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J. H. Coote, S. M. Hilton and A. W. Zbroyna

1. In anaesthetized cats the effects were investigated of electrical stimulation of regions in the caudal mesencephalon, pons and medulla on muscle blood flow, skin blood flow and arterial blood pressure.

2. It was found that within the dorsal part of the well known pressor area there is a narrow strip, 2·5 mm lateral from the mid line, starting ventral to the inferior colliculus and ending in the medulla close to the floor of the IV ventricle, from which vasodilatation in skeletal muscles is selectively obtained. This strip is quite separate from the more ventral, efferent pathway for active vasodilatation running from the hypothalamic and rostral mesencephalic `defence centre'.

3. As in the case of the hypothalamic and rostral mesencephalic `defence centre', the muscle vasodilatation obtained from the caudal strip is accompanied not only by a rise of arterial blood pressure, but also by tachycardia, vasoconstriction in the skin, pupillary dilatation and piloerection.

4. Stimulation, restricted to the caudal strip, via implanted electrodes in unanaesthetized animals, produced a behavioural response resembling the defence reaction. The strip, therefore, is probably a caudal extension of the `defence centre'.

5. Unlike the vasodilatation elicited from the more rostral part of the `defence centre' in the hypothalamus and mesencephalon, the muscle vasodilatation obtained on stimulation of the caudal strip was resistant to atropine, but was blocked by guanethidine.

6. It is suggested that during naturally occurring defence reactions in the normal animal the ponto-medullary area is activated together with the hypothalamo-mesencephalic area, inhibition of vasoconstrictor tone then accompanying activation of the vasodilator nerve fibres in skeletal muscle.

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