SENSE ORGANS AND NERVOUS COORDINATION 587 



of time; about 1,000 per second in the case of certain neurons with very 

 short refractory periods. 



When a neuron is artificially stimulated at some point near the 

 middle, the impulse spreads along the neuron in both directions. Im- 

 pulses can travel in either direction, but under normal conditions 

 neurons are stimulated only at their dendritic ends and impulses travel 

 only toward the axonal ends. A neuron is normally stimulated at its 

 dendritic end because this is the end that is related to the sense organs, 

 and because the nerve impulse can travel in only one direction across 

 a synapse— from the axon of one neuron to the dendrites or cell body of 

 another. 



The initiation of an impulse in a neuron, either by a sense organ 

 or by the transmission of an impulse across a synapse from an adjacent 

 neuron, is a complex phenomenon. A neuron will not initiate an impulse 

 unless the stimulus that it receives from the sense organ or presynaptic 

 neuron is strong enough to cause the chemical changes that underlie an 

 impulse to reach a certain threshold level. In this way, too, the neuron is 

 analogous to a fuse which does not burn until the temperature (stimulus) 

 reaches a certain threshold level. The threshold levels of neurons vary. 

 Some postsynaptic neurons with a low threshold will fire if a single im- 

 pulse reaches them. But most postsynaptic neurons have a higher thresh- 

 old, and a single impulse reaching them is insufficient to initiate an 

 impulse in them. Such neurons will not fire unless several impulses reach 

 them simultaneously from several presynaptic neurons, or in rapid succes- 

 sion from one. It must not be thought, however, that because a single pre- 

 synaptic impulse is subthreshold it has no effect upon the postsynaptic 

 neuron. It initiates certain changes leading toward the firing of the 

 neuron, and if enough subthreshold stimuli reach the neuron at the 

 same time, or before the effects of the first are worn off, their effects are 

 added to those of the first and the threshold of stimulation may be 

 reached. 



The impulses in some presynaptic axons have an inhibitory rather 

 than an excitatory effect upon the postsynaptic neuron. Whether or 

 not the threshold of stimulation is reached and a neuron fires is a 

 product of the interaction of all of the inhibitory and excitatory in- 

 fluences that reach it at any given time. If synaptic transmission were 

 simply an electrical phenomenon, excitation and inhibition would be 

 difficult to understand. The consensus at present is that synaptic trans- 

 mission involves the secretion by the presynaptic ending of hormone-like 

 substances. Some endings may produce an excitatory substance (possibly 

 acetylcholine), and others an inhibitory hormone. It is known that this 

 happens at the junction between neuron and muscle (myoneural junc- 

 tion) in the autonomic system; some of the autonomic neurons are ex- 

 citatory and others inhibitory (p. 596). This theory of synaptic trans- 

 mission is consistent with the observed delay in the transmission of an 

 impulse across a synapse and with one-way transmission across a syn- 

 apse. One-way transmission across a synapse is a very important inte- 

 grating factor for it enables the presynaptic neuron to modify the 

 activity of the postsynaptic neuron without being affected itself. 



