positive ions, and therefore 

 becomes negative relative to other parts of the surface. The opposite 

 condition is found on the inside. Although the membrane is said 

 to be "depolarized," the polarity of the membrane is actually re- 

 versed at the point of stimulation. This reversed polarity increases 

 the permeability of adjacent parts of the plasma membrane, ions 

 move freely through, the polarity of these regions becomes reversed, and 

 this reversal in turn affects the next adjacent parts of the membrane. The 

 impulse continues in this way along the neuron in both directions from 

 the point of stimulation. 



The electrical changes that accompany the nerve impulse are known 

 as the acHon potential. These changes can be measured, and it has been 

 found that impulses travel along mammalian neurons at speeds ranging 

 from 0.5 to 140 meters per second. Myelinated fibers and fibers with 

 relatively large diameters transmit impulses faster than nonmyelinated 

 and small fibers. Even 140 meters per second is very slow compared to 

 the speed of an electric current flowing through copper wire. The elec- 



