In contrast to the wealth of data relating to strain differences in the HPA axis activity, much less data is available with respect to differences in activity of the ANS. This may in part be due to the more complex techniques required for measuring the activity of the ANS. As discussed in the introduction the ANS consists of a sympathetic and a parasympathetic limb and the overall activity depends on the relative strength of both systems. Moreover, the ANS affects virtually all internal organs, and the functional consequences is, therefore, difficult to assess. Advantages in the methodology have made it possible to quantify heart rate and blood pressure in freely moving rats with relative ease. Since both parameters are under tight control of the ANS, these parameters can be used to (indirectly) assess the activity of this second stress system.
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