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NPB 101
Lecture 5
| Question | Answer |
|---|---|
| What is a spike? Where does it occur? What are the steps in a spike? | A spike is the action potential that occurs after voltage is generated from the ions passing through a cell membrane. It occurs in the axon of a neuron. The cell depolarizes, the Em reaches its peak, and the cell hyperpolarizes. |
| What happens in a spike when a cell depolarizes? | The Em raises from a lower number to a higher number, up to its peak |
| What happens in a spike when a cell hyperpolarizes? | The Em decreases from its peak back to to the resting Em, sometimes it goes to an even lower number than that |
| What happens in a spike when a cell repolarizes? | This is any return back to the resting Em. This most often occurs when the spike was hyperpolarized to far under the resting Em after the spike and the cell needs to return back to the resting Em |
| How does depolarization occur? | An influx of positive charged ions into the cell removes the polarization of the cell |
| How does hyperpolarization occur? | An efflux of the positive charged ions out of the cell will return the original charge back to the cell. |
| What ions flow in to the cell and what ions flow out of the cell? | Na and Ca in and K out |
| How do these cells suddenly flow into and out of a cell to generate voltage? How do these ion changes affect the cell? | There is a change in the membrane permeability for each of the ions. When Na flows in then an action potential (spike) is generated. After that the membrane is made more permeable to K so that the cell can re-polarize |
| What equation is used to calculate the amount of an ion in a cell that will then lead to an action potential? | Vm = RT/F ln (Pk[K]o +PNa [Na]o / Pk[K]I + PNa [Na]i) |
| What is the order of events to when a spike is being generated? | It first is triggered by a stimuli where it must surpass the threshold in order to spike. This spike is very brief. It then re-polarizes back down to below the threshold. |
| What does it mean that the depolarization happens regeneratively? How does it happen? | It means that one depolarization leads to another.This is a case of positive feedback. When spiking, Na voltage channels open and depolarize the cell, which then opens more Na channels. |
| What are the nine steps that occur during a spike? | 1: Resting Em 2: Stimuli (depol) 3: Threshold, Na chan open, K chan slowly open 4: Rapid Na entry 5: Peak, Na chan close, K open 6: K leaves cell 7: Below Em, more K leaves 8: K closes 9: Return to Em |
| In cells that use K and Na ions, what is the resting Em and why is it that number? | -70, its there because the K is at -90 and Na is at +60 so it balances between the two |
| What are the 5 ion channels in a cell with K and Na ions and what are they open? | Leak K: Always, Leak Na: Always, Stimulus gated: when a stimulus triggers a spike, Voltage-gated Na: During spike, Voltage-gated K: During spike |
| What does it mean when spikes are "sustained"? | They occur repeatedly, like a heart beat |
| What happens to a spike if the stimuli intensity increases? | The number of spikes that occur in a certain time period increases |
| What determines that maximum spike frequency? | Absolute refractory period |
| If the stimulus in a cell is AT the threshold, how long does it take for the cell to spike again? | The absolute refractory period + the relative refractory period |
| If the stimulus is ABOVE the threshold, how long does it take for the cell to spike again? How does this occur? | The cell can now ignore the relative refractory period and spike right after the absolute refractory period. Stronger stimuli depolarize the cell to threshold faster if they are stronger. |
| In what way does the spike move around the neuron? | Down the axon from the axon hillock towards the axon terminals |
| How do spikes travel down the axon? | The first segment spikes and at the peak of the spike, the ions spread to the next section of the axon. These ions depolarize the cells in this section and cause them to spike. |
| Why doesn't the spike move backwards on the axon? | The previous segment is in the absolute refractory period so it is incapable of spiking |
| Why are axons with smaller diameters worse at sending a signal than axons with larger diameters? | The axons with smaller diameters can't send as many ions down the core of the axon at once, increasing the resistance, decreasing the conduction |
| What is myelin made of and where is it located? | It is flattened glial cells that are wrapped around the axons in different segments |
| How does the myelin help the axons deliver a charge? | By wrapping around them, axons can no longer leak. It also separates ion containing solutions so the charge can quickly move down the axon core. |
| What is capacitance? Where do we see this in the nervous system? | It is the ability to hold a charge. The axons that are wrapped in myelin have a low capacitance. |
| Why do axons that have a lower capacitance spread the spikes faster? | If an axon segment has a lower capacitance than it requires less charge in to reach threshold and eventually spike in the cells farther down the axon. |
| What is the formula to calculate voltage and capacitance? | Q=CV, where Q is the number of charge going through the membrane |
| How would longer segments of axon affect the speed of spikes? | It would decrease it. A longer segment with a low capacitance could send a signal in half the time of shorter segments and use less charge to produce the same voltage. |
| How does myelin prevent leaks in the axon? | There are no Na channels and some K channels are missing |
| How fast can spikes travel? | 6-100 mph |
| What is a sub-threshold? | When the stimulus opens the ion channels, but not enough to reach threshold and generate a spike |
| Do spikes move electrostatically or diffuse down the axon? | Electrostaticaly, they do not diffuse |
Created by:
24rory
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