Anatomy: How The Muscles Work Essay

2004 words - 9 pages

In order for a skeletal muscle to contract, a signal must be first sent from the primary motor cortex which is located in the precentral gyrus of the cerebral cortex. This is considered to be an Upper motor neuron. These fibers travel through the brain via the internal capsule until they get to the lower portion of the medulla oblongata. At this point, the neuron crosses over to the other side of the body. This means that if a neuron is to inervate the right bicep muscle, it originated in the left side of the precentral gyrus and crossed over to the right side at the medulla oblongata. This neuron is traveling down the corticospinal tract in the spinal cord. The neuron travels down ...view middle of the document...

This is called the resting potential. The sodium has the "potential" to rush in if it is given the opportunity, but it can't right now so its considered "resting". Basically another way to say it is that there is just a lot more positive charge outside than inside.
When a neurotransmitter activates a neuron, it opens up special channels that only allow sodium in. This is like opening a flood gate. Some of the sodium that is outside the cell rushes in. Think of it like people in a crowded theater and no one outside. Once the door opens, people want to go from the high concentration to the area of low concentration. Because the sodium is a positive charge each time a sodium rushes it, it makes the outside of the membrane at that particular point slightly less positive and the inside of the membrane at this spot slightly more positive. The more sodium that rushes in the more the inside of the membrane becomes positive, The charge inside compared to outside become closer to the same, its not actually 0 because remember there is still potassium inside, but if you think about it like this it makes it a little easier. (remember it was positive before when all the sodium were outside trying to get in, now that everyone is going inside, the sodium doesnt' care as much about getting in so its charge is "lower". This is called "depolarization" and is the main force behind the action potential. It is called depolarization because before the neuron was considered polarized (sodium on the outside, potassium on the inside) after the channel is opened and sodium rushed in, it is no longer polorized things have more or less balanced out and so it is depolarized. Now there is a rush of positive ions (sodium) inside the cell, which causes this little area on the inside of the neuron to be positively charged.
(keep in mind that this is happening at one small spot in the membrane and that neurons are big compared to the little tiny sodium ions and their channels) When the membrane charge crosses a threshold this activates the sodium channel right next to it, causing this area of the neuron to have a rush of sodium into it. when this part gets approaches positive charge, it activates the next channel...etc etc until this "signal" travels down the whole neuron. But again, dont' think of it like a signal, think of it like a domino, once one channel "knocks open" the next channel, its charge crosses a threshold and knocks open the one next to it. eventually this signal will make it to the end of the neuron where it will signal the release of a neurotransmitter which goes across the gap to the next neuron.

Now, lets go back to where we said the sodiums rushed in. after the sodium has rushed in, and the membrane charge on the inside is more positive, this is the start of the "refractory period". This means is that no mater how many times you open the sodium channels at this point, no more sodiums wants to rush in (both because the concentration gradient between the...

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