220 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



TABLE I . Norepinephrine Content of Beef Nervous Tissue 



in tig per gm (129) 



cell constituent to form what was termed inhibitory 

 (I) or excitatory (E) sympathin or both. These find- 

 ings are readily explained on the assumption that 

 the actions observed were either due to the true 

 adrenergic neurotransmitter, norepinephrine, or to 

 epinephrine released from other sources, presumably 

 chromaffin cells, or a mixture of both, as suggested 

 i)y Bacq in 1934 and subsequently proved by the 

 demonstration of both amines in autonomically in- 

 nervated organs (126). The term sympathin should 

 preferably be abandoned in the physiological litera- 

 ture since it does not discriminate between the neuro- 

 transmitter and the hormones released as a result of 

 preganglionic stimulation of chromaffin cells. 



Occurrence, Biosynthesis and Storage oj Adrenergic 

 Nerve Transmitter 



Unless it is assumed that the chemical transmitters 

 are being formed and released at the moment of 

 nervous excitation it must be concluded that they 

 are present in the axon and relea.sed from .some kind 

 of store. Systematic studies of the content of trans- 

 mitter substances in postganglionic nerves have been 

 made both for the cholinergic and for the adrenergic 

 system. Such experiments have shown that the content 

 of norepinephrine in a nerve correlates well with the 

 number of unmyelinated fibers of autonomic origin 

 (i 1 1). As seen in table i the amount of norepineph- 

 rine varies greatly and is highest in the splenic nerves 

 which are known to contain practically only post- 

 ganglionic sympathetic fibers. In other nerves, such 

 as the vagus, the amount is quite small and this is 

 true also for most motor nerves and the majority of 

 sensory nerves. For technical reasons it is impossible 

 to prepare nerves in their most peripheral parts, 

 hence it has not been possible to study directly the 

 content of the transmitters in the immediate vicinitv 



of the target cell, which for many reasons would 

 have been desirable. On the other hand it has been 

 possible partly to overcome this difficulty by making 

 extracts of whole organs and estimating their trans- 

 mitter content, thus measuring the total amount 

 present in the tissue including the finest nerve rami- 

 fications (iio, 129). Proof that the transmitter sub- 

 stances so found are actually due to the presence of 

 postganglionic nerve fibers has been obtained by 

 studying the effect of denervation. If the postgangli- 

 onic nerves are severed and allowed to degenerate, 

 the amount of norepinephrine in the peripheral ti.ssue 

 falls to very low figures or disappears completely. 

 This indicates that the tissue is not able to store the 

 transmitter by itself but does so by means of its 

 autonomic nerve fibers. Further support for this 

 opinion is provided by experiments showing that 

 some 4 to 8 weeks after degeneration of the cardiac 

 nerves the content of adrenergic transmitter in the 

 sheep heart increases again and after the lapse of a 

 few months reaches the original value (fig. 7) (55). 

 .Similar results have been oljtained for other organs 



FIG. 7. Norepinephrine content of sheep hearts before and 

 various times after svmpathetic denervation. [From Goodall 



(55)-] 



T.\BLE 2. Norepinephrine Content in Beef Organs 



in fig per gm (129) 



Spleen 



Lymph glands 



Heart 



Ciliary body and iris 



Liver 



Arteries and veins 



Lung 



Intestine 



Uterus 



Testicle 



Skeletal muscle 



Bone marrow 



1-5-3-5 



0.5-0.8 



o . 3-0 . 6 



0.4 



o . 25 



o. I-I 



0.15 



0.15 

 0.15 



0.04 

 0.04 

 0.0 



