GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 161 



Irreversible Conduction. In all this maze of connections and 

 interconnections within the central nervous system, how is it 

 that the impulses coming in at a sensory neuron always come 

 out finally at a motor neuron instead of becoming switched some- 

 times to another sensory neuron? The orderly progress of im- 

 pulses is insured by a very simple arrangement, namely, that 

 impulses can pass freely across a synapse from end arborization 

 to dendrites but can never pass in the reverse direction, from 

 dendrites to end arborization. When a sensory neuron delivers 

 its impulses to an association neuron the impulses doubtless spread 

 to all parts of the latter. They can leave it, however, only by way 

 of its end arborizations, and these communicate only with the 

 dendrites of motor neurons or of other association neurons. The 

 final outcome is bound to be a motor neuron since all association 

 neurons lead ultimately to them. Sensory neurons never receive 

 impulses from other neurons because they have no dendrites 

 within the central nervous system by which impulses might be 

 received. The portion of a sensory neuron which corresponds to 

 the dendrites of a motor neuron is the long axon-like process 

 communicating with the receptor. 



Graded Synaptic Resistance. Another question which nat- 

 urally arises when one considers the innumerable courses which 

 an impulse may take within the central nervous system is what 

 determines the course it actually does take? Why, for instance, 

 when my eye is threatened do I wink instead of opening my mouth, 

 or why do I sometimes wink and sometimes dodge? A complete 

 answer to this question cannot be made in the present state of our 

 knowledge, but we have a fairly good general idea of the way in 

 which nerve impulses are probably guided. A sensory neuron has 

 several collaterals, each with its synaptic connection with another 

 neuron. If we suppose these synapses are not all alike, but that 

 certain ones transmit the sort of stream of impulses generated by 

 feeble stimuli more readily than do the others, such a stream 

 spreading over the sensory neuron will pass most easily to that 

 connecting neuron whose synapse offers least resistance to its pas- 

 sage. Thus we may imagine a stream of impulses spreading from 

 neuron to neuron following always the path of least resistance until 

 it finally terminates in a muscle which it arouses to activity. In 

 the central nervous system the various paths of least resistance are 



