You're just sitting at your desk and suddenly tic, tic, tic. Small muscles in your legs start twitching, seemingly taking on a mind of their own.

The zings, fizzles and ever-so-slight thumps of a twitching muscle feel weird, kind of unsettling. And if they happen to you frequently, you might worry whether they're normal.

"Fasciculations, which are random, involuntary muscle twitches, are extremely common," says Dr. William Ondo, a neurologist who specializes in movement disorders at Houston Methodist. "About 70% of people report experiencing them."

The most common places to experience muscle twitching includes the lower eyelid and legs, but muscles throughout your body, including the ones in your arms, feet and lower abdomen, can twitch as well.

"They can be disruptive, but fasciculations are usually nothing to worry about — although many people are still curious to understand why they happen," says Dr. Ondo. "Sometimes fasciculations can be a sign of an underlying health condition, but rarely. This does, however, make it important to know when to take them seriously."

What causes muscle twitching?

The human nervous system is divided into the central nervous system and the peripheral nervous system.

"The central nervous system includes the brain and the spinal cord," explains Dr. Ondo. "These then connect to a series of different nerves that travel throughout the body — to the arms, legs, face and everywhere else. This is the peripheral nervous system."

You're likely already familiar with a few components of the peripheral nervous system, such as the median nerve, which is the nerve compressed during carpal tunnel syndrome, and the ulnar nerve, which is — incongruously — referred to as your funny bone. (Related: Why Does Hitting Your Funny Bone Hurt So Much?)

The peripheral nervous system also helps control voluntary muscle movement, when motor nerves stimulate muscle contractions as needed and on demand.

These nerves trigger fine muscle movements day in and day out, so they're incredibly sensitive. And, sometimes, they misfire.

"Fasciculations occur when innervation from the peripheral nervous system to the muscle is not working correctly and a muscle is triggered involuntarily, causing it to twitch," says Dr. Ondo. "This is very common and these fasciculations usually go unnoticed, but in some cases, people do feel the muscle twitch."

As for what exactly causes the peripheral nervous system to involuntarily trigger a muscle, Dr. Ondo says the biochemical mechanism isn't fully understood.

"There's definitely stimulation of the body involved, so things like caffeine, excitement, stress — anything that increases adrenaline — may make fasciculations more likely," says Dr. Ondo. "In the case of eye twitching, it could even be due to general fatigue of the eye muscles."

How to stop muscle twitching

Because they're rarely serious and not fully understood, there aren't any FDA-approved treatments to stop muscle twitching.

"In cases where muscle twitching is affecting a person's career — for instance, facial twitching in someone who's frequently on TV — there are medications we can use to relax the muscles and thereby reduce the twitching," says Dr. Ondo.

For the harmless, occasional muscle twitching the rest of us experience, Dr. Ondo says the medications that can help stop muscle twitching aren't worth it.

Are muscle twitches the same as muscle cramps?

The painful cramping you experience during a charley horse, those random body jerks that startle you awake as you're drifting to sleep: Are these also caused by muscle twitching? Or are they something else?

"If the muscle contracts to such an extent that the whole limb or body moves, it's not a fasciculation," says Dr. Ondo. "That's either a muscle cramp or spasm — which are similar to fasciculations, yet different. Muscle twitching is a very slight, often repetitive triggering of the muscle, but it doesn't bring about a complete muscle contraction. The majority of the time they're just something that's visible, not necessarily felt. A cramp is usually obvious and painful but will eventually stop."

Another curious body phenomenon that's related to but different from muscle twitching are hiccups. They're caused by involuntary, spasmic contractions of the diaphragm, a muscle important for breathing.

When should you worry about muscle twitching?

A twitching muscle can be annoying, to be sure, but, fortunately, it's rarely serious.

"Some people are just more prone to experiencing fasciculations," says Dr. Ondo. "If you've had them for many years and haven't noticed any other changes in your muscle, there's likely nothing to be concerned about."

If muscle twitching is new and you're experiencing additional symptoms, however, Dr. Ondo says this is when muscle twitching becomes more concerning.

"We start to worry about fasciculations when they're of relatively sudden onset and there's accompanying weakness, loss of tone and shrinkage in the muscle," says Dr. Ondo.

That's because fasciculations in conjunction with other muscle-related symptoms can be indicative of a serious neurologic illness — like amyotrophic lateral sclerosis (ALS), also called Lou Gehrig's disease — or anything else that damages nerves.

"When there's a degeneration of motor neurons, one of the first features is fasciculations where before there were none, typically in the legs and also sometimes in the tongue," warns Dr. Ondo. "In fact, twitching in the tongue muscle is almost always abnormal."

If you're experiencing new muscle twitching as well as other issues in the same muscle, Dr. Ondo recommends discussing your symptoms with your doctor.