MR S4 - Electrical Excitability Flashcards
(22 cards)
How is sodium involved in action potentials?
Once membrane potential reaches threshold, voltage gated Na channels open
Influx of Na ions as they attempt to reach their equilibrium potential of +61mV
This further depolarises membrane, triggering more Na channels to open
Briefly describe an action potential
A change in voltage across a membrane
Depends on ionic gradients and the relative permeability of a membrane
Binary (all or nothing/1 or 0)
Propagated without loss of amplitude
How does depolarisation come about?
Combination of:
Na channels close by inactivation
Voltage gated K channels open in depolarisation, allowing an efflux of K ions as they try to reach their equilibrium potential of -88mV
How are action potentials “all or nothing”?
The Na channels that cause depolarisation are voltage gated
Therefore positive feedback occurs and more channels open until all channels are either open or inactive
This means depolarisation cannot stop part-way, so the action potential is “all or nothing”
What is the absolute refractory period?
When nearly all Na channels are in the inactive state
Excitability is at 0
What is the relative refractory period?
When Na channels are recovering from inactivation
Excitability returns to normal as the number of channels in the inactive state decreases
What is accommodation?
The longer the stimulus is, the larger the depolarisation necessary to initiate an action potential
Occurs because Na channels become inactivated and are in the absolute refractory period
Describe the molecular structure and properties of Na and Ca voltage gated channels
Function of channel requires one subunit
Main pore-forming subunit is one peptide of four homologous repeats
Each repeat consists of six transmembrane domains
One of these detects voltage
Describe the structure and molecular properties of a K voltage gated channel
Functional channel requires four subunits
Four homologous polypeptide subunits
Each subunit has six transmembrane domains
One of these detects voltage
What is the main difference between Na and K voltage gated channels?
Hint: structural
K channels have their four homologous repeats as separate peptide subunits
Na channels have their four homologous repeats in one subunit
Describe the action of local anaesthetics and give an example
Eg procaine
Act by binding to and blocking the action of Na channels, preventing action potential generation
Weak bases so pass through membranes unionised
Block open channels
Have higher affinity to inactive channels
In what order of nerves do local anaesthetics block conduction? What does this mean physiologically?
Small myelinated axons
Non myelinated axons
Large myelinated axons
This means sensory neurones are affected before motor
How may conduction velocity be measured?
Electrodes are used to raise the membrane potential to threshold
Record changes between the stimulating and recording electrodes
Use conductance velocity= distance/time
How are impulses propagated locally?
The local circuit theory of propagation
Depolarisation of a small area of membrane produces transmembrane currents to neighbouring regions
Na channels are voltage gated so these open in those regions
What factors affect conduction velocity?
Membrane resistance
Axon diameter
Membrane capacitance
Cytoplasmic resistance
How does membrane resistance affect action potential conduction velocity?
Ohms law, V=IR states that the higher the resistance, the greater the potential difference across it
Greater potential difference means more Na channels open
So easier to reach threshold for AP
Therefore conduction velocity is increased
How does axon diameter affect action potential conduction velocity?
Ohms law, I=V/R states that the lower the resistance (cytoplasmic),the larger the current
Therefore AP travels further
Therefore conduction velocity is increased
How does membrane capacitance affect action potential conduction velocity?
Capacitance is the ability to store charge
So a membrane with low capacitance will take less time to charge
Therefore low capacitance increases conduction speed
How does axon myelination affect action potential conduction velocity?
Myelination reduces capacitance and increases membrane resistance
Both of these increase conduction velocity
Describe saltatory conduction
Where the AP “jumps” between nodes of Ranvier
Myelin sheath is a very good insulator so cause local conduction circuits to depolarise the next node to threshold
Nodes have very high Na channel density
Describe the distribution of myelination
Motor neurones are usually larger so tend to be myelinated
Sensory neurones are commonly smaller so aren’t normally myelinated
Schwann cells myelinate peripheral axons
Oligodendrocytes myelinate axons in the CNS
Describe the consequences of demyelination?
Several diseases demyelinate axons eg MS
Demyelination can lead to fewer, slower or completely blocked action potentials
