PSY290H5 Lecture Notes - Lecture 4: Multiple Sclerosis, Axon Hillock, Neuroglia
PSY290
Lecture 4
Neural Communication 1
Transmission Within a Neuron
Chapter 3: Pages 55-67
How a neuron actually functions
An action potential takes approximately 2 seconds
The starting point of the action potential is slightly above the minimum: threshold (40 mv)
Question: Where does the action potential occur? Axons only (no dendrites or cell body)
The axon is not connected to anything else
Action potential is used for conduction WITHIN a neuron not communication BETWEEN neurons
Integration of Postsynaptic Potentials
Action potentials are initiated at a very specific part in the neuron
Axon hillock: where the action potential is initiated (as previously thought, WRONG now)
oStarted when studies in invertebrate that showed that the axon hillock initiates action potential
2015: the axon hillock does NOT actually generate action potentials, it does not have the capacity to do
so because it relies on something really special to happen: Voltage-gated Ion Channel once you reach
a certain threshold of depolarization, they open and allow ions to rush in
in 2015, they realized that there are NO voltage-gated ion channels in the axon hillock
action potential is initiated in the initial segment of the axon (integration zone)
Test question: where does the action potential start? Initial segment of the axon NOT the axon hillock since it
lacks the voltage-gated channel
ACTION POTENTIALS ARE GENERATED IN THE INTIAL AXONAL SEGMENT
The Voltage Gated Sodium Channel
we need an initial event that is change in the polarity in an axon
othis might be created with generator neurons: generate action potentials
oour sensory cells are currently generating action potentials
when you touch something, the sensory neurons have action potentials that open the gates upon touch
owhen you touch something, sodium will flood from the outside of a neuron to the inside of a
neuron which changes the membrane potential of that neuron
2 types of voltage gated channels
oPotassium gated channels
very slow opening and closing (2 milliseconds) in the initial phase of depolarization
oSodium gated channels
very quick opening and closing
the action potential is triggered by sodium ions rushing inside the cell
as soon as Na comes in, this results in depolarization in the neighboring voltage gated
channel in a consecutive manner (refer to the feedback loop) (orange)
extends all the way along the axon and takes less than a millisecond
when sodium no longer enters, the positive feedback loop will be terminated
we now see potassium channels opening up more slowly allowing potassium ions to flow
back out (pink)
othe two gates open up at the same time with the same potential
sensory activity depolarization gates open
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Action Potential Summary
Refractory Period: period where the action
potential can’t fire anymore
Question: How many action potentials
should we be able to fit into a single second?
1000/2 MS long per action potential =
500/second (based on calculation)
we can have a maximum of 500 action
potentials per second
however, it was established that 1000
action potentials (roughly double of the
calculation we already learned) per second
is possible, it turns out there is more than
one refractory period in a neuron
First refractory period: Absolute refractory period
bounds the orange area on the figure below
Second refractory period: Relative refractory period
channels can re-open but they need to overcome the state of hyperpolarization in phase 4 in the diagram below
this is why we can fire 1000 instead of 500
Question: why is it said that the neuron can fire 1000 AP instead of 500?
Speed of Action Potential Propagation
the conduction from one gated channel to another
the action potential is constantly regenerated across the axon itself
othis conduction is altered by several factors such as myelin sheath
Myelinated Axons
the vast majority of axons in our nervous system (CNS and PNS) are myelinated axons have glial cells
oCNS: oligodendrocytes
oPNS: Schwann cells
Unmyelinated neurons lack the glial cells
oOlfactory neurons are unmyelinated (conduct a lot more slowly)
oSome pain fibres are myelinated and others are NOT
If you pinch between your fingers
Sharp pain: Alpha myelinated neurons (quick reaction)
Prolonged pain: C unmyelinated neurons (prolonged pain)
Question: what are the 2 examples of unmyelinated axons?
oPotential travels at about 10 meters per second
At every point down the length of the axon where we have voltage gated channels, the
signal is being conducted down
What slows down an axon?
The number of voltage gated channels makes a difference
There are a couple of things working against action potential propagation in an axon: (TEST Q)
1. Leaky axons
a. Axons have passive and active ion channels along them
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b. Those passive ion channels (permeable to potassium and some to sodium)
c. They’re called leaky channels in that they’re under no specific active control
d. When they leak ions, they dissipate any membrane potential that actually exists
e. That leakiness happens everywhere on an axon and it really hinders the action potential
f. “membrane resistance” how likely the membrane is to rest the passage of ions through it
g. therefore, this makes the membrane very low resistant and leaks ions easily
h. LOW MEMBRANE RESISTANCE
2. Sticky axons
a. Ions of opposite charge tend to stick to one another
b. If neiboring positive and negatve ions are close, they’re going to stick
c. Axons are s thin that it is possible for ions on the inside to stick to ions on the outside
d. That stickness slows down the flow of ions an dmakes it difficult for ions to flow through
e. Membrane capacitance = stickiness
f. HIGH MEMBRANE CAPACITANCE
g. Ions are not flpwing in the proper way
3. Thin axons
a. Axons are full of microfilaments, fluids, etc
b. There is not a lot of free space
c. As a result, the flow of ions tends to be a little slower than otherwise given more room for ions to
flow freely
d. Because axons are thin, they tend to resist the flow of ions inside of them
e. Axoplasmic resistance = resistance of fluid to make ions flow
f. HIGH AXOPLASMIC RESISTANCE
What determines the speed of an action potential?
how many voltage gated channels do we actually need to propagate the action potential?
Leaky axons: low resistance, leak ions easily
oThat leakiness happens everywhere on an axon and is relevant when you’re trying to generate an
action potential
oThis resists the passage of ions through it
oLeakiness = membrane resistance
oThose membranes do a poor job in letting ions out
Sticky axons: ions of opposite charge tend to stick to one another
oIf neighboring positive and negative are close, they’re going to stick
oIons on the inside can stick to ions on the outside which hinders the propagation
oHigh membrane capacitance: the stickiness across a membrane
Thin axons: there is not a lot of free space in an axon and thus the flow of ions might be a little bit
slower: high axoplasmic resistance
oThe plasma (the stuff inside the axon) hinders the rate of flow
Invertebrates have a much larger representation of unmyelinated axons in their nervous system
oSome invertebrates have developed a clever solution to help overcome the flow resistance
oGIANT unmyelinated axons (squid and octopus)
oThe functional significance of those giant axons solves one of the problems about the speed of
conduction
oIf you have more space for the ions to flow, you have less axoplasmic resistance
oIf you have more space for ions to flow, you address the problem of stickiness. Those ions
flowing have more space to get away from the membrane
Question: what are the 2 problems that the giant axons adaptation solves?
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Document Summary
An action potential takes approximately 2 seconds. The starting point of the action potential is slightly above the minimum: threshold (40 mv) Axons only (no dendrites or cell body) The axon is not connected to anything else. Action potential is used for conduction within a neuron not communication between neurons. Action potentials are initiated at a very specific part in the neuron. Axon hillock: where the action potential is initiated (as previously thought, wrong now: started when studies in invertebrate that showed that the axon hillock initiates action potential. Initial segment of the axon not the axon hillock since it lacks the voltage-gated channel. Action potentials are generated in the intial axonal segment. We need an initial event that is change in the polarity in an axon: this might be created with generator neurons: generate action potentials, our sensory cells are currently generating action potentials. When sodium no longer enters, the positive feedback loop will be terminated.
