6B - Nervous Coordination Flashcards
Describe the polarisation of a neurone at rest.
The membrane is polarised.
For a neurone at rest, which part of the membrane is more positive: inside or outside?
Outside
At rest, are the charge across a neurone membrane the same?
No, there are more positive charges outside compare to inside the neurone.
Define resting potential.
The potential difference across a neurone membrane when it is at rest.
What is the value of the resting potential for a neurone?
About -70mV.
What causes the resting potential across a neurone membrane?
- Na⁺-K⁺ pumps move 3 sodium ions out of the cell for every 2 potassium into the cell
- Na⁺ ions can’t move back in, but K⁺ ions can move back out of the neurone using potassium ion channels
- There is more positive charge outside of the neurone compared to inside it, which causes there to be a resting potential
How does a sodium-potassium pump work?
Pumps 3 Na⁺ ions out of the cell for every 2 K⁺ ions that go into the neurone.
Do sodium-potassium pumps require energy?
Yes
What sort of transport is involved in potassium ion channels?
Facilitated diffusion
Which way do Na⁺-K⁺ pumps move sodium and potassium?
- Sodium -> Out of the cell
* Potassium -> Into the cell
What role do potassium ion channels have in a neurone membrane?
They allow potassium to move out of the cell.
What are the 3 types of transport protein involved in neurone membranes?
- Na⁺-K⁺ pump
- K⁺ channel
- Na⁺ channel
Describe the polarisation of a neurone when stimulated.
Depolarised
What is an action potential?
When a stimulus triggers sodium ion channels to open, causing a rapid change in potential difference.
What are the stages of an action potential?
1) At rest
2) Stimulus
3) Depolarisation
4) Repolarisation
5) Hyperpolarisation
6) Resting potential
Describe an action potential (including membrane potentials).
1) At rest:
• The membrane is polarised at a constant -70mV
• Na⁺ and K⁺ channels are closed
2) Stimulus:
• The neurone cell membrane is excited, causing Na⁺ channels to open
• Sodium ions diffuse into the neurone
• This causes the potential difference to become less negative
3) Depolarisation:
• If the potential difference reaches the threshold (-55mV), more Na⁺ channels open
• More sodium ions diffuse into the neurone
• The potential difference becomes rapidly more positive
4) Repolarisation:
• At about +30mV, Na⁺ channels close, while K⁺ channels open
• Potassium ions can diffuse out of the neurone
• The potential difference becomes more negative
5) Hyperpolarisation:
• K⁺ channels are slow to close, so there’s some “overshoot” when too many potassium ions diffuse out of the neurone
• Potential difference becomes slightly more negative than resting potential (-90mV)
6) Resting potential
• Ion channels are reset and the Na⁺-K⁺ pump returns the potential difference to the resting potential, then maintains it
What is the order of the events and channel openings in an action potential?
- Stimulus
- Na⁺ channels open
- Depolarisation
- Na⁺ channels close and K⁺ channels open
- Repolarisation
- Hyperpolarisation and K⁺ channels close
- Resting potential
What is the usual threshold voltage in an action potential?
-55mV
What is the usual peak voltage in an action potential?
+30mV
What is the usual hyperpolarisation voltage in an action potential?
-90mV
Give all of the important voltages in an action potential.
- Resting potential = -70mV
- Threshold potential = -55mV
- Peak voltage = +30mV
- Hyperpolarisation = -90mV
Remember to practise drawing out the shape of an action potential.
See diagram pg 146 of revision guide.
What is the refractory period?
The period after an action potential, during which the neurone cell membrane can’t be excited again.
What causes the refractory period?
The ion channels are recovering and can’t be made to open.
Where on the graph of an action potential is the refractory period?
From the peak voltage to the start of the resting potential.
Remember to practise writing out the order of an action potential.
Pg 146 of revision guide
To what order is the potential difference across a cell membrane?
mV
Describe how an action potential moves along a neurone.
1) When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways.
2) This causes sodium ion channels in the next region to open, so sodium ions diffuse into that part.
3) This causes a wave of depolarisation to travel along the membrane.
4) The wave moves away from the parts of the membrane in the refractory period, because these can’t fire an action potential.
Why does the wave of repolarisation not move the wrong way along a neurone (i.e. back along where it has just travelled)?
The part that is has just covered is in its refractory period, so it cannot trigger an action potential.
What is the term for the movement of an action potential along a neurone?
Wave of depolarisation
What is the purpose of the refractory period?
- Action potentials remain as discrete impulses
- Limit to the frequency at which nerve impulses can be transmitted
- Action potentials are unidirectional
Does the strength of a stimulus have an effect on the peak voltage of the action potential across a membrane?
No, once the threshold is reached, an action potential will always fire with the same change in voltage.
What happens if the threshold voltage in an action potential is not reached?
The action potential will not fire.
How does the strength of a stimulus affect the action potential difference?
- It has NO effect on the peak voltage reached
* But a bigger signal causes action potentials to fire more frequently
Remember to practise drawing the graphs for an action potential by a small and large stimulus.
See diagram pg 147 of revision guide
What 3 factors affect the speed of conduction of action potentials?
1) Myelination
2) Axon diameter
3) Temperature
What is myelination of a neurone?
When the neurone has a myelin sheath, which is an electrical insulator.
What is a Schwann cell?
The type of cell that myelin sheaths are made of.
Where are Schwann cells found?
On the neurones of the peripheral nervous system.
What are the spaces between Schwann cells called?
Nodes of Ranvier
What are nodes of Ranvier?
Small patches of bare membrane between Schwann cells where Na⁺ channels are concentrated.
Describe the structure of a myelinated motor neurone.
- Cell body with nucleus
- Dendrites spread out on one side of the cell body
- Axon is on the other side of the cell body
- Schwann cells are along the axon, with small gaps between them (nodes of Ranvier)
- Axon ends in axon terminal (which joins to the effector)
Remember to practise drawing out the structure of a myelinated motor neurone.
Pg 148 of revision guide
On a myelinated motor neurone, what do the dendrites do?
Connect with other neurones to receive the impulse.
On a myelinated motor neurone, what does the axon terminal do?
Connects to the effector.
How does myelination affect the speed of conduction of an action potential and why?
Speeds it up, because:
• Along the axon are Schwann cells that are insulators
• Depolarisation can only happen at the nodes of Ranvier between them, which have many sodium channels
• The cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse jumps between them by saltatory conduction
• This is faster than normal conduction
What is the term for the impulse jumping between nodes of Ranvier in a myelinated neurone?
Saltatory conduction
What allows the impulse to jump between nodes of Ranvier in a myelinated neurone?
The neurone’s cytoplasm conducts enough charge to depolarise the next node.
Does wide axon diameter speed up or slow down conduction along a neurone?
Speeds up
How does wide axon diameter affect the speed of conduction along a neurone and why?
Speeds it up, because:
• There is less resistance to the flow of ions in the cytoplasm
• So depolarisation reaches other parts of the neurone quicker, transmitting the impulse quicker
How does temperature affect the speed of conduction along a neurone and why?
Speeds it up (until about 40°C), because:
• Ions diffuse faster
• BUT above 40°C, the proteins begin to denature
Why does the speed of conduction along a neurone decrease above 40°C?
The proteins involved begin to denature.
What is a synapse?
A junction between:
• A neurone and another neurone
OR
• A neurone and an effector call
What is the gap between the cells in a synapse called?
Synaptic cleft
What is the neurone before a synapse called?
Presynaptic membrane
What is the swelling on the presynaptic neurone called?
Presynaptic knob
What is found in the presynaptic membrane?
Synaptic vesicles filled with neurotransmitters.
What is on the surface of the postsynaptic membrane?
Receptors
Give two examples of neurotransmitters.
- Acetylcholine
* Noradrenaline
Describe briefly how a synapse works in general.
1) Action potential reaches end of neurone
2) This causes neurotransmitters to be released into the synaptic cleft, which diffuse across it
3) These then bind to specific receptors on the postsynaptic membrane
4) This triggers an action potential, muscle contraction or hormone secretion
5) The neurotransmitter is removed from the cleft to stop the response happening over and over
What type of synapse do you need to know about?
Cholinergic synapse (one that uses acetylcholine as a neurotransmitter)
Describe how a cholinergic synapse works.
1) Action potential arrives at synaptic knob of presynaptic neurone
2) This stimulates voltage-gated calcium ion channels to open
3) Ca²⁺ ions diffuse into the synaptic knob
4) Influx of Ca²⁺ ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic membrane, where they then fuse with it
5) The vesicles release acetylcholine into the synaptic cleft
6) Acetylcholine diffuses across the synaptic cleft and binds to the cholinergic receptors on the postsynaptic membrane
7) This causes Na⁺ channels in be postsynaptic membrane to open
8) Influx of Na⁺ ions causes depolarisation, which triggers an action potential if the threshold is reached
9) Acetylcholine is removed by acetylcholinesterase and the products are re-absorbed by the presynaptic neurone
What are voltage-caged ion channels?
Ion channels that only open at a certain voltage.
What is the technical term for vesicles releasing acetylcholine into the synaptic cleft?
Exocytosis
What is the name of the enzyme that breaks down acetylcholine?
Acetylcholinesterase
What is the shorthand for acetylcholine?
ACh