AUTONOMIC NEUROEFFECTOR TRANSMISSION 



217 



region in question, in this case the structural relation- 

 ships between the autonomic postganglionic nerve 

 endings and the target cells. These cells in principle 

 include the heart muscle cells and the secretory cells 

 of the glands in addition to those of smooth muscle. 

 Much conflicting evidence has been presented with 

 regard to the innervation of smooth muscle cells by 

 autonomic ner\e fibers. It inay sufhce to mention that 

 an histologist as experienced as Slohr (121) found 

 that less than one cell in a hundred was inner\ated. 

 The numerous reports on intracellular nerve endings 

 in smooth muscle cells seem to require reconsideration 

 since an ingrowth of axonal endings into a cell ap- 

 pears for many reasons unlikely, and even unneces- 

 sary, especially in view of the probable distribution of 

 the transmitter in the terminal parts of the axons, 

 to be discussed later. It must therefore be seriously 

 considered whether the alleged findings are not due to 

 misinterpretation of the histological pictures. It is 

 well known that smooth muscle cells may .serve their 

 proper function without innervation, and unless it 

 can be shown that each smooth muscle cell receives 

 intracellular nerve twigs there is every reason to 

 regard the few exceptions known at present as interest- 

 ing special cases of unknown functional significance. 

 The finding of numerous endings in the ciliary muscle 

 of the eye does not alter the general picture. There is 

 nothing known so far to indicate any kind of motor 

 end plate' on the sinooth muscle cell. Knoblike 

 thickenings ending at or near the cell surface have 

 been described, however, both by older histologists 

 and more recently. Similar structures, sometimes 

 assuming the picture of bead-strings, have been re- 

 peatedly found at autonomic nerve endings (54, 62, 

 72). Garven & Gairns suggest "that the small beads 

 on the course of the finest fibrils represent the actual 

 release points of the humoral agents within the cyto- 

 plasmic continuum provided by cells other than the 

 neurones. " 



As will be discussed in the following section the 

 results of studies of electrical phenoinena in the 

 siTiooth muscles do not suggest direct innervation of 

 such cells. 



Cannon & Rosenblueth (20) have regarded the 

 few innervated cells as having special functions and 

 have named them 'key cells.' Their contention was 

 that by chemical transmission concentrated to these, 

 the neighboring cells will be affected by the diflfusing 

 neurotransmitter. There is little evidence to support 

 this hypothesis, however. Moreover, since it is known 

 that the autonomic nerve trans mitters are p resept 



FIG. 3. Dale's schpmatic representation of the autonomic 

 nervous system. A, adrenergic; C, cholinergic elements. 

 [From Dale (28).] 



all along the axons, it is unlikely that they should 

 be released only at one point of the axon in a small 

 nuinber of special cells. 



The question of the innervation of the smooth mus- 

 cle cell cannot be answered with coinplete certainty 

 but the best evidence points at a peripheral branching 

 system of the postganglionic autonomic nerve fibers 

 extending to the inynediate neighborhood of each 

 snipoth muscle cell (62). By release of the chemical 

 transmitter during nerve stimulation, the cells will 

 be reached by the active chemical substance through 

 diffusion. The proportion of cells activated in an organ 

 and the degree of activation will clearly depend upon 

 the amount of transmitter set free, which in its turn 

 is a function of the frequency and strength of the 

 stimulus applied to the nerve. 



HUMORAL VERSUS ELECTRICAL TRANSMISSION 



The bulk of evidence points to the conclusion that 

 denervated smooth muscle is electrically inexcitable 

 (100, 114). Even if direct stimulation of denervated 

 smooth iTiuscle may lead to contraction, this is weak 

 and differs in several respects from that produced by 

 the chemical stimuli. It appears likely that the direct 

 stimulation effect is unspecific and due to a direct 

 gross action on the contractile material. An important 



