Astronauts Help Us Understand Our Nervous System’s Tolerance

If you don’t know what the autonomic nervous system is – commonly referred to simply as the nervous system – astronauts and pilots offer an interesting angle on the subject.

Instead of just saying the autonomic nervous system (ANS) affects everything – because it does – we’re going to let you experience its effects through the lives of some interesting people.  But first, let’s go over the two important parts of the nervous system: 1) the parasympathetic and 2) the sympathetic system.

The parasympathetic nervous system is responsible for resting and digestion while the sympathetic nervous system is responsible for reaction to physical activity.  Tightening blood vessels, increasing heart rate, speeding up breathing patterns – these are all sympathetic activities.

Because the ANS is responsible for so many functions, it’s tedious to review every single one.  However, it’s interesting to see how tolerant the nervous system is to different activities – the near-mach-3 speeds of a fighter pilot and the floating, relaxed movements of an astronaut in outer space.

G-force and zero gravity.  Both have largely different effects on the human nervous system.

Let’s take a look.

Astronauts: It’s Easy to Let the Parasympathetic Triumph

While on land, before the big take-off, astronauts work night and day conditioning themselves for the launch into the ether.

They’re men and women with complete autonomic balance, which means the sympathetic and parasympathetic systems work in perfect harmony.  When an astronaut pushes off the ground in a zero-gravity training room, their heart rate responds appropriately.  And when they go to rest for the night, their body relaxes into a parasympathetic state ideal for healthy sleeping.

Everything is perfectly balanced.  That is, until it isn’t.

After months and months of being in space – sitting around, working the controls, eating freeze-dried ice cream and whatnot – deconditioning, a person’s adaptation to a less physically demanding environment, becomes a reality.  In this case, the parasympathetic system becomes dominant and the sympathetic system weakens.  This imbalance can lead to debilitating effects when the astronaut has to perform physically demanding tasks, in space and post-flight on ground.

Luckily, this rarely happens to astronauts.  They have various exercises they do and there is medicine now to prevent the debilitating side-effects of deconditioning.  There are also ANS tests performed on astronauts that test for any imbalance pre-flight.

Pilots: Mach 3 Speeds Control Body Circulation, Not the ANS

The fastest fighter jet can travel close to Mach 3 speeds.  That’s 3 times the speed of sound and about 2,550 miles per hour!

When fighter pilots fly this fast, they wear something called a G-suit to protect them from losing consciousness.  Without a G-suit, blood would pool in their feet and legs and they’d lose consciousness.  When positive G’s are reached during flight, the suit inflates and maintains the extravascular pressure in their lower limbs.  This maintains the autonomic balance of the parasympathetic and sympathetic talked about above.

The suit has also been used by a unique person with Orthostatic Intolerance (OI) to help control his irregular circulation.  In August, 2011, he began wearing a G-suit to combat the same problems fighter pilots were prone to while flying.  Like a pilot flying fast without a G-suit, someone with orthostatic intolerance has a body that’s unable to provide healthy circulation to all limbs.

The reason this unique person’s website has a giraffe theme is for this reason:

According to an encyclopedia article about giraffes, “the blood vessels in the [giraffe’s] lower legs are under great pressure (because of the weight of fluid pressing down on them). To solve this problem, the skin of the lower legs is thick and tight; preventing too much blood from pouring into them.”

… very much like a G-suit.

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