Propagation potential and conduction velocity Flashcards
How often are leak channels open?
All the time
How often are gated channels open?
Open in response to particular events e.g. a voltage change
What are some functional elements of voltage-gated ion channels?
Selectivity filter
Voltage sensor
Activation gate
Inactivation gate
What is the selectivity filter for?
Na+ and K+
What is the voltage sensor for ?
Na and K channels
What is the activation gate for?
Na and K channels
What is the inactivation gate for?
Na channel only
What are the different states for a voltage-gated channels for Na+?
- Closed
- Open
- Inactivated
- Closed
What are the different states for a voltage-gated channels for K+?
- Closed
- Open (1ms delay)
- Closed
When do the Na+ channels close?
When the K+ channels open
What is patch clamp recording?
Electrode up against the cell measuring a patch of membrane without entering the cell
How does the electrode measure the membrane without entering the cell?
By applying suction down the membrane to pull some of the membrane against the electrode
What do you use the electrode to do?
Create a small depolarisation
During a patch clamp recording what is the state of the cell at rest?
Na channels closed
K channels closed
During a patch clamp recording what is the state of the cell at on depolarisation immediately
Na channels open (activation)
pNA increases
Em is driven down towards ENa
K channels still closed
During a patch clamp recording what is the state of the cell at depolarisation with a delay?
Na inactivation, gates close Na channels are closed K channels open pK increases Em is driven back to Ek
During a patch clamp recording what is the state of the cell has gone through the method?
Na activation gates closing
Na inactivation gates re-opening
K channels closing
What does the Na+ K+ pump act in the background to do?
Move Na and K ions back to their original places
Is the Na+/K+ essential?
No
A nerve can support about 10,000 action potentials even when the pump is poisoned
What can be seen by recording 2 points along an axon?
Than an action potential (AP) moves away from the point of electrical stimulation (stimulus electrode)
What is an electrotonic currrent/ local current?
Current flowing passively along an axon decays with distance beyond the block
What do cable properties of the membrane dictate?
Decay of this current
What is electrotonic spread?
Detriment and voltage faults
Tells time
What is the stimulus applied to the axon hillock to create an action potential?
Graded potential
What are graded potentials?
A flow of positive charge
Once an action potential occurs in the axon hillock what happens?
A flow of local currents allowing each new part of the membrane to progress along the membrane until the full membrane has been propagated
Why doesn’t the local current flow backwards and create an action potential in the backward direction?
Flows backwards but doesn’t trigger an action potential because the voltage gated channels in the previous segments are in a refractory period meaning they can’t be activated again
Why can’t channels be re-activated once in the refractory period?
Channels closed by ball and chain mechanism
When ball is in the channel the channel is inactive and can’t be triggered again
What is the AP of the membrane at the peak?
Negative inside to the outside
What occurs as a result after the peak of a AP?
A longitudinal electrical gradient inside the axon
What does the longitudinal electrical gradient cause?
An electronic current to flow away from the point of the AP
What happens as a result of the electronic current flowing away into the next segment of the membrane?
The membrane ahead of the AP becomes depolarised
What then happens to this new depolarised part of the membrane?
Reaches threshold and a new AP exists
What is repetition of this process known as?
Action Potential (AP) Propagation
How fast and often do AP potentials occur during AP propagation?
Occur rapidly, so that the AP also conducts rapidly (rates in the range 1-10 m s-1 are typical)
What does the electronic current release?
The potential energy that exists all along the nerve in the form of the electrical gradient and the concentration gradients for Na+ (and later K+)
Which direction does electronic current flow if a point is electrically depolarised using an electrode?
Both directions resulting in an AP propagation along both directions of the axon
What factors affect AP conduction velocity?
The effect of axon diameter
The effect of myelination
What are possible effects of the axon diameter?
Conduction velocity is a function of axon diameter axons present less internal resistance to current flow (before it eventually leaks out)
This is the basis for the high conduction velocities in giant axons
What are the possible effects of myelination?
Conduction velocity also depends on the extent of electrical insulation around the axon. This increases the rate of spread of electrotonic current
This is the basis for the higher conduction velocities in myelinated nerves
Myelin is formed by Schwann cells, which wrap tightly around the axons (the internodes) and leave only small uninsulated patches (nodes of Ranvier)
What does the salutatory mechanism rely on?
Voltage-gated channels at the nodes
Where are the density channels highest?
Density channels at nodes is extremely high (3000 micrometers2) compared with an unmyelinated nerve (300micrometers2)
What does high density also increase?
Conduction velocity since the membrane comes to threshold a greater rate
Why is there no voltage-gated channels along the internode?
They are not involved in AP production
What does the density of voltage-gated channels in a patch of membrane determine?
Electrical excitability
What is the membrane of an unmyelinated axon?
Electrically excitable
What are patches with even greater densities of channels?
Particularly sensitive to electrotonic currents
What is the membrane of an internode?
Electrically inexcitable and cannot transmit an AP19
Where are extremely sensitive patches with high densities also found?
Where an AP is initiated naturally in a neurone
E.g. Axon hillock or spike initation zone (SIZ)
Sensory ending of a mechano-sensory neurone
What are the dendritic membrane and soma of nerve cells?
Electrically inexcitable
Site where decision is made whether or not to initiate an AP
What type of events are APs?
Discrete
What does an AP represent?
An AP represents one pulse in a frequency code, which is very effective mechanism for transmitting information, and is resistant to perturbation (noise)
How is information contained in an AP?
Both frequency and the pattern of activity
