Synaptic transmission

• Synapse


  The point where two neurons almost but not quite touch
• Synapse


  • The electrical impulses can't cross from the axon terminal to the adjacent dendrite, so it must pass information on as chemical messages
  • The gap between the presynaptic and postsynaptic neurons is tiny, about 20 to 40 nanometers
• Neurotransmitters


  The chemical messages that pass information across the synapse
• Synaptic transmission


  1. Action potential arrives at axon terminal

  2. Vesicles merge with presynaptic membrane, releasing neurotransmitters into synaptic cleft

  3. Neurotransmitters diffuse across synaptic cleft and bind to receptors on postsynaptic neuron

  4. Summation of excitatory and inhibitory effects on postsynaptic neuron

  5. If threshold reached, new action potential forms and travels down next axon

  6. Neurotransmitters are broken down or recycled back into presynaptic neuron
• Summation


  The effects of all the excitatory and inhibitory neurotransmitter influences on the postsynaptic cell, which are added and subtracted
• Excitatory neurotransmitters


  Make the electrical charge more positive in the postsynaptic cell, increasing the likelihood of a new action potential
• Inhibitory neurotransmitters


  Make the electrical charge more negative in the postsynaptic cell, decreasing the likelihood of a new action potential
• Communication across the synapse is unidirectional, meaning information signaling cannot happen in reverse
• Neurons can fire between 5 and 50 times per second
• Psychoactive drugs can interfere with the synaptic transmission system, either inhibiting or increasing the transmission of certain neurotransmitters
• The human brain has around 100 billion neurons and 170 trillion synapses, which is more than the number of stars in the Milky Way galaxy
• It would take around 5,500 years to count all the synapses in the brain at a rate of 100 per second