January 23, 2024	PSY 340 Brain and Behavior Class 06: Neural Impulse (part II) OUTLINE

NA+ & K+ channels are voltage-activated gates Resting potential is -70 mV    Threshold of excitation (about -40 mV). Both Na+ & K+ gates open, but the K+ gates open more slowly. Some researchers report different values for the threshold of excitation (e.g., -50, -55, or, even, -65 mV). In all cases, the threshold is less negative than the resting potential. 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?

Nerve poisons (e.g., scorpion or sea anemone venom; "Red tide"; Puffer fish [Fugu rubripes])
 

• The puffer fish, considered in Japan to be a real delicacy (it is called "Fugu"), contains the poison, tetrodotoxin (CDC site), which binds to the Na+ channel for 10-40 seconds and causes no signal to propagate. This paralyzes a victim and causes the loss of breathing.
  

Local anesthetic drugs (Lidocaine [brand name: Xylocaine®]

General anesthetics 

All or None Law

• Axons either "fire" or they don't "fire". For most neurons, there is no such thing as a partial action potential (except, see below, about "local neurons").
    
• Frequency (i.e., the number of action potentials per second) conveys the strength of the original stimulus
   
  

Myelin: Two Sources

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

An action potential is actually self-propagating. Triple Spiral Falling Domino video (YouTube; 15,000 dominoes) 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 = 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

• Local neurons contain very short dendrites and short (or absent) axons.
   
  
• They exchange information with only with neighboring neurons.
   
  
• They generate depolarizations (become less negatively charged) or hyperpolarizations (more negatively charged). These are called graded potentials and do not follow the All-or-None Law. These charges are conveyed by contact with neighboring neurons.
   
  
• Astrocytes also act a bit like local neurons by exchanging chemicals back and forth with nearby neurons. Therefore, they have some control over whether neuron will fire or not.
    

MISCONCEPTION: The 10% Brain Use Myth

 

• The myth takes different forms:
• Most of the brain is inactive most of the time (only 10% working & 90% inactive)
• Only 10% of the brain is used for consciousness; the rest is part of the unconscious
• Only 10% of the brain's areas have been mapped and found to have a role/function
   
  
• The myth is a myth: It is UNTRUE!!! There is no evidence whatsoever to support the myth in any of its forms.
  
  

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This page was first posted January 27, 2005