Twitches
We are all a little spastic sometimes, and there’s nothing to make you feel more like a spazz than the uncontrollable twitch of an eye or a thigh or a thumb. These little jumps under the skin, called fasciculations, are not well understood but apparently result from nerve abnormalities, which in turn trigger a response in muscle groups associated with that nerve. According to the National Institutes of Health, fasciculations can be prompted by caffeine intake, stress, lack of sleep and even by healthy exercise. You can get a twitch just about anywhere you have a muscle, and it’s not uncommon to feel them—and see them—in a calf, in the small muscle between the thumb and forefinger, or in the tongue.

Funny bone
Smacking the so-called funny bone is amusing only to other people. The pain and numbness that follows is due to hitting the ulnar nerve, which is protected by nothing other than a thin layer of skin where it passes around the outside of the elbow. The same nerve gives sensation all the way down the arm, and if you press on it hard enough you will feel your pinky and ring fingers go numb.

Hiccups
Hiccups are caused by spasms of the diaphragm, and like other spasms they are impossible to turn on voluntarily and troublesome to turn off. The diaphragm is the muscle at the base of the lungs, and a spasm causes a sharp intake of air. The characteristic sound is the glottis (the opening between the vocal chords) closing reflexively. All this can make it difficult to eat, talk, sleep or sneak up on anyone.

Shivers and chatters
Just like rubbing two sticks together creates friction and fire, the body shakes to generate more heat. Shivering hands, trembling legs and chattering teeth are all localized reactions to the body’s attempt at raising the temperature.

Shivers are a common symptom of the flu. Raising body temperature helps mobilize the antibodies that fight infections and viruses, which is why the body also creates a fever.

Brain freeze
No one understands exactly why we experience acute (though short-lived) brain pain from swallowing something too cold too quickly. In truth, the sensation is around our sinuses and face; there are no pain receptors in the brain, so we can’t feel pain there. According to Dr. David Biondi, a headache expert from Massachusetts General Hospital, the ice headache may be due to chilling the trigeminal nerve, which is located near the back of the mouth and is responsible for all sensation to the head and face. He explains, “In people with migraines, the threshold at which the trigeminal nerve is activated is lower than it should be. So we can speculate that anything that stimulates the face or other areas of the trigeminal nerve—dental work, getting hit in the head or face, or maybe drinking something that’s too cold and stimulates the trigeminal nerve through the throat—all can be potential triggers.”

Sleep starts
One moment you’re drifting off to la-la land, and the next instant—whoa!—you’re dropping off a cliff. Sleep experts believe the brain sometimes gets the wrong message from the body during the transition into sleep, and associates the lack of muscle tension with being in midair. “The usual explanation for sleep starts is that the loss of muscle tone at sleep onset is sometimes accompanied by the feeling of falling,” says Dr. Rosalind Cartwright, chairman of Behavioral Sciences at Rush-Presbyterian-St.Luke’s Medical Center in Chicago. “So it is reacted to by a sudden jerking of arms and legs to ‘save’ oneself.”

Jumping when startled
A useful leftover from our primal fight-or-flight days is how we instinctively defend ourselves from unforeseen conflict in a split second. The body prepares to deal with a threat in several ways before the brain even has time to assess the situation. When surprised, we instantly distance ourselves by jumping away and facing the threat. Muscles stiffen and we steady our feet in a fighting stance. Adrenaline surges, increasing our strength, speed and heart rate. Other reactions to sudden fear or surprise include widening of the eyes, clenching of the fists and jaw and, in men, the retreat of genitalia—kind of a “first things first” response.

Hair standing on end
Each hair on your body is held in place by a follicle, which is like a tiny tube in the skin. Sometimes, in response to fear, cold or even a light touch, muscle fibers connected to the follicle contract and cause the follicle to stiffen—making the hair in there stand up straight. Now imagine hundreds of those little follicles standing up straight and what do you have? Goose bumps.

This little body oddity is most likely another component of the fight-or-flight response, but no one is sure what good it actually does. If something intends to eat you, it probably won’t be stopped by the sight of a thousand tiny hairs standing on end. Although, it works for porcupines.

*This information was taken from the MSN Health and Fitness website, http://health.msn.com