L5 Action potentials Flashcards
a) What is an action potential?
b) What are the properties of an action potential?
a) The change in voltage/ electrical potential across a membrane
b) - Depend on ionic gradients and relative permeability
- Only occur if a threshold level is reached
- Are propagated without loss of amplitude
a) If the conductance to an ion is increased what happens to the membrane potential?
b) What is conductance dependent on?
a) The membrane potential will move closer to the equilibrium potential for that ion
b) The number of channels for the ion that are open
Explain how the ionic permeability of the membrane alters with time and how this generates the action potential
- The resting membrane of a cell is about -70mV (near to eqm potential of K+)
- Threshold potential is reached and Na+ channels open causing Na+ ions to move in and hence the membrane potential moves closer to Eqm potential of Na (less negative i.e. depolarisation)
- The na+ channels then become inactivated and K+ channels open causing K+ to move out and the membrane potential moves closer to the eqm potential of k+ becomes more negative i.e. repolarisation then hyperpolarisation
Explain the term threshold potential
- Threshold potential is the critical level to which a membrane potential must be depolarised to initiate an action potential
- Once this membrane potential is reached, a positive feedback occurs as Na+ channels begin to open
Depolarisation is followed by repolarisation.
What happens during this process?
- Potassium channels open causing an efflux of K+
- Sodium channels inactivate stopping the influx of Na+
All or nothing principle?
- no action potential is generated unless the threshold is reached
- when it is reached every action potential will reach the same peak membrane potential (40mV) no matter how strong the stimulus
What is the difference between the a)Absolute refractory period and the b)Relative refractory period?
a) ARP: Period immediately following a fire and no new AP can be generated no matter the stimulus
- nearly all Na+ channels are in the inactivated state
b) RRP: contains recovering Na+ channels and voltage gated K+ channels begin to close
- it is possible to induce an AP but it has to be a very large stimulus to depolarise
Outline the basic structure of a voltage-gated Na+ channel
- contains only one α subunit
- the α has 4 similar sections or repeats (I-IV)
- each containing six membrane-spanning segments, labelled S1 through S6.
- the S4 segment acts as the channel’s voltage sensor.
- The voltage sensitivity of this channel is due to positive amino acids located at every third position.
- when there is a change in transmembrane voltage, this segment moves toward the extracellular side of the cell membrane, allowing the channel to become permeable to ions. (open)
- There is a pore region and the inactivation particle will go into the pore to stop movement of ions –> inactivation
Outline the basic structure of a voltage-gates K+ channel
- one α subunit is 1/4 of a channel (i.e. only 1/4 of the na+)
- a channel consists of 4 individual α subunits
- each α subunit consists of 6 membrane-spanning domains
- s4 voltage sensing region also detects the voltage and opens or closes the channel pore
- no inactivation particle
How to local anaesthetics work ?
- Most local anaesthetics are weak bases and cross the membrane in their unionised form (membrane permeable)
- They bing to and block Na+ channels easily when the channel is open
- They also have a higher affinity for the inactivated state of the Na+ channel
- Stopping action potential generation
a) Examples of local anaesthetics?
b) Order of blocking for local anaesthetics?
a) Lidocaine
b)
1. Small myelinated axons
2. un-myelinated axons
3. large myelinated axons
What is electrical stimulation and how is it done?
- Electrical stimulation is used to stimulate an axon / group of axons to threshold potential, thus intiating an action potential
- Stimulation occurs under a cathode (negatively charged)
How does one calculate conduction velocity?
Conduction velocity = distance / time
How is an action potential conducted along an axon?
- Local current theory
- A change in membrane potential in one part can spread to adjacent areas of the axon by local currents
- When local current spread causes depolarisation of part of the axon to threshold potential then an action potential is initiated in that location
a) What is membrane capacitance?
b) What is membrane resistance?
a) The ability to store charge
b) It is a function of the number of open ion channels
- lower resistance - more ion channels open
- higher resistance - less ion channels open
What factors increase conduction velocity of an AP and explain why?
- Large axon diameter
- less resistance to ION flow (they have more space to travel and less likely to bump into something) - LOW membrane capacitance
- a high capacitance takes more current to charge (voltage changes more slowly) and would cause a decrease in local current spread - HIGH MEMBRANE resistance
- there would be less ion channels open and so less local currents would be lost across the membrane and the change in voltage would be able to spread further along the axon - Myelination
- AP jumps from node to node
- increases memb resistance and decreases memb capacitance
How does myelination affect the conduction velocity of a neurone?
- Myelinated axon has faster conduction velocity:
- -> axon surrounded by myelin sheath (comprised of schwann cells)
- -> sheath is electrically insulating, it increases the speed of transmission of AP
- -> there are gaps called nodes of ranvier which have a high density of Na+ channels where the AP is propagated by saltatory conduction: Local circuit currents depolarise the next node above threshold and initiate an AP
Identify the cells which form myelin and describe their actions
- Schwann cells myelinate peripheral axons
- Oligodendrocytes myelinate axons in the CNS
a) What is demyelination?
b) What diseases cause this in the:
i) Central nervous system
ii) Peripheral nervous system
a) Breakdown or damage of the myelin sheath
b)
i) Multiple sclerosis (all CNS nerves) and Devic’s disease (optic and spinal cord nerves only)
ii) Landry-guillain-barre syndrome and charcot-marie-tooth disease
Explain the affect that demyelination has on conduction velocity?
- Demyelination causes inefficient saltatory conduction as depolarisation must also occur at the gaps in the myelinated sheath, decreasing the speed of impulse transmission.
- failure to reach threshold
In terms of myelination & conduction velocity, describe what occurs in multiple sclerosis?
Multiple sclerosis is an autoimmune disease wherein the myelin is destroyed in certain areas of the CNS, leading to decreased conduction velocity and/or the complete block of action potentials
What is the relationship with fibre diameter and conduction velocity?
- Myelinated fibres: velocity proportional to diameter
- Unmyelinated fibres: velocity proportional diameter1/2
What is the Neuromuscular Junction?
The synapse between a nerve and a skeletal muscle fibre
Outline the steps that occur at the nerve terminal (the presynaptic neurone).
- An AP arrives at the nerve terminal
- AP causes voltage-gated Ca2+ channels to open
- Ca2+ enters the nerve terminal down conc gradient
- Increased concentration of Ca2+ inside
- stimulates neurotransmitter vesicles to move towards and fuse with the presynaptic membrane
- Neurotransmitter (e.g. Ach) released from vesicles via exocytosis
