Transmission of Impulse to the Next Cell

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Transmission of Impulse to the Next Cell

• Synapses

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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

3
Synapse

• Presynaptic axon end (synaptic knob)
• Gap synaptic cleft
• Postsynaptic membrane

4
Locations of synapses

• Neuromuscular junction
• axon to muscle fiber
• Axon to dendrite
• Axon to cell body
• Axon to axon

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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.

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Synapse

• Neurotransmitter can't remain in cleft (would
  continue to stimulate uncontrollably)
• ACh removed by acetylcholinesterase (AChE)
• Acetate choline reabsorbed by axon end,
  resynthesized to ACh
• Other neurotransmitters taken back by axon or
  diffuse away

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Neurotransmitters may be

• Excitatory
• depolarize postsynaptic membrane
• Excitatory Postsynaptic Potential (EPSP)
• Inhibitory
• hyperpolarize postsynaptic membrane
• Inhibitory Postsynaptic Potential (IPSP)

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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.

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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 gt threshold, then yields
  Action Potential

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Temporal summation

• Add effects of neurotransmitters over short times
• at one synapse
• at 2 synapses

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Spatial summation

• Add effects of neurotransmitters adjacent in
  space (one post-synaptic neuron)
• at 2 synapses
• at same time

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NEUROTRANSMITTERS

• Six classes
• Acetylcholine
• Widespread in body most studied, best understood.

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NEUROTRANSMITTERS

• Six classes
• Biogenic amines

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NEUROTRANSMITTERS

• Six classes
• Amino acids

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NEUROTRANSMITTERS

• Six classes
• Peptides (small proteins)

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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

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CIRCUITS

• Groups of neurons work together.

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Serial processing

• Reflex arc
• Sensory neuron to
• Association neuron to
• Motor neuron to
• Effector

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Diverging circuits

• One input,
• Multiple outputs
• May amplify signal

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Converging circuits

• Multiple input,
• One output.
• Multiple sources of sensation converge on one
  response
• Logic ??

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Reverberating circuit

• One input,
• One output,
• Axon collaterals cycle impulses to previous
  neurons
• impulses "cycle
• Regular cyclic activity breathing, short-term
  memory

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Parallel after-discharge circuit

• One input,
• One output,
• several parallel pathways with impulses arriving
  over extended time.