Neurones

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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.
    • 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.
      • 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.
    • 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.
    • 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.
    • 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.
      • 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
      • Temperature
        • As temp increases, speed of conduction increases
          • Ions diffuse faster
            • But after about 40 degrees C, proteins begin to denature and speed decreases

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