Changing Membrane Potential Flashcards
Give 3 examples of how changing membrane potentials leads to signalling between and within cells
- Action potentials in nerve and muscle cells (e.g. knee-jerk reaction)
- Triggering and control of muscle contraction
- Control of hormone and neurotransmitter secretion
(4. Transduction of sensory information into electrical activity by receptors (ie photoreceptors in the eye and sensory receptors in the ear)
5. Postsynaptic actions of fast synaptic transmitters)
Depolarisation
Decrease in size of membrane potential from it’s normal value
Cell interior = less negative (more positive)
- Action Potential upstroke in graph
Eg -70 to -50
Hyperpolarisation
Increase in size of membrane potential from its normal value
Cell interior becomes more negative
Eg -70 to -90
What is meant by repolarisation?
What shouldn’t it be confused with?
- Membrane potential returns to resting membrane potential
- Not to be confused with hyperpolarisation
(Hyper, the graph goes below resting)
Identify the points of repolarisation in the graph
- Turquoise arrow
- Blue dot
- As the membrane potential is returning to resting
Why do membrane potentials arise?
- As a result of selective ionic permeability within the membrane ( SIP)
How can we change membrane potential?
By changing ion selectivity
Changing permeability of an ion
What happens if you increase the membrane permeability to a particular ion in regards to the equilibrium potential for that ion?
Moves the membrane potential towards the equilibrium potential for that ion
State the equilibrium potentials for K+ Na+ Cl- and Ca2+
What causes hyperpolarisation?
Opening K+
K+ leaks out
What causes depolarisation?
Opening Na+ or Ca2+ channels
What effect does opening Cl- channels have on the membrane potential?
- In some cells, opening chloride channels can be depolarising
- In other cells it can be hyper polarising
- It all depends on what Ecl (equilibrium potential of chloride) is in those cells
What equation outlines the imperfect selectivity of cell membranes?
GHK (Goldman-Hodgkin-Katz) equation
(Real cell membranes have channels open for more than 1 type of ion)
(How permeable the membrane is to that ion depends on the number of open channels for each ion)
Give an example of some ion channels that are less selective
- At the neuromuscular junction, motor neurone nerve endings release acetylcholine (ACh).
- Acetylcholine binds to receptors on the muscle membrane
- These are Nicotinic acetylcholine receptors
(These are ligand gated ion channels and they are pentameric (5 subunits))
Ligand = Acetylcholine
There are 2 alpha subunits and they have binding sites for Ach
Binding of 2 Ach to alpha subuntis = cause the channel open
-ve pore within channels so only allows cations through (Na+, Ca2+ and K+) down their electrochemical gradients
When these channels are open, they drive the membrane potential towards 0mV (between equilibriums for Na+, Ca2+and K+, all the ions that pass through it) —> This is called the reversal potential for this channel
When these channels open, they will cause a depolarisation because 0mV is a lot more positive than the resting membrane potential
Summarise the properties of Nicotinic acetylcholine receptors
- Nicotinic acetylcholine receptors Have an intrinsic ion channel
- Opened by binding of acetylcholine causing a conformational change
- Channel lets Na+, Ca2+ and K+ through, but not anions due to the negative charge of the pore
- Moves the membrane potential towards 0 mV, intermediate between ENa, ECa and EK
How is channel activity controlled?
Channels can open and close - they are gated
3 Types of gating:
Ligand gating
Voltage gating
Mechanical gating
Describe Ligand gating channels (1)
Give 2 examples of ligand gating
- The channels Opens/closes in response to binding of a chemical ligand
- E.g. Channels at synapses that respond to extracellular transmitters (eg Ach)
- Channels that respond to intracellular messangers like calcium
Describe Voltage gated channels
Give an example
- Channels open/close in response to changes in membrane potential
- E.g. Channels involved in action potentials
Describe Mechanical gating
Give an example
- Channels that Open/close in response to membrane deformation e.g when cells swell
- E.g. channels in Mechanoreceptors (carotid sinus stretch receptor/hair cells)
- Tend to be involved in volume regulation
Describe Synaptic trasnmission (5)
- At the synapse, a chemical transmitter is released from presynaptic cell/bouton
- The transmitter diffuses in the synaptic cleft
- It then binds to receptors on post synaptic membrane
- Those receptors are gated by the neurotransmitter
- Can be FAST or SLOW
What do chemical synapses occur between? / Where can Synaptic connections occur between?
Nerve cell - nerve cell
Nerve cell - muscle cell (e.g. at neuromuscular junction)
Nerve cell - gland cell
Nerve cell and sensory cell (e.g. photoreceptor cell)
Describe Fast synaptic transmission (5)
Give an example
- Transmission that occurs quickly with a very short delay, because
- The receptor protein is also an ion channel
- Transmitter binding causes channel to open
- And you get ions flowing across
- Can be excitatory or inhibitory
Nicotinic Ach receptor
Name the 2 different classes of fast synaptic transmission
- Excitatory
- Inhibitory
Describe Excitatory synapses (4)
- Excitatory synapses are where you have ligand-gated channels that open to cause membrane depolarisation
- They can be permeable to Na+, Ca2+ or general cations
- ‘Excitatory’ Because it is moving the membrane potential closer to the threshold for firing an action potential
- Whether it is excitatory or inhibitory depends on the receptor
What makes synapses either inhibitory or excitatory?
The receptors
Facts about excitory synapses (2)
Longer time course than action potential
Graded with amount of transmitter- more transmitter = more depolarisation (more = more channels)
Examples of Excitory transmitters/receptors found in excitatory synapses (2)
- Nicotinic Acetylcholine receptors
- Some Glutamate receptors
Describe Inhibitory synapses (3)
- Where you have ligand gated channels that open to cause hyperpolarisation
- K+ or Cl- permeable
(Inhibitory because membrane potential is being taken further away from action potential threshold potential/cell is less likely to fire an action potential)
Name 2 receptors found in inhibitory synapses
- Glycine receptors
- Gamma aminobuytyric acid A receptors (GABA-A receptors)
What is change in membrane potential called in excitory synapse?/What do you call the resulting depolarisation of membrane potential in an excitatory transmitter?
What is it called in inhibitory?
Excitory post synaptic potential (EPSP)
Inhibitory post synaptic potential (IPSP)
Describe Slow synaptic transmission
- Receptor and channel are seperate proteins
Describe the 2 basic patterns that slow synaptic transmission follows
1) Direct G-protein gating
- Localised
- Quite rapid
2) Gating via an intracellular messanger or protein kinase - e.g slowing the heart rate by opening a potassium channel
- Throughout cell
- Amplification by cascade
Describe Direct G protein gating (G coupled receptor) (2)
What happens in it?
Localised, quite rapid
G protein activated and migrates to interact with channel
Describe the steps involved in Gating via intracellular messanger
- Neurotransmitter binds to g protein coupled receptor - Activates and releases G protein - Activates enzyme - Signalling cascade - Intracellular messanger or protein kinase produced and acts as a ligand to open/close the channel
Throughout cell, amplification by cascade
In synapses, what causes a change in membrane potential?
- Opening of an ion channel
Name 2 Factors that influence membrane potential
1) Changes in ion concentration
2) Electrogenic pumps - e.g. Na+ K+ ATPase
Describe how changes in ion concentration can influence membrane potential
- The most important change in ion concentration is a change in extracellular K+ conc.
- It is usually about 4.5 mM
- But it sometimes gets altered in clinical situations
- Can alter membrane excitability in the heart and lead to arrythmias
Describe how electrogenic pumps can influence membrane potential
Electrogenic pumps - Na+ K+ ATPase (sets up and maintains ionic gradients)
For each cycle:
- 3 Na+ pumped out of the cell
- 2 K+ pumped into the cell
- These ions are each being pumped against their electrochemical gradients
- That requires energy
- That energy comes from hydrolysing ATP
- Active transport of ions maintains ionic gradients
- One positive charge is moved out for each cycle
- That generates and electrical current and In some cells, this contributes a few mV directly to the membrane potential, making it more negative
(3, NA, get Out)
