Transcript Module 9: Neuron & Action Potential PowerPoint

The Neuron & Action Potential
Module 9: Biological Psychology & Neurotransmission
The basic building block of our
nervous system and how it sends
messages.
Cell Body & Nucleus
The Cell Body
Contains the cell’s nucleus
– round, centrally located
structure
– contains DNA
– controls protein
manufacturing
– directs metabolism
– no role in neural signaling
Dendrites
Dendrites
• RECEIVE info/signals from other neurons
• Inputs/receptor sites may number in thousands
• If enough inputs received it may cause the cell’s axon
to generate an electrical impulse (action potential)
Dendritic Growth
• As you learn, new dendrites can grow, creating more
connections to other neurons
• New connections are basis for learning
• People with higher education have more dendritic
connections than a high school dropout.
Axon
Axon
Axon
• SENDS an electrical message
when signaled by the dendrites
• Where all the action is
• Action Potential takes place
– electrical charge is sent down the
axon.
• One axon per cell, 2 distinct parts
– tube-like structure
– branches at end (axon
terminals) that branch out to
dendrites of other cells
Myelin Sheath & Nodes of Ranvier
Myelin Sheath
•
•
•
•
White fatty casing on axon
Acts as an electrical insulator
Not present on all cells
When present, increases the speed of neural
signals down the axon allowing the action
potential to “jump” to each Node of Ranvier
- like a paved highway (see video below to
compare mylenated axons vs. nonmylenated axons
• If this degenerates (dirt road), you have
multiple sclerosis and can’t control your
muscles.
Axon Terminal or Buttons
Axon
Terminals
Axon Terminal or Buttons
• End of Axon where
the electrical impulse
triggers synaptic
transmission sending
message to the
dendrites of a
receiving neuron.
• Let’s Review with this
Quick Video.
One Direction
Communication
Glial Cells
•They are the janitors of the neuron.
•Support cells that provide neurons
with structural support and
nutrition.
•They also remove cell wastes and
enhance the speed of the neuron
Action Potential
How neurons send an electrical message
How Neurons Communicate
• Neurons communicate by means of an electrical signal
called the Action Potential
• Action Potentials are based on movements of ions between
the outside and inside of the axon
• When an Action Potential occurs, a molecular message is
sent to neighboring neurons
• Action Potential is an All or Nothing Process
(like a gun firing)
Threshold:
Triggering Action Potential
•When a neuron is resting = balance of excitatory &
inhibitory signals.
•If one of these exceeds the other stimulus threshold is reached
•Triggering the neuron to transmit an electrical impulse down its axon (action
potential)
•How do you feel something that is intense?
•More neurons fire, the intensity of their electric impulse always stays the same.
•Lou Gehrig’s Disease - too many inhibitory stimuli cause
the muscles to freeze up.
•Parkinson’s Disease - too many excitatory stimuli cause the
muscles to move without control.
Steps to Action
Potential
Resting Potential
Axon at Resting Potential - fluid inside the axon is mostly
negatively charged with positive on the outside (polarized)
• At rest, the inside of the cell is at -70 microvolts
• With inputs to dendrites inside becomes more positive
• If resting potential rises above threshold, an action potential
starts to travel from cell body down the axon
• Figure shows resting axon being approached by an AP
Step 1: Threshold is Reached
• An impulse is triggered in the neuron’s
dendrite when stimulated by:
–
–
–
–
Pressure
Heat
Light
NT - chemical messenger from another
neuron (synaptic transmission)
• Minimal level of stimulation that causes
the axon to fire is called Stimulus
Threshold
Step 2: Action Potential Begins
• When neuron fires, its axon membrane is
selectively permeable.
• Gates in the axon called ion channels open
allowing positive sodium ions to enter the axon
while potassium ions leave causing a brief
positive electrical charge in the axon
(depolarized).
• The brief positive charge is action potential.
Depolarization Ahead of AP
• AP opens cell membrane to allow sodium (Na+) in
• Inside of cell rapidly becomes more positive than outside
• This depolarization travels down the axon
Step 3: Refractory Period
• As the next gates open allowing positive sodium ions in,
the previous gates close and begin to pump the positively
charged sodium ions out of the axon and potassium ions
back inside. (repolarized).
• This step is called the refractory period and the axon
cannot fire again until it returns to resting potential
(negative polarized state).
• The entire process is like falling dominoes all the way
down the axon except these dominoes can set themselves
back up as soon as they fall over.
• Why do you think the axon has to set itself back to a
resting state so quickly (3 milliseconds)?
• So the neuron can fire again and send another message
immediately after the last one.
Repolarization follows
• After depolarization potassium (K+) moves out restoring
the inside to a negative voltage
• This is called repolarization
• The rapid depolarization and repolarization produce a
pattern called a spike discharge
Finally, Hyperpolarization
• Repolarization leads to a voltage below the resting potential,
called hyperpolarization
• Now neuron cannot produce a new action potential
• It must return a resting state
• This is the refractory period
Action Potential Within a Neuron
1. Threshold is reached
2. +Na ions enter
beginning of axon
3. this triggers the next Na
gates to open.
4. As they open & allow in
Na+,
5. previous gates begin
pumping the Na+ out.
6. Before the action
potential has reached the
end, the beginning of the
axon is back at resting
potential & ready for
another firing.
A Review Action Potential
DAILY
DOUBLE
How can a toilet represent Action
Potential?
• Full Toilet – Resting Potential
• Push Flush Lever – Threshold Stimulus
triggering Action Potential.
• Toilet Refilling/Can’t Flush –
Repolarization/Refractory Period
• Sewer Pipes – One-way
communication like action potential
only goes from dendrite end to axon
terminal end.