PSY 340 Brain and Behavior
Class 06: Neural Impulse (part II) OUTLINE
- 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?
Nerve poisons (e.g., scorpion or sea anemone venom; "Red tide"; Puffer fish [Fugu rubripes])
Local anesthetic drugs (Lidocaine [brand name: Xylocaine®]
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.
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 NeuronsMISCONCEPTION: The 10% Brain Use Myth
- 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.
- 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.
This page was first posted January 27, 2005