How does an action potential travel down a neuron?… by Diana
What is a resting state? What kinds of changes occur when the impulse begins? Please help me. It's really hard to understand…
Best Answer:
Resting state is when the neuron is not activated nor excited. Just think of it sitting there doing nothing. =p
All cells have a membrane potential, which is the voltage across it's membranes. This is due to the difference in concentrations of ions inside and outside of the cells. (The main ions are Na+ which is mostly outside of the cells and K+ which is mainly in the cells) During the resting state, most cells have a membrane potential of roughly -70mV. This means that the inside of the cell is about 70mV more negative than the outside of the cell. Remember though that although this resting potential is stable in most cells (means that it doesn't fluctuate or change until something triggers the cell out of its resting state) the ions are constantly moving across the cell membrane. Only the NET movement of the ions is zero.
Anyway, when the neuron is excited, Na+ channels are triggered to open. These are fast channels, which means that they open and close quickly. This allows Na+ to start to leak into the neuron. The Na+ leaking into the cell (called the Na+ influx) causes the membrane potential to increase (become less negative). When the membrane potential crosses a threshold value (usually about -65mV), suddenly a whole lot more of Na+ channels are opened. There is a sudden rush of Na+ into the cell causing a rapid spike in the membrane potential. This can go up to 20mV. This influx of Na+ is the start of the action potential/impulse.
When the voltage reaches the peak, Na+ channels are closed off due to the voltage. At the same time, these closed Na+ channels are inactivated temporarily and cannot be stimulated to open again. This is known as the absolute refractory period. The same voltage which inactivates the Na+ channels also triggers the opening of slow K+ channels. These channels allow K+ to leak out of the cell (efflux) but its gates open and close slower compared to the Na+ fast channels. Without Na+ influx, the K+ efflux causes the cell membrane potential to drop as there is a net loss of positive ions out of the cell. Since the K+ channels are “slow” they tend to close too late, causing what is known as “undershoot” or “hyperpolarization”. This means that the membrane potential is lower than the initial resting potential. This causes what is known as relative refractory period, which means that the neuron is harder to stimulate than normal. After a while the voltage sensitive K+ channels finally close and the neuron readjusts its membrane potential to rest at -70mV, ready for another stimulation.
Once the impulse starts, everything is easier to understand. Think of the neuron as a continuous wire. If the action potential starts at the beginning of the wire (let's say the leftmost part) then the part of the neuron/wire that is slightly to the right of the impulse is also affected by the change in membrane potential due to the Na+ influx. Once this new part's membrane potential crosses the threshold value another impulse is generated at this new point. Thus, impulse generation is repeated an infinite number of times along the neuron on the side that is just adjacent to the initial impulse. So, if you look at the whole neuron/wire you would see that the impulse will move/propagate away from the direction of the source of stimulation.
You might wonder how come the impulse doesn't backtrack towards the source of stimulation as well. This is due to the refractory period I mentioned. Once the part of the neuron is stimulated to produce an impulse, it goes into the refractory period and is unable to be stimulated to produce another impulse (absolute refractory period makes it totally impossible to stimulate, while it is possible but much harder to stimulate the neuron during the relative refractory period). However, on the other side of the impulse, the neuron is still in its resting state and is ready to be stimulated. Thus, the impulse only travels in one direction instead of both ways.
In certain neurons in the human body, they are covered in myelin sheaths. Basically just think of these neurons as a wire that has been rolled up in a carpet made of fat. These are called myelinated neurons. They are special since they contain nodes of Ranvier, which are places along the neurons where the myelin sheath is missing. These allow what is known as staltatory conduction, which increases the speed of impulse propagation many fold. Basically what happens is exactly the same as before (in the non-myelinated neurons) but instead of the adjacent area of the neuron being activated to produce impulses, far away areas of the neurons are being activated instead. This is due to the myelin acting as insulation that prevents the ions from flowing across the cell membrane. In a way, the impulse looks as if it “jumps” across the length of the neuron, thus allowing the impulse to propagate much faster and cover greater distances compared to the action potential of non-myelinated neurons.
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