Neurones and Action Potentials - Nervous System

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  • The Nervous System
    • Made up of neurones
      • Sensory neurones
        • Transmit nerve impulses from receptors to CNS (brain and spinal cord)
          • Convert energy of a stimulus into electrical energy
            • They are transducers (convert one form of energy to another)
      • Motor neurones
        • transmit nerve impulses from the CNS to effectors
          • Cell body at the end of the cell
            • Many short dendrites off the cell body that carry nerve impulses from CNS to cell body
              • One long axon to carry nerve impulses from cell body to effector cells
      • Relay neurones
        • transmit nerve impulses between sensory neurones and motor neurones
      • Receptor
      • Resting state
        • Inside is -70mv as the outside is more positive due to more positive ions outside
          • The membrane is polarised
            • Resting potential
              • Created and maintained by sodium-potassium pumps, sodium ion channels and potassium ion channels
      • Myelinated neurones
        • Some neurones have a myelin sheath made of Schwann cells
          • These cells are electrical insulators
        • Tiny gaps where there is bare membrane are nodes of Ranvier
          • Sodium ion channels are concentrated at the nodes
            • Depolarisation only occurs at these nodes
              • Neurones cytoplasm conducts electrical charge to polarise next node - so they jump from node to node
                • This is called saltatory conduction
                  • It is very fast
    • Action Potentials
    • How an Action Potential moves along neurone
      • Sodium ions diffuse along the neurone sideways
        • This causes sodium ion channels in the next region of the neurone to open
          • sodium ions diffuse into that part of the neurone
            • Causing a wave of depolarisation to travel along neurone
  • Nervous system receptor in resting state
    • Difference in charge between inside cell vs outside cell
      • Generated by ion pumps and ion channels
        • Voltage across membrane = potential difference
          • Potential difference at rest = resting state
            • Membrane becomes more permeable when stimulus is detected
              • More ions move in and out altering potential difference and this change = generator potential
  • Bigger stimulus = more excited = bigger generator potential
    • If generator potential is big enough it triggers an action potential but only if it reaches the threshold level
  • Cell body is in the middle of the sensory neurone
  • Cell body have dendrite - carrying nerve impulses towards the cell body from the receptor cells
    • Also has axons to carry the nerve impulse away from the cell body and to the axon terminal
    • Long dendrite in the sensory neurone
  • Short axon
  • Neurone cell membranes become depolarised when stimulated
    • Stimulus reaches threshold value and causes voltage- gated sodium ion channels to open
      • Sodium diffuses into the neurone down sodium ion electrochemical gradient
        • Inside of the neurone is less negative
          • At around +40mV the sodium ion channels close
            • Voltage gated potassium ion channels open
              • Membrane is more permeable to potassium and so they diffuse out of the neurone down the potassium ion concentration gradient
                • Starts to return to resting potential
                  • Voltage-gated potassium ion channels are too slow to close
                    • Overshoot and the neurone becomes more negative the resting potential
                      • Sodium-potassium pump returns the membrane to resting potential by moving 3 Potassium and 2 Sodium ions at a time
  • Depolarisation
  • Repolarisation
  • Hyperpolarisation
    • Refractory Period
  • After an action potential, the neurone cannot be excited straight away as they are recovering
  • It moves away from parts in the refractory period because it can't fire an action potential there

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