Changes in muscle sympathetic nerve activity and vascular responses evoked in the spinotrapezius muscle of the rat by systemic hypoxia.

Non-technical summary  Hitherto, activity in sympathetic vasoconstrictor nerves that supply arterial vessels of skeletal muscle has been deduced from recordings made on mixed nerves that may be supplying skin, muscle or other tissue. We describe new methodology that allows direct recordings from nerve fibres on the surface of arterial vessels of muscle. The impulses occurred irregularly, but with rhythms that reflected heart rate and breathing rate. When oxygen levels in the blood were progressively lowered (hypoxia), muscle sympathetic nerve activity progressively increased from an average impulse frequency of 0.2 to one of 0.62 per second. Simultaneously, heart rate and respiratory rate increased, while our recordings of blood flow into the muscle showed that the arterial vessels dilated. We deduce that hypoxia causes an increase in the activity of the sympathetic nerves that supply muscle blood vessels that is mainly dependent on the stimulus for breathing, but their normal vasoconstrictor effect is overcome by local dilator influences.