1. The electrical properties of the plasma membrane (L3)

2. Ion Channels
Ion channels are transmembrane proteins that transport inorganic ions in
or out of the cell
They transport ions down concentration gradient (require no energy)
and 1000 times faster than carriers
Ion channels have two important properties:
Selectivity: allowing some ions to pass but not others
K+ channels
Na+ channels
Ca2+ channels
Cl- channels
Gating : ion channels open and close in response to specific stimuli
some channels inactivate before they close (open,
inactivated, close)
Movement of ions through ion channels constitute electrical current
I (current) = V (voltage) / R(resistance) This is Ohms law

3. Copyright 2009 John Wiley & Sons, Inc.
Gating of Ion Channels
The gating (opening and closing) of ion channels occurs in response to a specific stimulus:
Leakage (non-gated,) channels are always open
nerve cells have more K+ than Na+ leakage channels
as a result, membrane permeability to K+ is higher
explains resting membrane potential of -70mV in nerve tissue
Ligand-gated channels open and close in response to :
- extracellular mediator (excitatory and inhibitory neurotransmitters).
- intracellular mediators (Ca2+, ATP)
Voltage-gated channels respond to a direct change in the membrane potential (voltage).
There are Na+, K+ and Ca2+ voltage gated channels.
Responsible for generation of action potential
Mechanically gated ion channels respond to mechanical vibration or pressure.
Copyright 2009 John Wiley & Sons, Inc.

4. Copyright 2009 John Wiley & Sons, Inc.
Gating of Ion channels
Copyright 2009 John Wiley & Sons, Inc.

5. ATP sensitive potassium channel causes Insulin secretion
ATP…… closure of KATP channels
Depolarization
Opening of Ca2+ channels
Release of insulin of exocytosis
Compounds that close KATP channels cause insulin secretion

6. Copyright 2009 John Wiley & Sons, Inc.
Graded potentials in response to opening mechanically-gated channels or ligand-gated channels
Copyright 2009 John Wiley & Sons, Inc.

7. Ionic composition of the ICF and ECF
ECF ICF
Na mM > mM Na+
K mM < mM K+
Cl mM > mM Cl-
A meq/L < meq/L A-
Ca mM > µM Ca2+
Mg mM mM Mg2+
HCO mM > mM HCO3-
pH > pH 7.0
A- ARE IMPERMEABLE ANIONS

8. Membrane electrical charges

10. MEMBRANE POTENTIAL
Extra cell Intra cell
[K+] = 5mM [K+] = 150mM
There is a tendency for K+ to leave the cell because of the concentration gradient.
The loss of a cation leaves behind a small residual -ve charge. There is thus a tendency for K+ to enter the cell

11. Equilibrium Potential
Extra cell Intra cell
[K+] = 5mM [K+] = 150mM
Diffusion
Electro-
static
At equilibrium the diffusive forces will equal the electro-static forces so that there is no net movement of K+.
This is termed an
Equilibrium Potential.
The diffusion force E = RT/zF
[K+]e
[K+]i
The Nernst equation
R is the Gas Constant J.K-1.mol-1 T is the absolute temperature = °C
z is the ionic valency - for K+ = F is the Faraday Constant = C.mol-1

12. Because the log of 1/30 + -1.477 Ek = 61(-1.477) = - 90 mV [K+]e
Ek = 61 log
[K+]e
[K+]i
Ek = 61 log
[5 mM]e
[150 mMK+]i
Ek = 61 log
[1]e
[30]i
Ek = 61 log 1 30
Because the log of 1/
Ek = 61(-1.477) = - 90 mV

13. GRAPHICAL REPRESENTATION OF THE NERNST EQUATION
The relationship between the extracellular [K] and membrane potential, Em, is very non-linear. In particular small changes of [K] around the normal level have large effects on Em.

15. Equilibrium Potential
Membrane Potential when electrical and chemical forces are balanced therefore there is no net movement of an ion
Nearnst equation:
Ek = 61 log [K+] outside / [K+] inside = 61 log 5 mM/ 150 mM
= -90mV (Equilibrium potential for K+)
ENa = 61 log [Na+] outside/ [Na+] inside = 61 log 150 / 15 =
+ 60 mV (Equilibrium potential for Na+)

17. Resting Membrane Potential (RMP)
If K+ moves alone RMP = -90 mV
If Na+ moves alone RMP = +60 mV
RMP is closer to EK because the membrane at rest is more permeable to K+

20. Resting membrane potential

21. Resting Membrane Potential
There is an electrical voltage difference between inside and outside of the
cell membrane.
There is of small build up of –ve charges at the cytosol side of the membrane
Separation of –ve and +ve charges is measured in Volts (mV) and is
called membrane potential.

22. Factors that contribute to resting membrane potential
Unequal distribution of ions in the ECF and cytosol.
Inability of most anions to leave the cell
Na-K pump (electrogenic pump)

23. Resting Membrane Potential
Negative ions along inside of cell membrane & positive ions along outside
potential energy difference at rest is -70 mV
cell is “polarized”
Resting potential exists because
concentration of ions are different inside & outside
extracellular fluid rich in Na+ and Cl
cytosol full of K+, organic phosphate & amino acids
membrane permeability differs for Na+ and K+
greater permeability for K+
inward flow of Na+ can’t keep up with outward flow of K+
Na+/K+ pump removes Na+ as fast as it leaks in

24. Membrane Potential Depolarization Cell is depolarized (-50mV)
RMP (-70mV)
Cell is polarized
Cell is hyperpolarized
Hyperpolarization
(-90mV)

25. Membrane Potential Depolarization Cell is depolarized (-50mV)
Na+ ions move into the cell (influx)
RMP (-70mV)
Cell is polarized
K+ ions move out the cell (efflux)
Cl- ions move into the cell (influx)
Cell is hyperpolarized
Hyperpolarization
(-90mV)
In many cases depolarization occurs if K+ is prevented to move outside the cell

26. Changes in membrane potentials
Graded potentials

27. Stimulus strength and graded potentials

28. Graded Potentials Inhibitory Post Synaptic Potential
Small deviations from resting potential of -70mV
hyperpolarization = membrane has become more negative
Excitatory Post Synaptic Potential
depolarization = membrane has become more positive
The signals are graded, meaning they vary in amplitude (size), depending on the strength of the stimulus and localized.
Graded potentials occur most often in the dendrites and cell body of a neuron.

29. Electrical signals in neurons
Neurons are electrically excitable due to the voltage difference across their membrane
Communicate with 2 types of electric signals
action potentials that can travel long distances
graded potentials that are local membrane changes only

30. EPSP and IPSP

31. Summation Spatial Summation
If several presynaptic end bulbs release their neurotransmitter at about the same time, the combined effect may generate a nerve impulse due to summation
Summation may be spatial or temporal.
Spatial Summation
Summation of effects of neurotransmitters released from several end bulbs onto one neuron
Temporal summation:
Summation of effect of neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuron