PRINCIPLES OF PHYSIOLOGY \QJ 



inactive, and thus the continued stimulation of the adjacent neuron is 

 prevented. 



The mechanism of synaptic transmission in other types of nerves is 

 the subject of controversy. There is evidence tliat acetylchohne plays 

 some role, perhaps the major one, in synaptic conduction in the central 

 nervous system of vertebrates and certain invertebrates. Synaptic trans- 

 mission is greatly affected by the concentration of cations such as potas- 

 sium and calcium, and these ions may play some direct role in 

 transmission. There is evidence from certain types of nerves that the 

 electrical disturbance which accompanies the nerve impulse in one 

 neuron may be sufficient in itself to elicit a nerve impulse in the next 

 neuron. Each of these agents may, under certain conditions, be shown 

 to stimulate a nerve cell; which one or ones actually function in the 

 intact animal is not yet clear. One currently popular theory states that 

 transmission along the axon and across the synapse are fundamentally 

 the same sort of electrical phenomenon and that liberation of acetyl- 

 choline is an essential part of the transmission mechanism of each. 

 Synapses are important functionally because they are points at which 

 the flow of impulses through the nervous system is regulated. Not every 

 impulse reaching a synapse is transmitted to the next neuron. The 

 synapses, by regulating the route of nerve impulses through the nervous 

 system, determine the response of the organism to a specific stimulus. 



The important details of the arrangement of the neurons to form 

 the central nervous systems of the higher invertebrates and of the verte- 

 brates will be discussed in later chapters. The invertebrate nervous 

 system consists of one or more pairs of ganglia— collections of nerve cell 

 bodies— at the anterior end of the body and one or more nerve cords 

 extending posteriorly. The invertebrate nerve cord is solid and is 

 typically located on the ventral side of the body; the vertebrate nerve 

 cord is single, hollow, and located on the dorsal side of the body. 



Sense Organs. Physiologic experiments show that nerve fibers can 

 be stimulated directly by a variety of treatments, by electric shocks, by 

 the application of chemicals, or by mechanical cutting or crushing. 

 In the intact organism, of course, sensory nerve fibers are activated by 

 the sense organs to which they are connected. Sense organs, like nerve 

 fibers, respond to a variety of treatments, but each is specialized so that 

 it is extremely sensitive to one particular kind of stimulus. The negli- 

 gible amount of vinegar which can be tasted, or the least amount of 

 vanillin which can be smelled, has no effect when applied directly to 

 a nerve. 



Sense organs may be classified according to the type of stimulus to 

 which they are sensitive. We can distinguish (1) chemoreceptors— smell 

 and taste; (2) mechanoreceptors-touch, pressure, hearing and balance; 

 (3) photoreceptors-sight; (1) thermoreceptors-hot and cold; and (5) 

 undifferentiated nerve endings which serve the pain sense. Sense organs 

 may also be classified by the location of the stimulus: thus extero- 

 ceptors supply information about the surface of the body (touch, pres- 

 sure, taste, heat, cold); proprioceptors supply information about the 

 position of the body (stretch receptors in muscles and joints, equilibrium 



