Lecture 8 - Part 1 - Neural Transmission Flashcards
What is an action potential
Response to neurone to depolerisation of 15mV - all or nothing
What happens if threshold is reached
Action potential transmitted along neurone = depolerisation
What is the 1st positive shape of action potential due to
Influx of sodium into neuron
What is charge outside neuron
Positive
What is charge inside
Negative
How does neuron become negative after being positive by Na entering
Potassium moves out of neuron by opening potassium ion channels
How is resting potential restored
Sodium potassium pump - pumps K+ back in neurone and Na+ back out
What forces are acting on K+ when the neuron is depolarised during the action potential?
Both the electrical and chemical forces are acting to push potassium
out of the neuron.
Electrical force is pushing it out as inside has become positive and K is positive
Chemical force - a lot of K inside, not outside
What act as ion channels
Proteins inside plasma membrane of a cell which change shape to let ion through
Examples of ion channels
Sodium and potassium ion channels - open and close depending on voltage
What happens to ion channels when membrane is depolerised
Changes shape
open Na channels = Na into neuron
Then Na channel shuts
K channel opens - K out of neuron
What happens when sodium conductance increases
Na+ channel open
What happens when potassium conductance increases
K+ channel open
What happens when sodium conductance decreases
Na+ channel closes
What happens when K+ rises much later
Fall of action potential
What do Na+/k+ channels form
Alpha helices
What are alpha helices
Long chains of proteins that change shape to open/close - pore/hole in middle = ion goes through
How do channels open/close
Due to charge = repulsion/attraction within amino acids makes repulsion
What happens to channels during negative resting neuron and why
Close
Negative attract the positive
What happens to channels during action potential and how is triggered
Inside becomes + = opens Na+ channel
Positive charge inside neuron repels positive bit of protein = molecule flip = open channel = Na move
What property does axoplasm have
High resistance to current spread
What happens as a result of axoplasm having high resistance to current spread
Local (electrotonic) potentials will therefore diminish in size and not be propagated along (longer) neurons = die out
Small potential at one end of neuron will never get to far end of neuron
Do action potentials diminish in size as they travel along neuron
No
What happens to action potentials as they go along nueron
Regenerated
How is action potential transmitted - moved
Inside = positive = Na channel opens = Na pushes in = positive inside = action potential
What happens to action potential when neuron positive
It freezes
What are local circuits
Action potential triggers next bit of axon to produce new action potential
How are local circuits trasnmitted
Positive ions surrounding axoplasm attracted forward to part of neuron that is not stimulated ( opposites attract ) = action potential triggers next bit of axon to produce new action potential
Na+ moves in = attracted to next bit of neuron = opens more Na+ channels = another action potential
How is the speed of impulse propagation increased
- Increasing the diameter of the axon
- Myelinisation
How does myelinisation increase speed of impulse propagation
Action potentials are only regenerated at the nodes of Ranvier – saltatory conduction
How does Increasing the diameter of
the axon increase speed of impulse propagation
Faster conduction of action potential
What is saltatory conduction
Action potentials jump from node to node
What are nodes of ranvier and what happens here
Gaps = no myelin
Action potential is regenerated = opens Na channels
Where do sodium ion channels open
At nodes of ranvier
What wrap around axon
Schwann cells
What is myelin sheath
Insulator -no current flows across membrane
What happens when sodium enters in
Axon potential regenerated at nodes of ranvier
What does myelination do
Save a lot of space - can’t have big neurons like squids
What happens at unmyelinated axon
Regenerated whole way, Na+ goes to next node of ranvier = opens channels = another action potential
What is Refractory period
The interval after an action potential during which a second action potential absolutely cannot be initiated no matter how intense the stimulus
Can’t keep firing action potentials in neuron - need to rest between action potentials
How is threshold stimulus strength and axon diameter related
The threshold stimulus strength needed to activate a neuron is inversely proportional to its axon diameter (i.e. larger axons are more easily stimulated)
What size axons are more easily stimulated
Larger axons - decrease threshold - increase sensitivity - conduct faster - easier to stimulate
Big axons need less voltage to stimulate action potentials
What are types of neural transmission
Action potential
Electrotonic current spread - common in short neurons
Action potentials using calcium
Use Ca2+ rather than Na+ for initial depolarisation - coming into neuron
Example of action potentials using calcium
Slow jelly fish swimming
Action potential due to Ca coming in = small action potential
What is all or nothing principle?
Either an action potential is produced, or it is not.
What is first part of action potential
Threshold reached = depolarisation = sodium ion channels opened = sodium rushes into neuron until eqbm reached
Proof of sodium rushing into neuron
Alter sodium concentration outside neuron experimentally
How does amount of sodium affect size of action potential and what does that tell you
Less Na outside neuron = smaller action potential = 1st phase of action potential is definitely due to Na rushing into neuron
What happens once sodium reaches equilibrium potential
Neuron becomes negative again
What is second part of action potential
Potassium leaves inside of neuron, goes out due to electrical and chemical force pushing it out. This is by potassium ion channels opening
What happens when potassium is pushed out
Inside becomes negative again
When will potassium stop moving
Until it reaches equilibrium potential
Summary of action potential
- Resting neuron negative inside
- Threshold reached = depolarisation
- Opens Na channels = Na rushes in until equilibrium reached
- Closes Na channels
- Opens K channels = it leaves neuron
- Na/K pump restores resting potential
What are sodium/potassium ion channels
Long chain of amino acids which when go through neuron form alpha helices - go in/out membrane - bunch together
What is each alpha helix and what does it have
A tube
It has a pore/hole in the middle
What causes channels to open and shut
Due to charge because of repulsion of attraction within amino acids that make up protein that makes up channel
Do local potentials die out
Yes - diminish in size and not propogated along neuron
What moves action potential
By local circuits
What happens in local circuit
Sodium goes into neuron at start of action potential Sodium attracted to next bit of neuron That opens more Na channels Another action potential... All the way down the neuron
Giant squid axon - diameter
- Giant axon = axon that runs down side of squid = conducts action potential quickly = squid contract mantle to escape
- 1mm across - conduct it at 20ms-1 approx
Why can squid axon get away with having neurone 1mm in diameter
It doesn’t have many neurons
How many neurons does human brain have
10^12
Why can’t human brain have a 1mm wide axon
Brain would be too big
What causes refractory period
Sodium potassium pump restoring and sorting the ions out back to resting level
What do you need to depolarise axon by to trigger action potential
15mV
What does threshold depend on
Size of axon
Which neurons conduct slower and why
Tiny neurons
Less sensitive
Conduct slower
Which neurons faster slower and why
Bigger neurons
More sensitive
Conduct faster
What does sodium potassium pump do
Restores negative resting potential by pumping potassium back in and sodium back out
