The Synapse

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The Synapse

• Charles Scott Sherrington (1852-1952)
• Studied with Cajal
• Proposed that neurons communicate at their
  junctions, which he named the SYNAPSE

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The Synapse

• How did Sherrington learn about the synapse?
• Did he mostly study the BRAIN or BEHAVIOR?
• He studied the REFLEX
• Stimulation to skin --gt muscle movement
• Brain not necessary

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The Synapse
Whats missing?
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The Synapse

• Sherrington might pinch the foot of a dog
• Dog raised that foot, and extended others?
• What happened when he cut the spinal connection
  from this reflex arc to the brain?
• What about the reflex arc helped him understand
  how the synapse operates?

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The Synapse

• Does the action potential travel across the
  synapse?
• How might you be able to tell?
• What if multiple inputs to the reflex arc occur
  (suppose you could pinch 3 or 4 places on the
  dogs foot)? Does each cause the same reflex?
• How could you test it?

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The Synapse

• One test if the action potential travels across
  the synapse, then the time from A to B should
  relate how to the time from C to D

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The Synapse

• Another test if multiple reflex arcs are
  simulated, what kind of response occurs?

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The Synapse

• Answer to 1 if the time from A to B is NOT the
  same as from C to D, we can infer that the action
  potential does not travel across the synapse, but
  some other kind of communication occurs
• Answer to 2 the second case occurs. This means
  the inputs are SUMMED

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The Synapse

• What is information in the brain?
• Inputs are summed.
• What does summed mean?
• Summed in intensity?
• Summed in time?
• How does time vary?
• Inputs
• Output

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The Synapse

• Sherrington also observed that as the dogs foot
  is pinched and some muscles are excited, others
  relax
• What is information?
• A balance between competing information flows
• Examples at higher levels?
• Humans have GREATER CONTROL
• What part of the brain do they use?

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The Synapse

• What about the system Sherrington studied allowed
  him to infer so much about neuron behavior,
  without seeing or touching specific neurons?
• Anatomy of the system
• A perfect laboratory
• Input neuron, interneuron, ouput neuron

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The Synapse

• Communication across a synapse is different than
  communication along a neuron
• A single pinch might not always cause the reflex
  to happen THRESHOLD
• Temporal summation of multiple stimuli
• In temporal summation, time matters
• The time between stimuli tells us several things

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The Synapse

• The input decays
• How fast it decays
• The inputs must be more rapid than the decay rate
  to summate
• Remember spike trains and information?
• Remember depolarization?
• John Eccles later confirmed Sherringtons
  findings with electrodes in cells measuring
  changes in potentials (electrical stimulation and
  recording)

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The Synapse

• Graded potential
• Not all or nothing like an action potential
• Positive (depolarization)
• Negative (hyperpolarization)
• What is mechanism?
• What elements are involed?

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The Synapse

• EPSP excitatory postsynaptic potential (positive
  change, depolarization)
• What IONS? Sodium ions move INTO the cell

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The Synapse

• Magnitude decreases
• Across time
• Across space
• Information?
• (illustrate next slide)
• Summation, competition and timing

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Spatial and Temporal summation