Neurones
- Created by: Sophie
- Created on: 05-04-15 15:22
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- Nervous System- Neurones
- Neurone cell membranes are polarised at rest
- In its resting state, the outside of a membrane is positively charged compared to the inside.
- More +ve ions outside the cell than inside
- Hence the membrane is polarised- there is a difference in charge.
- Resting potential = -70mV
- Resting potential maintained by sodium- potassium pumps and potassium ion channels in a neurone's membrane.
- Na/K pump: Uses active transport to move 3 sodium ions out for every 2 potassium ions moved into the membrane. ATP is required.
- The membrane isn't permeable to sodium ions, so sodium ions cannot diffuse back in, creating a sodium ion electrochemical gradient (more +ve sodium ions outside cell than inside).
- Membrane is permeable to potassium ions so potassium ions diffuse back out through potassium ion channels.
- Potassium ion channel: Allows facilitated diffusion of potassium ions out of the membrane, down their concentration gradient.
- Na/K pump: Uses active transport to move 3 sodium ions out for every 2 potassium ions moved into the membrane. ATP is required.
- In its resting state, the outside of a membrane is positively charged compared to the inside.
- Neurone cell membranes become depolarised when they're stimulated.
- Stimulus triggers sodium ion channels to open
- If stimulus is big enough, it will trigger a rapid change in potential difference
- Membrane becomes more permeable to sodium ions so the ions diffused into the neurone down the sodium ion electrochemical gradient
- Depolarisation occurs if the potential difference reaches the threshold (around -55mV)
- More sodium channels open, more sodium diffuses into membrane.
- Repolarisation occurs, at a potential difference of around -30mV the sodium ion channels close and potassium ion channels open.
- Membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient.
- Hyperpolarisation then occurs- 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.
- Resting potential is reached again, the ion channels are reset. The sodium potassium pumps returns the membrane to its resting potential.
- Hyperpolarisation then occurs- 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.
- Membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient.
- Repolarisation occurs, at a potential difference of around -30mV the sodium ion channels close and potassium ion channels open.
- More sodium channels open, more sodium diffuses into membrane.
- Depolarisation occurs if the potential difference reaches the threshold (around -55mV)
- The refractory period: Ion channels are recovering and can't be made to open, sodium ion channels are closed during repolarisation and potassium channels are closed during hyperpolarisation.
- Stimulus triggers sodium ion channels to open
- An action potential moves along the neurone
- During an AP, some of the sodium ions diffuse sideways.
- Sodium ion channels in the next region of the neurone open and diffuse into that part.
- Wave of depolarisation along neurone.
- Wave moves away from parts of the neurone in the refractory period as these parts can't fire an AP.
- Wave of depolarisation along neurone.
- Sodium ion channels in the next region of the neurone open and diffuse into that part.
- During an AP, some of the sodium ions diffuse sideways.
- Refractory period produces discrete impulses
- Ion channels are recovering and can't be opened.
- RP acts as a time delay between APs. Hence APs don't overlap but pass along as discrete impulses.
- Ensures APs are unidirectional.
- Action potentials have an 'all or nothing' nature
- Once the threshold is reached, an AP will always fire with the same change in voltage.
- If the threshold isn't reached, the AP won't fire.
- A bigger stimulus won't cause a bigger AP, only causes them to fire more frequently.
- Once the threshold is reached, an AP will always fire with the same change in voltage.
- Factors affecting the speed of an action potential
- Myelination
- Myelin sheaths are electrical insulators
- These are made from Schwann cells
- Sodium ion channels are concentrated at the Nodes of Ranvier
- In a myelinated neurone, depolarisation only occurs at the Nodes
- Impulse jumps from node to node due to conductivity of cytoplasm
- 'Saltatory conduction' - fast.
- In a non-myelinated neurone, the impulse travels as a wave along the whole axon membrane.
- 'Saltatory conduction' - fast.
- Impulse jumps from node to node due to conductivity of cytoplasm
- In a myelinated neurone, depolarisation only occurs at the Nodes
- Sodium ion channels are concentrated at the Nodes of Ranvier
- These are made from Schwann cells
- Myelin sheaths are electrical insulators
- Axon diameter
- APs conduct faster along thicker axon diameters, as there is less resistance to the flow of ions than in the cytoplasm of a smaller axon
- Less resistance = depolarisation reaches other parts of the neurone quicker
- APs conduct faster along thicker axon diameters, as there is less resistance to the flow of ions than in the cytoplasm of a smaller axon
- Temperature
- As temp increases, speed of conduction increases
- Ions diffuse faster
- But after about 40 degrees C, proteins begin to denature and speed decreases
- Ions diffuse faster
- As temp increases, speed of conduction increases
- Myelination
- Neurone cell membranes are polarised at rest
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