Biology of Man 501 



impulse, it uses more oxygen, gives off more carbon dioxide and heat, 

 and expends more energy than when it is not transmitting impulses, which 

 suggests the oxidative nature of the phenomenon. (4) Normally the 

 rate of travel of a nerve impulse is independent of the intensity and 

 nature of the stimulus, providing the stimulus is of a certain minimum 

 intensity; in other words, stronger stimuli do not cause impulses to travel 

 faster, because the energy for im.pulse conduction comes from the nerve 

 and not the stimulus. This is known as the all-or-none law which implies 

 that a stimulus of sufficient intensity results in an impulse independent 

 of the strength or nature of the stimulus. In other words, stimuli result 

 in an impulse — or they do not. The rate of travel of a nerve impulse 

 may be dependent on the state of the nerve fiber, because certain drugs 

 may retard or even prevent impulse transmissions. Advantage is taken 

 of this in "blocking off" and preventing the transmission of certain im- 

 pulses by the use of certain drugs. The speed of nerve impulses is much 

 slower than the speed of electricity; hence they are not electric currents 

 even though certain electrical phenomena may be associated with them. 

 Injured or dead nerves are capable of conducting electrical currents, but 

 they cannot transmit nerve impulses. The rate of impulse travel over a 

 given nerve is the same whether the stimulus be chemical, heat, touch, 

 electrical, etc. (5) It is thought that all types of nerve cells (sensory, 

 motor, etc.) conduct impulses in a similar way and that the end result 

 depends on the nature of the specific structure to which the impulses 

 travel; impulses traveling from the ear to a certain part of the brain 

 result is a sensation of sound; proper impulses traveling to muscles result 

 in movements, etc. (6) Although impulses may be initiated anywhere 

 along a nerve, they usually originate at one end only and travel toward 

 the opposite end; that is, from the dendrite toward the axon. (7) Nerve 

 fibers do not seem to fatigue so long as a sufficient supply of oxygen is 

 present. (8) Neurons (nerve cells) consist of one or more dentrites, one 

 or more axons, and a cell body with its nucleus. Adjacent neurons do 

 not quite contact each other, and the small area between them is called 

 a synapse (sin' aps) (Gr. synapsis, union). It has been proved experi- 

 mentally that in certain instances an impulse travels across the synapse 

 from the tip of the axon of one neuron to the dendrite of an adjacent 

 neuron because of a chemical secretion known as a neurohumor, pro- 

 duced by the tip of the axon. Because this neurohumor is produced by 

 the axon and stimulates the adjacent dendrite, the impulse travels from 

 the axon of one neuron to the dendrite of another neuron and not the 

 reverse because dendrites are unable to secrete this substance. In cer- 



