Textbook Notes (380,765)
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UTSC (19,296)
Neuroscience (295)
NROB60H3 (157)
Chapter 4

Chapter 4 Notes

5 Pages
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Department
Neuroscience
Course Code
NROB60H3
Professor
Janelle Leboutillier

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Chapter 4 Notes
Action Potential: Conveys information over distances in the nervous system
-Often called a spike, nerve impulse, or a discharge
-Frequency and pattern of action potentials constitute the code used by neurons to transfer
information from one location to another
Four Components to An Action Potential
Rising Phase: Characterized by a rapid depolarization of the membrane; continues to peak until Vm
reaches 40 mV
Overshoot: Phases where inside of the neuron is positively charged with respect to the outside
Falling Phase: Rapid repolarization until the membrane is actually more negative than the resting potential
Undershoot/After-Hyperpolarization: Gradual restoration of the resting potential
Generation of Action Potential
-Perception of sharp pain when a thumbtack enters your foot is caused by the generation of
action potentials in certain nerve fibers in the skin
oChain of events:
Thumbtack enters skin
Membrane of nerve fibers is stretched (Causes activation of specialized ion
channels sensitive to membrane stretching)
Na+ - permeable channels open; depolarizes membrane beyond threshold
-Applying increasing depolarization to a neuron has no effect until it crosses threshold;
therefore APs are all-or-none
Generation of Multiple Action Potentials
-Action potentials in succession can be generated by passing continuous depolarizing current
into a neuron
-Rate of action potential generation depends on the magnitude/intensity of the continuous
depolarization current
-Firing frequency of action potentials reflects the magnitude of the depolarizing current
oStimulus intensity is coded this way in NS
oThere is a limit to rate at which neuron can generate AP (1000Hz)
-Absolute Refractory Period: It is impossible to initiate another AP for about 1msec after an
AP has been initiated
-Relative Refractory Period: The amount of current required to depolarize the neuron to
action potential threshold is elevated above normal
The Action Potential
Membrane Currents and Conductances
Neuron consists of 3 protein molecules:
-Sodium-potassium pump – Works continuously to establish and maintain concentration
gradients
-Sodium channels
-Potassium channels
-K+ is concentrated 20-fold inside the cell, Ek = -80mV
www.notesolution.com
-Na+ is concentrated 10-fold outside the cell, ENa = 62mV
Assuming only K+ channels are open, K+ ions will flow out of the cell:
1. Net movement of K+ ions across membrane is an electrical current (Ik)
2. The number of open K+ channels is proportional to conductance (gK)
3. Membrane potassium current Ik, will flow only is Vm does not equal Ek. Driving force of K+
is Vm-Ek
Net movement of K+ (Iion) = gion (Vm-Eion)
- Current flow is in the direction that takes Vm towards Ek
- When Vm=Ek there is no longer any net driving force on K+ ions even if there is a K+ conductance (gK >
0)
Voltage Clamp: Device invented by Kenneth C. Cole enabling one to “clamp” the membrane potential of
an axon at any value you choose
-Experiments performed by Hodgkin and Huxley
The Voltage-Gated Sodium Channel
-Protein forms a pore in the membrane that is highly selective to Na+ ions, and pore is
opened/closed by changes in the electrical potential of the membrane
Sodium Channel Structure
-Created from a single long polypeptide
-Molecule has four distinct domains I-IV, clump together to form a pore
-Each domain consists of six transmembrane alpha helices S1-S6
-When depolarized, molecule twists into configuration that allows the passage of Na+ through
the pore
-Contains pore loops assembled into a selectivity filter
oFilter makes Na+ channel 12-times more permeable to Na+ than it is to K+
oNa+ ions are stripped of most of their associated water molecules as they pass
through channel
Ion-water complex can be used to select Na+ and exclude K+
Water accompanies the ions as they pass through the channel
Hydrated Na+ fits, K+ does not through sodium channel selectivity filter
oRetained water serves as a molecular chaperone for ion, and necessary for the ion to
pass the selectivity filter
oDomain contains a voltage sensor (on alpha helix S4)
Positively charged amino acids residues are regularly spaced along coils of
helix
Entire segment can be forced to move by changing the membrane potential
Depolarization twists S4 and this conformational change causes gate to
open
oDomain contains a pore loop, which contributes to selectivity filter
Functional Properties of Sodium Channel
Patch Clamp: Technique used to study the ionic currents passing through individual ion channels
-Entails sealing the tip of an electrode to a very small path of neuronal membrane
www.notesolution.com

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Description
Chapter 4 Notes Action Potential: Conveys information over distances in the nervous system - Often called a spike, nerve impulse, or a discharge - Frequency and pattern of action potentials constitute the code used by neurons to transfer information from one location to another Four Components to An Action Potential Rising Phase: Characterized by a rapid depolarization of the membrane; continues to peak until Vm reaches 40 mV Overshoot: Phases where inside of the neuron is positively charged with respect to the outside Falling Phase: Rapid repolarization until the membrane is actually more negative than the resting potential UndershootAfter-Hyperpolarization: Gradual restoration of the resting potential Generation of Action Potential - Perception of sharp pain when a thumbtack enters your foot is caused by the generation of action potentials in certain nerve fibers in the skin o Chain of events: Thumbtack enters skin Membrane of nerve fibers is stretched (Causes activation of specialized ion channels sensitive to membrane stretching) Na+ - permeable channels open; depolarizes membrane beyond threshold - Applying increasing depolarization to a neuron has no effect until it crosses threshold; therefore APs are all-or-none Generation of Multiple Action Potentials - Action potentials in succession can be generated by passing continuous depolarizing current into a neuron - Rate of action potential generation depends on the magnitudeintensity of the continuous depolarization current - Firing frequency of action potentials reflects the magnitude of the depolarizing current o Stimulus intensity is coded this way in NS o There is a limit to rate at which neuron can generate AP (1000Hz) - Absolute Refractory Period: It is impossible to initiate another AP for about 1msec after an AP has been initiated - Relative Refractory Period: The amount of current required to depolarize the neuron to action potential threshold is elevated above normal The Action Potential Membrane Currents and Conductances Neuron consists of 3 protein molecules: - Sodium-potassium pump Works continuously to establish and maintain concentration gradients - Sodium channels - Potassium channels - K+ is concentrated 20-fold inside the cell, Ek = -80mV www.notesolution.com
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