Introduction to Neurochemistry I

1
Introduction to Neurochemistry I

• Presentation by Josh Morrison for Biochemistry II
• February 21, 2005

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The Membrane Potential

• Vm is the symbol for Membrane Potential
• Vm is the electrical charge of a cell
• Present in all cells

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Importance of Vm

• Source of potential energy for transporting ions
  and molecules across cell membrane i.e.
  Na/glucose cotransporter
• Determines if ion will be actively or passively
  transported across membrane
• Thus, to some extent, determines how cell will
  spend energy

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Origin of Vm

• Vm is complex interaction between ion
  concentrations and the channels and pumps through
  which ions enter and exit cell
• To help understand the factors involved, let us
  look at a simple cell with only one positive and
  one negative ion

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Simple Cell

• Recall physics. For electricity to exist, there
  must be a complete pathway for electron flow.
• Ioncharged particle (like electron). Thus, flow
  of ions equals electric flow.

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Simple cell cont

• Lets say that the positive ion flows through
  channel from high concentration to low
  concentration (selective permeability)
• However, flow of charged particle causes
  electrical charge of cell to shift (from neutral
  to positive)
• Shift in electrical properties of cell disfavors
  flow of positive charge out of cell

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Flow of ions in simple cell
Lecture 2 From Dr. James A. Murrays Website
http//faculty.uca.edu/7Ejmurray/BIOL4425/lec/lec
tures.asp
8
Nernst Equation

• Eion(RT)/(zF) log iono / ioni
• Or
• Eion58/charge log iono / ioni

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Nernst Equation

• Predicts Vm value for cell whose membrane is
  permeable to one ion
• Example calculation for K
• EK 58/1 log 5mM / 125mM -81 mV
• Limits of Nernst

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Wrap Up of Vm

• Real cells are permeable to many different ions
• Membranes permeability to ions major factor in
  determining what Vm (illustrated by
  Goldman-Hodgkin-Katz Eq.)
• Thus, the most conductant ion will have the
  greatest effect on Vm

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Resting Potential
Lecture 2 From Dr. James A. Murrays Website
http//faculty.uca.edu/7Ejmurray/BIOL4425/lec/lec
tures.asp
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Spike Initiation Zone
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Myelin is actually Schwann cell wrapped around
axon multiple times
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The Action Potential
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Action Potentials

• Mode of Communication in Neurons
• Intensity (frequency) determines magnitude of
  response
• Initiation-Propagation-Termination

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Initiation

• Occurs only in SIZ
• Initially, only leak channels open (K)
• Slight depolarization to threshold opens
  Voltage-gated Na channels (VGNaC)
• Na flows with electrochemical gradient, causing
  further depolarization
• All-or-none response

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Action potential initiation
S.I.Z.
Lecture 5 From Dr. James A. Murrays Website
http//faculty.uca.edu/7Ejmurray/BIOL4425/lec/lec
tures.asp
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Propagation

• Local Circuit CurrentsNa diffuses down axon and
  depolarizes other places in axon
• AP Initiate in these nearby areas
• Saltatory conduction of AP due to myelin

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Propagation
Lecture 5 From Dr. James A. Murrays Website
http//faculty.uca.edu/7Ejmurray/BIOL4425/lec/lec
tures.asp
20
VGNaC found only on the nodes
Potassium leak channels present throughout neuron
Voltage Gated K channels also in area around node
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Termination

• VGNaC inactivatecause repolarization
• At the same time, the depolarization has cause
  VGKC to openspeed repol
• Flow of K out of cell causes hyperpolarization
• Refractory Periodprevents backwards movement
  of AP

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Action potential termination
Lecture 5 From Dr. James A. Murrays Website
http//faculty.uca.edu/7Ejmurray/BIOL4425/lec/lec
tures.asp
23
Ig loop (H-gate)
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Role of S4 helix in gating
http//wilkes-fs1.wilkes.edu/terzaghi/BIO-226/lec
tures/13.html
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Activation of sodium channel through S4 movement
(M-gate)
Outside
depol
Cytosol
Ready state No Na entry (Vm-70 mV)
Active State Na enters (Vmthreshold)
time
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H-gate inactivates sodium channel once Vm cytosol
becomes positive
Inactive
Ready
Active
Na
H-gate
M-gate
Lecture 5 From Dr. James A. Murrays Website
http//faculty.uca.edu/7Ejmurray/BIOL4425/lec/lec
tures.asp