Brain & Nervous System


Action Potentials

  • Membrane permeability changes producing a change in the potential difference between the intra and extra-cellular fluids
  • Cell briefly becomes positively charged then the cell membrane returns the cell to its resting state
  • Opposite charges attract (like charges repel) which balances the diffusion gradient
  • Na channels open and Na flows in; then K channels open and K flows out; Na channels close = RISING PHASE
  • As K flows out, the cell is returned to its resting state; membrane potential decreases = REPOLARISATION
  • K channels start to close = HYPERPOLARISATION
  • High intensity, rapid process lasting 4 - 5 ms
  • All or nothing process
  • Fixed amplitude and high frequency
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Ion Channels

  • Cell membrane is a double layer of lipid molecules with different protein structures, e.g. ion channels 
  • Cell changes its permeability for different ions
  • Na channels are voltage-dependent
  • These open when the cell reaches its action potential threshold
  • This allows Na into the cell membrane
  • This causes a rapid positive shift in the membrane which happens at the start of an action potential
  • A closed ion channel is impermeable
  • Sodium-potassium pump/transporter is an additional protein structure that maintains the distribution of ions needed for the resting state and normal functioning of the cell
  • Continually at work through cycles
  • 3 Na ions pumped out and 2 K ions pumped in
  • Energy for this comes from ATP, which is produced by mitochondria
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Ionotropic Receptors

  • Ligand gated ion channels respond to neurotransmitters, causing either excitatory or inhibitory post synaptic potentials
  • Ligand = a chemical that binds with a receptor
  • Inflow of Na+ = depolarisation / excitatory postsynaptic potential (EPSP)
  • Outflow of K+ = hyperpolarisation / inhibitory postsynaptic potential (IPSP)
  • Inflow of Cl- = hyperpolarisation / IPSP
  • Negatively charged ions entering and positively charged ions leaving are inhibitory 
  • This makes an action potential less likely if the cell is at its resting state because the potential difference has moved away from the threshold value that would trigger an action potential
  • Positively charged ions entering and negatively charged ions leaving are excitatory
  • This makes an action potential more likely if the cell is at its resting state because the potential difference has moved towards the threshold value needed to trigger an action potential
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Metabotropic Receptors

  • Metabotropic receptors = more complex, slower and less direct than iontropic
  • Two potential routes
  • 1: Transmitter substance binds with receptor, activating G protein
  • Alpha subunit breaks away from G protein and binds with the ion channel, opening it
  • Ions enter the cell, producing a postsynaptic potential
  • 2: Transmitter substance binds with receptor, activating G protein
  • Alpha subunit breaks away
  • Alpha subunit interacts with an enzyme, producing a second messenger
  • The second messenger either opens the ion channel or goes to other parts of the cell
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Lifecycle of a Transmitter

5 stages

1. Synthesis - series of chemical reactions that convert substances into the final neurotransmitter

2. Storage - in vesticles

3. Release

4. Binding

5. Inactivation - breakdown of a neurotransmitter, or reuptaken/repacked

  • Agonist and antagonist drugs either work with or work against neurotransmitters
  • They can do this at any stage in the lifecycle of a neurotransmitter
  • Transmitters are inactivated by being reuptaken into the pre-synaptic neuron by specialised transporter channels in the cell membrane
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  • Act as the 'glue' in the CNS, making up 85% of the cells in the brain
  • Control the supply of nutrients, hold neurons in place and remove damaged neurons
  • 3 types:
  • Astrocytes = star-shaped; transport nutrients
  • Oligodendrocytes = star-shaped; produce myelin that insulates the axon and speeds up the conduction of an action potential via saltatory conduction
  • Microglia = in the immune system; cleans up waste and debris
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Blood Brain Barrier

  • Discovered by Erlich 
  • Small gaps in the capillaries allow for the exchange of certain substances between the blood plasma and the fluiid outside the capillaries
  • However, there are no gaps in the capillaries in the brain so most substances cannot leave the blood
  • L-dopa can cross the blood brain barrier
  • For messages to be sent/received to/from the brain there must be the right balance between substances within neurons and the fluid that surrounds them, otherwise the messages get disrupted
  • The blood brain barrier makes it easier to control this balance of fluids 
  • The barrier also prevents chemicals from certain foods that interrupt messages from reaching the brain
  • Its permeability is not the same throughout the nervous system, e.g. some substances not allowed to enter in a certain area may be able to in another
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