Action potential: generation and transmission Flashcards
When the potential of a neuron become more negative, what is this called? What is the polarisation of the cell tending towards?
Hyperpolarisation Tending towards the RMP of K+
When the potential of a neuron become more positive, what is this called? What is the polarisation of the cell tending towards?
Depolarisation Tending towards the RMP of Na+
What causes the potential of a neuron to change?
The permeability of the neuron changes
What are some other terms for action potential?
Spike, nerve impulse, discharge
What is an action potential?
A brief fluctuation in membrane potential caused by a transient opening of voltage-gated ion channels which spread like a wave along gan axon
When does an action potential occur?
After the membrane potential reaches a certain voltage threshold
What is the voltage threshold?
-55mV
What is the significance of action potential?
Information is coded in the frequency of action potentials between neurons
What can action potentials be considered as between neurons?
A language which neurons use to communicate
What are action potential a key process of?
Signal transmission along axons
What are the stages of initiating and creating an action potentials
0 [this causes the action potential so not included] - a simulus shifts the cell membrane potential by causing a slow depolarising it to -55mV 1 - there is very rapid depolarisation that causes an overshoot making the cell membrane to become +30mV 2 - there is depolarisation where the cell potential becomes negative again but more negative than the RMP (i.e. hyper-polarised) 3 - After hyperpolarisation the potential return back to RMP
What are stages 1 and 2 considered?
Absolute refractory periods
What is stage 3 considered?
Relative refractory period
What happens to a cell if it produces too many action potential?
Can cause the ion gradient to dissipate resulting in the neuron not being able to respond to changes in the RMP by depolarising
What is the refractory period?
A mechanism that prevents the neuron from changing RMP too rapidly prevent depolarisation
How does the refractory period protect the ion gradient?
It prevents an action potential from being produced in stages 1 and 2 by the neuron and it requires a stronger signal during stage 3
What are some types of stimulus?
Physical changes (e.g. electric, light or stretch) or chemical stimuli (drugs or synaptic excitation)
What is the most important step in action potential activation?
Opening of the voltage gated Na+ channels
What are voltage gated channel responsive to?
Small voltage changes outside the cell
How does the permeability of Na and K change from stimulus to stage 1? What does this result in for the cell potential?
Na:K permeability stage 0: 1:40 Na:K permeability stage 1: 20:1 Causes the cell to to become depolarised and the membrane potential shifts towards the Equilibrium potential of Na+
What does the membrane potential get up to during stage 1?
+30mV
Why does stage 1 not get all the way to the Na membrane potential of 60mV? Which is the most important factor?
-Opening of voltage gated Na channels is only short lasting as they are quickly inactivated preventing the potential from depolarising completely (most important) -During stage 2 the voltage gated K channels activate which changes the permeability to 100:1, K/Na which causes the potential to quickly decrease -As more Na+ move into the cell it causes the cell potential to become more positive, eventually the electrical gradient opposes the concentration gradient slowing/stopping Na flow
How does the membrane potential return to the RMP after hyper-polarisation?
Some of K leak channels become inactivated causing the permeability to decrease from K/Na 100:1 to 40:1 making the potential slightly less negative
How are voltage gated channels able to detect the change in potential?
the proteins of the channels are made of polar amino acids, apart of the activation gate, which responds to small changes in the charge. When the membrane potential decreases the polar amino acids of the activation gate change arrangement and open the channel
What causes the Na to flow very fast into the cell membrane when the voltage-gated channel opens?
It is moving down its concentration and electrical gradient
How are voltage gated Na channels deactivated?
An inactivation gate rapidly stops the Na flowing through channels
How do inactivation gates on Na channels work?
Inactivation gates which are like a ball and chain are made of polar amino acids similar to the channels. However they respond to large changes in membrane potential (i.e. when it goes from - to + ) and when there is a large chain it blocks the channel
What happens once inactivation gates on Na channels are activated?
Na+ stops flowing through channel and the activation gate become inactivated returning the membrane potential back to a - charge and deactivating the inactivation gate
Why is it important to prevent too much Na getting into the neuron?
Prevents the neuron from depolarising completely enabling it respond to future signals
What is the amplitude of the action potential?
100mV
How does the amplitude of the action potential change depending on the stimulus intensity? What is necessary from the stimulus?
It doesn’t really, it is 100mV regardless of intensity of the stiumuls Stimulus must be suprathreshold (i.e. meet the threshold potential of -55mv to stimulate response)
What do anodes do on an electrode?
Attract anions (i.e. it is positively charged)
What do cathodes do on an electrode?
Attract cations (i.e. it is negatively charged)
What is the neuron bathed in?
Cerebrospinal fluid
How does the cerebrospinal fluid contribute to the electrode structure of the neurons? Why this structure?
It is the electrode as it contains many ions able to conduct charges
When an axon is stimulated by a current sufficient for an action potential, what direction does the signal go? What condition is this created in?
Goes in both ways through the axon When an electrical current is induced in the middle of an axon
In real life how does the action potential flow through the axon? Why
Flows one way down the axon as it is generated in the cell body at on end of the axon
How does the direction of the current determine the polarisation of the axon?
It the current is going into the cell (i.e. + charge into cell) then it causes local hyper-polarisation –> causes the membrane potential to become more negative If the current is going away fro the cell (i.e. + charge away from cell) then it cases local depolarisation –> causes the membrane potential to become more positive
What is the direction of current convention?
The movement of cations (+ ions)
Where are the most Na voltage gated channels? How does this affect its function?
Axon initial segment / Axon hillock The threshold for action potential generation is the lowest here
What are dendrites covered in?
Synapse ends
Where are action potential first generated?
In the axon initial segment / axon hillock
What is the function of axon hillocks?
It is the trigger zone for action potentials
How is depolarisation to threshold evoked?
by excitatory postsynaptic potentials (EPSPs)
How doe EPSP spread? Where do the speed from and to?
Spread passively
From the dendrites to the cell body
How are axon potentials generated physiologically?
By synaptic currents which are generated by synapses contacting dendrites and EPSP depolarising the neurons to initiate an action potential
What are the types of axons?
Un-myelinated and myelinated axons
What is the structure of unmyelinated axons? What is the speed and signalling pattern of their action potentials?
Small diameter, ~1µm Slow but continuous
What is the structure of myelinated axons? What is the speed and signalling pattern of their action potentials?
Larger diameter, ~10µm Fast but saltatory (jumpy signal)
What are the two stages of action potential transmission in both types of axons?
1-Passive spread 2-generation of action potential
How does the speed of a potential through an axon compare to electricity flowing through a cable?
It is much slower
Why is the speed of potential through an axon much slower than a cable?
It is the movement of ion not electrons and there are other processes inside the cell which slow these ions down
What is the process of passive spread of current?
1 - there is a sub threshold depolarisation at a local region of the membrane 2 - this sub threshold depolarisation is not enough to generate an action potential however it does create passive current flow inside and outside the axon due to the more positive inner membrane/more negative outer membrane relative to the rest of it 3 - This causes sub-threshold depolarisation of adjacent parts of the membrane
How far can passive spread of current transit a signal? Why this far?
Not very far, usually dissipate less than 1mm The further away from the local point of depolarisation the more cumulative resistance from the axon, decreasing current
When is passive current spread useful?
Between axons in the brain which may only be 0.5mm apart
What is used to transmit signals over long distance? Why is this more effective
Action potentials There is a larger depolarisation (100mV)
What are the processes of an action potential in unmyelinated axons?
1 - Action potential is generated at a local point on the axon due to stimulus 2 - there is a LARGE passive current flow (due to the large potential difference generated from the action potential) 3 - this causes threshold depolarisation of adjacent parts of the membrane 4 - voltage gated Na+ channels i adjacent parts of the membrane are opened 5 - a new full sized action potential in adjacent parts of the membrane generated (no loss in signal strength FYI)
What is the speed of action potential transmission in unmyelinated axons?
1msec-1
What makes action potential so slow in unmyelinated axons? How does this compare to passive current flow?
An action potential has to be generated at every point on the membrane which is slow Passive current flow is faster
What is the speed of transmission in myelinated axons?
20-100msec-1
Knowing that unmyelinated fibres are much slower at transmitting action potentials than myelinated fibres, why are they apart of the nervous system?
They are much thinner so inside the brain where volume is limited, having much thinner fibres is more advantageous
Why do myelinated fibres transmit AP (action potentials) faster than unmyelinated fibres?
It increases the distance which passive spread of current can occur
What forms the myelin sheath around the axons?
Oligodendrocytes in the CNS and the schwann cells in the PNS
Label the diagram
How is the myeline distributed over the axon?
It is discontinuous where it is interrupted at nodes of Ranvier
What is the function of the Node of Ranvier?
It is the source of the Na and K leak channels and where the AP is regenerated
What is the function of the myelin? What does this allow?
It reduces the current that dissipates as it flows along the axon Allows the passive current to travel much further form the source of AP activation
What direction does passive conduction occur?
It occurs in both directions
What does myelin do?
It increases the action potential conducting velocity
What do myelinated axons do differently compared to unmyelinated axons? What is this method of AP transport called?
AP do not need to be regenerated at every part of the cell membrane, just at the nodes of Ranvier, allowing for passive conducting for longer parts of the axon Saltatory conduction
Under test conditions what direction does the AP go? Under physiological conditions what direction does the AP go?
Both ways One way away from the cell body
What causes the AP to conduct in only one direction?
The absolute refractory period
How does the refractory period prevent the AP moving in both directions?
It takes 1-2ms for the neuron to restore its RMP after an AP is generated therefore it is non-responsive to the refracted AP ahead. By the time it has restored its RMP the AP has moved down the axon 3 Ranvier nodes head
What are the parts of the PNS?
Motor neuron axons, automatic nervous system and the sensory neurons
What are the motor neuron axons?
The axons of the neurons that start in the CNS and the axons transit signals to the body
What does the automatic nervous system control?
subconscious actions (e.g. heart, digestion etc.)
What do sensory neurons do?
Transmit information from the periphery to the CNS
What is the relative direction of nervous signals from sensory and motor neurons?
They send signals in the opposite direction (sensory to CNS, motor from CNS)
What kind neuron is used in sensory neurons?
Unipolar neurons
What do unipolar neurons NOT have?
No dendrites or synaptic contacts
What do dendrites do?
Increase the surface area for synaptic contacts
Where is the cell body located on a unipolar neuron?
Located outside the CNS in a group of ganglion (structure containing a number of cell bodies) outside the spinal cord
What are the two parts of a unipolar neuron?
Proximal and distal axon
What is the relative length of a proximal and distal axon?
Proximal is shorter
What does the distal axon project to?
Somewhere to the periphery (e.g. surface of the skin, cochlea (i.e. hearing) )
What part of the unipolar neuron is myelinated?
The distal end
What do muscle spindles do?
Detect how much the muscles stretch
What part of the unipolar neuron goes into the CNS?
The proximal end
What activates muscle spindles?
Stretch gated ion channels
What kind of channels are on the muscle spindles?
Non-selective cationic channels
What are non-selective cationic channels?
Leak channels that allows the movement of all cations (i.e. K and Na)
What does the flow of both Na and K out of the cell from non-selective cationic channels do?
It causes depolarisation as both the ions leave, the potential of the cell becomes the average of both the Na and K equilibrium potentials
What process are APs generated in the sensory neurons?
By the receptor potential process
Explain how APs are generated in sensory neurons
1 - a stimulus acts causing the stretch gated non-selective cationic channels to open 2 - The opening of these ions channel causes graded depolarisation that is known as receptor potential: the larger the stretch of the ion channels the more that flow through therefore the larger and faster the depolarisation 3 - The receptor potential spreads passively to the distal end of the unipolar neuron where it then generates an action potential in a trigger zone. 4 - AP then spreads along the axon of the unipolar neuron (goes through myelinated distal end and unmyelinated proximal end FYI) into the CNS
