Action Potentials and Synaptic Transmission

Goals for this lesson:• Explain the formation and transmission of an action potential, including all-

or-none response and intensity of response"

• Explain the transmission of a signal across a synapse and the main chemicals and transmitters involved

Keywords• Action potential"• Resting potential"• Facilitated diffusion"• Gated ion channel"• Sodium-potassium

pump"• Active transport"

• Depolarization"• Repolarization"• Hyperpolarization"• Saltatory

conduction"• All-or-none

response

• Refractory period"• Threshold level"• Synapse"• Neurotransmitter"• Presynaptic neuron"• Postsynaptic neuron"• Acetylcholine

• Cholinesterase"• Summation"• Norepinephrine"• Dopamine"• Serotonin"• GAMA (gamma-

aminobutric acid)

Let’s start with some definitions:

The nervous system uses ELECTROCHEMICAL IMPULSES to transport information throughout the body.!

Action potential:!

Resting potential:

The charge is caused by sodium (Na+) and potassium (K+) ions!

The potassium ions are the main cause in creating the electrical potential!

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Let’s learn more about the Sodium-Potassium Pump!

To the whiteboard!

The Action Potential: more definitions

When the neuron receives a stimulus the sodium channels are all opened while the potassium channels close causing sodium ions to rush into the cell.!

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Depolarization: !

Once the inside of the cell gains a positive charge the sodium channels close and the potassium channels open causing potassium to rush out of the cell!

Repolarization:

The sodium channels close much slower than the potassium channels open, thus the charge inside the cell becomes even more negative than it was at resting potential, this is called:!

The sodium-potassium pumps works to restore the cell to resting potential and the time it takes to do this is called the REFRACTORY PERIOD

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Once the threshold is reached there will be a response, this is an ALL-OR-NONE RESPONSE, it does not get stronger with a stronger stimulus, it either happens, or it doesn’t and when it happens it is always the same response

What about the myelin sheath?

The myelin sheath that covers the axon do not allow for sodium or potassium channels to open.!

Along the axon, channels are only found at the nodes of Ranvier!

The action potential must “jump” from one node to the next, creating a much faster transmission!

This process is called:

Why doesn’t the action potential travel backwards?

Let’s get a visual… or two…To the whiteboard!!

How is this triggered? Through Synaptic transmission of course!!

Synapse:!

Neurotransmitter: !

Presynaptic neuron:!

Postsynaptic neuron:

Common Neurotransmitters

Neurotransmitter Action Secretion sites Major effects

Acetylcholineexcitatory to skeletal muscles; excitatory or

inhibitory at other locations

Neuromuscular, CNS, PNS

skeletal muscle contraction

Norepinepherine excitatory or inhibitory

CNS, PNS wakefulness

Dopamine generally excitatory

CNS, PNS voluntary movement and emotions

Seratonin generally inhibitory

CNS sleep

GABA inhibitory CNS motor behaviour

Summation:

Let’s look at the BIG picture

One more time… To the whiteboard!!!