Psych3 The Biological Basis of Psychology

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Psych3 - The Biological Basis of Psychology
Lecture 3
Neurotransmission
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Lecture 3 Objectives
Define an ion and an ion channel

Define membrane potential and describe the
distribution of ions across the axon

membrane that contributes to the resting membrane
potential
Identify and describe the 4 factors contributing
to the establishment of the resting

membrane potential.
Explain how each of the factors contributes to
the distribution of Na, K and Cl-

across the membrane at rest
Compare and contrast depolarization and
hyperpolarization in terms of the

membrane potential
Define a postsynaptic potential, distinguish
between EPSPs and IPSPs, and explain

whey they are graded responses
Define the threshold for activation and explain
what happens in the neuron when

the threshold is reached
Define temporal and spatial summation and
understand how they can influence the

likelihood that a neuron will fire an action
potential
Explain how the movement of ions contributes to
the generation and propagation

of the action potential
Explain why action potentials usually only travel
in one direction

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Lecture 3 Objectives
Explain the ionic basis for the refractory
period. Compare absolute versus relative
refractory period in terms of the likelihood a
neuron will fire
Describe what is meant by the action potential
being All-or-None and being non-

decrimental in its propagation
Describe the 2 factors contributing to the
velocity of the action potential

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NEURON 1
AXON
NEURON 2
How can NEURON 1 send a message to NEURON 2?
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Where does the voltage come from?
NEURON MEMBRANE
NEURON BODY
Lipid bilayer 2 layers of fat molecules
Ion channels Pores that enable ions to pass
across the membrane
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Resting Membrane Potential
At rest, there is an uneven distribution of
and - ions across the cell membrane

Sodium (Na) and chloride (Cl-) are more
concentrated outside Potassium (K) and large -
charged proteins are more concentrated inside
This sets up a difference in electrical charge
between the inside and outside of the neuron
(membrane potential)
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The membrane resting potential
OUTSIDE

-70 mV
RESTING POTENTIAL
-
INSIDE
The inside the membrane is negatively charged
with respect to the outside
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What establishes the Resting Membrane Potential?
4 factors
1) Random motionè concentration gradients
2) Electrostatic pressure

like charges repel each other ( repels -
repels -)
opposite charges attract each other ( attracts
-)

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What establishes the Resting Membrane Potential?
3) Permeability of the Membrane
to the Ionsè
Ion channels
The ion channels contributing to the membrane
potential are voltage-gated
At rest
Na channels mostly closed (require membrane
potential to be about -50 mV to open)
K channels are open (will be opened at -90 mV or
higher) Cl- channels are open (opened by -70 mV
or higher)
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What establishes the Resting Membrane Potential?
4) Active ion transportè
Na/K pump
A transporter that uses energy to pump 3 Na out
for every 2 K in
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What is really happening at rest Na
Na wants to enter
Passive concentration gradient (more Na outside)
Electrostatic pressure (Na is () and the
interior of the cell is (-).
Most stay out because Na channels are closed
What gets in is pumped out by the Na/K pump
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What is really happening at rest K
K wants to leave
Passive concentration gradient (more K inside).
However, Electrostatic pressure to keep them in
K channels are leaky so less resistance to
them leaving
BUT Na/K pump keeps pumping them back inside
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What is really happening at rest Cl-
Cl- in equilibrium
Electrostatic pressure forces Cl- out (more - on
inside)
As Cl- accumulates outside, passive concentration
gradient moves it back inside
Cl- channels are open at rest so no resistance to
them crossing the membrane
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Where it all begins the Synapse

• The Synapse the point where one neuron comes
  into contact with another neuron

• Neurotransmitters are released from the
  presynaptic neuron to the synaptic cleft and
  reach receptors on the dendrites of the
  postsynaptic neuron

Presynaptic
Synaptic Cleft
Postsynaptic
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Exciting electrical effects of receptor
activation
Some PSPs can make the inside of the cell
more positive relative to the outside
depolarization Excitatory PostSynaptic
Potentials or EPSPs

• EPSPs are graded (their size depends upon the
  intensity of the stimulus that elicited them)
• Amount of neurotransmitter released
• Number of receptors activated
• EPSPs degrade as they travel along the
  dendrite

EPSPs encourage the neuron to generate and
pass on an action potential
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Inhibiting electrical effects of receptor
activation
Some PSPs can make the inside of the cell
more negative relative to the outside
hyperpolarization Inhibitory Postsynaptic
Potentials or IPSPs
Similar to EPSs, IPSPs are degraded as they
travel along the dendrite
IPSPs discourage the neuron from generating
an action potential
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The exciting electrical effects of receptor
activation
The dendrites and cell bodies are constantly
integrating EPSPs and IPSPs
If the sum of the EPSPs and IPSPs reaches the
threshold of activation in the vicinity of the
axon hillock, the neuron will fire an action
potential
Threshold of activationdepolarization of
about 5-20 mV (so the neuron goes from -70 mV to
-65 mV or -50 mV)
Action potentialmassive momentary (1 msec)
reversal of the membrane potential (from -70 mV
to 50 mV)
- Not graded! Its All-or-None!
Occurs to its full extent or not at all
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Spatial Summation The integration of PSPs
arriving at different parts of the neuron
Scenario
You have 4 presynaptic neurons synapsing on a
postsynaptic neuron.
Neurons A and B activate excitatory receptors
EPSP
Neurons C and D activate inhibitory receptors
IPSP
What happens when Neurons A and C fire
simultaneously?
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Temporal Summation The integration of PSPs
arriving at different times
Scenario
You have 2 presynaptic neurons synapsing on a
postsynaptic neuron.
Neuron A activates
excitatory receptors EPSP
What happens when neuron A fires rapidly?
Neuron B activates
inhibitory receptors IPSP
What happens when Neuron B fires rapidly?
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Review of EPSPs and IPSPs
The size, type and effect of PSPs depend upon
several factors
The neurotransmitter/receptor combination
Number of a particular type of receptor on
the postsynaptic neuron
Distance of receptors from the axon hillock
which is the area where the cell body becomes the
axon
Frequency of firing and neurotransmitter
release from the presynaptic neuron
Number and identity of presynaptic neurons
firing at the same time on a postsynaptic neuron
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The Ionic Basis of the Action Potential
Action potentialmassive momentary
(1 msec) reversal of the membrane potential
(from -70 mV to 50 mV)
Action potentials are generated when the
threshold of activation occurs around the axon
hillock
The threshold of activationvoltage that opens
voltage-gated Na channels. Na thus comes
rushing in, making the membrane potential
depolarize up to 50 mV
Action potential is not graded but rather an
All-or-none event!
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The Ionic Basis of the Action Potential
The depolarization due to the influx of
Na (Na moving into the neuron) activates
voltage- dependent K channels that allow K
to flow out
The influx of Na is so large that the efflux
(moving out) of K does not alter the effect of
Na on the membrane potential

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The Ionic Basis of the Action Potential
When the neuronal membrane becomes very
depolarized (about 50 mV), the Na channels
close
K continues to leave the neuron causing
repolarization because positive charge is leaving
the cell
Unlike Na channels, K channels are slow to
close keep spitting out K, dropping the
membrane potential BELOW the resting membrane
potential hyperpolarization
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The Ionic Basis of the Action Potential
When the neuronal membrane is
hyperpolarized, the neuron is extremely below the
threshold of action
The period of time when the neuron is
hyperpolarized is call the Refractory Period
Absolute refractory period
Refractory periods keep the action potential
going in one direction
Relative refractory period
Why is the refractory period important?
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Propagation of the Action Potential
Once generated at the axon hillock, the action
potential travels WITHOUT DEGRADING down the
length of the axon
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Velocity of the Action Potential
The speed of the action potential Down the axon
depends upon 2 Main factors
1-thickness of the axon
2-Myelination
saltatory conduction