Introduction to Neurochemistry I Presentation by Josh Morrison for Biochemistry II February 21,...

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Introduction to Neurochemistry I

Presentation by Josh Morrison for Biochemistry II

February 21, 2005

The Membrane Potential

• Vm is the symbol for Membrane Potential

• Vm is the electrical charge of a cell

• Present in all cells

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

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

Simple Cell

• Recall physics. For electricity to exist, there must be a complete pathway for electron flow.

• Ion=charged particle (like electron). Thus, flow of ions equals electric flow.

Simple cell con’t

• Let’s 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

Flow of ions in simple cell

Lecture #2 From Dr. James A. Murray’s Website http://faculty.uca.edu/%7Ejmurray/BIOL4425/lec/lectures.asp

Nernst Equation

Eion=(RT)/(zF) log [ion]o / [ion]i

Eion=58/charge log [ion]o / [ion]i

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

Wrap Up of Vm

• Real cells are permeable to many different ions

• Membrane’s 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

Resting Potential

Spike Initiation Zone

Myelin is actually Schwann cell wrapped around axon multiple times

The Action Potential

Action Potentials

• Mode of Communication in Neurons

• Intensity (frequency) determines magnitude of response

• Initiation-Propagation-Termination

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

Action potential initiation

S.I.Z.

Propagation

• Local Circuit Currents—Na diffuses down axon and depolarizes other places in axon

• AP Initiate in these nearby areas

• Saltatory conduction of AP due to myelin

Propagation

Voltage Gated K channels also in area around node

Potassium leak channels present throughout neuron

VGNaC found only on the nodes

Termination

• VGNaC inactivate—cause repolarization

• At the same time, the depolarization has cause VGKC to open—speed repol

• Flow of K out of cell causes hyperpolarization

• Refractory Period—prevents “backwards” movement of AP

Action potential termination

Ig loop (H-gate)

http://wilkes-fs1.wilkes.edu/~terzaghi/BIO-226/lectures/13.html

Role of S4 helix in gating

Activation of sodium channel through S4 movement (M-gate)

Outside

Cytosol

Ready state: No Na entry

(Vm=-70 mV)

Active State: Na enters

(Vm=threshold)

H-gate inactivates sodium channel once Vm cytosol becomes positive

Ready InactiveActive

H-gateM-gate

Introduction to Neurochemistry I Presentation by Josh Morrison for Biochemistry II February 21,...

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