last updated:

Feb 16, 2021


[Brain Image]    

PSY 340 Brain and Behavior

Class 06: Neural Impulse (part II) OUTLINE


The Action Potential

(continuation of PowerPoint presentation)
 Action Potential
  • NA+ & K+ channels are voltage-activated gates
  • Threshold of excitation (about -40 mV).
    • Both Na+ & K+ gates open, but the K+ gates open more slowly.
  • The movement of positive ions into the membrane (1 to 2) is called depolarization 
  • Hyperpolarization (at #3) = -90mV
  • Refractory period (at #4)
  • Action potential equals the rapid depolarization followed by brief hyperpolarization at a location along the axon membrane.

What are some real-world implications of this?

Puffer FishNerve poisons (e.g., scorpion or sea anemone venom; "Red tide"; Puffer fish [Fugu rubripes])
Local anesthetic drugs (Lidocaine [brand name: Xylocaine®]
General anesthetics 
All or None Law

Myelin: Two Sources
Myelin in CNS & PNS

Myelin = insulation along the axon. Where does it come from?

Inside the Central Nervous System (CNS) from oligodendrocytes (a glial cell)

Outside the CNS, i.e., in the Peripheral Nervous System from Schwann cells


Propagating an Action Potential: Method and Speed

 [Action Potential

An action potential is actually self-propagating.

How fast does an action potential move along an axon?

  • Thinnest axons = slow (at less than 1 meter per second)
  • Thick axons = fast (about 10 m/s) 

However, myelin sheaths permit speeds up to 100 m/s. How?
See below for Saltatory conduction

 [Saltatory Conduction]

Saltatory conduction = action potential jumps from one Node of Ranvier to the next.

In the PNS (Peripheral NS, e.g., your leg or arm), speed reaches 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