{"id":8561,"date":"2012-01-24T00:01:32","date_gmt":"2012-01-24T08:01:32","guid":{"rendered":"http:\/\/www.todayifoundout.com\/?p=8561"},"modified":"2015-06-05T00:46:23","modified_gmt":"2015-06-05T07:46:23","slug":"what-causes-arms-legs-and-feet-to-fall-asleep","status":"publish","type":"post","link":"http:\/\/www.todayifoundout.com\/index.php\/2012\/01\/what-causes-arms-legs-and-feet-to-fall-asleep\/","title":{"rendered":"What Causes Arms, Legs, and Feet to “Fall Asleep”"},"content":{"rendered":"

\"\"<\/a>Today I found out<\/a> what causes limbs to \u201cfall asleep\u201d.<\/p>\n

Technically known as “paresthesia”, this syndrome is caused by the compression of specific nerves. When you sit cross-legged, sleep with your arm above your head, or position any limb in such a way to put excess pressure on a nerve, that nerve will stop sending impulses normally. Should the pressure be great, or the duration be long, the nerve will eventually stop sending impulses altogether.\u00a0 Any area the nerve services will essentially then \u201cfall asleep\u201d.\u00a0 Think of a person standing on a garden hose. It’s hard for the water to get to the nozzle when the person’s feet get in the way. If the person’s heavy enough, or she stands on the hose for too long and the hose fully compresses, water will eventually stop flowing entirely. Once this pressure is relieved, your nerve will start to function normally again (hopefully) and you can now move your hand\/legs\/arms\/feet. Nerves, like a well worn hose, may take some time to work properly (expand) and you may feel some tingling, “pins and needles”, during the process.<\/p>\n

Nerve cells, for the most part, have their main bodies located in the spinal cord itself. They have what are known as “axons” that branch out to your limbs (and other parts of the body, but we will focus on the limbs) and carry nerve impulses out from the spinal cord. Together with another cellular protrusion known as a dendrite, these projections allow us to feel the world around us.<\/p>\n

Nerve impulses require a healthy energy supply, known as the axonal transport system.\u00a0 This well developed micro-vascular distribution method provides the blood flow needed to maintain the cells in good working order. If pressure is put on the correct spot, though, all the tiny arteries, veins and capillaries that supply the nerves’ nutrients become pinched off and the nerve cells begin to function abnormally. If you doubt me, hit the Ulnar nerve (the funny bone) on something and see how sudden, extreme pressure makes you scream!<\/p>\n

Studies have shown that it doesn\u2019t take much to cause the axonal transport system go haywire. External pressures of as little as 20 mmHg (about \u00bd pound per square inch) caused intermittent paresthesia due to the veins supplying the nerve having their blood flow reduced.\u00a0 When you get to around 1 pound per square inch for around 2 minutes, this can cause numbness, impaired dexterity, and result in muscle weakness.<\/p>\n

Compression isn’t the only thing that can cause limbs to fall asleep. Excessive vibration will also lead to paresthesia. Operating hand-held vibrating tools is an extremely common cause of this.\u00a0 For example, dirt bike racers who need to grip handlebars tightly, while jumping frequently, experience this and commonly refer to these symptoms as \u201carm pump\u201d. The process that causes these symptoms in the presence of vibration hasn’t yet been fully clarified, but these pulsations have been shown to injure the entire neuron.<\/p>\n

Any discussion of what causes limbs to fall asleep would be remiss without talking about the common misconception that a limb falling asleep is caused by a lack of blood flow to the entire limb involved. While blood flow is blocked to the nerve specifically, if the entire limb were to be cut off from blood, severe life threatening problems would arise every time we sat \u201ccrisscross applesauce\u201d in grade-school (see the problems with tourniquet use in the bonus factoids<\/a> below).<\/p>\n

In medical terms, this is referred to as compartment syndrome. When blood flow is halted or reduced, life sustaining PH balance is in jeopardy. The area affected begins to become extremely acidic and cells begin to break down. Waste products and lethal levels of electrolytes, like potassium, begin to build up. Once the pressure on the limb is released, this \u201cacid blood\u201d is then sent back to the heart with potentially lethal consequences. Ask all those people on death row if excessive potassium is a good thing.\u00a0 They might want to reconsider investing in a portfolio that backs the production of potassium chloride.\u00a0 Cardiac arrest anyone!<\/p>\n

If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (iTunes<\/a>, Spotify<\/a>, Google Play Music<\/a>, Feed<\/a>), as well as:<\/p>\n