Anatomy of a Neuron and Saltatory Conduction

Neurons have 3 principal regionals: a nucleated cell body, dendrites, and an axon.

The cell body is the “nutritional center” of the neuron and contains densely staining areas of rouge endoplasmic reticulum.

Dendrites are thin, branched processes that extend from the cytoplasm of the cell body, providing a receptive area that collects and transmits electrical impulses to the cell body.

The axon is usually a longer process that conducts impulses away from the cell body.

Nerve impulses originate in the axon hillock, an expanded regions where the cell body and axon meet.

Schwann cells (specialized glial cells) form the neurons’ myelin sheaths outside the brain and spinal cord. In the CNS (brain and spinal cord), they are called oligodendrocytes). The glial cells wrap the axon in layers or sheaths of tightly compacted plasma membrane, which form the myelin. Each new layer of Schwann cells overlaps the previous ones.

Gaps of axon are exposed b/w adjacent myelinated segments called nodes of Ranvier. They are important to the speed at which a neuron transmits messages. At the nodes, there are Na+ and K+ gated channels that can open to allow Na+ to come in. Once its in, it can jump from one node to the next rapidly. So, it doesn’t have to wait for voltage gated Na+ channels to open along the axon.

The myelin sheath is an insulating outer layer on the axon. It acts as an electrical insulator, allowing the nerve impulse to travel quickly and passively b/w the nodes of Ranvier. Myelin sheath also prevents electrical charages to leak through the membrane.  But the myelin sheath is bare of voltage-gated channels. At the nodes, impulses are actively regenerated. This is b/c the myelin sheath prevents action potential from continuing so a local electrical circuit is generated. The circuit stimilates action potential at the next node, which is why there is a jump.

So, the nerve impulse jumps successively from one node to the next in a type of transmission called saltatory conduction (saltare = to jump). It is around 100 times faster than signal or action potential conduction in unmyelinated axon of the same diameter.

Action potential conduction (unmyelinated sheath): Basically, as long as the action potential occurs somewhere on the axon, it will run all the way down the axon.  Here’s a figure that shows this:

apconduct.jpg (54553 bytes)

Saltatory Conduction: This manner of conduction is a lot faster because only the nodes of Ranvier are involved in action potential conduction.

In order for an action potential to occur, you saw that sodium and potassium ions have to move across the axonal membrane.  Remember?   Well, wherever the Schwann cells (in yellow in the animation) wrap around the axon, the sodium and potassium ions cannot cross the membrane; the Schwann cells wrap too tightly around the axonal membrane for there to be any extracellular space underneath them.  Therefore, the only place that an action potential can occur is at the node of Ranvier– the space between the Schwann cells.  Because of this, the action potential seems to jump from node to node along the axon.  “Jumping” is what the word “saltatory” means.

    So, saltatory conduction is when the action potential jumps down the axon from node to node.

Pearson Animation: http://wps.aw.com/bc_goodenough_boh_3/104/26721/6840613.cw/content/index.html

Interactive Biology: http://www.youtube.com/watch?v=M3u7v0RXkok

Saltatory conduction (myelinated sheath) vs. Action potential conduction (unmyelinated sheath): http://www.brainviews.com/abFiles/AniSalt.htm

Wisegeek: http://www.wisegeek.org/what-is-the-saltatory-conduction.htm

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