1 138 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



CHEMICAL TRANSMISSION. Acetylcholine is believed to 

 be the transmitter at all parasympathetic nerve end- 

 ings. Bacq {21) found, in conformity with this view, 

 that vasodilatation produced in the canine penis by 

 stimulation of the sacral nerves was potentiated by 

 physostigmine and blocked by atropine. It is worthy 

 of note that vasodilatation elicited in the tongue by 

 stimulation of the chorda tympani [Erici & Uvnas 

 (78)] is not blocked by atropine. The cause of this re- 

 sistance to atropine is unknown, and further investiga- 

 tions into the chemical transmission at vasodilator 

 nerve terminals might be well worthwhile. 



PHYSIOLOGIC PROPERTIES. Very little is known aljotu 

 the physiologic properties of the parasympathetic 

 vasodilator fibers. Stimulating the lingual ner\e of 

 dogs with sinusoidal waves, Maltesos & Schneider 

 (161) found vasodilator fibers with various time con- 

 stants. The clironaxie values varied between o. i and 

 6 msec, with a tendency to grouping around four 

 values, 0.2, 0.8, Q.3 and 5.5 msec. 



The pliysiologic range of the discharge rate is un- 

 known, but it seems reasonable to assume that the 

 relation between firing frequency and vasodilator 

 effect is about the same for the parasympatlietic vaso- 

 dilator fibers as for the other efferent \asomotor 

 nerves. 



Heinbecker & Bishop (115) reported that in the 

 turtle vagus impulses having a negative inotropic 

 effect passed along the fine myelinated B3 filaers, 

 whereas a negative chronotropic action was obtained 

 on activ'ation of unmyelinated C fillers. In the cat the 

 negative inotropic fiijers may be mostly, and the nega- 

 tive chronotropic fibers are always, thin myelinated 

 fillers. The cardiac efferent fibers are thus \ery fine 

 and their potentials are difficult or impossible to 

 record. Schacfer (188) was occasionally able to see 

 spontaneous volleys of slow impulses of low voltage 

 in B fibers. He ol)ser\ed a rhvthmicitv in the discharge 

 which, according to him, reflected the effect of the 

 afferent cardiac impulses (jn tiie medullarv cardio- 

 va.scular center. 



CENTRAL REPRESENTATION OF VASOCONSTRICTOR 

 AND CARDIAC NERVES 



Spinal Cord 



SPINAL VASOMOTOR TONE. As far back as 1864, Goltz 

 (102) demonstrated convincingly that nervous vaso- 

 constrictor tone was present in a spinal animal. The 



nervous structures responsible for this tonic discharge 

 are located in the lateral horn. 



Govaerts (103-105) and Alexander (6j reported 

 direct recording of the activity in the sympathetic 

 cardiac outflow of spinal animals. Both of them found 

 a persisting discharge after section of the buffer nerves 

 (the sinus and vagus nerves). A surprising observation 

 was Alexander's finding that a continuous discharge 

 still occurred in the inferior cardiac nerve fibers after 

 total deaiTerentation of the thoracic spinal cord. The 

 cord was transected in the lower cervical and mid- 

 thoracic regions. In addition, all afferent fibers to 

 the isolated thoracic part of the cord were cut. The 

 tonic activity of this spinal preparation was fairly 

 slight but unquestionable. It was depressed by hyper- 

 \'entilation or by ventilation with a mixture of 90 

 per cent oxygen and 10 per cent carbon dioxide, and 

 stimulated by asphyxiation or ventilation with pure 

 nitrogen. Alexander therefore suggested that even in 

 the normal animal the oxygen tension may con- 

 tribute to the excitatory states of the spinal cardio- 

 vascular centers and thereby reinforce the buffer 

 reflexes that are integrated at higher levels of the 

 nervous system. This idea is at variance with the com- 

 monly accepted notion that the carbon dioxide ten- 

 sion is the main local chemical factor e.xciting the 

 vasomotor centers. 



SPINAL V-^sOMOTOR REFLEXES. Numerous workers have 

 observed segmental and intersegmental vasomotor 

 reflexes in both acute and chronic spinal animals. 

 Brooks (43) observed increments of arterial pressure 

 and heart rate while Downman & McSwiney (72) 

 found pressor responses to nociceptive stimuli in the 

 acute spinal cat. 



In spinal man, Gilliatt and co-workers (98 1 ob- 

 served that deep inspiration elicits vasoconstriction 

 in the fingers both in intact subjects and in patients 

 with a complete break in the functional continuity 

 of the spinal cord above the level of the sympathetic 

 outflow to the hands. Distention of the urinary bladder 

 similarly elicited vasoconstriction in the fingers. 



Increase of the arterial pressure in the spinal dog 

 results in an increase of the volume of the spleen 

 [Heymans et al. (119)]. Heating of an extremity in 

 spinal monkeys was reported to elicit cutaneous 

 va.sodilatation contralaterally [Fulton (94)]. Since 

 the abdominal viscera and the skin of the animals ob- 

 served have no centrally controlled vasodilator nerve 

 outflow (see p. 1136), the observations point to the 

 presence of vasoconstrictor inhibition in the spinal 

 animals. 



