tivity travels over the cell surface. Apparently 

 the changing potential in each localized 

 area touches off excitation in neighboring 

 areas and the excitation is propagated with 

 undiminished vigor. 



Apparently the maintenance of an elec- 

 trically polarized membrane is an essential 

 factor in maintaining reactivity in living cells. 

 Whenever polarization is locally abolished, 

 excitation occurs; conversely, whenever ex- 

 citation occurs, the polarization is momen- 

 tarily abolished. These changes are not neces- 

 sarily confined to the plasma membrane. They 

 may extend via the membranes of the endo- 

 plasmic reticulum to deeper parts of the cell. 



Excitation Metabolism. An excited cell 

 immediately begins to liberate extra heat, 

 but it does not immediately begin to con- 

 sume extra oxygen. In fact, many cells, such 

 as muscle and nerve cells, can continue to 

 respond to stimulation for quite a while after 

 their oxygen has been used up. This excita- 

 tion metabolism should not be confused with 

 the recovery metabolism, which follows ex- 

 citation. Recovery always involves an extra 

 consumption of oxygen and an extra produc- 

 tion of carbon dioxide. In short, the recovery 

 reactions supply the energy required to re- 

 store the cell to its original condition (aside 

 from the loss of a small quantity of oxidized 

 fuel). When recovery metabolism is inhib- 

 ited, as by lack of oxygen, the cell more 

 quickly reaches a state of total fatigue — that 

 is, a state in which it can no longer respond 

 to stimulation. It may respond later, how- 

 ever, if it has been allowed to recover in the 

 presence of an adequate supply of oxygen. 



The precise nature and role of the excita- 

 tion metabolism in relation to membrane 

 structure and potential remains an open 

 question. In nerve cells it has been proposed 

 by David Nachmansohn of Columbia Uni- 

 versity that excitation involves the liberation 

 of an excitatory substance, acetylcholine (p. 

 455), which then is hydrolyzed by a special 

 enzyme, cholinesterose, as the excitation sub- 

 sides. More evidence is needed, however, be- 



Responsiveness in Single Cells - 1 93 



fore the question can be resolved completely. 



The Refractory Period. Immediately follow- 

 ing excitation, before the membrane has re- 

 turned to its original state, there is a brief 

 absolute refractory period, during which the 

 cell cannot be re-excited by any stimulus, 

 however strong; and following this, there is 

 a relative refractory period, during which an 

 unusually strong stimulus is required to elicit 

 excitation. The duration of the total refrac- 

 tory period differs in different cells, from 

 less than 0.001 second (in nerve cells) to a 

 number of seconds (in plant cells). Due to the 

 refractory period, the state of excitation is 

 self-limiting: a cell cannot remain continu- 

 ously in the excited state. 



The refractory period also has an impor- 

 tant bearing on the rhythmicity of certain 

 responses, such as the beating of heart muscle 

 or of cilia, since the refractory period im- 

 poses a definite limit upon the frequency at 

 w ; hich the responses are able to recur. Many 

 responses that appear to endure for some 

 time, like the prolonged contraction of a 

 body muscle, actually represent a series of 

 rapidly recurring responses. In the case of 

 the muscle this can be seen in the electrical 

 records, for the electromyogram always shows 

 a series of action potentials occurring syn- 

 chronously with the excitations. 



STIMULI AND STIMULATION 



Any physical or chemical change occur- 

 ring within or around a living body may act 

 as a stimulus, provided the quantity, quality, 

 and rate of the change are properly adjusted 

 to the sensitivities of the cell or cells that are 

 to be excited. The various kinds of stimuli 

 capable of exciting cells and organisms gen- 

 erally may be classified as follows: 



Mechanical stimuli: Contact, pressure, sound. 



Thermal stimuli: Changes of temperature. 



Concentration stimuli: Changes in the concen- 

 tration of substances. 



Chemical stimuli: Changes in the kinds of sub- 

 stances present in or around the organism. 



