Introduction to excitable cells Flashcards
What functions rely on channels/receptors?
Cell signalling Muscle contraction Immune responses Volume regulation pH balance Fertilisation Electrical signals in neurons and muscles Secretion (hormones, neurotransmitters)
What provide routes for charged molecules to enter phospholipid bilayer membrane?
Ion channels
Membrane transporters
Gap junctions
- Ion channels and transporters have different properties
Characteristics of ion channels
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
What is Ohm’s law?
The relationship among membrane potential (voltage), current and conductance (the inverse of resistance)
Formula for Ohm’s law
I= CV
What is membrane potential?
The difference in electrical potential between the interior and exterior of a biological cell
Resting membrane potential of neuron?
-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
Why do neurons and myocytes have a larger RMP?(-30 to -70mV)
They have a larger number of K+ channels open at rest
Function of neuronal Na+/K+ ATPase? (sodium-potassium pump)
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.
What is a equilibrium/reverse potential for an ion?
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
How can equilibrium/reverse potential (Erev) be calculated?
Nernst equation
Nernst equation formula
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)
What must we do to calculate RMP?
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
cell membrane- phospholipid bilayer
made up of glycerophospholipids that have hydrophilic heads and 2 hydrophobic tails
prevents the movement of charged molecules across the membrane
What does the cell membrane have a high resistance to?
Current
What can diffuse easily through membrane?
Gases (O2, CO2)
Some neurotransmitters
Ampihilic compounds (mostly general anaesthetics)
these can diffuse easily through membrane
What are the 3 types of membrane proteins that allow entry of charged molecules?
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 diffusionTypes of diffusion
Simple (passive)
Facilitated
both travel from high conc to low conc
What is facilitated diffusion?
Occurs via specific transport proteins (“permeases”) which will only allow specific ions/molecules to pass through the membrane
2 forms of active transport
- Primary- utilises energy source (ATP)
2. Secondary- Uses ion gradients made by primary active transport process
3 types of membrane transport proteins
UNIPORTS: move a single molecule
SYMPORTS: move multiple molecules in the same direction
ANTIPORTS: move multiple molecules in opposing directions
Differences between channels and pumps
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
Meaning of depolarised
Moving to a more positive membrane potential
Meaning of polarised
Moving to a more negative membrane potential
How can membrane potential be measured?
Using a microelectrode - includes reference electrode and voltmeter
Meaning of threshold potential?
The amount of potential change required to generate an action potential
What happens when action potential is generated?
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
What happens if threshold potential is reached?
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
What is repolarisation?
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
What is hyperpolarization?
Membrane potential becomes more negative than resting membrane potential
happens as there is an influx of K+ ions leaving cell
What is the refractory period?
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
What is the membrane potential like under normal physiological conditions?
Constant range
What happens to membrane and equilibrium potential if there is an electrolyte balance?
Membrane and equilibrium potential change
What is hypokalemia?
More K+ ions leak out the cell during resting state
so membrane potential becomes more negative (-90mV)
What is hyperkalemia?
elevated plasma (extracellular fluid) K+ concentration Fewer K+ ions move out the cell through leaky potassium channels So membrane potential= less negative
Hyperpolarization
Efflux of positive ions
Depolarisation
Influx of positive ions
Changing intracellular/extracellular K+ concentration effect?
effects resting membrane potential
and therefore ability of neurons and muscle cells to reach action potential threshold
What is hyperkalemia caused by?
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)
What are angiotensin converting enzyme inhibitors and angiotensin II receptor blockers
medicines which widen your blood vessels and so lower your blood pressure
treated for high blood pressure
