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Feb 16, 2021
  

[Brain Image]    

PSY/BSC 340 Brain and Behavior

Class 06: Neural Impulse (Part II)

   

The Action Potential
(this page is a continuation of PowerPoint presentation)



What are some real-world implications of this?
Puffer FishNerve poisons (e.g., scorpion or sea anemone venom; "Red tide"; Puffer fish [Fugu rubripes]) often have a variety of effects on the Na+ and K+ channels. Some open Na+ channels and shut K+ channels & this leads to the disruption of any action potentials.
Local anesthetic drugs (Lidocaine [brand name: Xylocaine®]; Prilocaine; Novocaine® is almost never used any more) used by dentists and doctors block the Na+ channels and prevent action potentials along sensory neurons.
General anesthetics used in hospitals (ether, chloroform) open some K+ channels in the brain a bit wider than usual. This counter-acts the effects of Na+ channels being opened and prevents action potentials from propagating, too.



All or None Law


Myelin: Two Sources

Myelin in the CNS & PNS

What is myelin? Here’s one description: "“Myelin is an insulating layer, or sheath that forms around nerves, including those in the brain and spinal cord. It is made up of protein and fatty substances. This myelin sheath allows electrical impulses to transmit quickly and efficiently along the nerve cells. If myelin is damaged, these impulses slow down.” [from https://medlineplus.gov/ency/article/002261.htm]”

Where does the myelin come from? Within the Central Nervous System (CNS, that is, the brain and the spinal cord), the myelin comes from glial cells, specifically oligodendrocytes. Outside the CNS, that is, in the Peripheral Nervous System (PNS), the myelin comes from Schwann cells which wrap themselves around the axon of neurons in the PNS. See the diagram on the right.








Propagating an Action Potential: Method and Speed

So far, we've looked at an action potential as it is generated at one place: the axon hillock. How does it propagate or "move" down the axon?

 [Action Potential
                    Propagation]

Look at the diagram to the left. An action potential propagates along an axon by a process in which, once triggered, the influx of positive ions causes the adjacent Na+ gate to open and, in turn, this causes the next Na+ gate to open, and so on. Hence, an action potential is actually self-propagating.

How fast does an action potential move along an axon?

  • the thinnest axons propagate an action potential at less than 1 meter per second (1 m/s).
      
  • Thick axons propagate action potentials at about 10 m/s.  
[Giraffe]

However, myelin sheaths permit speeds up to 100 m/s. How? 

 [Saltatory Conduction]

Saltatory conduction: When the Na+ ions enter the inside of the axon, they quickly spread. The insulation of the myelin allows the ions to move quickly to the next Na+ gate at a node of Ranvier. The movement of ions from one node to the next is known as local current flow. And, so, the action potential jumps from one node to the next.


In the peripheral nervous system (e.g., your leg or arm), this allows the signal to more fare more speedily than if there was no myelin insulation, i.e., 100 meters per second.

By the way, the Latin word, saltus, which is the origin of "saltatory," means "a jump".

Local Neurons

MISCONCEPTION: The 10% Brain Use Myth

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This page was first posted January 27, 2005