Animation 3.5: Conduction along Unmyelinated vs. Myelinated Axons - Script

INTRODUCTION

Getting critical information about the environment to the central nervous system quickly is essential for responding to threats and for preventing serious injury. The action potential is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell’s axonal membranes to specific ions. The action potential strongly depolarizes the adjacent patch of axonal membrane, causing it to generate its own action potential. In this regenerative manner, the action potential spreads down the axon.

In this animation we will review the mechanism of axon potential propagation, and examine the mechanisms that have evolved to increase the speed of action potential conduction along the length of an axon.

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  1. Action potential conduction requires both active and passive current flow. The passive electrical properties of a nerve cell axon can be determined by measuring the voltage change resulting from a current pulse passed across the axonal membrane. First, we can use a current-passing electrode to produce a subthreshold change in membrane potential, which spreads passively along the axon.
  2. If we then record potential responses at various distances from the site of current injection, we see that with increasing distance, the amplitude of the potential change is attenuated.
  3. If this current pulse is not large enough to generate an action potential, the magnitude of the resulting potential change decays exponentially with increasing distance from the site of current injection. Typically, the potential falls to a small fraction of its initial value at a distance of no more than a couple of millimeters away from the site of injection.
  4. The progressive decrease in the amplitude of the induced potential change occurs because the injected current leaks out across the axonal membrane; accordingly, less current is available to change the membrane potential farther along the axon.
  5. If the experiment is repeated with a depolarizing current pulse large enough to produce an action potential, the result is dramatically different. In this case, an action potential occurs without decrement along the entire length of the axon. Thus, action potentials somehow circumvent the inherent leakiness of neurons.
  6. Note that the amplitude of the action potential is constant along the length of the axon. This all-or-none behavior indicates that more than simple passive flow of current must be involved in action potential propagation.
  7. The mechanism of action potential propagation is easy to grasp once one understands how action potentials are generated and how current passively flows along an axon.
  8. A depolarizing stimulus—a synaptic potential or a receptor potential in an intact neuron, or an injected current pulse in an experiment—locally depolarizes the axon, opening the voltage-gated Na+ channels in that region. The opening of Na+ channels causes inward movement of Na+, and the resultant depolarization of the membrane potential generates an action potential at that site.
  9. Some of the local current generated by the action potential then flows passively down the axon. This passive current flow depolarizes the membrane potential in the adjacent region of the axon, thus opening the Na+ channels in the neighboring membrane. The local depolarization triggers an action potential in this region, which then spreads again in a continuing cycle until the end of the axon is reached.
  10. The regenerative properties of Na+ channel opening allow action potentials to propagate in an all-or-none fashion by acting as a booster at each point along the axon. Note that as the action potential spreads, the membrane potential repolarizes due to K+ channel opening and Na+ channel inactivation, leaving a “wake” of refractoriness behind the action potential that prevents its backward propagation.
  11. The rate of action potential conduction limits the flow of information within the nervous system. It is not surprising, then, that various mechanisms have evolved that optimize the propagation of action potentials. One way of improving passive current flow is to increase the diameter of an axon, which effectively decreases the internal resistance to passive current flow.
  12. Another strategy to improve the passive flow of current is to insulate the axonal membrane, reducing the ability of current to leak out of the axon and thus increasing the distance along the axon that a given local current can flow passively. Among vertebrates, this strategy is evident in the myelination of axons, a process by which glial cells wrap the axon in myelin, which consists of multiple layers of closely opposed membranes.
  13. By acting as an electrical insulator, myelin greatly speeds up action potential conduction. The major reason underlying this marked increase in speed is that the time-consuming process of action potential generation occurs only at specific points along the axon, called nodes of Ranvier, where there is a gap in the myelin wrapping.
  14. An action potential generated at one node of Ranvier induces current that flows passively within the myelinated segment until the next node is reached. This local current flow then generates an action potential in the neighboring segment, and the cycle is repeated along the length of the axon.
  15. Because current flows across the neuronal membrane only at the nodes, this type of propagation is called saltatory, meaning that the action potential appears to jump from node to node.
  16. The result is a greatly enhanced velocity of action potential conduction. For example, whereas unmyelinated axon conduction velocities range from about 0.5 to 10 m/s, myelinated axons can conduct at velocities of up to 150 m/s.

CONCLUSION

Given the importance of the action potential in transmitting signals between nerve cells, it is not surprising that various strategies have evolved to increase the efficiency and speed of transmission. These include increasing the diameter of axons as well as insulating axonal membranes to reduce current leakage across the membrane.

The evolution of rapid saltatory conduction in vertebrates has given them a major behavioral advantage over invertebrates, in which axons are unmyelinated and mostly small in diameter, and thus slower in conduction. To conduct action potentials as swiftly as a myelinated vertebrate axon does, an unmyelinated axon would have to be 100 times larger in volume. It has been estimated that at least 10% of the volume of the human brain is occupied by myelinated axons. To maintain the conduction velocity of our cerebral neurons without the help of myelin, our brains would have to be 10 times as large as they are. This fact helps explain why myelination is an important index of maturation of the developing nervous system.

Textbook Reference: Electrical Signals Are the Vocabulary of the Nervous System, pp. 57–59