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Psychology 304:
Brain and Behaviour
Lecture 12
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The Generation 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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What is the neuron’s resting potential?
• In order to understand how a neuron generates and
transmits an electrochemical signal, it is necessary to
consider the neuron’s membrane 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 ranges
from -50 mV 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 contains ion channels that
allow ions to pass through the membrane. The
membrane is most permeable to K+ and Cl-, and last
permeable to negatively charged protein ions.
2. The action of sodium-potassium pumps. These
pumps 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; that is, it
either occurs to its full extent or does not occur at all.
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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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Stages Associated with an Action Potential
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Membrane Permeability to Na+ and K+
During an Action Potential
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• Immediately following the action potential, the neuron is
refractory:
Absolute refractory period: Another action potential
cannot be generated. Occurs immediately after the
sodium channels open.
Relative refractory period: Another action potential can
only be generated with greater than normal
stimulation. Occurs when the neuron is
hyperpolarized.
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The Generation 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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