412 



ORGAN SYSTEMS OF MAN 



which is covered by a tough sheet of tissue. 

 This is remarkably similar to a telephone 

 cable where each wire is insulated from all 

 the others. 



There are several major kinds of neurons 

 located in specific parts of the nervous sys- 

 tem. In the brain and cord they are highly 

 specialized and occur only in certain loca- 

 tions. These neurons are called association 

 neurons, and they function in connecting 

 various parts of the brain and cord. Those 

 that conduct impulses from the distal parts 

 of the body to the brain or cord are called 

 sensory or afferent neurons and those that 

 conduct the impulses away from the brain 

 and cord are called motor or efferent neu- 

 rons. Some nerve trunks are composed 

 entirely of one or tlie other, in which case 

 they are called sensory or motor nerves. 

 Most, however, carry both kinds of fibers 

 and are called mixed nerves. In the verte- 

 brates the cytons for the sensory neurons lie 

 on each spinal nerve in a swelling, the dor- 

 sal root ganglion, just outside the cord, 

 whereas those for the motor neurons lie 

 within the cord itself in a region that is 

 grayish in color (Fig. 16-16). 



In order for a nerve impulse to complete 

 its circuit, it must pass over more than one 

 neuron; in fact, a great many are probably 

 involved even in the most simple action. 

 Neurons are not directly connected with 

 each other but come in close association 

 only. The region or area where the dendrite 

 of one neuron is in close proximity with the 

 axon of another is known as the synapse 

 (Fig. 16-12). This is a very important part 

 of the nervous system because it is here 

 that a selection is made as to whether or 

 not an impulse is permitted to pass on to 

 the next neuron. The impulse can travel 

 both ways within a neuron but where the 

 neurons are in a series, as they always are, 

 the impulse travels toward the cell body on 

 the dendrites and away from it on the axon. 

 The synapse, therefore, acts like a traffic 

 signal on a one-way street. 



Nature of the nerve impulse. Up to the 



present time, attempts to solve the nature 

 of the nerve impulse have been made only 

 with peripheral nerves, that is, those out- 

 side the cord and brain. Very little progress 

 has been made toward an understanding 

 of how they work within the brain itself, 

 though there is no reason to doubt that in 

 a general way the functioning is similar. 

 However, our notions of how conscious- 

 ness, reasoning, memory, thought, and so 

 forth, are carried on is purely in the con- 

 jectural stage today. Perhaps an under- 

 standing will be reached some time; if and 

 when that happens an understanding of life 

 itself will undoubtedly be had. One can 

 only conjecture to what extent the human 

 brain may be able to comprehend its own 

 mechanism. 



The simplest way to study the nerve im- 

 pulse is to observe the action of a muscle 

 to which it is attached. The classic setup 

 for such study is the sciatic nerve of the 

 frog attached to the large gastrocnemius 

 (calf) muscle and a mechanical device for 

 recording the contraction of the muscle 

 ( Fig. 15-9 ) . Various stimuli can be used to 

 stimulate the nerve but an electrical one 

 is the best and most convenient. When- 

 ever the nerve is stimulated the muscle 

 twitches, indicating that some change set 

 up in the nerve has traveled along the nerve 

 to the muscle, causing it to contract. This 

 change is called the nerve impulse. 



If a special instrument designed to detect 

 minute electrical currents is placed on a 

 nerve over which an impulse passes, there 

 will be a definite response, indicating that 

 the impulse has an electrical aspect (Fig. 

 16-13 ) . If it were merely electricity, it would 

 travel with the speed of electricity, namely, 

 186,000 miles per second. An early experi- 

 ment, however, demonstrated that it trav- 

 eled at a much slower speed, 30 meters, or 

 about 100 feet, per second in the frog and 

 only four times that rate in man. The nature 

 of stimulation bears no relation to the speed 

 of transmission and whether the nerve is 

 stimulated with heat, pressure, or electric- 



