The action potential Flashcards
What is an AP?
Transient impulse triggered by a depolarisation beyond threshold
Length of AP?
1-2 Ms
What three things can trigger an AP?
Electrode
Synaptic activity
Propagation of AP
How much is the Em displaced in an action potential?
Up to 100mv
What underpins an action potential?
Changes in membrane conductance to different ions
VGIC
Describe how pd changes in AP?
RMP at -70mV
Depolarisation til threshold of -55mV
Rapid upstroke to 40mV
Hyperpolarisation below RMP (-80mv)
What happens in initial depolarisation of AP?
Gentle increase in Em
Cation (Na+) channels open by chemical/physical stimulus
Or current has spread from neighbouring already excited tissues
What happens in upstroke?
Threshold reached, VGNC open and Na+ goes down electrochemical gradient
Once some open, the depolarisation initiates opening of others causing propagation (+ve feedback)
What happens in repolarisation?
Na+ channels inactivated
VGKC open, leads to K+ efflux down their electrochemical gradient
When do VGKC get stimulated to open and when do they open, why?
Stimulated at the same time as VGNC, but slow to open so open later
Why does hyperpolarisation occur?
VGKC slow to close
What occurs in the hyperpolarisation stage?
Hyperpolarisation of the membrane - raised K+ permeability takes the Em closer to Ek.
The Na+ channels are restored from inactivated to closed.
How is the hyperpolarised membrane returned to RMP?
Mainly action of Na/KATPase
What are the K+ channels called?
Delayed rectifier channels
What is the difference between absolute and relative refractory period?
Absolute: no upstroke can be initiated regardless of depolarisation. VGNC recover from activation (time and voltage dependent)
Relative refractory period: large stimulus required to elicit AP. Slightly after closure of K+ channels
What is the refractory period?
Neurone cannot fire another action potential
When the voltage-gated sodium ion channels are in their inactivated state the
What can the cell membrane be imagined as and why (physics)?
A capacitor, it separates charge
How is AP propagated in unmyelinated neurones?
Na+ influx generates local depolarisation
Negative region draws +ve charge from areas behind and ahead
Depol in one region is stimulus for depol and opening of VGNC in the other
What does the refractory period provide?
Unidirectional flow
What is the term for one way AP propagation?
Orthrodromic
How can nerve propagation be sped up (overall)?
Capacitance or resistance must be reduced, to decrease the time period of depolarisation.
What are gaps between myelination called?
Nodes of ranvier
Effect of myelination on length and time constant
Decrease capacitance - decrease time constant
Increase membrane resistance - increase length constant
Describe AP propagation in myelinated neurones
Saltatory conduction
Depolarisation wave jumps from node to node as the Na+ channels are packed into the nodes.
What effect does myelination have on speed of propagation?
Increase the speed from 1-2m/s to up to 120m/s.
What neurones are myelinated?
Motor neurones
Preganglionic (ANS)
Why does myelination increase the length constant?
Larger Rm
Why does myelination decrease time constant?
Decreases capacitance (increases the distance between the cations on the outside of the axon and the Na⁺-ions that enter the axon at the nodes of Ranvier during an action potential)
What is the time constant?
Time taken for voltage to fall to 1/e of its initial value
Faster time constant…
Membrane ahead of impulse reaches threshold quicker so faster conduction velocity
What is length constant?
Distance needed for voltage to drop to 1/e
Long length constant means…
More distant areas of membrane ahead of impulse can be depolarised to threshold - faster conduction velocity
Equation for length constant
Square root: membrane resistance/internal resistance
Equation for time constant
Membrane resistance x capacitance
What factors affect conduction velocity?
Myelination (increases)
Nerve diameter (larger, faster)
Temperature (higher, faster)
How does increased nerve diameter affect length constant?
Decreased internal resistance so increased length constant
Why does increased temperature increase conduction velocity?
High temp ions diffuse faster into neurone and along axon
What is internal resistance inversely proportional to?
Cross sectional area
Describe a alpha nerve fibres?
100m/s
Myelinated
15-20 micrometer
Innervate skelelal muscle
Describe a gamma nerve fibres?
2-5 micrometer
Sensory (pain and temp)
Myelinated
Describe b nerve fibres?
Moderately myelinated Conduction velocity is 3 to 14 m/s
General visceral afferent fibers and preganglionic nerve fibers of the autonomic nervous system.
Describe c nerve fibres?
0.5-1 micrometer
1m/s
Unmyelinated sensory (transmit info e.g. pain temp to CNS)
What is membrane resistance inversely dependent on?
Axon circumference
Compare the stimulus needed to initiate AP in small vs large diameter axons
Stimulus needed is larger in small axons
Is the Na+/K+ ATPase pump necessary to restore the membrane potential to its resting value at the end of an action potential?
No
What four changes increase velocity of AP?
Decrease in external resistance
Decrease in internal resistance
Decrease in membrane capacitance
Increase in membrane resistance
The initial (local) depolarization (immediately after point A) is mediated by…
Ligand-gated ion channels
How would TTX affect the threshold of AP?
Make it higher, less negative
