1 THE AUTONOMIC PHYSIOLOGY OF TERROR MANAGEMENT ...

so PNS instigated chronotropic effects that result from innervation of the SA node would
seem also to indirectly affect the force of ventricular contractility. Further, as will be
discussed in the next section, in contrast to most descriptions (e.g., Brownley, Hurwitz, &
Schneiderman, 2000), research has demonstrated that the PNS does indeed innervate the
ventricles, though its effects only emerge clearly in interaction with SNS activity.
Parasympathetic-sympathetic interaction
First shown by Rosenblueth and Simeone (1934), Levy and colleagues, working
generally with anesthetized dogs, have provided a good deal of evidence that the effects
of the PNS and SNS do not simply summate, but instead interact to affect the end state of
the heart. To investigate this process, they have stimulated parasympathetic and
sympathetic nerves that innervate the heart at different frequencies and in different
combinations. One might expect that if, for instance, they stimulated the PNS and the
SNS equally, both should equally oppose each other and cancel each other out. Further,
one might expect that if stimulated at differing levels, the end state of the organ would be
determined by calculating how much more active one system was than the other. But this
did not occur. Activity in one system did not serve to negate a comparable level of
influence on the heart by the other system. Instead, the PNS appeared dominant, to hold a
trump card, termed by Levy, vagal preponderance or accentuated antagonism. The term
vagal preponderance stems from the name of the nerve that serves as the main vehicle
for PNS influence on the heart: the vagus nerve. With the PNS or vagus nerve highly
active or stimulated, SNS activity generally plays a lesser role in affecting the heart. Only
when the PNS is less active does the SNS play a substantial role.
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