Session 4- AP and its properties Flashcards
What effect on Vm does an increase in conductance of a particular ion have?
The Vm will move closer to the membrane potential for that ion
What happens to Na+ conductance during the upstroke?
It increases
What happens to K+ conductance during depolarisation?
In increases
What is the use of a voltage clamp?
It prevents Vm from changing in response to changes in membrane current, so allows ionic currents to be measured.
What is the threshold potential?
The level to which Vm must be increased in cause an action potential
What is the axon hillock?
Area where AP is initiated from synaptic inputs
What is the “all or nothing response”?
If threshold potential is exceeded then an AP will be completely generated regardless of starting voltage. This is due to +ve feedback on voltage gated Na+ channels
What happens during the upstroke?
V-gated Na+ channels open
Na+ enters cell
Membrane depolarises
More Na+ channels open (+ve feedback)
What happens during the downstroke?
Na+ channels are inactivated K+ open Efflux of K+ Na+ influx stops Repolarisation
What is the absolute refractory period?
Most (nearly all) Na+ channels are inactivated. Another AP cannot occur during the ARP
What is the relative refractory period?
Na+ channels are recovering and the electrical excitability of the membrane returns to normal as fewer Na+ channels are inactivated
What is accommodation?
The longer the stimulus, the larger the depolarisation needed to initiate an AP - shown by the threshold curve
What is the function of the S4 region of K+ and Na+ channels?
Voltage sensitivity
What is the function of the P region of Na+ and K+ channels?
Contributes to ionic selectivity
What is the MoA of local anaesthetics such as procaine?
Blocks Na+ channels, preventing Na+ influx, so preventing an AP from being generated
How can the conduction velocity of axons be measured?
By recording changes in potential between the stimulating (Cathode –‘ve) and recording (Anode, +’ve) electrodes along an axon, conduction velocity can be calculated using the equation Conduction Velocity = Distance/Time
What is the local current theory?
Depolarisation of a small region of membrane produces a local current which then propagates by activating nearby v-gated Na+ channels causing the propagation of an AP
What factors affect the propagation of the current?
Membrane resistance
Axon diameter (so membrane area)
Membrane capacitance
What is the effect of myelination on conduction velocity?
Increased resistance, and decreased capacitance so increases conduction velocity
What is saltatory conduction?
AP jumps from one node of ranvier to the next; the mechanism of AP conduction in myelinated neurones
What are some examples of myelinated neurones?
Motor neurones
What are some examples of unmyelinated neurones?
Small diameter sensory fibers e.g C-type neurones
What are some examples of demyelinating disease?
Multiple Sclerosis : autoimmune demyelination in some areas of the CNS
Guillain-Barre syndrome = peripheral nerve autoimmune demyelination, usually triggered by an infection
