How do neurons work?
How do neurons conduct electrical impulses?Neurons conduct electrical impulses by using the Action Potential. This phenomenon is generated through the flow of positively charged ions across the neuronal membrane. I'll explain.......
Neurons, like all cells, maintain different concentrations of certain ions (charged atoms) across their cell membranes. Imagine the case of a boat with a small leak below the water line. In order to keep the boat afloat, the small amount of water entering through the leak has to be pumped out, which maintains a lower water level relative to the open sea. Neurons do the same thing, but they pump out positively charged sodium ions. In addition, they pump in positively charged potassium ions (potash to the gardeners out there!!) Thus there is a high concentration of sodium ions present outside the neuron, and a high concentration of potassium ions inside. The neuronal membrane also contains specialised proteins called channels, which form pores in the membrane that are selectively permeable to particular ions. Thus sodium channels allow sodium ions through the membrane while potassium channels allow potassium ions through.
OK, so far so good. Now, under resting conditions, the potassium channel is more permeable to potassium ions than the sodium channel is to sodium ions. So there is a slow outward leak of potassium ions that is larger than the inward leak of sodium ions. This means that the membrane has a charge on the inside face that is negative relative to the outside, as more positively charged ions flow out of the neuron than flow in. This difference in the concentrations of ions on either side of the membrane gives rise to the membrane potential and the membrane is said to be polarised.
The Action PotentialLet's go back to the boat. Now, in the boat, there is a pressure for water to enter and if a big hole is punched in the side, the rate at which water flows into the boat in massively increased. Similarly, there is a pressure for the sodium ions to enter the neuron, but they are prevented from doing so by the membrane and the pumping mechanisms that remove any ions that manage to get in. However, if the sodium channels are opened, positively charged sodium ions flood into the neuron, and making the inside of the cell momentarily positively charged - the cell is said to be depolarized. This has the effect of opening the potassium channels, allowing potassium ions to leave the cell. Thus, there is first an influx of sodium ions (leading to massive depolarization) followed by a rapid efflux of potassium ions from the neuron (leading to repolarisation). Excess ions are subsequently pumped in/out of the neuron.
This transient switch in membrane potential is the action potential. The cycle of depolarization and repolarization is extremely rapid, taking only about 2 milliseconds (0.002 seconds) and thus allows neurons to fire action potentials in rapid bursts, a common feature in neuronal communication.
How does the action potential propagate along the axon?
If this were all there was to it, then the action potential would propagate in all directions along an axon. But action potentials move in one direction. This is achieved because the sodium channels have a refractory period following activation, during which they cannot open again. This ensures that the action potential is propagated in a specific direction along the axon.
The speed of propagation is related to the size of the axon.
The speed of action potential propagation is usually directly related to the size of the axon. Big axons result in fast transmission rates. For example, the squid has an axon nearly 1 mm in diameter that initiates a rapid escape reflex. Increasing the size of the axon retains more of the sodium ions that form the internal depolarisation wave inside the axon.
However, if we had to have axons the size of the squid giant axon in our brains, doorways would have to be substantially widened to accommodate our heads!!! We could only have a few muscles located at any great distance from our brains - so we'd all be extremely short with very large heads....not really feasible, is it? The answer is to insulate the axonal membrane to prevent the dissipation of the internal depolarisation in small axons - myelin.
So what does Myelin do?Myelin is the fatty membranes of cells called Oligodendroglia (in the CNS) and Schwann Cells (in the PNS) that wraps around the axon and acts as an insulator, preventing the dissipation of the depolarisation wave. The sodium and potassium ion channels, pumps and all the other paraphernalia associated with action potential propagation are concentrated at sites between blocks of myelin called the Nodes of Ranvier. This myelin sheath allows the action potential to jump from one node to another, greatly increasing the rate of transmission.
Without the myelin sheath, we cannot function. This is demonstrated by the devastating effects of Multiple Sclerosis, a demyelinating disease that affects bundles of axons in the brain, spinal cord and optic nerve, leading to lack of co-ordination and muscle control as well as difficulties with speech and vision. For further information on this disease, visit the MS Society's web site.