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Lecture 4

PSY290H5 Lecture Notes - Lecture 4: Axon Hillock, Sodium Channel, Resting Potential

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Alison Fleming

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Neural Communication: Transmission Within a Neuron
The Neuron
- Neurons communicate based on a resting potential
Action Potentials vs. Graded Potentials
- There’s two different ways that axons communicate
- How do neurons communicate between different neurons?
- How do neurons transmit information inside of itself?
The Action Potential
Where do ‘Action Potentials’ take place? Action potentials only exist in axons.
Integration of Postsynaptic Potentials and Generation of Action Potentials
- Action potentials are generated at the initial axonal segment
- The axon initial segment is the first point where information gets met by a voltage gated channel
- There are no voltage gated channels in the axon hillock, meaning that the voltage gated channels must begin
beside it, or slightly upstream on the axon
- Voltage gated channels in the initial axonal segment are of two types; the types that let sodium (Na+) go through,
and the type that let potassium (K+) go through
- These channels are normally closed, and only open up under the right conditions (the right voltage)
- Any electrical potential can result in changes in the channels, so long as the electrical potential is enough
The Voltage Gated Sodium Channel
1. Sensory activity
2. Depolarization
3. Channel opens
- Interactions with touch receptors open up o the deflection,
allowing sodium to enter a neuron

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- Generator potential is the start of transduction (taking energy
from the environment and turning it into a neural impulse in our
- When the generator potential in initiated (a graded potential) is
part of the little hill in the action potential representing the
depolarization curve right before the main action potential
- If the depolarization is enough (around -45 mV), it reaches
threshold, and it’s sufficient enough to allow voltage-gated
channels to open, resulting in a depolarization curve and causing
sodium ions to flood in
- As one voltage gated channel floods with sodium, it triggers other
sodium-gated channels, causing other channels to open, resulting
in a massive influx of sodium to the inside of a cell (the state of
- Depolarization phase represents the massive opening of voltage-
gated sodium channels
- Voltage-gated sodium channels are very fast (possible shorter than
a millisecond)
- Once a voltage-gated channel opens, it opens to it’s full extent (all-
or-none principle), then quickly closes up again (this is the
mechanism that turns off the positive feedback loop)
- Positive Feedback Loop
The action potential rising phase occurs because the neuronal
membrane contains voltage-gated Na+ channels that open in a regenerative manner (the opening of sodium
channels forces other channels to open,
causing more and more channels to open) if
depolarization passes the threshold level
The action potential rising phase ends
because the voltage-gated Na+ channels
The Voltage Gated Potassium Channel
1. Sensory activity
2. Depolarization
3. Channel opens
4. K+ channel opens
- Potassium channels are relatively slow to depolarize (about a millisecond), so
it lags behind
Action Potential Falling Phase
- Potassium ions are positive and work against the concentration gradient
(inside the cell), and don’t want to be around other positive cells
- When positive ions flood out of the cell (the intracellular space) there is a
drop in the depolarization (resulting in repolarization)
- The action potential falling phase occurs because the voltage-gated K+
channel opens (with a delay) in response to depolarization
- The hyperpolarization (including the after-shoot, going below -70 mV) is a
product of potassium ions leaving the intracellular space
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