In the last post Neurons: Introduction, I described the structure and type of the neurons. We saw that the structure of the neuron is very uniquely divided into three distinct parts: dendrites, cell body and axon. This design is specifically for the conduction of electric impulse called an action potential, which gives rise to the impulse, explained in this post.
The impulse takes place in form of a repeated change in the charge (and subsequently the potential) inside the neuron, as mentioned below:
1 Resting membrane potential: The presence of large anions like nucleic acids and proteins makes the interior of the neuron cell more negative. Also, the sodium potassium pump propels in two K+ ions for every three Na+ ions pumped out, creating an imbalance in the charge across the plasma membrane. Hence there exists a potential difference (voltage) across the plasma membrane, with the cytoplasmic side being more negative (negative pole) than extracellular side (positive pole). In most of the cells the net negativity of the cell remains same, however the net charge of the neuron (and muscle cell) changes in response to various stimuli. In the absence of any external stimuli, the negativity inside the cell with respect to that outside, that is the resting membrane potential is around -70mV. At this potential K+ freely moves in and out of the cell and maintain equilibrium.
(Just for information: Read about Sodium-Potassium pump)
2. Depolarization: external stimulus, causes Na+ ions to enter into the cell and the cell depolarizes. When action potential threshold (-55mV) is reached all the voltage gated Na+ and the K+ channels open and inside becomes more positive (+30mV).
3. Repolarisation: K+ ions flows out of the neuron and Cl- ions into the neuron and the neuronhyperpolarizes (̴ -93 mV).
4. Resume resting potential: Sodium-potassium pumpchanges the concentration of Na+ and K+ ions to achieve resting membrane potential
In the fig,
- Resting membrane potential: around -70mV due to large anions, Na-K pump.
- Depolarization: external stimulus, causes Na+ ions to enter into the cell and the cell depolarizes. When action potential threshold (-55mV) is reached all the voltage gated Na+ and the K+ channels open and inside becomes more positive (+30mV).
- Repolarisation: K+ ions flows out of the neuron and Cl- ions into the neuron and the neuronhyperpolarizes ~(-93 mV).
- Resume resting potential: Sodium-potassium pumpchanges the concentration of Na+ and K+ ions to achieve resting membrane potential.
The entire sequence of events (steps 1 to 4) in an action potential is over in a few milliseconds. The production of an action potential results entirely from the passive diffusion of ions. After hyperpolarization, Sodium-potassium pumps bring back the resting membrane potential, hence active transport isrequired only to maintain the ion gradients after action potential.
- Action Potentials proceeds uni-directionally
Action potential begins with the entry of positive Na+ ions, leading to depolarization (decrease in difference of charges across the plasma membrane). Also, the depolarization leads to more Na channels to open up (in the nearby region) and allow more Na ions inside leading to action potential in the next region. Hence action potential leads to depolarization and depolarization leads to action potential in the next successive region. However, the in the previous region, that is the region where action potential has already taken place, voltage-gated Na+ channels become inactive for a brief moment. Hence the action potential cannot continue on the previous region and only the next region, making the propagation uni-directional.
- Saltatory Conduction: (saltare (latin) – to jump/ hop).
Saltatory conduction is the impulse in the myelinated axon (read the previous post)…. These myelinated axons conduct impulses more rapidly than the non-myelinated ones. In these axons, the action potentials happen only at the nodes of Ranvier, where the axons are not insulated. Hence the action potential (entering of the Na+ ions) happen at nodes to nodes. Hence the impulses appear to jump or hop from node to node. This process is called saltatory conduction. As the entire length of the axon doesn’t have to get depolarised but only at nodes, the conduction of the impulse is more rapid.
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Ref:
Raven, Peter H. & Johnson, George B. (2002). Biology (6th ed). McGraw-Hill, Boston
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