Review of Membrane & Action Potentials

Textbook Chapter 4
CD Modules NervousSystem I & II

Neuron Structure
Be sure to examine Figures 6.1, 2, 4, & 5. and Table 6.1
It is critical to understand the concepts behind the terms in this table.

‘Nerve vs. Neuron’


Equilibrium Membrane Potential (Vm) Made Easy
Membrane Potential (Vm)
See Figs 6.6 - 6.8

To Approximate Vm:
NERNST Equation
                              E Ion = R T     ln   [Ion]1
                                           z F    [Ion]2
R = gas constant = 1.987 cal/mol-deg
T = degrees K
z = ion valence
F = Faraday’s Constant 23,062 cal/V-mol

Constants in the Nernst Equation
Log10 = (ln/2.303)

At 37 o C,

EIon (in mV) =  - 61.5  log10  [Ion]1
                               z          [Ion]2

At 20 o C,

EIon (in mV) = - 58     log10   [Ion]1
                              z   [Ion]2

What does EIon mean?

Equilibrium Potential (Vm)

Predicting the sign of Vm:

When does Eion approximate Vm?

Use the permeable ion in the Nernst
If it doesn’t diffuse across the membrane, it doesn’t contribute to Vm

Ohm’s Law
V = I R
I = V/R
When ions (charges) move through membrane channels, current flows!
Amperes (A)
The flow of ions (I) is impeded by R
We use 1/R or conductance (g) to express the permeability of a membrane to an ion

If GNa+ increases, Vm will……?
If GK+ decreases, Vm will…….?
If GNa+ decreases, Vm will……?
If GK+ increases, Vm will…….?
If [K+]out increases, Vm will…..?
If [K+]in increases, Vm will…..?
If [K+]out decreases, Vm will…..?
If [Na+]out increases, Vm will…..?
If [Na+]out decreases, Vm will…..?

To Calculate Vm:  Goldman Equation  Text, page 182

Membrane Potential:  Changes in Vm:
• Depolarization:  change in resting Vm to a more positive value
• Hyperpolarization:  change in resting Vm to a more negative value

Membrane Potential (Fig 6.9)

Review Graded Potential Fig 6.10
• Variable amplitude
• Variable duration
• Can be summed
• Hyper- or depolarization
• Decremtnatl; not propagated; decreased amplitude as it passes
  along a membrane

Excitable Cells
Conduct action potentials along entire cell membrane
In humans, nerves, skeletal and cardiac muscle cells
All possess voltage-gated ion channels
Channels that change between open/closed states as Vm changes
Voltage-gated ion channels:
Open <---> Closed states determined by voltage difference across membrane (Vm)
Threshold:  Closed state often referred to as channel inactivation
Has a voltage sensor region
Inactivation region
For Voltage - gated ion channel, see Fig 6.14

Action Potential (Fig 6.13; Fig 6.15)

Most nerve cells have voltage gated Na+ channels
At resting Vm, these channels are closed, Vm is close to E of K+
Voltage gated Na+ channels open when Vm depolarizes to threshold

Action Potential
• The G of Na+ increases many fold and Vm goes towards E of Na+
  (usually around +30 to +60 mV)
• Na+ channels close and inactivate
• Voltage gated K+ channels open
• Vm returns towards E of K+

Action Potential
• All or None (threshold)
• Same amplitude on a given neuron
• Cannot be summed
• One direction (dendrites to terminal)
• Refractory period
• Propagated (non-decremental)

How does the action potential move along the axon?


How is a depolarization generated at the dendrites of a neuron?
• Neurotransmitter binding to receptor which is linked to an ion channel
• Mechanical or other stimulus activating a sensory structure which
  leads to opening an ion channel

How to increase speed at which an action potential travels down a neuron?
• Increase axon diameter
• Larger diameter has less interior resistance to current flow
• Depolarization to threshold occurs farther down the axon, so AP moves
  faster in a given period of time

How to increase speed at which an action potential travels down a neuron?
• Myelinate the axon
    • Saltatory conduction Fig 6.20

Neurotransmiiters & Receptors
• Exocytosis of signal molecule
• Receptors
• [NT] + R  <------>  [NT]*R
• NT removal
• Graded Potential
        • epsp & ipsp

Post Synaptic Events:
IPSP -->