Lecture 4

Question 1

Membrane potential - what is this?

Answer 1

• All cells have an electrical potential (voltage) difference across their plasma membrane (resting potentials - negative inside, positive outside)
• This Membrane Potential provides the basis of signalling in the nervous system as well as in many other types of cells

Question 2

How to measure membrane potential?

Answer 2

Membrane potential can be measured with a voltmeter, one micro-electrode outside of the cell, one microelectrode inside the cell.

• The graph at -70mV, shows that the inside of the cell in negative to the outside of the cell

Question 3

What are resting potentials?

Answer 3

Membrane potential is the electrical charge that exists across a membrane (e.g. plasma membrane, mitochondrial membrane) and is always expressed as the potential INSIDE the cell relative to the extracellular solution

Membrane potentials are measured in millivolts (mV). I mV = 10-3 V

Animal cells have negative membrane potentials at rest that range from –20 to – 90 mV

Question 4

What are the resting membrane potentials for:

• Cardiac myocytes
• Neurones
• Skeletal muscle myocytes
• Smooth muscle myocytes

(NEED TO LEARN THESE VALUES)

Answer 4

• Cardiac myocytes -80mV
• Neurones -70mV
• Skeletal muscle myocytes -90mV
• Smooth muscle myocytes -50mV

Question 5

How are membranes selectively permeable?

Answer 5

(REMEMBER - JUST CONCERNED ABOUT IONS HERE - AS THEY HAVE CHARGES SO AFFECT THE MEMBRANE POTENTIAL)

1. The phospholipid bilayer
   Hydrophobic interior
   Permeable to small uncharged molecules (O2 , CO2, H2O, ethanol)
   Very impermeable to charged molecules (ions)
2. Ion channels
   Proteins that enable ions to cross cell membranes.
   Channel properties:
   - Selectivity: for one (or a few) ion species. Na⁺, K⁺, Ca²⁺, Cl^{<span>-</span>}, cation channels (cation channels are slightly less seletive)
   - Gating: the pore can open or close by a conformational change in the protein
   - Rapid ion flow: always down the electrochemical gradient

Question 6

Recap (from lecture 2) - ionic concentrations intracellularly (inside the cell) and extracellularly (plasma)

Answer 6

Question 7

Setting up the resting potential… in cells (MOST IMPORTANT ION)

Answer 7

For most cells, open K+ channels dominate the membrane ionic permeability at rest. The resting membrane potential arises because the membrane is selectively to K⁺

1. Concentration gradient for K⁺ moving outsides (from inside to outside of the cell) is SET UP by Na⁺/K⁺-ATPase. This is where 3 Na⁺ move out and 2K⁺ enters the cell
2. K⁺ ions move out of the cell along it’s concentration graident (shown by blue arrow above)
3. Anions (A^-) as left behind in the cell. This creates a membrane potential as the anions are negative and the potassium is positive
4. The membrane potential (electrical gradient) as it develops will act to inhibit the further outward movement of K⁺ as the K⁺ will be pulled back by the negative charge inside the cell, therefore an opposite electrical gradient builds for potassium ions.

Question 8

Equilibrium potential for K⁺

(How to work this out…. using what…)

Answer 8

Use Nernst equation

At equilibrium, the electrical and chemical gradients for K+ balance, so that there is no net driving force on K+ across the membrane:

Left side of the equation: Electrical driving force on K⁺
Right side of the equation: Chemical gradient
When electrical gradient = chemical gradient, there is no net movement of K⁺ ions

Question 9

The nernst equation - learn this…

Answer 9

The Nernst equation allows you to calculate the membrane potential at which K⁺ will be in equilibrium, given the extracellular and intracellular K⁺ concentrations.

• The amount of K⁺ ions that move to set up the voltage is tiny.
• If a membrane is selectively permeable to K⁺ alone, its membrane potential will be at E_K
• You can write the Nernst equation for any ion: e.g. Na+ , Cl- , Ca2+

How to work it out -
E_ion = 61/z log₁₀ [ion]out / [ion]in

z is the valancy e.g. Na¹⁺ this is 1+ , Cl^- this is 1+, Ca²⁺ this is 2+

Step 1: Substitute in values -
E_K = 61/+1 log₁₀ (4.5)/(160)
Step 2: Simplify
EK log₁₀ (0.028125)
Step 3: Work out log
EK = 61 x 1.54299527
Step 4: last bits!
EK = -94.3mM

(REMEMBER TO WRITE THE CHARGE - IF IT IS NEGATIVE OR POSITIVE)

Question 10

The real cell - not just based on the E_k of potassium ion

Answer 10

If the membrane was perfectively selectable for K⁺ only:
- If only potassium was determining the resting membrane potential, it would be -95mV (this is what the Nernst equation would equal)

However, it also allows other ions to leak in and out:

• Na⁺ leaks (moves inwards) down it’s conc gradient. This reduces the size of the negative charge made by K⁺, as sodium is positive
• Ca²⁺ also leaks inwards (conc. gradient inwards). Also reduces the size of the negative charge of the resting potential (charge inside the cell)

These are all voltage sensitive channels

NB: Some cells have other channels in them, e.g. skeletal muscle cells have Cl^- to move into the cell. This will increase the negative charge, inside of the cell:
- Cl^1- : in skeletal muscles, the presence of these channels means the resting potential of the cell is even more negative than -70mV, it is -90mV

Question 11

Explain cardiac muscle and nerve cell resting muscle

Answer 11

• Cardiac muscle: -80mV
• Nerve cells: -70mV
  Resting potential is quite close to E_K
  Not exactly at E_K (less negative): membrane not perfectly selective for K^₊

Question 12

Explain skeletal muscle resting potential

Answer 12

Skeletal muscle: -90mV
Many Cl^- and K⁺ channels open in resting membrane
Close to both ECl and EK (equilibrium potential for Cl^-)

Question 13

Meaning of: perfectively selectable for K⁺

Answer 13

If a membrane is perfectly selective for K⁺, it wouldn’t allow any other ions to affect the resting potential

Question 14

Checkpoint - What do I need to know?

Answer 14

1. Outline what a membrane potential is, how the resting potential of a cell may be measured, and the range of values found
2. Understand the concept of selective permeability, and explain how the selective permeability of cell membranes arises
3. Describe how the resting potential is set up given the distribution of ions across cell membranes.
4. Understand the term equilibrium potential for an ion, and calculate its value from the ionic concentrations on either side of the membrane

Question 15

Recap - overview…

Answer 15

If perfectively soluble for K⁺ , the resting potential wold be -95mV. But this doesn’t often happen, because there is leaking of other ions like Na⁺ and Ca²⁺ which makes the membrane a little less negative than what it would be with just K⁺

Question 16

Changing membrane potentials - what would occur if membrane potential is changed?

Answer 16

1. Action potentials in nerve and muscle cells
2. Triggering and control of muscle contraction
3. Control of secretion of hormones and neurotransmitters
4. Transduction of sensory information into electrical activity by receptors
5. Postsynaptic actions of fast synaptic transmitters

Question 17

Changing membrane potential - what is depolarisation?

Answer 17

A decrease in the size of the membrane potential from its normal value Cell interior becomes less negative e.g. a change from – 70 mV to – 50 mV

Question 18

Changing membrane potential - what is hyperpolarisation?

Answer 18

An increase in the size of the membrane potential from its normal value Cell interior becomes more negative e.g. a change from – 70 mV to – 90 mV

Question 19

Changing membrane ion permeability - (for the 4 types of membrane ions, does this lead to hyperpolarisation or depolarisation?)

Answer 19

• Membrane potentials arise as a result of selective ionic permeability
• Changing the selectivity between ions will change membrane potential
• Increasing membrane permeability to a particular ion moves the membrane potential towards the Equilibrium Potential for that ion

Equilibrium potentials for physiological ions
K⁺ : E_K = - 95 mV
Ca²⁺: E_Ca = + 122 mV

Na⁺ : E_Na = + 70 mV
Cl^- : E_Cl = - 96 mV

• *If the resting potential for a cell is -70mV:**
• > Opening K+ or Cl- channels will cause hyperpolarization (becomes more negative)
• > Opening Na+ or Ca2+ channels will cause depolarization (becomes more positive)

This means:
Thus changes in membrane potential are caused by changes in the activity of ion channels

Question 20

Real cell membranes: Imperfect seletivity
-> need to take in consideration of each ion to the membrane potential…

Answer 20

Membrane potentials are not perfectively selective for one species
Real cell membranes have channels open for >1 type of ion.
The contribution of each ion to the membrane potential will depend on how permeable the membrane is to that ion.
Membrane potential at one moment, depends on the number of open channels for each ion
The real membrane potential, takes into consideration all of the ion permeabilities

Question 21

Controlling channel activity - i.e. types of channel gating (ions moving in or out will affect membrane potential)

Answer 21

Channels can open and close: they are Gated Types of Gating

• *1.** Ligand Gating (ligand binds to the channel)
  a. The channel opens or closes in response to binding of a chemical ligand
  b. e.g. Channels at synapses that respond to extracellular transmitters
  c. Channels that respond to intracellular messengers
• *2. Voltage Gating**
  a. Channel opens or closes in response to changes in membrane potential
  b. e.g. Channels involved in action potentials
• *3. Mechanical Gating**
  a. Channel opens or closes in response to membrane deformation
  b. e.g. Channels in mechanoreceptors: carotid sinus stretch receptors, hair cells
  e. g. stretch of skin, can open or close membrane channels

Question 22

Synaptic transmission - synaptic connections occur between…

Answer 22

Synaptic connections occur between:
nerve cell – nerve cell
nerve cell – muscle cell
nerve cell – gland cell
sensory cell – nerve cell

General for all the above:
At the synapse, a chemical transmitter released from the presynaptic cell binds to receptors on the postsynaptic membrane

We can distinguish Fast and Slow synaptic transmission

Question 23

Fast synaptic transmission - what type of transporter?

Answer 23

In fast synaptic transmission, the receptor protein is also an ion channel

Transmitter binding causes the channel to open

This process is very rapid

e.g. Nicotine acetylcholine receptor

red ligand = Acetylcholine

Question 24

Fast synaptic transmission - channel example… (selectivity, example, how it works)

Answer 24

Example of fast transmission -
At the neuromuscular junction, motor neurone terminals release acetylcholine, ACh. ACh binds to receptors on the muscle membrane

• *Nicotinic Acetylcholine Receptors (can be activated by nicotine, acetyl choline):**
  1. Have an intrinsic ion channel
  2. Opened by binding of acetylcholine
  3. Channel lets Na+, K+, Ca2+ through, but not anions (look at pic - ring of negative charged, so selective for cations)
  4. Moves the membrane potential towards 0 mV, intermediate between E_Na (+70mV) and E_K (-95mV)

Question 25

Slow synaptic transmission - example of type of a receptor

Answer 25

The receptor and channel are separate proteins. Two basic patterns:

1. Direct G-protein gating:
   - Localised
   - Quite rapid (not as rapid as fast - Agonist binds to the receptor. G protein is activated. The G protein migrates in the plane of the membrane to interact with a channel, leading to the channel opening)
2. Gating via an intracellular messenger:
   - Throughout cell (more dispersed)
   - Amplification by cascade (intracellular messenger or protein kinase can go on to activate many channels).
   Binding of agonist to G protein receptor. Activates the G protein. G protein then activates the enzyme. This activated enzyme may start a signalling cascade, this results in the production of an intracellular messanger molecule or activation of protein kinase. These then act on the channel and lead to the channel opening.

Question 26

Excitatory synapse - how does this work?

Answer 26

Excitatory transmitters open ligand-gated channels that cause membrane depolarization

• Can be permeable to Na+ , Ca2+, sometimes cations in general (nAChR)
• The resulting change in membrane potential is called an:
• *Excitatory post-synaptic potential (EPSP) THE POTENTIAL GETS CLOSER TO 0mV**
  1. Longer time course than AP
  2. Graded with amount of transmitter
  3. Transmitters include: Acetylcholine, Glutamate

Question 27

Inhibitory synapse - how does this work?

Answer 27

Inhibitory transmitters open ligand-gated channels that cause hyperpolarization (makes the membrane potential more negative)
Permeable to K⁺ (Ek is -95mV) or Cl^- (ECl is -95mV)

Inhibitory post-synaptic potential (IPSP)

• Moving the membrane potential in this direction moves it away from any threshold of an action potential. It is therefore inhibitory.

Question 28

Factors that can influence membrane potential (changes in ion concentration)

Answer 28

• Most important is extracellular K+ concentration (~4.5 mM normally) - very small changes can have huge consequencies clinicially
• Sometimes altered in clinical situations
• K⁺ can alter membrane excitability (make the membrane more depolarised), e.g. in heart, in nerve tissues

Question 29

Other factors can influence membrane potential…

Answer 29

Electrogenic pumps

E.g. Na⁺/K⁺-ATPase

• Role is to generate the chemical gradient to then set up the resting potential
• Only makes a slight difference to electrifal gradient

HAS A VERY SMALL EFFECT ON MEMBRANE POTENTIAL:
• One positive charge is moved out for each cycle
• In some cells, this contributes a few mV directly to the membrane potential, making it more negative
• Indirectly, active transport of ions is responsible for the entire membrane potential, because it sets up and maintains the ionic gradients (it doesn’t contribute a huge amount to the electrical gradient)

Question 30

What is hyperkalaemia? (cause and effect and clinical links)

Answer 30

• Hyperkalaemia is where there is too much potassium ions in the blood plasma and interstitial fluid, so too little potassium ions in the cell.

• Therefore, as potassium ions have a very negative EK value, this will depolarise the cell
*it is not about physical charges, but electrical charges*

• Using the Nernst equation you can illustrate this as the value of EK at resting potential is -94.3mM (worked this out in another question). Now the extracellular conc of potassium ions has increased to 7.5mM, this means the new EK is -81mM (just illustrating that the cell has become depolarised)
• During an action potential, cells are depolarised. Therefore, this change has made action potentials much more easily occur
• As a result, hyperkalaemia can make cells depolarise inappropriately.

CLINICAL LINKS:
• this can cause irregular heart rhythm (arrhythmia) and increase heart rate

• Explanation of arrhythmia - break it down: ‘rhymia’ means rhythm, ‘a’ in medicine often means irregular, therefore means irregular rhythm

Question 31

What is hypokalaemia? (cause and effect and clinical links)

Answer 31

• Opposite to hyperkalaemia
• Too much potassium ion in the cells lowers the membrane potential inside the cell

LOW LEVEL OF K⁺ IN THE BLOOD

• Makes action potentials more difficult to occur

• CLINICAL LINK: this can lower heart rate and cause arthymia

Question 32

Checkpoint - What do I need to know list?

Answer 32

1. Define depolarization and hyperpolarization, and explain the mechanisms that may lead to each of these
2. Explain how changes in ion channel activity can lead to changes in membrane potential, and outline some of the roles of the membrane potential in signalling within and between cells.
3. Outline how ligand-gated channels can give rise to synaptic potentials.