Specialized Cells: Muscles, Nerves and Sense Organs 1 19 



like substance consisting of proteins and inorganic salts which is 

 in fact continuous with the cytoplasm of the nerve cell, and contains 

 mitochondria and other particles necessary to maintain the life of 

 the cell (Plate 15). 



During recent years many experiments have been made, especially 

 by A. L. Hodgkin and A. F. Huxley, using the giant nerve fibres 

 of the squid, which are so large (0-5 mm. or more in diameter) that it 

 is possible to put electrodes under the axon jelly and so detect the 

 changes which occur when the nerve impulse passes. These fibres 

 are not myelinated, but they do possess a lipo-protein covering, which 

 plays a very important part in the action. The composition of the 

 fluid inside the fibre is very different to that of the blood plasma in 

 which is usually immersed (see Fig. 22). In fact, while the blood 

 plasma contains about twenty times as much sodium (Na) as potas- 

 sium (K) the interior of the fibre contains ten times as much potassium 

 as sodium. There is also a marked difference between the concentra- 

 tions of chloride (CI) ions inside and outside. 



— — — — — _«.'' 

 Axoplasm I 



Na K CI I 



49 410 40 \ 



440 22 560 



FIG. 22. Distribution of inorganic 

 ions inside and outside the nerve fibre 



These differences persist in an unexcited fibre because although 

 potassium can pass freely through the membrane, sodium cannot 

 and there is also an efficient mechanism (a sodium pump) for re- 

 moving sodium from inside the membrane when its concentration 

 is too high. 



Because of the inequalities in the amounts of the sodium, potas- 

 sium and chloride (which are present as electrically charged ions) 

 inside and outside the fibre, there is a difference of electrical potential 

 across the fibre membrane and as a result the outside has a positive 

 charge and the inside a negative charge. (Fig. 22.) 



When a nerve impulse passes down the fibre, the permeability of 

 the membrane to sodium ions is momentarily increased. As they 

 are at a higher concentration outside, they immediately pass through 

 into the interior, and as a consequence the negative charge on the 

 inner surface is reduced and it may be reversed. (Fig. 23.) 



When the impulse has passed, the membrane properties are re- 



