The skeletal muscles that move your body all depend on your nervous system to tell them when to contract. Messages pass from your nervous system to your muscles in a zone called the "neuromuscular junction." Nerve cells release acetylcholine, a chemical, into the neuromuscular junction to signal your muscles to contract. Nerve cells depend on sodium and potassium to be able to release acetylcholine and conduct messages to your muscles. Your muscles also depend on sodium and potassium to start and end the process of contracting. If your body has too much or too little sodium or potassium, your muscles will be unable to function.
Action Potentials
Your motor nerve cells use a process called an "action potential" to send messages to your muscles. An action potential causes the nerve cell to reach a specific electrical charge that will cause it to "fire" and release acetylcholine. Nerve cells normally keep themselves in a resting charge of about minus 70 millivolts (mV) by maintaining the right number of ions inside the cell membrane. When a nerve cell receives enough stimulation from other nerve cells, the cell pumps more positive ions into itself and its electrical charge becomes more positive. Once the cell's charge reaches a threshold of about minus 55 mV, an action potential occurs. The action potential opens ion channels and allows positive ions to rush into the cell, causing it to release acetylcholine.
The Neuromuscular Junction
Like nerve cells, muscle fibers have a normal electrical charge while they are at rest. For muscle fibers, this resting potential is about minus 95 mV. During an action potential, the nerve cell releases acetylcholine into the neuromuscular junction. When the acetylcholine comes in contact with muscle fibers, it causes specialized channels on the surface of the fibers to open up and allow sodium to flow into the fibers. When this happens, the interior electrical charge of the muscle fibers becomes more positive. When a muscle fiber's charge reaches about minus 50 mV, an action potential travels down the fiber. The action potential triggers a process that causes the muscle to contract.
Sodium's Role
Sodium, or Na+, is a positively charged ion which plays several key roles in muscle contraction. First, Na+ pumps located in the cell membranes of nerve cells pump sodium into the cell and cause an action potential which releases acetylcholine into the neuromuscular junction. Sodium also triggers action potentials within muscle fibers. When an action potential occurs within a muscle fiber, it causes special channels in a part of the fiber called the "sarcolemma" to open and release calcium. When the calcium comes into contact with the thin filaments of the muscle fiber, it binds with them and causes the muscle fiber to contract. Without the proper amount of sodium, nerve cells could not send messages to your muscles and your muscles could not release calcium and contract.
Potassium's Role
Potassium, or K+, is also a positive ion. While Na+ causes action potentials in nerve and muscle cells and helps trigger muscle contraction, potassium's role is to return your nerve and muscle cells to their resting state. After a motor neuron experiences an action potential, its charge rises rapidly due to many Na+ ions rushing into the cell. When the nerve cell reaches its maximum voltage special proteins within the cell membrane pump K+ into the cell. This causes the cell's charge to return to its normal resting state. A nerve cell cannot "fire" again until it has reached its resting state, so nerve cells would not be able to function without K+. Potassium also returns muscle fibers to their resting charge after an action potential, making them able to contract again.
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