Synapses

• TYPES OF SYNAPSES

  • point in which two neurons form a connections
  • aim --> transmit singals from one neuron to another
  • presynaptic neuron --> before the synapse
  • postsynaptic neuron --> after the synapse
  • synaptic cleft --> gap between pre and post
  • pos-synaptic potential --> excitatory or inhibitory protentials 
  • one way transmission --.> rectifying 
  • two way transmission --> non-rectifying
• ELECTRICAL SYNAPSES

  • formed where two neurons have a really close connection --> forms a gap junction --> joined together through connnecons
  • direct transfer of ions
  • non-rectifying
  • fast transmission
  • single often attenuated --> signal is smaller in second neuron
  • gap junction made up of 2 connexons  each connexon has 6 connexins
  • highly reliable --> useful in systems in escape reflex or coordinate large number of cells in a tissue (heart)
  • aren't flexible --> not as common
• CHEMICAL SYNAPSE

  • very flexible 
  • synaptic delay  time taken for it to cross the synaptic cleft
  • vesicular release of neurotransmitter is released into the synaptic cleft

  1. snare proteins
  2. synaptobrevin --> on the vesicLes
  3. SNAP-25 --> on the nerve terminal membrane
  4. syntaxin --> on the nerve terminal membrane
  5. SNARE proteins interact and bind to each other, drawing the vesicle close to the membrane
  6. when calcium increase  vesicle fuses with the membrane release ach into the synapstic cleft
  7. calcium is sensed by synaptotagmin --> causes a conformational change in the snare proteins 
• VESICLE RECYCLING

  • clathrin coat the outside of the vesicle  bind to the vesicle but form interactions with outher clathrin molecules
  • forms a cage like structure that coats the vesicle and drag it away form the membrane
  • size of the cage is defined by the size of clathrin  vesicles are usually abojut the same size  constant number of vesicle
  • maintains constant size of the terminal
• ACTIVE ZONE

  • where neurotransmitters are being released
  • energy-consuming --> lots of mitochondria
  • signals need to be turned off:

  1. enzyme (ach) can break down the neurotransmitter
  2. there is an uptake system for neurotransmitters 
• QUANTAL HYPOTHESIS

  • neurotransmitter release is quantal
  • miniture end plate potential vary in a step way manner  distribution of the amplitude vary in a integer multiplier
  • 1 quantum --> amount of transmitter per vesicle 
  • quantal content = epp / 1 quantum current 

  1. average zize of a full end plate potential

  • miniture postsynaptic potentials

  1. occur spontaneously even when no calcium is present
  2. have amplitudes that are multiples of a quantal unit
  3. sue to release of one or a few quanta 
• QUANTAL HYPOTHESIS cont.

  • evoked release (epp)

  1. as extracellular calcium is lowered --> epsp amplitude decreases in a step-wise manner
  2. quantal analysis of ap-evoked epsps show that they involve release of up to 200 quanta per ap
  3. each quantum contains several thousands molecules of ach

  
• TETANUS

  • caused by anaerobic bacterium --> clostridium tetani
  • toxin cleaves snare proteins  --> causes muscle spasm 
  • binds to and enters the presynaptic nerve terminal --> in the skeletal muscle meuromuscular junction --> released in the dendrietes and destroy snare proteins in inhibitory neurotransmitter --> stops the release of gaba or glycine