Action potentials can jump down axons

In vertebrate nervous systems, increasing the speed of action potentials by increasing the diameter of axons is not feasible because of the huge number of axons in these organisms. Each of our eyes, for example, has about a million axons extending from it. Evolution has increased action potential velocity in vertebrate axons in a way that does not require large size.

When we described glial cells earlier in the chapter, we saw that certain glial cells wrap themselves around axons, covering them with concentric layers of myelin (see Figure 44.3). These myelin wrappings are not continuous along the length of the axon, but have regularly spaced gaps, called nodes of Ranvier, where the axon is not covered (Figure 44.12).

Myelin electrically insulates the axon; that is, charged ions cannot cross the regions of the plasma membrane that are

Patch Clamping Technique
44.11 Patch Clamping The patch clamping technique can record the opening and closing of a single ion channel.

wrapped in myelin. Additionally, ion channels are clustered at the nodes of Ranvier. Thus an axon can fire an action potential only at a node, and that action potential cannot be propagated through the adjacent patch of membrane covered with myelin. The positive charges that flow into the axon at the node, however, spread down the inside of the axon. When the spread of current causes the plasma membrane at the next node to depolarize to threshold, an action potential is fired at that node. Action potentials therefore appear to jump from node to node down the axon.

The speed of conduction is increased in these myelin-wrapped axons because electric current flows very fast through the cytoplasm in comparison to the time required for channels to open and close. This form of impulse propagation is called saltatory ( jumping) conduction and is much quicker than continuous propagation of action potentials down an unmyelinated axon.

Saltatory Jumping Conduction

Point C

| Upstream Na+ channels inactivate, making the membrane refractory. K+ channels open, repolarizing the axon.

Point C

| Upstream Na+ channels inactivate, making the membrane refractory. K+ channels open, repolarizing the axon.

| The action potential jumps quickly to the new node and...

| The action potential jumps quickly to the new node and...

Action Potential Refractory
44.12 Saltatory Action Potentials Action potentials appear to jump from node to node in myelinated axons.

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Responses

  • marcus
    Why action potentials jump down axons?
    3 years ago
  • Ryan
    Does an myelinated axon skip down the length of the axon, node to node?
    2 years ago
  • kibra luwam
    Why do action potential jump down an action?
    2 years ago

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