146 LLOYD M. BEIDLER 



This situation raises a fundamental problem of neural organization 

 because we know that the pattern of chemical sensitivity varies from one 

 cell to another. Different patterns of response to salt, sweet, bitter and 

 acid substances are demonstrated for different neurons and also, by their 

 receptor potentials, for different receptor cells. But if the connections 

 between receptor cells and neurons are continually changing the patterns 

 of central activity should become disorganized. A predominantly " sweet " 

 fiber would become " bitter", "bitter" might become "salt", and so on. 



Only two escapes from this dilemma appear possible. One is that the 

 afferent neurons continually change their central connections to match the 

 sensitivity of their receptor cells. This possibility I dismiss as fantastic. 

 The other possibility is that the pattern of sensitivity of ea^h re:eptor cell is 

 determined by the neuron or set of neurons by which it happens to b^ 

 innervated. This possibility is in general accord with ac::epl;ed prin::iple3 

 of morphogenesis. I understand that Dr. Beidler shares this interpreta- 

 tion with me. 



But now let us pursue the implications of this important hypothesis, 

 which is new, as far as 1 am aware, in relation to the gustatory system. It 

 implies that the specific sensitivity of a receptor cell can be changed and 

 changed rapidly as synaptic contact is made with a new fiber or fibers or 

 as old contacts are lost. There must be a continual flow of" information " 

 in the form of chemical material, very specific chemical material, outward 

 from the cell body of the afferent neuron. This information establishes the 

 coc^e according to which the receptor cell will excite the neuron in response 

 to a particular class of chemical stimuli applied to the taste bud. 



Now we also know that each receptor cell and consequently its neuro:i(s) 

 respond to not only one class of chemical substance but to at least three or 

 four, to different degrees. The variety of patter.is of sensitivity is actually 

 so great that it suggests a continuance in each of several dimensions as if 

 these were a random or partially random distribution of sensitivity to 

 each class of stimuli independently of one another. 



The type of chemical information that is transmitted outward by each 

 afferent neuron is presumably relatively constant for that neuron. Each 

 neuron may, perhaps, impose only a single class of sensitivity on a receptor 

 cell. This is not a necessary assumption, however. A neuron might trans- 

 mit more than one chemical sensitizer. The essential assumption is that 

 the character and quantity of the outflowing chemical information are 

 determined by the central connections of the peripheral sensory neuron. 

 We assume that the amount of chemical sensitizer of each class is propor- 

 tional to the richness of the central connections of a neuron to the ultimate 

 " center " for that class of gustatory sensation. Note that this is almost 

 exactly the same assumption that we have already made for the receptor 

 cells. It is not a new major assumption. 



