Flashcards in Membranes and receptors 4 Deck (81):
What is an action potential?
A momentary change in electrical potential across a membrane of a cell, especially of a nerve or muscle cell, that occurs when it is stimulated, resulting in the transmission of an electrical impulse. It occurs when a threshold level of depolarisation has been reached and therefore is an all or nothing state and can be propagated without loss of amplitude.
Stimulation of an action potential depends on what properties of the cell membrane and surrounding solution?
1. The relative permeability of the membrane
2. The ionic gradient across that membrane
What is the effect of reducing extracellular Na+ concentration on the equilibrium potential of sodium? What is that effect on the peak of an AP?
The equilibrium potential becomes more negative. Therefore the peak of the action potential also becomes increasingly negative (smaller peak).
Give the Nernst equation you would use to calculate the equilibrium constant for an ion:
E(i) = (61/Z) x log10 ([ion]o/[ion]i)
What happens to the membrane potential if the conductance for any particular ion is increased?
The membrane potential will move closer to the equilibrium potential of that ion.
What does the conductance of the membrane to a particular ion depend upon?
The number of channels for that ion that are open.
What happens to the conductance of sodium ions once the threshold of depolarisation has been reached and an action potential is triggered?
The conductance of Na+ rapidly increases as more and more Na+ channels open.
In general, what amount of ions need to flow to generate an action potential?
Only a small amount e.g. only ~ 0.4% increase in intracellular axon [Na+] occurs during an action potential
What is voltage-clamping? How does it enable you the investigated the mechanism of action potential generation?
Voltage clamping allows you to control membrane potential and therefore you can measure the currents flowing through the membrane at given membrane potentials. This gives a much clearer measurement of the effect of voltage on the number of Na+ and K+ channels open at different membrane potentials.
What is patch-clamping? How does it enable you the investigated the mechanism of action potential generation?
Patch clamping is a technique that enables currents flowing through individual ion channels to be measured. It allows you to investigate the time-frames of opening and closing of these ion channels during the action potential.
How does using different ionic concentrations enable you to investigate action potential generation?
It enables you to investigate the different contributions of different ions to the action potential.
What is the difference in the opening and closing of Na+ and K+ voltage-gated channels during an action potential?
Na+ channels opening increases rapidly and then decreases rapidly as they become inactivated. K+ channels open more slowly and close more slowly, therefore they do not close immediately upon repolarisation.
Describe the ion channels at the axon hillock. What is their purpose?
There is a high density of voltage-gated Na+ channels, enabling the triggering of action potentials.
What is the basis of the all or nothing characteristic of action potentials?
The positive feedback that occurs once depolarisation to threshold has happened. Beyond this membrane potential Na+ channels open, Na+ enters the cells and depolarises the membrane potential further, so more Na+ channels open.
What determines the threshold value of an action potential?
This is the membrane potential which opens up enough Na+ channels to overcome the hyperpolarising effect of the (non-voltage gated) K+ channels and therefore causes depolarisation which open up more Na+ channels (positive feedback occurs).
What happens during the upstroke of an action potential?
Depolarisation to threshold ->
1. Na+ channels open
2. Na+ enter the cell
3. Membrane depolarises
cycle begins again
What happens during the downstroke of an action potential?
Depolarisation causes ->
1. Voltage-gated K+ channels to open and inactivates Na+ channels
2. K+ efflux starts and Na+ influx stops
What is the role of the Na+-K+ pump in repolarisation of the action potential?
None! It just works in the background to maintain the Na+/K+ gradient.
What is meant by the absolute refractory period?
The period when nearly all Na+ channels are in their inactivated state. Therefore it does not matter how much stimulus is now being received a new action potential will not be generated as there are not enough Na+ channels able to open.
What is meant by the relative refractory period?
Na+ channels are recovering from inactivation, the excitability returns towards normal as the number of channels in the inactivated state decreases. Therefore a new action potential can be triggered in this period.
What is accommodation?
It is a state that occurs after a membrane experiences a long period of stimulus. This causes an increasing number of Na+ channels to be in their inactivated state and consequently a larger depolarisation of the membrane becomes necessary to trigger an action potential (the threshold potential becomes more positive).
What happens to the peak of an action potential when the membrane becomes increasingly accommodated?
Peak decreases in amplitude because there are less Na+ channels available to open and therefore a smaller influx of Na+ into the cell.
How would accommodation occur physiologically?
In an axon which has received prolonged stimuli from excitatory neurotransmitters but none that have depolarised the axon hillock enough to cause an action potential.
What is the basic structure of voltage-gated Na+ channels?
1. 6 transmembrane domains, repeated 4 times
2. Form a functional channel with a pore in the middle
What is the function of transmembrane domain 4 in voltage-gated Na+ channels?
It contains positively charged amino acids which lie in the membrane and detect the voltage field. It causes a voltage-dependent conformation change which opens the pore.
What is the function of the pore/ H5 region?
It is a strip of amino acids which form a beta-sheet that dips into the membrane and contributes to the pore selectivity.
What is the function of the inactivation particle?
It is a region of the channel which covers the pore of the channel when it is in its inactive state.
What is the difference in structure between a Na+ voltage-gated channel and a K+ voltage-gated channel?
Na+ channel is a monomer whereas K+ channel is a tetramer, however each K+ monomer contains the same 6 transmembrane structure as the Na+ repeat unit.
How does patch clamping work?
You touch the membrane, creating a seal around the glass pipette and therefore can measure the current passing though that small part of the membrane which is hopefully just one channel.
How in general do local anaesthetics, such as procaine, work?
All local anaesthetics are membrane stabilizing drugs; they reversibly decrease the rate of depolarization and repolarization of excitable membranes (like nociceptors). Local anesthetic drugs act mainly by inhibiting sodium influx through sodium-specific ion channels in the neuronal cell membrane, in particular the voltage-gated sodium channels. When the influx of sodium is interrupted, an action potential cannot arise and signal conduction is inhibited. The receptor site is thought to be located at the cytoplasmic (inner) portion of the sodium channel.
The hydrophilic pathway of local anaesthetics is use-dependent, what is an important consequence of this?
This is because this pathway only occurs when Na+ channels are open. The more you work on a wound the more Na+ channels will open and therefore the more local anaesthetic you will get into the cell and therefore the more Na+ channels will get blocked. Local anaesthetic has the most afinity for channels in their inactive state.
What are the two pathways for inhibiting Na+ channels that local anaesthetics take?
There are two pathways for local anaesthetics to block Na+ channels: hydrophobic and hydrophilic pathway. The hydrophobic pathway is used by uncharged molecules which can directly diffuse through the phospholipid membrane (from inside or out of the cell) and into the channel. The hydrophilic pathway (thought to be the major pathway) is used by charged anaesthetic molecules INSIDE the cell that use the OPEN (cannot use this pathway when Na+ channels are closed) Na+ channels to bind to the inside of the channels.
What is meant by 'ion trapping'?
Local anaesthetics are weak bases. For the local anaesthetic to diffuse into the cell you want it to be in its uncharged state, unprotonated state. At a pH equal to the pKa of the protonated base, half the base will be in its unprotonated (uncharged state). Once the uncharged base diffuses across the membrane it will once again be at equilibrium with its protonated form. Once inside the cell you want them to be in their protonated state because that traps them inside the cell (cannot diffuse across membrane) and it is in this state that they bind best to the ion channel (using the hydrophilic pathway).
Due to a combination of diameter and myelination, local anaesthetics block neurons in the following order:
1. small myelinated axons
2. un-myelinated axons
3. Large myelinated axons
Name two disease that affect the myelin sheath of neurones in the CNS:
1. Multiple schlerosis
2. Delvic's disease - optic and spinal cord nerves only
Name two disease that affect the myelin sheath of neurones in the peripheral nervous system:
1. Landry-Guillain-Barre syndrome
2. Charcot-Marie-Tooth disease
What are examples of the functions of the A-alpha class of peripheral axon?
1. Sensory fibres from muscle spindles
2. Motor neurones to skeletal muscles
As you go from peripheral axon class A-alpha to C what happens to their diameter and conduction velocity?
Fibre diameter decreases and conduction velocity decreases.
What are examples of the functions of the A-delta class of peripheral axon?
Sensory fibres from pain and temperature receptors (sharp localised pain).
What are examples of the functions of the B class of peripheral axon?
Preganglionic neurones of the autonomic system.
What are examples of the functions of the C class of peripheral axon?
Sensory fibres from pain, temperature and itch receptors (diffuse pain).
How can the conduction velocity of axons be measured?
Extracellular recording of action potentials - induced by electrodes (excitability is induced under the cathode and therefore can be used to trigger an AP) - at different locations along the length of an axon, either:
(i) normal axon -> diphasic recording
(ii) partly damaged axon -> monophasic recording
Why is there such a difference in conduction velocity?
Diameter of the axons and myelination
Why are multiple peaks in membrane potential seen at different time points when measuring a single action potential in a single nerve FIBRE?
Because a nerve fibre comprises several axons with different diameters and some axons transmit signals faster than others. Each peak therefore corresponds to a population of neurones which conduct at different velocities.
What is local current theory?
It is the theory that injection of positive current into an axon (e.g. during an action potential) will spread charge along the axon (as positive-positive charges repel and positive-negative charges attract) and cause an IMMEDIATE LOCAL CHANGE in the membrane potential.
What is meant by the capacitance of a membrane?
It is the ability of a lipid bilayer to store charge/
What is meant by membrane resistance? What factors affect it?
The membrane's opposition to the flow of electrical current. The resistance decreases when more ion channels are opened and vice versa.
Properties of an axon that lead to high conduction velocity include:
1. High membrane resistance
2. Low membrane capacitance
3. A large axon diameter
How does capacitance affect the speed at which the maximal voltage of local currents is acheived?
Increased capacitance, increased time to Vmax.
How does resistance affect the distance that local currents can travel?
As membrane resistance increases the distance that local currents can travel along an axon increases.
What happens to the size of Vmax due to local currents the further away from the propagating AP that you travel?
Vmax decreases (amplitude becomes smaller).
Why does an AP only travel in one direction along an axon?
Because the portion of axon behind the portion actively firing an action potential is in its refractory period - Na+ channels are inactive.
Why is the length constant (lamda - the distance it takes for the relative membrane potential to fall to 37% of its original value, a value that quantifies how far the depolarisation has spread) steeper behind the action potential?
Because voltage-gated K+ channels are still open and therefore there is less resistance to charge and therefore charge can travel less far.
How is myelin sheath formed?
Like a reverse swiss roll - the axon is enveloped by a Scwann cell which rotates around the axon wrapping it in layers and layers of cytoplasmic projections.
Describe the distribution of Na+ channels in myelinated axons:
There are very few Na+ channels under the myelin sheath but a high density at the nodes of Ranvier.
What proportion of axon diameter/ myelin+axon diameter, gives the optimum conduction velocity?
Describe the distribution of Na+ channels in unmyelinated axons:
Na+ channels are evenly distributed along their length.
Describe the mechanism of saltatory conduction
The myelin sheath acts as a good insulator thereby causing the local circuit currents to depolarise the next node above threshold and initiate an action potential. The action potential "jumps" from node to node allowing a much faster conduction velocity.
How does the myelin sheath improve conduction?
1. ~100X increase in membrane resistance
2.~100X decreases in membrane capacitance
Both of these increase lamda (the length constant) and cause a slight decrease in the time constant.
Conduction velocity approximately equals lamda over the time constant.
Where do action potentials occur in saltatory conduction?
Only at the nodes of Ranvier. Underneath the myelin sheath only local spread of depolarisation occurs.
What is the effect of demyelination on an action potential?
Demyelination increases capacitance and decreases resistance in those regions. This can cause failure to reach the threshold for triggering an action potential. it can lead to decreased conduction velocity, complete block or cases where only some action potentials are transmitted.
How are action potentials generated?
By an increase in membrane permeability to Na+ which brings the membrane closer to the Na+ equilibrium potential.
What two things must happen for membrane repolarisation to occur?
1. Inactivation of Na+ voltage-gated channels
2. Activation of K+ voltage-gated channels
The basic structure of voltage-gated Ca2+ channels is the same as what other type of voltage-gated channel?
Local anaesthetics have a higher affinity for Na+ channels in which state?
Inactive state of Na+ channels.
How is conduction velocity measured?
Distance between stimulating electrode and recording electrode/ time gap between the stimulus and registration of AP at the recording electrode.
What is the relationship between the spread of local currents and conduction velocity?
Conduction velocity is determined by how far along the axon these local currents can spread.
Why does a high capacitance cause a decrease in the spread of local current, especially with brief current pulses?
A high capacitance takes more current to charge or a longer time for a given current.
Why does a low resistance cause a decrease in spread of local current?
The lower the resistance the more ion channels are open and the more loss of the local current occurs across the membrane, thus limiting the local current effect.
What cells myelinate axons of the CNS?
Give an example of a myelinated axon:
Give an example of a unmyelinated axon:
C sensory fibre
What is the relationship between axon diameter and conduction velocity in myelinated axons?
Velocity is proportional to diameter.
What is the relationship between axon diameter and conduction velocity in unmyelinated axons?
Velocity is proportional to the square root of diameter.
What is the effect of Tetrodoxin, from a japanese puffer fish?
It blocks voltage-gated Na+ channels and therefore prevents the upstroke of an action potential and therefore an action potential itself form occurring. This causes total paralysis (including respiratory and heart muscles) and therefore death if ingested.
What is the effect of 4-aminopyridine, a substance which blocks K+ channels?
This slows down repolarisation because voltage-gated K+ channels are the main channel causing repolarisation. Repolarisation however will still happen due to leak K+ channels, but there will no longer be a period of hyperpolarisation.
Why do myelinated axons have a faster conduction velocity than unmyelinated axons?
Because local axonal current spreads faster than action potentials along a membrane.
What is the composition of the myelin sheath?
80% lipid and 20% protein - it is mainly the lipid bilayer of the Schwann cells (or oligodendrocytes in the CNS). Cholesterol is an essential component.
What is the difference between longitudinal and membrane electrical resistance?
Longitudinal resistance is related to the axon diameter. The greater the cross sectional area of the axon, πa², the greater the number of paths for the charge to flow through its axoplasm, and the lower the axoplasmic resistance.
Membrane electrical resistance is the resistance across the membrane which correlates with the number of ion channels - the more ions that leak out of the cell the less charge there is to spread along the axon.
Why is the relationship between fibre diameter and conduction velocity a linear relationship in myelinated axons?
Becuase the longitudinal resistance decreases with increased fibre diameter and longitudinal resistance is proportional to conduction velocity.