synapses

• transmissions by chemicals called neurotransmitters, these are made in the golgi apparatus, which requires energy from the mitochondria
• structure of a synapse:

  • presynaptic neurone
  • synaptic knob
  • vesicles
  • synaptic cleft
  • postsynaptic neurone
  • presynaptic membane
  • postsynaptic membrane
• neurotransmitters

  acetylcholine - used in voluntary (motor neurone = muscles) and the parasympathetic nervous system

  noradrenaline - used in the sympathetic nervous system
• What triggers the arrival of an action potential at the pre-synaptic knob?

  Action potential
• What happens to voltage-gated calcium channels when depolarization occurs?


  They open, allowing calcium to flow into the presynaptic knob
• What is the effect of calcium influx on synaptic vesicles?


  It causes synaptic vesicles to move towards and fuse with the presynaptic membrane
• How do neurotransmitters enter the synaptic cleft?


  Through exocytosis from the synaptic vesicles
• What happens to neurotransmitters after they are released into the synaptic cleft?


  They diffuse across the synaptic cleft
• What is the role of neurotransmitters in the postsynaptic membrane?


  They bind to receptor proteins on sodium channels
• What occurs when neurotransmitters bind to receptor proteins on the postsynaptic membrane?


  The receptors change shape, opening sodium ion channels
• What is the result of sodium ion channels opening in the postsynaptic membrane?


  Sodium ions diffuse across the membrane, leading to an action potential
• What happens to the action potential after it is generated in the postsynaptic membrane?


  It is propagated down the postsynaptic membrane
• What role does an enzyme play at the synapse after neurotransmitter release?


  It breaks down the neurotransmitter
• What happens to the breakdown products of neurotransmitters at the synapse?


  They are reabsorbed into the presynaptic neurone for recycling
• types of synapse:

  • excitatory
  • inhibitory
• excitatory synapse

  • neurotransmitter - acetylcholine
  • binding of neurotransmitter to post synaptic neurone, opens sodium gated channel, sodium diffuses in causing depolarisation
  • action potential is created
  • neurotransmitter recycled and process repeated
• inhibitory synapse

  • neurotransmitter GABA found in the brain
  • binding of neurotransmitter to post synaptic neurone, opens potassium gated channel, potassium ions diffuse out
  • inside of the neurone becomes even more negative and so is impossible to depolarise - hyper-polarised
  • no action potentials
• functions of the synapse:

  • enables impulses to be transmitted from one neurone to another, so enables circuits to function
  • neurotransmitter only released from pre-synaptic neurone, so nerve impulses can only be transmitted in one direction around a nerve circuit
• types of summation:

  • temporal
  • spatial
• summation - the addition of the neurotransmitter e.g acetylcholine
• temporal = 1 neurone to 1 neurone
• spatial = several neurones to 1 neurone
• temporal summation

  • single presynaptic neurone releases neurotransmitter many times over a short period of time
  • if the total amount of neurotransmitter exceeds the threshold value an action potential is set
• spatial summation

  • a number of different presynaptic neurones share the same synaptic cleft
  • together they can release enough neurotransmitter to create an action potential
  • multiple neurones
• acclimatisation

  • after repeated stimulation, a synpase could run out of vesicles containing the neurotransmitter
  • If this occurs, the nervous system no longer responds to the stimulus - fatigued
  • this is why you get used to a smell or noise, protects over stimulation of the effector, preventing damage
• acetylcholine broken down by acetylcholinesterase into choline and acetic acid