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wrote...
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9 years ago
The purpose is for depolarization. Sodium, the positive ion, is on the outside of the axon membrane. When the sodium ion channels open, positively charged sodium ions flood the axon's insides and switches the polarity, this then causes an action potential.
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wrote...
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9 years ago
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Sorry, I should've made the question clearer. But what is the purpose for depolarisation and action potential? Does the changing of polarity allow for energy to pass through? Or does the passing energy change polarity? And for what purpose?
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wrote...
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9 years ago
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The movement of ions causes the propagation of an action potential. Action potentials are initiated at the axon hillock (where sodium enters the neuron). It is the initial segment of the neuron. The transmembrane potential can rise and fall due to excitatory and inhibiting inputs, respectively. When the potential becomes more positive a depolarization has occurred, and when it becomes more negative a hyperpolarization has occurred. There is a special transmembrane potential known as threshold, at about -60mV to -55mV for a human axon. If the membrane is depolarized to threshold, then a sudden and drastic change occurs: many voltage-gated sodium channels in the membrane open up and a vast number of sodium ions rush into the cell. This inrush of positive charge raises the transmembrane potential greatly, making it actually become positive (about +30mV to +40mV). This initiates an action potential.
An inrush of sodium ions (Na+) spreads in both directions under the plasma membrane, increasing the positive charge in the segments adjacent to the one whose voltage-gated sodium channels just opened. There is enough depolarization on both adjacent segments to raise the membranes to threshold. However, action potentials are propagated in only one direction – from axon hillock towards synaptic terminals - because a segment that has already had an action potential needs time to recover, which is called the refractory period. So only the adjacent downstream segment's voltage-gated ion channels open, causing a huge inrush of sodium ions there. A chain reaction of one segment depolarizing the next downstream segment to threshold occurs, and the action potential is propagated along the entire length of the axon.
When the action potential reaches the synaptic knobs at the synaptic terminals, it causes calcium ion channels to open, allowing calcium ions (Ca2+) to flow into the cell. The calcium ions cause synaptic vesicles, each containing a large number of neurotransmitter molecules, to fuse with the plasma membrane, dumping their contents. The neurotransmitters diffuse across the gap, or synaptic cleft, separating the presynaptic membrane from the postsynaptic membrane. On the other side of the cleft, the neurotransmitters bind with receptors, thereby influencing the postsynaptic cell.
NOTE: Soon after the sodium channels open, they close, preventing more Na+ ions from flowing in. In addition, the opening of voltage-gated potassium channels in the membrane begins allowing K+ ions to flow out of the cell. With no more positively charged ions flowing in, but with positively charged ions flowing out, the transmembrane potential falls back down to about (actually, just below) the resting membrane potential. The sodium-potassium pump then works to reestablish the appropriate resting-state concentrations of sodium and potassium.
Does that make sense?
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wrote...
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9 years ago
What Is the Purpose of an Action Potential The brain is made up of two types of cells. Glial cells act as support cells for neurons, the cells responsible for sending signals in the brain. Action potentials — electrical signals that travel across neurons — are the means for receiving, analyzing, and conveying information in the brain. They have an amplitude of about 100 millivolt (mV) and last for around 1 millisecond (ms).
A neuron is made up of four distinct parts. The cell body contains the nucleus and other cell structures. Dendrites branch out from the cell body like the branches on a tree, and receive information from other neurons. The axon is a long extension on one side of the cell body, similar to the trunk of the tree, and it ends in the presynaptic terminals.
This type of cell is polarized, meaning the electrical charge inside the neuron is different from the charge outside the cell. Dendrites receive signals from other neurons that can change the charge inside the cell. A neuron at rest is more negatively charged than the surrounding area. Excitatory postsynaptic potentials bring the charge closer to zero, and inhibitory postsynaptic potentials make the charge even more negative.
At the axon hillock, all of these potentials are averaged in one of two ways: across time or across space. The further away from the axon hillock a potential is, the less effect it has. The longer a potential lasts, the more effect it has on the axon hillock.
If the averaged charge from the postsynaptic potentials reaches a certain threshold, an action potential is generated. Postsynaptic potentials can be different sizes depending on the signals received by the dendrites, but the action potential operates on an all-or-none principle, meaning there is no gradient — either there is one or there is not.
The action potential is an electrical signal that travels down the neuron’s axon. The axon is coated in a myelin sheath, which, similar to the insulation on an electrical wire, allows the signal to travel faster. It carries the electrical signal to the presynaptic terminals, which then communicate to another neuron.
Between two neurons there is a gap called a synapse. When the presynaptic terminal on a neuron receives a signal from an action potential, it sends chemicals called neurotransmitters into the synapse. These chemicals are absorbed by another neuron. Neurotransmitters are the mechanism for sending signals between neurons. |
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