Neuroscience/Cellular Neurobiology/Action Potentials

Characteristics of Action Potentials

There are a few components to the action potential. On a graph, plotting membrane potential vs. time, the slope become steep and the potential goes over 0 mV in the overshoot phase. After it comes back down, the potential undershoots, and the resting potential returns.

Experimental Generation

Action potentials are generated at the axon hillock. Their frequency can vary but usually not the amplitude. If a recording electrode is placed close to the cell, it allows recording the action potentials across the membrane. Charge may be injected via the electrode, which causes a depolarization that, when the membrane potential reaches the threshold, causes a spike. In regular firing neurons, the cell continues to generate action potentials as long as the membrane potential stays above threshold. Also, if the depolarization is increased to a more positive potential, these cells fire at a faster rate. Other neurons cease firing action potentials during prolonged depolarization, and are therefore termed "bursters". Their firing mode is a very simple form of adaptation.

Action Potential Current

A current clamp can be used to monitor current. The injection electrode will trigger an action potential by injecting a suprathreshold voltage, but the potential across the membrane is held steady. The negative deflection represents the influx of a positive charge and the positive deflection represents the efflux of positive charge. Huxley used TTX(terodotoxin) to block voltage gated Na channels. They found that that blocked the inward component of the action potential. Using TEA(tetraethylammonium, an antagonist for K channels) they suppressed the outward component of the action potential, and they could look at the current vs. time graph of an action potential and extrapolate the current v. time graphs for K and Na individually. When suprathreshold potential is applied, Na channels open and then close even though membrane is depolarized. K v-channels do not close as long as depolarization is sustained. Na v-channel current is a 'non-inactivating'/'trangent' current and the K v-channel current a 'sustained' and 'inactivating' current.

Positive Na+/Negative K+ Feedback

If both Na and K channels are open, there is flow in both directions but the permeability is different. During the depolarization, Na rapidly depolarizes the membrane and then stops. It would gradually leak back to equilibrium. K flow repolarizes the membrane but is slower to flow. When enough Na channels let in enough Na that the efflux of K can't balance the depolarization, the depolarization continues and action potential occurs. Subthreshold, K channels open to increase the K+ current. At threshold depolarization, the negative feedback cycle doesn't stop, but is simply overwhelmed. If there were no Na flooding in, K wouldn't move across the membrane very quickly.

Measurement Techniques

Current Clamp Technique

A capillary pipet can be fired on the end to melt the tip to a micrometer size opening to give a useful capillary and then filled with a salt solution. The pipet tip is put toward a membrane, and using negative pressure, it can pull in a area of the membrane. An impermeable seal is created, prevented ions from entering or leaving. Enables the measurement of the current of a single channel. Graphed, the current is a square shape. Positive or negative current indicates direct of current flow and can identify which ion is flowing. Depending on the type of channel, one can use voltage or neurotransmitter to control the opening and closing of a particular channel. Channels are either open or closed. The more the channel is open, the longer charge flows across the membrane.

Voltage Clamp Technique

The rapid, large change in potential is the result of the activities of ion channels. The purpose of the voltage clamp technique allows the recording of membrane current (as opposed to potential. Uses two electrodes: one to inject voltage and one to monitor membrane potential. Connecting these two electrodes is a error-correcting amplifier. This amplifier injects a set voltage into the cell and uses the monitor electrode to detect changes and compensate at the injection electrode. The purpose to electrically force the membrane to stay at one potential and then monitor the feedback to see what currents pass through the membrane.

Mechanisms of Action Potentials

Voltage-Gated Channels

They have a voltage sensor in their protein structure. They contain charged amino acids that react to a change in membrane voltage. When there is a more positive charge on the membrane, positively amino acids within the protein are repelled creating a force internal to the protein that will cause a change in conformation. When membrane is at -10mV, the channel is open 10-15% of the time. When at -10mV 80%, at 20mV 95%.

Whole-Cell Currents

A sum of all the single-channel currents. If recording a single Na channel, depolarization increases the occurrence of an open channel. Add two more channels to the sum, and the larger possible total current increases. As the number goes up, the curve represents the whole cell current curve of the Na portion of the total curve. Three graphs showing the current of K, the current of Na and the total current during one action potential; the graph lines up action potential curve with graph of current of Na and K channels independently so one can see which portion of the action potential is caused by a particular ion channel. After AP leak channels return resting potential. When the peak depolarization is reached, the Na channels have closed and the K channels have opened, quickly falling from the peak. Influx of Na causes rising part of action potential and the efflux of K brings the membrane potential back down

K and Na Channel Differences

Na Channel Has a quick onset and quickly turns off: inactivating channel. K is the opposite with slow onset and slow turn off: noninactivating channel. This difference is caused by differences in the two proteins. The Na channel has two gates: the activation gate and the inactivation gate. The activation gate is sensitive to the potential. When the activation gate is closed the inactivation gate is closed. When the inside of the cell is depolarized, the activation gate quickly opens. The inactivation gate, composed of negative proteins, slowly is repelled by the positive change in the intracellular environment, so it closes the channel by find its way into the pore of the channel.

K Channel. Has a single gate activated by the membrane potential, so it will stay open all the time the membrane has a certain potential. Na and K Conductances Na's conductance is characterized by a rapid onset and a rapid offset. K's conductance is characterized by a slow onset and a slow offset. Membrane potential line up temporally with a graph of conductance of K and Na on different lines. The upward rise of the AP is corresponds to the Na conductance change and the downward fall of the AP corresponds to the K conductance. K's slow offset creates an undershoot. This sets up a refractory period during which another AP cannot be generated because an action potential during the undershoot requires a much greater Na influx than during resting potential. Also, if the ball on the Na channel hasn't been released, the Na channel cannot conduct current creating a absolute refractory period during which no action potential can be generated, regardless of membrane potential. After the protein complex blocking the Na channel has been released is the relative refractory period where is more difficult.

Other types of Voltage-Gated Channels

Ca ion can keep the peak for a relatively long time before K brings the potential back. Ca can activate Ca-activated Na channels that help generate action potential waveforms.