How neurons communicateACTION POTENTIALS

• Researchers have used the axons of squids to study action potentials

• The axons are large (~1mm) and extend the length of the squid’s body

Electrochemical messages

• Neurons use electrochemicals to communicate

• The cells use ions - most important ones;

Na+, Cl-, Ca2+, and K+.

Neuron at Rest

• At rest the neuron is said to have a resting potential.

• Intracellular (inside) area is negative relative to extracellular (outside) area.

Resting Potential

• The cell membrane selectively allows some ions to pass through (ion channels)

• Passive transport• K+, pass freely• Na+ and Cl- less freely• A- protein cannot pass

Resting Potential

• Active transport also occurs

• It moves 3 Na+ out of cell for every 2 K+ that move in to the cell

• Resting potential is

-70mV (millivolts)

• At rest there are more Na ions outside and more K ions inside

ACTION POTENTIALS

• Passed down axon - away from cell body

• They are created when a stimulus causes a depolarizing current

• The resting potential moves toward 0mV

• When it reaches a level of -55 mV, it will “fire” an action potential

All or None

• Once an axon’s threshold potential has been reached it will fire an action potential of a fixed size.

• This size will not vary• Thus it is called the

All or None Principle

Action Potentials:

• Caused by the exchange of ions across the neuron membrane

1. Na channels open, and Na+ rushes into the cell (due to the negative charge and [] difference).

2. Inside the neuron becomes more positive and depolarizes

3. K channels open, causing K to rush out of cell (follows [] gradient)

Action Potentials:

4. The K leaving the cell reverses the depolarization

5. At this time the Na channels close, and the neurons begins a repolarization.

6. The potential actually depresses below -70mv for a short period because the K channels are still open.

7. Gradually the potential returns to resting levels and the cell is ready to fire again. (refractory period)

ttp://faculty.washington.edu/chudler/ap.html

http://www.youtube.com/watch?v=pRyR5NNLaig

What are some real-world implications of this?•

Some nerve poisons (e.g., scorpion venom) open Na+ channels and shut K+ channels & disrupts any action potentials.

Local anesthetic drugs (Novocain, Xylocaine) block the Na+ channels and prevent action potentials along sensory neurons.

General anesthetics used in hospitals,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.

Using your textbook, (page 418-420) find and define the following terms:

•Action potential

•Polarized membrane

•Depolarization, repolarization

•Sodium-potassium pump

•Refractory period

Explain the contributions of the following researchers:

•Galvani, Einthoven, Dubois-Raymond, Berger, Bernstein, and Cole and Curtis

Propagating an Action Potential: Method and Speed

• Domino effect:

• Influx of positive ions causes 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.

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

• How?• Saltatory

conduction

• Saltus is Latin for jump

Mylin Sheath

• Produced by Glial Support Cells

• Myelin wraps around the axon and helps electrical current flow

• does not cover the entire axon – nodes of Ranvier

• distance between these nodes is between 0.2 and 2 mm

• Allows AP to jump

•The action potential jumps from one node to the next

•Myelin allows the Na+ ions to move quickly to the next Na+ gate at a node of Ranvier

•http://www.youtube.com/watch?v=7EyhsOewnH4

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.  

WHAT YOU SHOULD KNOW

•Identify the three major components of a neuron and their function

•Identify the benefits of myelination to a neuron

•Distinguish between the basic types of neurons

•Explain the role of sodium and potassium in an action potential

•Explain the components of an action potential

•Explain the concept of threshold

•Explain how salutatory conduction enhances neuron activity

Neuron to Neuron Communication

• Two types of events 1. Electrical

2. Chemical

• Electrical events propagate a signal within a neuron

• Chemical processes transmit the signal from one neuron to another or to a muscle cell.

Chemical process occurs at end of axon in a structure called a synapse

• Touching very close against the dendrite of another cell

• The axon  releases chemical substances called neurotransmitters

• Neurotransmitters attach to chemical receptors in the next neuron and promote excitatory or inhibitory changes in its membrane.

Chemical Synapses

NEUROTRANSMITTER

1. The release of a neurotransmitter is triggered by the arrival of an action potential

2. AP causes cellular secretion, known as exocytosis:.

Neurotransmitter Release

A. Synaptic vesicles store neurotransmitters

B. When action potential arrives at presynaptic axon bulb, synaptic vesicles merge with presynaptic membrane.

C. Release chemicals into cleft

D. Neurotransmitter molecules diffuse across synaptic cleft to bind with receptors on postsynaptic membrane

E. The type of neurotransmitter and/or receptor determines if the response is excitation or inhibition.

• The action that follows activation of a receptor site may be either depolarization (an excitatory postsynaptic potential - EPSP) or hyperpolarization (an inhibitory postsynaptic potential - IPSP).