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Nerve impulses - the over all story

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Nerve impulses, synapses and neurotransmitters

Nerve impulses, synapses and neurotransmitters

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  • SummaryReceptorsResting potentialAction potentialAt the synapseNeurotransmitter
  • Action potentialResting potentialNeurotransmitter released
  • Others in the green book
  • The distribution of ions found in the solutions inside and outside a squid giant axon is unequal.
  • There is a higher concentration of sodium ions outside of the axon, so sodium ions flow rapidly inwards through the open voltage-dependent Na+ channels, causing a build-up of positive charges inside. This reverses the polarity of the membrane. This is where the potential difference reaches +40mV.
  • If hundreds of action potentials occur in the neurone, the sodium ion concentration inside the cell rises significantly. The sodium-potassium pumps start to function, restoring the original ion concentrations across the cell membrane. If a cell is not transmitting many action potentials, these pumps will not have to be used very frequently. At rest there is some slow leakage of sodium ions into the axon. These sodium ions are pumped back out of the cell.
  • The gap is about 20-50 nm and a nerve impulse cannot jump across it.
  • The neurotransmitter takes about 0.5 ms to diffuse across the synaptic cleft and reach the postsynaptic membrane. A single impulse will not usually be enough and several impulses are usually required to generate enough neurotransmitter to depolarise the postsynaptic membrane.The number of functioning receptors in the postsynaptic membrane will also influence the degree of depolarisation.
  • Transcript

    • 1. Nerve Impulses
      The Over all Story
      Katie Burke
    • 2. Summary
    • 3. Receptors
      A stimulus is received by the relevant receptors
      The main types of receptor are:
      Chemoreceptors
      This is for chemicals: for taste, smell and chemical concentration in the blood
      Mechanoreceptors
      For balance, touch and hearing
      Photoreceptors
      Light: for sight
      Thermoreceptors
      For temperature control and awareness of surroundings
    • 4. Sodium-potassium ion pump creates concentration gradients across the membrane
      Potassium ions diffuse out of the cell down the potassium ion concentration gradient, making the outside of the membrane positive and the inside negative
      The electrical gradient will pull potassium ions back into the cell
      At -70mV potential difference, the two gradients counteract each other and there is no net movement of potassium ions
      Resting Potential
    • 5. Action Potential
      What causes action potential?
      The change in the potential difference across the membrane causes a change in the shape of the Na+ gate, opening some of the voltage-dependent sodium ion channels
      As the sodium ions flow in, depolarisation increases, triggering more gates to open once a certain potential difference threshold is reached, thus increasing depolarisation (positive feedback)
      There is no way of controlling the degree of depolarisation of the membrane
      Action potentials are either there or they are not (all-or-nothing)
      Action potential is caused by changes in the permeability of the cell surface membrane to Na+ and K+ channels
      At the resting potential, these channels are blocked by gates preventing the flow of ions through them
      Changes in the voltage across the membrane cause the gates to open, and so they are referred to as voltage-dependent gated channels
      Depolarisation
    • 6. Action Potential
      Repolarisation
      • The voltage-dependent Na+ channels spontaneously close and Na+permeability of the membrane returns to its usual very low level
      • 7. Voltage-dependent K+ channels open due to the depolarisation of the membrane
      • 8. Potassium ions move out of the axon, down the electrochemical gradient, and the inside of the cell once again becomes more negative than the outside
      • 9. This is the falling phase of the oscilloscope trace
      Restoring the resting potential
      The membrane is now highly permeable to potassium ions, and more ions move out than occurs at resting potential, making the potential difference more negative than the normal resting potential (hyperpolarisation)
      The resting potential is re-established by closing of the voltage-dependent K+ channels and potassium ion diffusion into the axon.
    • 10. At the Synapse
      An action potential arrives
      The membrane depolarises. Calcium ion channels open. Calcium ions enter the neurone
      Calcium ions cause synaptic vesicles containing neurotransmitter to fuse with the presynaptic membrane
      Neurotransmitter is released into the synaptic cleft
      Neurotransmitter binds with receptors on the postsynaptic membrane. Cation channels open. Sodium ions flow through the channels
      The membrane depolarises and initiates an action potential
      When released the neurotransmitter will be taken up across the presynaptic membrane (whole or after being broken down), or it can diffuse away and be broken down
    • 11. Neurotransmitter
      Summary
      Neurotransmitters include acetylcholine and glutamate
      There are threestages leading to the nerve impulse passing along the postsynaptic neurone:
      Neurotransmitter release
      Stimulation of the postsynaptic membrane
      Inactivation of the neurotransmitter
      Neurotransmitter release
      When the presynaptic membrane is depolarised, channels open, increasing the permeability to calcium ions
      Calcium ions diffuse into the cytoplasm
      This causes synaptic vesicles with the neurotransmitter to diffuse with the membrane
      They release the neurotransmitter by exocytosis
    • 12. Neurotransmitter
      Stimulation of the postsynaptic membrane
      Specificreceptor proteins in the postsynaptic membrane bind to the neurotransmitter molecule
      When it binds it changes shape of the protein, opening cation channels
      The membrane is permeable to sodium ions, causing depolarisation
      The extent of depolarisation is caused by the amount of neurotransmitter which depends on the frequency of impulses
      Inactivation of the neurotransmitter
      It can be actively taken up by the presynaptic membrane and reused
      They can rapidly diffuse from the synaptic cleft or taken up by other cells
      An enzyme will break it down which can be reabsorbed and reused

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