Concept of Synapse

1. Synapse - close position between neurons [two types]
• Electrical - few in humans (in retina)
• Chemical - majority
2. Site of chemical release - Acts as one-way valve

3. Fatigues

4. Causes a delay in transmission

• 3 msec for first stimulus
• 0.6 msec after repeated stimulation
5. Decreases the probability of continued activity

6. Gives directionality to impulse

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Synaptic Transmission

• Neuronal influence
  • 1905 T. R. Elliott - adrenaline effects.
  • 1920 Otto Loewi - chemicals (heart decrease).
• Synaptic connection - flexibility
• Terminal ending contains small vesicles
  • Round vessels: excitatory neurotransmitter
  • Flat vessels: inhibitatory neurotransmitter
• Impulse: release of transmitter substance.
• An individual cell may have 3 different types of transmitter substance
• EPSP - IPSP: Neuromodulators - intermediator between hormones and neurotransmitters.
• Enzymes destroy the transmitter substance

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Three Types of Synapses

1. Axodendritic Synapse
   • Axon to dendrite
   • Excitatory
2. Axosomatic Synapse
   • Axon to cell body
   • Inhibitory
3. Axoaxonic Synapse
   • Axon to terminal endings
   • Presynaptic inhibition
   • Reduced polarity

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   Transmitter Substance

   • Acetylcholine, 5HT, Adrenaline, GABA
   • Fit with a receptor site
   • EPSP
   • IPSP (3mv Cl- movement into cell)
   • Occlusion
   • Facilitation

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   Release of Transmitter Substance

   Chem. Synapse figure

   Calcium controls the release of neurotransmitter

   • Vesicle Docking at Ca++ pore -- Most Ca++ outside cell
   • The amount released is directly related to the calcium concentration in the extracellular fluid.
   • High concentration - greater release of transmitter substance.
   • Action potential triggers opening of calcium-selective channels in membrane
   • With low concentration of calcium inside, this opening causes calcium ions to enter
   • Probably activates microtubules which force movement of vesicles.
   • This causes area of membrane to move apart
   • Vesicle membrane then fuses with cell membrane & releases transmitter (Exocytosis)
   • Vesicles are produced in cell body & are transported to terminal ending.
   • After considerable use, the vesicles are transported back to the cell for repair.
   • The excess membrane is removed at the axon-buton junction.

   Summation figure

   Transmitter Disposal & Termination of _PSP

   • Enzymatic degradation
     Acetylcholinesterase
     Destroys acetylcholine - broken down by enzymes
     Works mostly at neuromuscular junction
     
   • Reuptake
     Special carrier molecules in membrane - Capture molecules
     Transport transmitter back into cell; return for reuse
     More frequent process than Enzymatic degradation
     
     

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   Terms to Know

   Occlusion	Axosomatic synapse
   Facilitation	Axodendritic synapse
   IPSP	Axoaxonic synapse
   EPSP	Postsynaptic membrane
   Electrical synapse	Presynaptic membrane
   Transmitter substance

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   More Detailed Information on the Synapse

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