Neurones Flashcards
Describe a neurones resting state
The outside of the membrane is positively charged compared to the inside as there are more positively charged ions outside the cell than inside.
Why is the membrane during its resting state described as polarised?
There is a difference in charge
Define resting potential
The voltage across the membrane at rest
How is resting potential maintained?
By sodium potassium pumps and potassium ion channels
Describe how sodium potassium pumps and potassium ion channels work
- Sodium potassium pumps move sodium ions out of the neurone. The membrane isn’t permeable to sodium ions so they can’t diffuse back in. This creates a sodium ion electrochemical gradient as there are more positive sodium ions outside than inside.
- The sodium potassium pump moves potassium ions into the neurone but the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels.
- This makes the outside of the cell positively charged compared to the inside
What does a stimulus trigger?
Ion channels to open
What happens if the stimulus is big enough?
If it is big enough it will trigger a rapid change in potential difference (action potential)
What are the 5 stages of an action potential?
Stimulus
Depolarisation
Repolarisation
Hyperpolarisation
Resting potential
Describe the stimulus stage of an action potential
This excites the neurone cell membrane causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative.
Describe the depolarisation stage of an action potential
If the potential difference reaches the threshold more sodium ion channels open. More sodium ions diffuse rapidly into the neurone.
Describe the repolarisation stage of an action potential
At a potential difference of around +30mV the sodium ion channels close and potassium channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone getting the neurone back to resting potential
Describe the hyperpolarisation stage of an action potential
Potassium ion channels are slow to close so there is a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential
Describe the resting potential of an action potential
The ion channels are reset. The sodium potassium pump returns to the membrane to its resting potential and maintains it until the membranes get excited by another stimulus
What causes a wave of depolarisation to travel along the neurone?
When an action potential happens some of the sodium ions that enter the neurone diffuse sideways.
This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part
What happens during the refractory period?
Ion channels recover and cannot be opened
What does the refractory period do?
Acts as a time delay between one action potential and the next
What does the time delay as a result of the refractory period mean?
Action potentials don’t overlap but pass along as a discrete (separate) impulses
There is a limit to the frequency at which the nerve impulses can be transmitted
Action potentials are unidirectional
Explain the all or nothing law
Once the threshold is reached an action potential will always fire the same charge in voltage no matter how big the stimulus is. If the threshold isn’t reached an action potential won’t fire.
What will a bigger stimulus cause?
Action potentials to fire more frequently
What 3 factors affect the conduction speed of an action potential?
Myelination
Axon diameter
Temperature
What do myelinated neurones have?
Myelin sheath
In the peripheral nervous system what is the sheath made from?
Schwann cells
What are nodes of Ranvier?
Tiny patches of bare membrane located between Schwann cells
What is concentration at the nodes of Ranvier?
Sodium ion channels
Where does depolarisation happen in a myelinated sheath?
Nodes of Ranvier
What happens in a myelinated neurone?
The neurone’s cytoplasm conducts enough electrical charge to depolarise the next node so the impulse jumps from node to node. This is called saltatory conduction and it’s really fast
What happens in a non-myelinated neurone?
The impulse travels as a wave along the whole length of the axon membrane.
Where does depolarisation happen in a non-myelinated sheath?
Along the whole length of the membrane
What happens to axons with bigger diameters and why?
Action potentials are conducted quicker as there is less resistance to the flow of ions than in the cytoplasm of a smaller axon. This means that depolarisation reaches other parts of the neurone cell membrane quicker
What happens if you increase the temperature of a neurone and why?
The speed of conduction increase as ions diffuse faster. The speed only increase up to around 40 degrees and after that the proteins begin to denature and the speed decreases
