Introduction to excitable cells

Question 1

What functions rely on channels/receptors?

Answer 1

Cell signalling
Muscle contraction
Immune responses
Volume regulation
pH balance 
Fertilisation
Electrical signals in neurons and muscles
Secretion (hormones, neurotransmitters)

Question 2

What provide routes for charged molecules to enter phospholipid bilayer membrane?

Answer 2

Ion channels
Membrane transporters
Gap junctions
- Ion channels and transporters have different properties

Question 3

Characteristics of ion channels

Answer 3

Selectivity- each ion channel will have a different number and type of ion they’ll allow to pass through
Gating- can be mechanically/ligand/voltage gated ion channels
Some channels can also prevent flow of ions into channle

Question 4

What is Ohm’s law?

Answer 4

The relationship among membrane potential (voltage), current and conductance (the inverse of resistance)

Question 5

Formula for Ohm’s law

Answer 5

I= CV

Question 6

What is membrane potential?

Answer 6

The difference in electrical potential between the interior and exterior of a biological cell

Question 7

Resting membrane potential of neuron?

Answer 7

-70mV
Polarised
The resting potential is negative due to the accumulation of more sodium ions outside the cell than potassium ions inside the cell.
Potassium ions diffuse out of the cell at a much faster rate than sodium ions diffuse into the cell because neurons have many more potassium leakage channels than sodium leakage channels.
Sodium-potassium pumps move two potassium ions inside the cell as three sodium ions are pumped out to maintain the negatively-charged membrane inside the cell; this helps maintain the resting potential

Question 8

Why do neurons and myocytes have a larger RMP?(-30 to -70mV)

Answer 8

They have a larger number of K+ channels open at rest

Question 9

Function of neuronal Na+/K+ ATPase? (sodium-potassium pump)

Answer 9

To establish concentration gradients for Na+ and K+ that are needed to generate resting, graded and action potentials
3 sodium out for every 2 potassium pumped in
Uses ATP
Only mildly electrogenic: the net result of the actions of Na+/K+ ATPase=Vm of ~-5 to -12 mV.

Question 10

What is a equilibrium/reverse potential for an ion?

Answer 10

The membrane potential where the net flow of specific ions through an open channel across the cell membrane is zero (when equilibrium potential across CM is 0mV)- so basically membrane potential when there are no ions flowing through ion channel
When electrical and chemical forces are equal
E.g in mammalian neurons, the equilibrium potential for Na is +60 mV, while that of K is -88mV

Question 11

How can equilibrium/reverse potential (Erev) be calculated?

Answer 11

Nernst equation

Question 12

Nernst equation formula

Answer 12

E of ion= RT/ZF x ln [ion outside]/[ion inside]
R = gas constant
T = temperature (in Kelvins)
z = ion charge
F = Faraday's constant (96500J)

Question 13

What must we do to calculate RMP?

Answer 13

include RELATIVE contribution of each channel type, which is expressed in terms of permeability (P)
Resting membrane potential will be close in value to the reversal potential for the ion that carries the majority of the resting current

Question 14

cell membrane- phospholipid bilayer

Answer 14

made up of glycerophospholipids that have hydrophilic heads and 2 hydrophobic tails
prevents the movement of charged molecules across the membrane

Question 15

What does the cell membrane have a high resistance to?

Answer 15

Current

Question 16

What can diffuse easily through membrane?

Answer 16

Gases (O2, CO2)
Some neurotransmitters
Ampihilic compounds (mostly general anaesthetics)
these can diffuse easily through membrane

Question 17

What are the 3 types of membrane proteins that allow entry of charged molecules?

Answer 17

gap junctions: large pores that form between two adjacent cells and can pass ions and small molecules (including ATP)
electrical synapses are a specialized form of gap junctions
membrane transporters ("pumps"): integral membrane proteins that mediate either facilitated diffusion

Question 18

Types of diffusion

Answer 18

Simple (passive)
Facilitated
both travel from high conc to low conc

Question 19

What is facilitated diffusion?

Answer 19

Occurs via specific transport proteins (“permeases”) which will only allow specific ions/molecules to pass through the membrane

Question 20

2 forms of active transport

Answer 20

1. Primary- utilises energy source (ATP)

2. Secondary- Uses ion gradients made by primary active transport process

Question 21

3 types of membrane transport proteins

Answer 21

UNIPORTS: move a single molecule
SYMPORTS: move multiple molecules in the same direction
ANTIPORTS: move multiple molecules in opposing directions

Question 22

Differences between channels and pumps

Answer 22

1.Channels= direct connection between intracellular and extracellular spaces
Pumps= no
2. Channels= move small molecules (ions,water)
Pumps= big molecules (glucose)
3. Channels= move charged molecules down conc gradient
Pumps= can move molecules against conc gradient (AT)
4. Channels= conc gradients are dissipated (wasted)
Pumps= build up conc gradients
5. Channels= v high speed
Pumps= slower

Question 23

Meaning of depolarised

Answer 23

Moving to a more positive membrane potential

Question 24

Meaning of polarised

Answer 24

Moving to a more negative membrane potential

Question 25

How can membrane potential be measured?

Answer 25

Using a microelectrode - includes reference electrode and voltmeter

Question 26

Meaning of threshold potential?

Answer 26

The amount of potential change required to generate an action potential

Question 27

What happens when action potential is generated?

Answer 27

The nerve endings are stimulated by a change in the environment(stimulus)
Results in the opening of sodium channels
Sodium ions move into the cell=membrane is depolarized
membrane potential changes from -70mV to +40mV

Question 28

What happens if threshold potential is reached?

Answer 28

A stimulus opens some sodium ion channels(before threshold)=decrease the negativity inside the cell slightly
If the threshold potential is reached, all sodium channels open=more sodium ions move into the cell

Question 29

What is repolarisation?

Answer 29

Happens after depolarisation
Once an action potential is generated and the cell membrane is depolarized= the sodium channels close and the potassium channels open
This allows the potassium ions to move out of the cell= recreates negativity inside the cell
The cell membrane is repolarized

Question 30

What is hyperpolarization?

Answer 30

Membrane potential becomes more negative than resting membrane potential
happens as there is an influx of K+ ions leaving cell

Question 31

What is the refractory period?

Answer 31

During hyperpolarisation
New action potential can’t be regenerated
The cell membrane retains its polarized state by the sodium-potassium pump
Sodium ions, which moved into the cell during the action potential, are expelled against the concentration gradient= move out of cell
Similarly, potassium ions are moved into the cell. The cell membrane regains the potential of -70 mV.

If another stimulus arrives=membrane will be depolarized again, and action potential will be generated

Question 32

What is the membrane potential like under normal physiological conditions?

Answer 32

Constant range

Question 33

What happens to membrane and equilibrium potential if there is an electrolyte balance?

Answer 33

Membrane and equilibrium potential change

Question 34

What is hypokalemia?

Answer 34

More K+ ions leak out the cell during resting state

so membrane potential becomes more negative (-90mV)

Question 35

What is hyperkalemia?

Answer 35

elevated plasma (extracellular fluid) K+ concentration
Fewer K+ ions move out the cell through leaky potassium channels
So membrane potential= less negative

Question 36

Hyperpolarization

Answer 36

Efflux of positive ions

Question 37

Depolarisation

Answer 37

Influx of positive ions

Question 38

Changing intracellular/extracellular K+ concentration effect?

Answer 38

effects resting membrane potential

and therefore ability of neurons and muscle cells to reach action potential threshold

Question 39

What is hyperkalemia caused by?

Answer 39

increased intake
decreased renal elimination
renal failure
adrenal disease (alters the function of hormones that regulate kidney function)
medications that alter kidney function
angiotensin-converting enzyme (ACE) inhibitors
angiotensin II receptor blockers (ARBs)
potassium-sparing diuretics
non-steroidal anti-inflammatory drugs (NSAIDs)
increased release from intracellular stores due to tissue damage (trauma, burns, surgery, rhabdomyolysis, cancer treatment)

Question 40

What are angiotensin converting enzyme inhibitors and angiotensin II receptor blockers

Answer 40

medicines which widen your blood vessels and so lower your blood pressure
treated for high blood pressure