Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels ion channels. The negatively charged protein molecules A - inside the neuron cannot cross the membrane.
In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential.
At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron. The resting potential tells about what happens when a neuron is at rest. An action potential occurs when a neuron sends information down an axon, away from the cell body. Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential.
The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event a stimulus causes the resting potential to move toward 0 mV. When the depolarization reaches about mV a neuron will fire an action potential. This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire. Also, when the threshold level is reached, an action potential of a fixed sized will always fire There are no big or small action potentials in one nerve cell - all action potentials are the same size.
Action potentials are caused when different ions cross the neuron membrane. This electrode is connected to a battery and a device that can monitor the amount of current I that flows through the electrode. Changes in membrane potential are produced by closing the switch and by systematically changing both the size and polarity of the battery.
If the negative pole of the battery is connected to the inside of the cell as in Figure 1. This result should not be surprising.
The negative pole of the battery makes the inside of the cell more negative than it was before. A change in potential that increases the polarized state of a membrane is called a hyperpolarization. The cell is more polarized than it was normally. Use yet a larger battery and the potential becomes even larger. The resultant hyperpolarizations are graded functions of the magnitude of the stimuli used to produce them.
Now consider the case in which the positive pole of the battery is connected to the electrode Figure 1. When the positive pole of the battery is connected to the electrode, the potential of the cell becomes more positive when the switch is closed Figure 1.
Such potentials are called depolarizations. The polarized state of the membrane is decreased. Larger batteries produce even larger depolarizations. Again, the magnitude of the responses are proportional to the magnitude of the stimuli. However, an unusual event occurs when the magnitude of the depolarization reaches a level of membrane potential called the threshold. A totally new type of signal is initiated; the action potential. Note that if the size of the battery is increased even more, the amplitude of the action potential is the same as the previous one Figure 1.
The process of eliciting an action potential in a nerve cell is analogous to igniting a fuse with a heat source. A certain minimum temperature threshold is necessary. Temperatures less than the threshold fail to ignite the fuse.
Temperatures greater than the threshold ignite the fuse just as well as the threshold temperature and the fuse does not burn any brighter or hotter. If the suprathreshold current stimulus is long enough, however, a train of action potentials will be elicited.
In general, the action potentials will continue to fire as long as the stimulus continues, with the frequency of firing being proportional to the magnitude of the stimulus Figure 1. Action potentials are not only initiated in an all-or-nothing fashion, but they are also propagated in an all-or-nothing fashion. An action potential initiated in the cell body of a motor neuron in the spinal cord will propagate in an undecremented fashion all the way to the synaptic terminals of that motor neuron.
Again, the situation is analogous to a burning fuse. Once the fuse is ignited, the flame will spread to its end. The action potential consists of several components Figure 1. The threshold is the value of the membrane potential which, if reached, leads to the all-or-nothing initiation of an action potential.
The initial or rising phase of the action potential is called the depolarizing phase or the upstroke. The region of the action potential between the 0 mV level and the peak amplitude is the overshoot. The return of the membrane potential to the resting potential is called the repolarization phase. There is also a phase of the action potential during which time the membrane potential can be more negative than the resting potential. This phase of the action potential is called the undershoot or the hyperpolarizing afterpotential.
In Figure 1. Before examining the ionic mechanisms of action potentials, it is first necessary to understand the ionic mechanisms of the resting potential. The two phenomena are intimately related. The story of the resting potential goes back to the early 's when Julius Bernstein suggested that the resting potential V m was equal to the potassium equilibrium potential E K.
The key to understanding the resting potential is the fact that ions are distributed unequally on the inside and outside of cells, and that cell membranes are selectively permeable to different ions. Thus, there will be an electrical force directed inward that will tend to counterbalance the diffusional force directed outward.
Figure 1. Voltage-gated ion channels open in response to changes in membrane voltage. After activation, they become inactivated for a brief period and will no longer open in response to a signal. This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium.
Because ions cannot simply cross the membrane at will, there are different concentrations of several ions inside and outside the cell, as shown in Table 1. The resting membrane potential is a result of different concentrations inside and outside the cell.
Figure 2. The negative resting membrane potential is created and maintained by increasing the concentration of cations outside the cell in the extracellular fluid relative to inside the cell in the cytoplasm.
The negative charge within the cell is created by the cell membrane being more permeable to potassium ion movement than sodium ion movement. In neurons, potassium ions are maintained at high concentrations within the cell while sodium ions are maintained at high concentrations outside of the cell.
The cell possesses potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient.
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