Neurolab Experiments 
On Neurolab, crewmembers will carry out a comprehensive set of tests designed to investigate every aspect of autonomic circulatory control, with the overall goal of understanding how it has changed after space flight. Several key measurements will be made. In addition to measuring blood pressure and other basic cardiovascular parameters, blood flow to the brain will be estimated using the non-invasive transcranial Doppler technique.  

Measurement Technique 1: Sound waves 
With this device, high-frequency sound waves can be used to show how blood flow to the brain is regulated. This is important because not all people who have orthostatic intolerance after space flight experience a drop in blood pressure. They may have a problem with the control of brain blood flow, and the transcranial Doppler measurements can reveal this.   

Measurement Technique 2: Tiny needles 
Another innovative technique to be used on Neurolab is microneurography. With this procedure, a small needle the size of an acupuncture needle is placed in an easily accessible nerve just below the knee (see box). With this needle, the nerve signals traveling from the brain to the blood vessels can be measured directly. This provides a clear insight into how the autonomic nervous system is functioning.  

Test conditions:  
The measurements are made while the autonomic nervous system is challenged by a variety of tests. The controlled frequency breathing test allows for the natural oscillations in the control of blood pressure to be measured precisely. The Valsalva test stimulates the pressure receptors in the neck and chest and measures the responses. The lower body negative pressure test places a stress on the cardiovascular system very similar to the one experienced with standing in Earth's gravity. The cold pressor and hand grip tests also activate the blood pressure control system and raise blood pressure, but not by stimulating pressure receptors as the Valsalva test does. The cuff occlusion test and body impedance measurements show the fluid distribution in the body and how this may contribute to problems in blood pressure control. When the results of these tests are evaluated together, the part of the autonomic system that may be functioning improperly can be localized.   

Blood Samples: 
One suggested reason for why the blood pressure control system has changed is that it is underactive in space. The autonomic nervous system releases chemical messengers when it operates (norepinephrine is one). There has been some disputed evidence from previous space flightS that the levels of norepinephrine are lowered in space and that this indicates a low level of activity in the blood pressure control system. Looking only at the blood levels of norepinephrine, however, can lead to false conclusionS, since the blood level depends not only on how much is released, but also on how fast it is removed from the circulation. On Neurolab, crewmembers will infuse labeled norepinephrine into the blood stream and collect blood samples. Investigators will use these samples to assess whether the blood pressure control system is really underactive or not.  

Getting Results: 
Taken together, this comprehensive set of measurements should help to solve the puzzle of why astronauts have problems with blood pressure control after space flight. The answer is important not only for helping astronauts to re-enter gravity safely after being in space, but also may help with patients who suffer with diseases of their blood pressure control systems.