Conduction Of The Nerve Impulse Flashcards
At which electrode is the action potential initiated under experimental conditions: anode or cathode?
- CATHODE (negatively charged)
- Excitability will be reduced under an anode as it is positively charged
What is the length constant, λ?
Distance taken for the potential to fall to 37% of its original value
Describe how an action potential is conducted to adjacent areas of the axon
- Change in membrane potential at one point forms a LOCAL CURRENT
- When local current spreads it causes DEPOLARISATION of the adjacent part of the axon to threshold, which initiates the action potential along the axon
What is conduction velocity determined by?
- HOW FAR ALONG THE AXON THE LOCAL CURRENT SPREADS
- The further the local current spreads, the faster the conduction velocity
List 3 properties of an axon which give a high conduction velocity
- High membrane resistance
- Low membrane capacitance
- Large axon diameter (low cytoplasmic resistance)
How does a large axon diameter contribute to a faster conduction velocity?
- Leads to a low cytoplasmic resistance
- In comparison the resistance of the membrane is higher, therefore capacitance is lower, which contributes to a faster conduction velocity
How do you calculate the conduction velocity of an axon using electrodes?
- v = d/t where:
- d is the DISTANCE between the stimulating and recording electrode
- t is the TIME taken from stimulating the action potential from one electrode to recording the action potential at the other electrode
Describe how the membrane resistance can affect the spread of a local current
- Membrane resistance depends on the NO. OF OPEN ION CHANNELS
- Lower the resistance, more channels open so increases loss of local current
- Higher resistance means more channels close so allows spread of the local current to depolarise and inactivate channels
Define “capacitance” and state how it relates to the spread of a local current
- Capacitance is the ABILITY TO STORE CHARGE from a current
- High capacitance takes more current to charge (or a longer time for a given current)
- Causes a decrease in the spread of the local current
What is meant by ‘propagation of an action potential’ and how is this achieved?
- Spread of an action potential along the axon
- Achieved by the spread of local currents along the axon, causing depolarisation at adjacent nodal regions above the threshold, which generates an action potential
What is the relationship between the length constant, λ and the speed of conduction?
- Directly proportional
- The LONGER the length constant, the FASTER the conduction velocity
Which cells myelinate axons in the CNS?
Oligodendrocytes
Which cells are responsible for the myelination of peripheral nerves?
Schwann cells
What is the effect of myelination on the conduction velocity of an action potential?
- DECREASES CAPACITANCE
- INCREASES RESISTANCE
- Therefore the conduction velocity is INCREASED
Explain how myelination aids in propagating an action potential in a saltatory manner
- Reduced capacitance in intermodal region due to myelin results in an action potential being induced at a NODE OF RANVIER
- This can spread via local currents to adjacent nodes down the axon and depolarise the membrane potential above the threshold, thereby generating an action potential further down the axon
