1
Q
Electric-to chemical-to electric signaling.
A
Resting state of the neuron
2
Q
Phospholipid bilayer:
A
- Polar phosphate group
attached to the end - Long chains of carbon
atoms bound to hydrogen - Consists of two layers of phospholipids
Protects and supports the cell
Selective permeability: allows only certain substances to pass through
3
Q
Proteins: a brief review
A
Proteins are assembled from various combinations of 20 amino acids
An amino acid:
- Amino group (NH3+)
- carboxyl group (COO-).
- variable R group.
4
Q
Four levels of protein structure:
A
- Primary structure: the chain of amino acids
- Secondary structure: the structure of the chain (for example a helix)
- Tertiary structure: how proteins bend and fold in 3 dimensions
- Quaternary structure: how different chains can bond together to form
a larger molecule
————————————————— - ion channels are composed of proteins.
5
Q
The cell membrane is composed of a …
A
- phospholipid bilayer
- It contains ion channels composed of proteins.
- These channels are selective: they let only certain ions through the membrane.
6
Q
The movement of ions across the cell’s
membrane is determined by 2 forces:
A
- Diffusion
- Electricity
7
Q
Diffusion
A
- from high concentration to low concentration
- With ion channels,
an equilibrium is reached.
8
Q
Concentration gradient:
A
- difference in concentration.
- Ions flow down their concentration gradient until an equilibrium is established.
9
Q
Requirements for diffusion:
A
- There are open ion channels to allow ions to flow from one side of the membrane to the other.
- There is a concentration gradient.
10
Q
Electricity + Electrical current
A
- ions are electrically charged particles. Opposite charges attract. Like charges repel
- movement of electrical charge (I) measured in amps.
11
Q
Electrical potential or voltage =
A
- force exerted on a charged particle (V) in volts.
- f the voltage is increased, the force on the particles will increase and more
current will flow
12
Q
Electrical conductance
A
- relative ability of a charged particle to move (g) in siemens
- The ion channels determine the conductance.
13
Q
Electrical resistance
A
- the inability of a charged particle to move. (R) in ohms. R = 1/g
- Ohm’s law: V=IR or I=gV.
14
Q
The membrane potential + Why is the resting potential -65mV?
A
- the voltage difference across the neuronal membrane: Vm
- The potential difference acroiss the membrane. The inside of the membrane negative and outside positive. Inside the cell measurement means that the value is negative.
15
Q
Electrical current needs:
A
- An electrical potential difference across the membrane.
Ion channels that are permeable to the ion.
16
Q
Hypothetical cell + what happens
A
- K+ and A- are dissolved at a higher concentration inside the cell than out.
- No ion channels, nothing happens
17
Q
An equilibrium is reached when …
A
- the force of diffusion in one direction
equals the electrical force in the opposite direction. - When this happens, the electrical potential difference is called the Equilibrium Potential. For K+ , in a neuron, it is -75mV.
18
Q
Now add a K+ channel to hypothetical cell
A
- K+ flows down the concentration gradient (brings positive charge)
- at equilibrium, there is equal charge of K+ (After build-up of positive charge on the other side of the membrane)
19
Q
An equilibrium is reached when the force of diffusion…
A
- in one direction equals the electrical force in the opposite direction.
- When this happens, the electrical potential difference is called the Equilibrium Potential. For K+ , in a neuron, it is -75mV.
- The equilibrium potential for K+ is abbreviated EK.
20
Q
EK = -75 mV
A
- It is the membrane potential where K+ is at equilibrium.
21
Q
Start with more Na+ outside the cell…
1) Once a Na+ channel is added, in which direction will
Na+ ions flow?
2) As Na+ flows down its concentration gradient, will
the inside of the cell become positive or negative?
A
- (inside th cell)
- (positive)
22
Q
Forces of diffusion =
A
electrical force
- For a neuron, ENa = +58 mV. It is the membrane potential where Na+ is at equilibrium.
23
Q
Start with more Cl- outside the cell:
1) When you add a channel, which way will Cl- flow?
2) Will the inside of the cell become positive or negative?
A
- inside the cell
- negative
24
Q
Equilibrium potential:
A
- the potential (voltage) at which there is no net
driving force on the ion. - Each ion has its own equilibrium potential.
25
The Nernst equation:
- used to calculate the exact value of the equilibrium potential for each ion.
26
Components of Nernst Equation
- R = gas constant
- T = temperature (in Kelvin)
- F = Faraday constant
- z = charge of the ion
- [ion]o = concentration of the ion outside the membrane
- [ion]i = concentration of the ion inside the membrane
27
Concentration gradients are maintained by the sodium-potassium pump:
- Pushes Na+ out and K+ in
* Requires ATP
28
Concentration gradients are maintained by...
- astrocytes
“potassium spatial buffering”.
- shuttle from high concentration to low concentration
29
Relative ion permeabilities at rest
- In the Nernst equation, we assume that the membrane has only one type of ion channel.
- In reality, there are many different channels (K+, Na+, Cl- etc).
- The true resting membrane potential will depend on how permeable the membrane is to all of these ions.
30
The membrane is permeable to both K+ and Na+...
- Then Vm = some average of -75mV and +58mV.
31
Goldman equation + components
- Takes into consideration the relative permeability of the membrane to different ions
- PK = relative permeability of K+
- [K+]o = concentration of K+ outside the membrane
- [K+]i = concentration of K+ inside
32
Use the Nernst equation to:
- What happens to the membrane potential if the membrane is permeable to one ion.
- You can use the Nernst equation to calculate the equilibrium potential of one ion.
- The equilibrium potential is the membrane potential where there will be no net driving force on that one ion.
33
The Goldman equation shows us:
- What happens to the membrane potential when the membrane is permeable to more than one ion.
- It takes into consideration the fact that the membrane is more permeable to some ions more than others.
- You can use the Goldman equation to calculate the membrane potential if you know the concentrations of several ions AND their relative permeabilities.
34
The fact that the resting potential (-65mV) is close to the equilibrium potential for K+ (-75mV) implies that:
1) At rest, the membrane is more permeable to K+ than to other ions.
2) This permeability is the source of the resting membrane potential.
35
Alan Hodgkin and Bernard Katz (1949)
- The squid giant axon
- Change the concentration gradient of an ion (in this case, K+) and see what happens to the membrane potential (Vm).
- Change the concentration of K+ and measure what happens to the membrane potential of the cell.
36
General set-up for recording membrane potentials in a neuron
- axon
- electrode
- wire
- amplifier
- oscilloscope or computer
- ground
37
Hodgkin and Katz results:
- At normal levels of K+,
Vm = -65mV
- and graphs differ because there are more ion channels than just K+ in the cell.
38
When the concentration of K+ inside the cell = the concentration of K+ outside the cell...
What is EK?
What is Vm?
- 0
- 0
39
Experimental results (for Na+):
- axon
- electrode
- wire
- amplifier
- oscilloscope or computer
- ground
- Add Na+
40
When Hodgkin and Katz added more Na+ outside the cell, what was the effect on membrane potential? Why?
- No change, Na+ channels within the cell are closed
41
Conclusion of Hodgkin Katz experiments
- K+ channels open at rest
- Na+ channels closed at rest
42
Summary of Hodgkin and Katz:
The negative resting potential arises because:
1) The membrane of the resting neuron is more permeable to K+ than any of
the other ions present.
2) There is more K+ inside the neuron than outside.
3) At rest, K+ permeable channels are open.
4) At rest, Na+ permeable channels are closed.
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