MadSci Network: General Biology |
Hello Josh, The types of muscle spasms that you are referring to are caused by a repetitive twitching of the muscle. These can usually be stopped quite easily simply by using the affected muscle. To be precise I will only talk about muscles that one has conscious control of the movement, which are called skeletal muscles. The body contains many other non voluntary muscles called cardiac (heart) and smooth muscles but I will not talk about these since they have different regulatory systems and will only help to confuse the issue. In the absence of any conscious stimulation the tension in skeletal muscles should remain constant at all times. In the body skeletal muscles do not relax completely and under resting conditions there is a small amount of tension but this is constant. Muscles are activated by electrical impulses that travel down nerves to the muscle. When an impulse reaches the muscle it stimulates a contraction. The twitching that you experienced in your eye is caused by a repetitive electrical impulse being conducted by a nerve that stimulates the muscle repeatedly. For details on how nerve impulses stimulate muscle contraction you may like to look at the answer to ano ther Mad Scientist question but I will summarise this briefly. A single nerve impulse will result in a single quick twitch of a muscle. Repetitive impulses will result in repetitive muscle twitches. If the impulses are close enough together the following twitch may be larger than the previous one, and this can go on up to a rate of simulation where the twitches all combine together to form a tetanus, which is what we are normally aware of as a muscle contraction. So the spasms that you experience are repetitive stimulations by a nerve that are not close enough together to result in single contraction and are experienced as a repetitively twitching muscle. These twitches are very common and not normally an indication of anything wrong. Occasionally they can become problematic for people and this is an indication of an underlying condition that causes these tremors. What makes the nerve rapidly fire in this manner is not known in all cases but in more severe cases of muscle tremor underlying causes have been identified. In a syndrome known as hemifacial spasm the muscles on one side of the face undergo this tremor due to the facial nerve being trapped and squeezed. The squeezing of this nerve results in the rapid firing of electrical stimuli to the muscle resulting in spasm. Another form of painful muscle spasm is called reflex muscle spasm. This is caused by disease in the spinal chord or joints. One common reflex you may be aware of is the knee jerk or patellar reflex, where one strikes a tendon just below the knee resulting in a lower leg reflex. This reflex action occurs by the activation of a nerve circuit that starts with a stretch receptor in the tendon below the knee, goes via the spinal chord to the thigh muscle to stimulate contraction. There are many of these reflex circuits in the body and if one of these gets repetitively stimulated due to disease it can become painful. However I should point out that the occasional tremor in the eye or leg is nothing to worry about and is not an indication of anything wrong. Only when a tremor becomes persistent and painful is it a problem. Thank you for your interesting question, and good luck in the future. Dave Burton
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