lesson 2.3 workbook - tufts...

W o r k b o o kLesson 2.3 53

LESSON 2.3 WORKBOOKHow fast do our neurons signal?

What are glia cells, and what are some of their functions?____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

DEFINITIONS OF TERMS

Glial cell – several classes of non-neuronal cells of the

nervous system.

For a complete list of defined terms, see the Glossary.

Remember that winning goal you scored, that snowball you dodged or the cup of coffee you managed to catch before the cat knocked it all over your computer? Hundreds of times a day our quick reactions improve our performance or save us from disaster. Take a minute to think of something that happened to you this week. Often we react so quickly that we’ve reacted before we even know what has happened. How can your neurons signal so quickly? In this lesson we will find out, and to do so we need to learn about the other important type of cell in our nervous systems – the glial cell.

Glial Cells

There are actually far more glial cells (usually referred to as glia) than neurons in the CNS of vertebrates — between 10 to 50 times more in fact. Nerve cell bodies and axons are surrounded by them and be-cause of this they were named from the Greek word for glue. For a long time neuroscientists thought glial cells did behave like glue, and pretty much ignored them. Over the last few years though they have been found to be far more active than we thought, conducting their own signals and acting more as partners for neurons than the boring old structural cells we originally thought. Glia in fact have several vital roles in neuronal function:

• They provide firmness and structure to the brain. This isn’t trivial. Remember from the lesson on neural imaging that the brain has very low density. Glia beef up the density and make the neurons more resistant to trauma. That’s important because remember that if a brain neuron is damaged and dies it can’t be replaced.

• Two different types of glial cells act as insulation, which as we shall see, allows the ac-tion potential to travel faster – important if we want to move a signal quickly.

• When the brain is developing in the embryo, some glia act as guides so that the neural network forms its connections in the right place.

• Other glial cells help form an impermeable lining around the capillaries and venules of the brain that prevents toxic substances in the blood from entering the brain. This lining is called the blood-brain barrier.

http://sites.tufts.edu/greatdiseases/student/glossary/


LESSON READING How does myelination increase the conduc-tion velocity of the action potential?_________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Myelination increases the conduction speed of the action potential

In the last lesson we saw that if only one action potential occurred at the beginning of the axon, the depo-larizing current wouldn’t reach the axon terminal. This happens because as it travels down the axon some of the current leaks out of the axon across the membrane, and also because the materials in the axon (chiefly protein) offer resistance to the current. We also learned that some axons solve this problem by lin-ing up their voltage-gated Na+ channels along the axon membrane, so multiple action potentials can occur in rapid succession, ensuring that the signal is transmitted all the way down the axon.

This is not a great solution because the energy required to keep the Na+/K+ pump working to repolarize the axon membrane is huge. So axons have come up with another strategy, which is to have the action potential jump along the axon rather than progress down it (think of the action potential pogo-sticking down the axon rather than walking down). This how it works.

Remember that the problem with a single ac-tion potential was that the current would decay. To prevent that decay glial cells wrap around the axon like beads on a necklace covering the axon tightly except for the areas in between the beads called nodes of Ranvier which remain naked axon (Figure 17). Two things make this strategy work. First the glia make a substance called my-elin, which acts as an insulator. Now the parts of the axon that are wrapped around by the myelin are insulated and the depolarizing current can’t leak out. Second the sodium channels are con-centrated in the small areas of naked axon in between each myelin ‘bead’ so the action potential can hop down the axon like a pogo stick. Let’s have a

look in a bit more detail:

The glial cells wrap around the axon like paper wrap-ping around a pencil. The glial cell membrane attach-es so tightly to the axon, and to itself that there is no extracellular fluid in contact with the axon in that area (Figure 18). The only place where the axon comes into contact with extracellular fluid is at a node of Ran-vier, where the axon is naked. In the myelinated areas therefore, there can be no inward flow of Na+ into the axon because the myelin insulates the axon from the extracellular fluid.


Nodes of Ranvier – gaps between adjacent myelin

segments on an axon.

Demyelination – the loss of myelin insulating neurons.


Figure 17: Nodes of Ranvier. Myelin is formed from membranes of glial cells wrapping tightly around the axon, like beads on a necklace. Between the beads of myelin are spaces of naked axon, called the nodes of Ranvier.

Figure 18: Cross section of myelinat-ed axons. The glial cell membranes wrap so tightly around the axon that the cytoplasm is squeezed out of the glial cells.



LESSON READINGWhat are the advantages of myelination?__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Think about your big toe neuron. Imagine the axon starts under your armpit. How long will an action potential take to travel down to your big toe if is myelinated? ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

How then does the action potential travel along the area of an axon covered by a myelin sheath? The answer to this is by behaving like an electrical cable. Since the axon is covered in myelin, there is minimal leakage of depolarizing charge out of the axon so the depolarizing current is able to travel passively between the nodes of Ranvier. When the depolarizing current reaches the next node of Ranvier, it encounters both Na+ ions and Na+ channels, and so it can trigger another action po-tential at the node. The action potential gets retriggered, or repeated, at each node of Ranvier and the depolar-izing current moves passively along the myelinated por-tions of the axon to the next node. This type of conduc-tion, which appears to hop from node to node, is called saltatory conduction, from the Latin saltare, “to leap, to dance” (Figure 19).

Why is myelination an advantage for the axon?

We can immediately see two advantages of saltatory conduc-tion. The first is it saves energy. Sodium ions that enter axons during the action potential must eventually be removed. You’ll remember that the Na+ ions are removed by Na+/K+ pumps, which use significant amounts of energy. As we mentioned before, in axons that aren’t myelinated, these pumps must be located along the entire length of the axon, because Na+ ions can enter everywhere. However, in a myelinated axon, where Na+ ions can only enter at the nodes of Ranvier, much less Na+ gets in, and consequently, much less needs to be pumped out. Therefore, in myelinated axons much less energy is needed to remove Na+ ions and maintain the high extracellular Na+ con-centration.

The second advantage of myelin is speed. The action poten-tial is conducted much faster in a myelinated axon because transmission between the nodes, which occurs by means of the axon’s cable properties, is very fast (Figure 20). Increased speed enables us to react faster and undoubtedly to think fast-er. In fact, the fastest myelinated axon, 20 micrometers (µm) in diameter, can conduct action potentials at speeds of 150 m/s, or 335 mph!


Saltatory conduction – conduction of the action potential

from one node of Ranvier to the next along a myelinated axon.


Figure 19: Saltatory conduction. Action potentials are conducted down the myelin-ated axon via saltatory conduction. The depolarization “jumps” from one node to the next without decaying.

Figure 20: Comparing action poten-tial conduction in unmyelinated and myelinated axons. The black arrows represent current flowing down an unmyelinated axon and the red ar-rows represent current flowing down a myelinated axon. Notice how much faster the myelinated current travels.



LESSON READINGUnder what circumstances would it be ben-eficial not to have myelinated axons?________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________What if your big toe neuron wasn’t myelin-ated? How long would it take the action po-tantial to reach your toe then? Would this be an advantage or not?___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

At what age does our frontal lobe become myelinated?_________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Why aren’t all neurons myelinated?

Since myelin provides such important benefits – decreasing energy consumption and increasing speed – why aren’t all of our axons myelinated? In fact, most of our axons are myelinated, but later we’ll argue that having some unmyelinated axons is important.

Take for example the so-called C fibers (fibers is just another name for nerve). C fibers are sensory neu-rons located in the PNS and involved in the pain response. They are not myelinated and their conduction velocities are slow 2 m/s (or only 4.5 mph). But conducting pain information slowly, gives us an advantage because we can respond to the source of the pain before the pain sensation becomes intense. Sometimes it is actually beneficial for a signal to reach our brains more slowly.

What happens when myelin gets damaged?

Demyelination is the loss of the myelin sheath insulating neurons. As you might imagine, losing even a part of the myelin sheath disrupts action potential conduction. When myelin is disrupted, conduction along an axon may become desynchronized or even fail completely.

Demyelination is the hallmark of some neurodegenerative diseases including multiple sclerosis, (MS) and Charcot-Marie-Tooth disease. Demyelination results in a set of symptoms that will depend on which neu-rons are affected.

We’ll talk more about demyelinating diseases in the last lesson of this unit, but for now remember that the myelin sheath insulates the axon increasing the conduction velocity of the action potential, as well conserv-ing the axon’s energy.

When does myelination occur?

Recently, research has shown that our brains gradually add myelin as we mature. Figure 21 is taken from one of the studies on which that state-ment is based. Remember, grey matter is where neurons connect with each other and white mat-ter is where the myelinated axons are. The study analyzed changes in grey matter relative to white matter, so another way to look at the data is that not only does grey matter decrease, but white matter also increases as we mature. Take a look specifically our frontal lobes, which do not be-come fully myelinated until we are about 20. Some scientists have taken this further to argue that teenag-ers show poor judgment because their frontal lobes aren’t fully myelinated. This conclusion has been hotly debated in the field, and might be one you’d like to take a minute to think about.


Demyelination – the loss of myelin insulating neurons.

Saltatory conduction – conduction of the action potential

from one node of Ranvier to the next along a myelinated axon

Demyelination – the loss of myelin insulating neurons


Mostly grey ma-er

Mostly white ma-er

Figure 21: Loss of grey matter and gain of white matter from 5 – 20 years. Notice that our frontal lobes are the last areas to become heavily myelin-ated and thus be represented as mostly white matter.



Remember to identify your sources

STUDENT RESPONSES

You just read about research that shows that the human brain, specifically the frontal lobe, is not heavily myelinated until the age of 20. Some scientists argue that teenagers show poor judgment because their brains aren’t fully myelinated. What do you think? Do you agree with the scientists’ arguments? Do you think there could be another explanation?

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