Session 4 - Electrical Excitability Flashcards
What the events that occur to trigger an AP?
- a membrane will be depolarised up to the threshold value
- this will trigger voltage gated Na channels to open
- Sodium will rush into the cell
- Depolarisation will occur
What happens to re-polarise a cell?
- Sodium channels become inactivated
- Voltage gated Potassium channels open
- Large efflux of Potassium
- Repolarisation occurs
What is the Absolute Refractory Period?
Nearly all of the sodium ion channels are inactivated meaning that an AP cannot be stimulated
What is the Relative Refractory Period?
- Sodium ions are recovering from Inactivation
- An AP can be stimulated and becomes more easy as more sodium channels recover from inactivation
Describe accommodation
The longer the stimulus is, the larger the depolarisation necessary to initiate an AP
(Due to sodium ion channel inactivation)
Describe the molecular features of a voltage gated sodium/calcium ion channel. (They are similar to each other)
- 1 peptide
- 4 homologous repeats
- 6 transmembrane domains
- 1 domain is voltage sensitive in each repeat
- All four repeats come together to form the pore
Describe the molecular features of a voltage gated potassium ion channel
- 4 peptides
- 6 transmembrane domains
- 1 domain is voltage sensitive in each peptide
- function requires four sub-units to come together
How do anaesthetics such as Procaine work?
They bind to and block sodium ion channels, therefore stopping AP generation
In what order to local anaesthetics block conduction in different nerve fibres? Because of this which do they effect first, sensory or motor neurones?
- 1st: Small myelinated axons
- 2nd: Non-myelinated axons
- 3rd: Large myelinated axons
Sensory neurones are effected first
How can conduction velocity be measured?
- electrodes are used to raise membrane potential and generate an AP
- Changes are recorded between a stimulating (-ve) and recording (+ve) electrode
- then; Conduction Velocity = Distance/Time
Explain the local circuit theory of propagation?
- depolarisation of a small region of membrane produces transmembrane currents in neighbouring regions
- Sodium channels are voltage gated more and more open, hence propagating AP
- Further AP spreads faster the conduction velocity
What properties of an axon lead to a high conduction velocity?
- A high membrane resistance
- A high axon diameter
- A low membrane capacitance
How does a high membrane resistance result in a high conduction velocity?
- High resistance = High potential difference across it
- More voltage gated sodium channels will therefore open
- Easier to reach threshold = Easier to fire AP
How does a large axon diameter increase conduction velocity?
- High axon diameter lowers cytoplasmic resistance
- Increasing current therefore AP will travel further
- hence velocity increases
How does a low membrane capacitance increase conduction velocity? (Capacitance is the ability to store charge)
- less membrane capacitance means the cell takes less time to charge
- therefore increasing conduction velocity
