I/E: Neurones Flashcards
Describe the resting state of a neurone:
- outside?
- inside?
Why is this?
Membrane is polarised - outside is positively charged compared to the inside.
More positive ions outside the cell than inside.
What is the resting potential of a neurone?
-70mV
What is the resting potential of a neurone created and maintained by?
Sodium-potassium pumps and potassium ion channels.
What does the sodium-potassium pump transport?
Use active transport to move 3 sodium ions out of the neurone for every 2 potassium ions moved in.
Requires ATP.
What do potassium ion channels do?
Allow facilitated diffusion of potassium ions out of the neurone, down their concentration gradient.
Describe how the resting potential of a neurone is created:
- Sodium-potassium pump moves sodium ions out of neurone, but membrane isn’t permeable to sodium ions, so they can’t diffuse back in.
- Sodium ion electrochemical gradient is created as there are more positive sodium ions outside than inside the cell.
- SPP also moves potassium ions in to the neurone, but membrane is permeable to potassium ions so they diffuse out again.
- Makes outside of the cell positively charged compared to the inside.
Describe the permeability of the neurone membrane to sodium ions and potassium ions:
Sodium ions = not permeable
Potassium ions = permeable
What does a stimulus trigger the opening of?
Sodium ion channels
Describe the 5 events of an action potential:
- Stimulus excites neurone cell membrane, opening sodium ion channels.
- Depolarisation - if the potential differences reaches the threshold (-55mV).
- Repolarisation - +30mV sodium ion channels close and potassium ion channels open.
- Hyperpolarisation - K ions are slow to close.
- Resting potential - ion channels are reset
Refractory period
The period of time after an action potential where the neurone cell membrane can’t be excited again.
Why does the refractory period exist?
Ion channels are recovering and can’t be made to open.
Na channels are closed during repolarisation and K ion channels are closed during hyperpolarisation.
How does an action potential move along the neurone?
As a wave of depolarisation.
- Some Na ions enter the neurone sideways.
- Na ion channels in the next region of the neurone open and Na ions diffuse into that part.
- Causes a wave of depolarisation to travel along the neurone.
- Wave moves away from parts of the membrane in refractory period because these parts can’t fire an action potential.
Give three characteristics of a neurone that result from the refractory period:
- Action potentials don’t overlap and are discrete impulses.
- Limited frequency at which nerve impulses can be transmitted.
- Unidirectional.
Explain what is meant by the ‘all-or-nothing’ principle:
- Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is.
- If the threshold is never reached, an action potential won’t fire.
A bigger stimulus won’t cause a bigger action potential, but it will cause them to fire more frequently.
What 3 factors affect the speed of conduction of action potentials?
- Myelination
- Axon diameter
- Temperature
How does axon diameter affect the speed of conduction of action potentials?
- Quicker along axons with bigger diameters because there’s less resistance to the flow of ions than in the cytoplasm of a smaller axon.
- So, depolarisation reches other parts of the neurone cell membrane quicker.
How does temperature affect the speed of conduction of action potentials?
- Speed of conduction increases as the temperature increases too, because ions diffuse faster.
- Speed only incraeses up to around 40oC - after this proteins begin to denature and speed decreases.
How does myelination affect the speed of conduction of action potentials?
- Myelination increases the speed of conduction, as the impulse jumps from node to node (saltatory conduction).
What type of cell makes up the myelin sheath in the PNS?
Schwann cell
Describe the structure of a myelin sheath:
- Between Schwann cells are tiny patches of bare membrane - nodes of Ranvier.
- Na ion channels are concentrated at the nodes.
Nodes of Ranvier
Tiny patches of bare membrane on the myelin sheath of a neurone, where sodium ion channels are concentrated.
How does an action potential travel along a myelinated neurone?
- Depolarisation only happens at the nodes of Ranvier.
- Cytoplasm of neurone conducts enough electrical charge to depolarise the next node.
Saltator conduction.
How does an action potential travel along a non-myelinated neurone?
Impulse travels as a wave along the whole length of the axon membrane - this is slower than saltatory conduction.
Describe the first step of an action potential:
- Stimulus excites neurone cell membrane, opening sodium ion channels.
- Membrane becomes more permeable to sodium, so Na diffuses into the neurone down the Na ion electrochemical gradient.
- Inside of neurone is less negative.
Describe the second step of an action potential:
- Depolarisation - if the potential differences reaches the threshold (-55mV).
- More Na ion channels open, so more Na rapidly diffuses into neurone.
Describe the thrid step of an action potential:
- Repolarisation
- +30mV sodium ion channels close and potassium ion channels open.
- Membrane is more permable to K so K ions diffuse out of neurone, down the concentration gradient (starts membrane back to its resting potential).
Describe the fourth step of an action potential:
- Hyperpolarisation - K ions are slow to close.
- Slight ‘overshoot’ where too many K ions diffuse out of neurone.
- PD becomes more negative than resting potential.
Describe the fifth and last step of an action potential:
- Resting potential - ion channels are reset.
- Sodium-potassium pump returns the membrane to its resting potential and maintains it until it is excited by another stimulus.
