Transmission of Impulse to next cell by malj

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									                    Synapses

               Transmission of Impulse
                    to the Next Cell


24 Oct. 2012            Synapses.ppt     1
                      Synapses
  • Electrical synapses
        – “Gap junctions,” tiny holes connect
          cytoplasm of adjacent cells
        – A.P. continuous from cell to cell
  • Chemical synapses
        – Axon (& A.P.) ends short of next cell,
        – Synaptic knob (axon end) releases chemical
          transmitter

24 Oct. 2012              Synapses.ppt                 2
                      Synapse
  • Presynaptic
    axon end
    (synaptic knob)
  • Gap =
    synaptic cleft
  • Postsynaptic
    membrane



24 Oct. 2012           Synapses.ppt   3
                Locations of synapses
     • Neuromuscular
       junction
           – = axon to muscle
             fiber
     • Axon to dendrite
     • Axon to cell body
     • Axon to axon




24 Oct. 2012                    Synapses.ppt   4
                             Synapse
• When Action Potential
  reaches synaptic knob,
    Synaptic vesicles unite with
    membrane,
    Release neurotransmitter,
    Neurotransmitter diffuses
    across synaptic cleft,
    Neurotransmitter binds to
    receptors in postsynaptic
    membrane,
    Chemically gated channels
    open.

24 Oct. 2012                       Synapses.ppt   5
                            Synapse
• Neurotransmitter can't remain in
  cleft (would continue to stimulate
  uncontrollably)
     – ACh removed by acetylcholinesterase
       (AChE)
     – Choline reabsorbed by axon end,
       resynthesized to ACh
     – Other neurotransmitters taken back by
       axon or diffuse away




24 Oct. 2012                   Synapses.ppt    6
          Neurotransmitters may be …
       • Excitatory
               – depolarize postsynaptic membrane
               – Excitatory Postsynaptic Potential (EPSP)
       • Inhibitory
               – hyperpolarize postsynaptic membrane
               – Inhibitory Postsynaptic Potential (IPSP)




24 Oct. 2012                     Synapses.ppt               7
                      Neurotransmitters

 • Excitation (EPSP) must be post-synaptic
 • Inhibition (IPSP) may be
       – Post-synaptic (inhibitory neuron synapses
         with post-synaptic cell)
       – Pre-synaptic (inhibitory neuron synapses
         with pre-synaptic axon end)
               • blocks presynaptic A.P.


24 Oct. 2012                     Synapses.ppt        8
                   SUMMATION
• Several to many to very many axons synapse with
  one neuron
• Whether Action Potential occurs in postsynaptic
  neuron depends on
      – number of excitatory inputs (doses of transmitter)
      – number of inhibitory "
• If sum EPSP - IPSP > threshold, then yields
  Action Potential

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               Temporal summation:
• Add effects of neurotransmitters over short times
     – at one synapse
     – at 2+ synapses close together




24 Oct. 2012                Synapses.ppt              10
               Spatial summation:
  • Add effects of neurotransmitters adjacent in
    space (one post-synaptic neuron)
        – at 2+ synapses
        – at same time




24 Oct. 2012               Synapses.ppt            11
               NEUROTRANSMITTERS:
                • Six classes
                  – Acetylcholine
                     • Widespread in body; most studied,
                       best understood.




24 Oct. 2012                  Synapses.ppt                 12
               NEUROTRANSMITTERS:

   • Six classes
         – Biogenic amines
         – (Norepinephrine is usually a
           neurotransmitter; epinephrine
           [p. 388] is usually a hormone




24 Oct. 2012                     Synapses.ppt   13
               NEUROTRANSMITTERS:

      • Six classes
               – Amino acids




24 Oct. 2012                   Synapses.ppt   14
               NEUROTRANSMITTERS:

    • Six classes
          – Peptides (small
            proteins)




24 Oct. 2012                  Synapses.ppt   15
               NEUROTRANSMITTERS:
       • Six classes (Oops, 7)
               – Purines
                  • ATP & adenosine
               – Dissolved gases
                  • Nitric oxide (NO) “retrograde messenger”
                  • Carbon monoxide “retrograde messenger”
               – Endocannabinoid
                  • Binds to cannabinoid receptors
                      – “retrograde messenger”



24 Oct. 2012                       Synapses.ppt                16
                       CIRCUITS

               Groups of neurons work together.
                          See p. 413.


24 Oct. 2012                Synapses.ppt          17
                   Serial processing
• Reflex arc
     –   Receptor to
     –   Sensory neuron to
     –   Association neuron to
     –   Motor neuron to
     –   Effector




24 Oct. 2012                 Synapses.ppt   18
                   Diverging circuits
               • One input,
               • Multiple outputs
               • May amplify signal




24 Oct. 2012                Synapses.ppt   19
                 Converging circuits
    • Multiple input,
    • One output.
    • Multiple sources of
      sensation converge
      on one response
          – Logic ??




24 Oct. 2012            Synapses.ppt   20
                Reverberating circuit
   • One input,
   • One output,
   • Axon collaterals cycle impulses to previous neurons
         – impulses "cycle“
   • Regular cyclic activity; breathing, short-term memory




24 Oct. 2012                  Synapses.ppt                   21

								
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