588 VERTEBRATE LIFE AND ORGANIZATION 



The iminilse that is initiated in a neuron when the threshold is 

 reached is qualitatively the same regardless of what sort of a stimulus 

 initiated the impulse, or whether the stimulus was just at or far above 

 the threshold. In other words, the nerve impulse is an all-or-none 

 phcMiomenon. Nerves do not conduct "strong" impulses or "weak" im- 

 pulses correlated with the strength of the stimulus, yet we can dis- 

 tinguish between stimuli that are just at threshold and those that are 

 strong. The intensity of a stimulus does not afEect the quality of the 

 impulse, but it does affect the frequency of the impulse. A threshold 

 stimulus may generate one or two impulses per second, but as the 

 stimulus increases, the frequency of impulses increases up to a maximum 

 which cannot be exceeded no matter how much the stimulus is in- 

 creased. Neurons differ markedly in the number of impulses initiated 

 in response to a stimulus of a given strength and in the maximum fre- 

 quency of impulses that can be generated. 



Neuron Interrelations. The neurons in the body are so arranged 

 that it is possible to divide the nervous system grossly into a central 

 nervous system consisting of the brain and spinal cord, and a peripheral 

 nervous system which includes the nerves that extend between the 

 central nervous system and the receptors and effectors. The neurons 

 themselves can be grouped into three broad categories— (1) sensory or 

 afferent neurons, which carry impulses from the sense organs through 

 the nerves to the brain or cord; (2) motor or efferent neurons, which 

 carry impulses from the brain or cord through the nerves to the muscles 

 and other effectors of the body; and (3) connector or internuncio! neu- 

 rons, which lie entirely within the central nervous system and are 

 interposed between the other two. When you touch a hot stove, for 

 example (Fig. 29.8), a receptor in the skin is stimulated and it initiates 

 an impulse in an afferent neuron. This neuron is part of a spinal nerve 

 and extends into the spinal cord, where it ends in a synapse with one 

 or more internuncial neurons. An internuncial neuron, in turn, carries 

 the impulse to an appropriate efferent neuron, which extends from the 

 cord and carries the impulse back through the spinal nerve to a group of 

 extensor muscle fibers of the hand. Their contraction withdraws your 

 hand from the stove. For the movement to be effective, however, the 

 antagonistic flexor muscles should relax, and this relaxation would in- 

 volve the inhibition of impulses going to these muscles. Normally some 

 impulses go out to all of the muscles of the body continually, and 

 cause a partial contraction, a condition called muscle tonus. Inhibition 

 might be accomplished by impulses in another branch of the inter- 

 nuncial neuron in question, or in another internuncial neuron, passing 

 to the efferent neurons that innervate the flexor muscles. 



The stimulus and response just described is a simple spinal reflex, 

 and the neuronal pathway along which the impulse travels is called a 

 reflex arc. Reflexes are fixed patterns of response to stimuli and they 

 need not involve an awareness of the stimulus. The impulse need not 

 pass through any of the higher centers in the brain in order that the 

 response occur. An impulse may be carried to the cerebral cortex of 

 the brain by other connector neurons, afferent internuncial neurons. 



