Transcript What causes a neuron to produce an action potential?

Psychology 304:
Brain and Behaviour
Lecture 12
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Announcement
As indicated in the course syllabus, I anticipate that we
will complete the contents of Chapters 1 – 5 by the date
of the midterm exam. Please plan your readings
accordingly.
If a change in content for the exam is necessary, it will
be announced in class next week.
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The Generation and Conduction of Electrochemical
Neural Signals
1. What is the neuron’s resting potential?
2. What causes a neuron to produce an action potential?
3. What is the ionic basis of an action potential?
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By the end of today’s class, you should be able to:
1. explain how the resting potential of a neuron is
maintained.
2. distinguish between EPSPs, IPSPs and action
potentials.
3. describe the electrochemical changes that tigger an
action potential.
4. describe the electrochemical changes that occur during
an action potential.
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What is the neuron’s resting potential?
• A neuron’s membrane potential refers to the
difference in electrical charge between the inside and
the outside of the cell.
• The membrane potential of a resting neuron is about
-70 mV (-50 to -80 mV). Thus, the resting neuron is
polarized.
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• Resting neurons are polarized due to the distribution of
ions around the neuron’s membrane.
• Sodium ions (Na+), potassium ions (K+), chloride ions (Cl-)
and negatively charged protein ions are distributed
unevenly across the neuron’s membrane.
• The ratio of negative to positive charges is greater inside
the resting neuron than outside.
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The Resting Neuron
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• Two processes are responsible for the unequal
distribution of ions across the membrane of resting
neurons:
1. The differential permeability of the membrane to
the ions. The membrane is most permeable to K+
and Cl-, and last permeable to negatively charged
protein ions.
2. The action of sodium-potassium pumps that
continually exchange three Na+ ions inside the
neuron for two K+ ions outside of the neuron.
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A Sodium-Potassium Pump in a
Neuron Membrane
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What causes a neuron to produce an action potential?
• A neuron produces an action potential or “fires” when it
generates and conducts an electrochemical signal.
• A given neuron receives electrochemical signals from
thousands of adjacent neurons. The terminal buttons of
adjacent neurons “synapse” onto the dendrites or cell
body of the target neuron.
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Electron Micrograph of Synaptic Contact
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• The terminal buttons release chemicals or neurotransmitters that bind to receptors on the dendrites or cell
body of the target neuron.
• The neurotransmitters can excite or inhibit the target
neuron.
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• Neurotransmitters that excite the target neuron
depolarize its membrane, producing excitatory
postsynaptic potentials (EPSPs). EPSPs increase the
likelihood that the target neuron will fire.
• Neurotransmitters that inhibit the target neuron hyperpolarize its membrane, producing inhibitory postsynaptic
potentials (IPSPs). IPSPs reduce the likelihood that the
target neuron will fire.
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• The EPSPs and IPSPs are conducted passively (i.e.,
instantly and decrementally) to an area adjacent to the
axon hillock. In this area, the EPSPs and IPSPs are
integrated.
• If the integrated sum of the EPSPs and IPSPs is
sufficient to depolarize the membrane to a level referred
to as the threshold of activation (-40 to -65mV), an action
potential is generated and the neuron will fire.
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Neural Integration
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• An action potential is a momentary reversal of the
membrane potential from a highly negative value (e.g.,
-70mV) to a highly positive value (e.g., +50 mV).
• An action potential is an all-or-none response.
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What is the ionic basis of an action potential?
• When the membrane potential of a neuron reaches the
threshold of excitation, voltage-activated ion channels
open.
• Initially, voltage-activated sodium ion channels open,
allowing Na+ ions to rush in. The membrane potential
shifts to a value between +40 and +50 mV.
• Then, voltage-activated potassium channels open,
allowing K+ to rush out.
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• Thereafter, the sodium channels close, marking the end
of the rising phase and the beginning of the
repolarization phase of the action potential.
• During repolarization, the efflux of K+ continues,
causing the membrane potential to return to a negative
state.
• An excessive number of K+ flow out of the neuron,
leaving it hyperpolarized for a brief period.
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The Generation and Conduction of Electrochemical
Neural Signals
1. What is the neuron’s resting potential?
2. What causes a neuron to produce an action potential?
3. What is the ionic basis of an action potential?
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