Chapter 5

• How do neurons communicate?

  through synapses
• Neuron A = presynaptic

  Neuron B = postsynaptic

  
• What is the synapse?

  space between 2 neurons
• Neuron communication- image

  1. synaptic cleft 2. vessicles 3. reuptake pump 4. neurotransmitter 5. receptor

  
• What was Loewi's experiment?

  wanted to make sure that hearts couldn't communicate electrically (so: physically separated them)

  discovery: stimulating vagus nerve slowed down heartbeat from left AND right heart

  conclusion: transmission of information between neurons happens chemically

  
• What are neurotransmitters?

  Chemical messengers that transmit signals between neurons in the brain and nervous system

  !! only chemical substance!!
• What is the presynaptic terminal button?


  Endpoint of axon
• Where is the synaptic cleft located?

  between pre- & postsynaptic membrane
• What do synapses consist of?

  • presynaptic terminal button
  • synaptic cleft
  • postsynaptic membrane
• Neurochemical synapse components:

  • Action potential (generated by presynaptic neuron) leads to exocytosis of a neurotransmitter from presynaptic terminal button into synaptic cleft
  • neurotransmitter binds to postsynaptic membrane & causes change in resting potential of postsynaptic neuron (EPSP or IPSP)
• What is exocytosis?

  = form of active transport; cell transports molecules (e.g., neurotransmitters and proteins) out of the cell
• Neurochemical synapse components- image

  1. postsynaptic membrane 2. synaptic cleft 3. presynaptic membrane 4. synaptic vessicles 5. postsynaptic receptor

  
• What are the 4 steps of synaptic transmission?

  1. Synthesis & packaging
  2. Release
  3. Receptor action at postsynaptic membrane
  4. Inactivation
• What are & where do the first steps synthesos & packaging happen in the synaptic transmission?

  • cell body (DNA, mRNA)
  • axon terminal (precursor chemicals derived from food)
  • building blocks of transmitter substance -> imported into the terminal
  • in terminal: neurontransmitter is synthesized & packaged into vessicles
• What are vessicles?

  packages of neurotransmitters
• What happens during the 'release' in the synaptic transmission?

  • Calcium influx triggered by action potential
  • release into synaptic cleft (exocytosis)
• What happens during 'receptor action at postsynaptic membrane' in a synaptic transmission?

  • depolarization (excitation)
  • hyperpolarization (inhibition)
  • modulation (inhibit or excite otehr chemical reactions)
  • so: transmitter crosses synaptic cleft & binds to receptor
• What happens during 'inactivation' in a synaptic transmission?

  transmitter is either taken back into terminal OR inactivated in synaptic cleft

  • diffusion away from synaptic cleft
  • degradation by enzymes
  • reuptake in presynaptic cell
  • uptake by glial cells (astrocytes) -> glial cells eat up all of abandoned neurotransmitters
• synaptic transmission (lecture explanation)

  first: need neurotransmitters

  2 options of getting them (synthesis):

  1. from proteins 2. from substance in a fluid (e.g. from food)

  release: calcium necessary to get vessicles to fuse (= release their neurotransmitters)

  Inactivation: necessary to make process temporary (bc too much activation/ inhibition isn't good)

  degradation: enzymes actively eat neurotransmitters
• What is degradation?

  enzymes actively eat neurotransmitters
• synaptic transmission - image

  
• What is a Quantum?

  = contents of 1 synaptic vessicle (so: limit of vessicle)
• What does the amount of neurotransmitter being released depend on?

  • amount of Calcium entering axon terminal
  • number of vesicles docked at the membrane
• Is one Quantum enough to induce action potential at postsynaptic cell?

  NO, many quanta needed
• Neurotransmitter release - image

  
• Neurotransmitter release

  
• Variety of synapses

  all components can be connected with each other

  
• What is the connection for an axo-dendritic synapse?

  from axon to dendrite
• What is the connection for an axo-somatic synapse?

  from axon to cell body
• Varieties of synapses- focus on red boxes

  
• Excitatory & inhibitory synapses- Where does excitation take place?

  at dendrites
• Excitatory and inhibitory synapses- Where does inhibition take place?

  at the cell body
• What are characteristics of excitatory synapses?

  • at dendrites
  • round vesicles
  • high density (both pre- & postsynaptical)
  • wide synaptic cleft
  • large active zone
• What are characteristics of inhibitory synapses?

  • at cell body
  • flat vessicles
  • low density (both pre- & postsynaptical)
  • narrow synaptic cleft
  • small active zone
• Do neurotransmitters themselves determine excitation or inhibition?

  No, but the receptors do
• Excitatory and inhibitory synapses- Which image belongs to what type?

  Excitatory synapse (TYPE I): Image 1 Inhibitory synapse (TYPE II): Image 2

  
• What are the 4 'classical' criteria to determine whether a chemical substance is a neurotransmitter?

  1. Synthesized or present in the neuron
  2. When released, must produce response in target cell
  3. Experimental placement must result in same response
  4. Mechanism of removal must exist
• What are putative neurotransmitters?

  substances that do not (yet) fulfill the 4 classical criteria for neurotransmitters
• Neurotransmitters-image

  
• What does the 'classical' neurotransmitter do?

  Causes a potential difference at the postsynaptic membrane (EPSP, IPSP)