Transmission of nerve impulses

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  • Transmission of nerve impulses
    • Resting membrane potential
      • There is a difference between the charges on the inside and outside of a neuron
      • The membrane of a neuron at rest is said to be polarised
      • ICF has proteins (neg) and potassium, overall negative charge
      • ECF has sodium and chloride, overall positive charge
      • RMP is -70mV, potential of inside is less than out
    • Maintaining the RMP
      • The membrane of a neuron is selectively permeable - ions can pass in and out of cell
      • Potassium and sodium diffuse across the membrane using channel proteins - limited bc these are gated
      • Sodium-potassium pump works against concentration gradient to keep membrane polarized
        • Uses ATP - 3NA+ leave ICF, 2K+ leave ECF
      • Interior of cell remind negative due to the proteins that cannot leave
    • Generating an action potential
      • The depolarization and repolarisation of a neural membrane is caused by a change in permeability of the membrane to sodium
      • 4 main steps: Stimulus and slight depolarization, rapid depolarization, repolarisationand hyperpolarisation and RMP
      • AP 1 - stimulus and slight depolarisation
        • 3: Sodium spreads along inside of membrane (assisted by proteins)
        • 2: Channel opens
        • 4: Slight depolarization (-70 to -65mV)
          • This slight depolarisation triggers the rapid depolarization
          • Depolarisation only occurs at a certain threshold as this makes the membrane more permeable to sodium
        • 1: stimulus - acetylcholine bonds to a gated channel protein for sodium
      • AP 2 - Rapid depolarisation
        • 3: Sodium floods  the cell
        • 2: Lots of sodium gates open
        • 4: Membrane potential changes (-65 to -35mV)
        • 1: Enough sodium enters the cell to cause rapid depolarization
      • AP 3 - repolarisation
        • 1: Change in membrane potential causes the gates to close
        • 2: Potassium gates open
        • 3: Potassium diffuses from ICF to ECF
        • 4: Repolarisation occurs as the inside becomes more negative and the outside become more positive
      • AP 4 - hyperpolarisation and RMP
        • 1: sodium gates close
        • 2: potassium gates close - this slow response causes hyperpolarization
        • 3: Correct charges across the membrane exist but there is now more potassium outside the cell than in
        • 4: the sodium-potassium pump works to restore this
    • Conduction along an unmyelinated fibre
      • Fibres that don't have a fatty myelin sheath
      • Depolarisation of one area of neural membrane causes a local current in neighbouringareas
      • Process repeats itself along the length of the membrane
      • Action potential moves along the membrane away from point of stimulation (like domino line)
        • If stimulus occurs in middle of fibre impulses travel in both directions away from point of stimulation (impulses usually occur at end of fibre)
      • Nerve impulse prevented from going backwards bc of refractory period
        • Refractory period is time in which another action potential cannot be generated due to imbalance of ions
        • Need ion concentrations to return to normal before another action potential is generated
      • Speed of transmission is 2m/sec
    • Conduction along a myelinated fibre
      • Fibres covered in myelin sheath
      • Nodes of rangier at intervals
      • Myelin sheath insulates fibres from extracellular fluid so no ions can flow between ICF and ECF and an action potential cannot form
      • Action potential 'jumps' from one node of Ranvier to another as myelin is absent from the nodes
      • This jumping is known as saltatory conduction - significantly speeds up impulse transmission
      • Speed of transmission along myelinated fibres is 140m/sec
    • Transmission across a synapse
      • Neuratransmitters diffuse across gap
  • If stimulus occurs in middle of fibre impulses travel in both directions away from point of stimulation (impulses usually occur at end of fibre)


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