Nerve & Muscle - Action Potential Flashcards
What are action potentials?
The basis of neuronal communication. They are electrical impulses passing along axons
Axons are biologically…
active so the information sent down the axon is amplified to move along the full length of the axon
What are voltage gated sodium channels designed for?
So that the signal is brief
What is the action of the voltage gated sodium channel at resting membrane potential (-70mV)?
The activation gate is closed and the inactivation gate is open so the channel is closed
What is the action of the voltage gated sodium channel at the threshold (-60mV)?
Activation and inactivation gates are both open so the channel is open and sodium is able to enter
What is the action of the voltage gated sodium channel at +30mV?
The inactivation gate closes so that the channel becomes inactive
What occurs at the axon hillock?
The decision point
What happens at the decision point?
If enough signal for an action potential is received, the sodium channels in the initial segment of the axon will open.
What happens before the action potential reaches the initial segment?
A graded potential spreads through the axon hillock
What is AP propagation?
When voltage-gated sodium channels open at the axon initial segment
What is step 1?
Depolarisation to threshold
What happens during depolarisation to threshold?
A local change in membrane potential occurs sufficient to depolarise the cell and open voltage gated sodium channels
What is step 2?
Activation of sodium ion channels and rapid depolarisation
What happens during activation of sodium ion channels and rapid depolarisation?
Voltage gated sodium channels open causing rapid depolarisation
What is step 3?
Inactivation of sodium channels and activation of potassium ion channels
What happens during inactivation of sodium channels and activation of potassium ion channels?
The voltage gated sodium channels become inactive by closing the inactivation gate and the voltage gated potassium channels open. This causes repolarisation of the membrane which occurs slower than the depolarisation.
What is step 4?
Potassium ion channels close
What happens when potassium ion channels close?
Membrane potential returns to resting and both the voltage gated sodium and potassium channels close. However, a slight hyperpolarization occurs before resting membrane potential is reached because of the slow response time of the voltage gated potassium channel
What is the absolute refractory period?
The period when a new action potential cannot be received
Where does the absolute refractory period start and finish?
Begins when the threshold membrane potential is reached and ends around the middle of repolarisation
What is the relative refractory period?
The period when a new action potential can be received
Where does the relative refractory period begin and end?
Begins around the middle of the repolarisation and ends when the resting membrane potential has been restored
What is in step one of action potential propagation of an unmyelinated axon?
As an action potential develops at the initial segment, the membrane potential at this site depolarises to 30mV
What is in step two of action potential propagation of an unmyelinated axon?
A the sodium ions entering spread away from the voltage gated channels, a graded depolarisation quickly brings the membrane in segment 2 to threshold
What is in step three of action potential propagation of an unmyelinated axon?
An action potential develops in segment 2. The initial segment begins repolarisation and is refractory
What is in step four of action potential propagation of an unmyelinated axon?
As the sodium ions entering at segment 2 spread laterally, a graded depolarisation quickly brings the membrane in section 3 to a threshold
What does conduction velocity in an axon depend on?
The size of the organism
How does the axon conduct if the organism is small?
Slowly, for example approximately 0.5m/s in a fly
How does the axon conduct if the organism is large?
Fast, for example approximately 50-70m/s in mammals`
How does nature improve action potential conduction velocity?
Myelination of axons
What happens in step one of action potential propagation of myelinated axons?
An action potential develops at the initial segment. Then the action potential jumps from node to node to node, skipping the myelinated areas
What happens in step two of action potential propagation of myelinated axons?
A local current produces a graded depolarisation that brings the axolemma at node 1 to threshold
What is the axolemma?
The plasma membrane of an axon
What happens in step three of action potential propagation of myelinated axons?
An action potential develops at node 1. The initial segment begins depolarisation and is now refractory
What happens in step four of action potential propagation of myelinated axons?
A local current produces a graded depolarisation the brings the axolemma at node 2 to threshold
What do myelinated areas act as?
Excellent conductors
Where does chemical synaptic transmission occur?
At the plasma membrane of the post-synaptic neuron
What happens first at the pore during chemical synaptic transmission?
ACh binds the the specific binding site in the subunit
When is the pore closed?
When there is no ACh bound to the subunit of the pore
When is the pore open?
When there is ACh bound to the subunit of the pore
What passes through the pore?
Sodium until it closes and then waits for another ACh to bind so that it can reopen
Where is the post-synaptic neuron?
At the receiving end of the action potential
What does the pre-synaptic neuron do?
Release the action potential
What do regulatory neurons do?
Regulate the pre-synaptic neuron
Neurons can be both…
pre and post synaptic at the sometime if they are in a sequence
What is calcium?
A cation
How much calcium is in the ECF?
2-2.5mM (1.3mM in the fluid but the rest is bound to things such as surfaces and proteins)
What is calcium important for?
Not just bones but also nervous system health and function
What is the first step of chemical synaptic transmission?
Action potential triggers the opening of the voltage-gated calcium channels into the axon terminal
What happens after the voltage-gated calcium channels are opened?
Calcium ions diffuse into the axon terminal and trigger synaptic vesicles to release ACh by exocytosis. This is also helped by proteins
What happens once the ACh has been released?
ACh diffuses across the synaptic cleft, binds to ACh gated sodium ion channels and produces a graded depolarization
What happens once the graded depolarisation has occurred?
The depolarisation ends as ACh is broken down into acetate and choline boy AChe
What happens after ACh is broken down?
The axon terminal reabsorbs choline from the synaptic cleft and uses it to synthesise new molecules of ACh
Explain depolarization
Occurs when the chemical stimulus opens sodium ion channels. When the stimulus is removed and excess sodium ions are transported out of the cytosol repolarization occurs
What is another name for depolarisation?
Excitatory Post Synaptic Potential (EPSP)
Explain hyperpolarization
Occurs when the chemical stimulus opens potassium ion channels. When the stimulus is removed repolarization occurs
What is another name for hyperpolarization?
Inhibitory Post Synaptic Potential (IPSP)
Where do EPSP’s always reach the threshold?
At the neuromuscular junction
How can EPSP’s reach the threshold if they don’t individually?
If the EPSP doesn’t meet the threshold but another one arrives at the same time they may sum to reach the threshold and cause an action potential
What happens if an EPSP and an IPSP arrive at the same time?
There may be no action potential produced
How would synaptic transmission be described at the NMJ?
Excitatory and absolute
Why is there no axon hillock at the NMJ?
Because the EPSP always exceeds the threshold and therefore no IPSP/EPSP summation occurs
What is temporal summation?
Summation in time
Explain temporal summation
If the first stimulus doesn’t fade away before the second stimulus arrives the two will add to hopefully reach the threshold for causing an action potential
How can temporal summation be maximised?
The less time between the two stimuli arriving, the greater the summation and likelihood of reaching the threshold
What is spatial summation?
Summation in space
Explain spatial summation
If stimuli arrive close enough together at the same time they will add
