Download Lesson 4 Section 9.2 Electrochemical Impulse

Lesson 4 Section 9.2
Electrochemical Impulse – Sending a Message
Action Potential
•
•
•
•
•
•
•
•
•
•
Nerves conduct electrochemical impulses also known as action potential
This happens from the axon of one neuron to the dendrite of another
Neurons have a rich supply of positive (+) and negative (-) ions both inside and outside the cell
Negative ions are too large to pass through the cell membrane
The positive ions do have the ability to diffuse in and out of the cell
Potassium ions (K+) are abundant inside and diffuse out through K+ gates
Sodium (Na+) ions are abundant outside the cell and diffuse inside through Na+ gates
So as K+ is diffusing out, Na+ is diffusing in
The cell membrane is 50x more permeable to K+ than Na+ , so more K+ diffuses out than Na+
diffuses in
This is important because:
o The outside of the neuron has an overall + charge
o The inside of the neuron has an overall negative charge because less Na+ have entered
plus the negative ions are still inside the cell
o This means the membrane is charged and is called a polarized membrane
(See diagram sections 1 and 2)
•
•
•
•
When the neuron becomes excited, or switched on, the Na+ gates open and Na+ floods into the
neuron (also attracted by the negatives ions inside) while the K+ gates close
o So Na+ is flooding in; K+ is not moving anywhere
This causes a charge reversal, or depolarization
Once the overall charge becomes negative (more + than – on the inside of the membrane) the
Na+ gates close
The cell works to restore the original polarity by using a sodium/potassium pump
o 3 Na+ are pumped out, while 2 K+ are pumped in
o ATP fuels this
o The membrane is now repolarized, or back to normal
(See diagram sections 3 and 4)
•
•
Nerves cannot be activated again until they are finished sending an impulse (I.e. the membrane
is repolarized
This time it takes for repolarization to happen is called the refractory period
Lesson 4 Section 9.2
Activity: How well do I understand an “action potential”?
Task:
• Try to see if you can, in your own words, put the steps of an action potential in the proper
order.
• You can use a diagram to help you but you don’t have to
• You may find the following headings helpful to organize tour thoughts:
o What is happening along the neuron when the neuron is said to be at rest?
o What causes the neuron to be polarized?
o What causes the neuron to be depolarized?
o What causes the neuron to be repolarized or return to its resting phase?
o How would you describe the movement of an action potential along a neuron?
Threshold levels/All-or-None Response – What make a neuron fire?
•
•
•
In order to get a response from a nerve cell, the stimulus has to be at a strong enough intensity
The minimum level of stimulus that causes a response is known as the threshold level
Thresholds can vary for each neuron
o The intensity of the stimulus dictates the frequency of impulses:
The stronger the stimulus the greater the frequency of impulses
o Detecting something warm means that the impulses are sent slower than detecting
something hot
The greater number of impulses reaching the brain, the greater the intensity of
the response
Synaptic Transmission – Messages from one neuron to another
•
•
•
•
•
•
•
Neurons do not physically touch other neurons or effectors (muscles, organs, glands)
They are separated by a gap, or synapse
As the impulse (action potential; message) moves along the axon, neurotransmitters are
released
They are held in vesicles and go from the presynaptic neuron to the postsynaptic neuron
This gap slows down transmission speed
o So the greater number of synapses that have to be made, the slower the impulse (action
potential; message) is sent
Acetylcholine is one neurotransmitter that can have an excitatory response on a neuron (allows
the neuron to fire)
It excites the postsynaptic neuron by opening sodium channels, causing Na+ ions to rush in,
depolarizing the membrane
o Remember: the reversal of the charge on the membrane causes an action potential to
move along the axon
Lesson 4 Section 9.2
•
•
Cholinesterase (enzyme) reverses acetylcholine’s effects
o It closes the sodium channels
o This allows the membrane to recover or repolarize
o Insecticides block cholinesterase, and never allow the nerve to go back to rest (an
insect’s heart is controlled by nerve impulse, unlike the muscular human heart)
Acetylcholine can also be inhibitory (prevent post-synaptic neurons from becoming active)
o Inhibitory neurotransmitters make the postsynaptic neuron more permeable to K+,
causing K+ to diffuse out of the neuron
o This causes the outside of the neuron to be even more + than usual and the inside to be
more – than usual
o This causes the postsynaptic neuron to be hyperpolarized
o To reverse this even more Na+ channels must open to start an action potential
Lesson 4 Section 9.2