Signal Transduction Mechanisms: I' Electrical Signals in Nerve Cells II

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Signal Transduction Mechanisms I. Electrical
Signals in Nerve Cells II

• Reading Becker, ch. 9, pp. 236-253

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Action Potential

• Not all depolarizations result in an action
  potential
• Some do not reach threshold

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Subthreshold Depolarizations

• Neuron receives 2 depolarizing stimuli through
  the same synapse
• Each leads to subthreshold depolarization
• In between, membrane potential repolarizes back
  to resting level
• Why does the membrane repolarize?

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• Na flowing in after depolarizing stimulus causes
  the inside to become more positive
• Relieves the electrical constraint on K not
  attracted back in as readily
• Net outward leakage of K increases during a
  depolarizing event
• When ligand-gated Na channels close, increased
  outward leakage of K continues until membrane
  potential returns back to resting level

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Temporal and Spatial Summation
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Repolarization and Hyperpolarization
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Neurons Work In Pathways (Or Circuits)
Neuronal Cell Body
Synaptic Cleft
Axon
Axon Hillock
Synapse
Pre-Synaptic Neuron
Post-Synaptic Neuron
Dendrite
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Passive Spread of Depolarization
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Propagation Of The Action Potential
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The Myelin Sheath

• Glial cells wrapped tightly around axon
• Oligodendrocytes in CNS Schwann cells in PNS
• Function to insulate the axon
• Nodes of Ranvier spaces in between glial cells
  where axonal membrane is exposed
• Nodes of Ranvier contain high concentrations of
  ligand-gated Na channels only spots along axon
  where action potential can be generated!

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Myelination Speeds Up Conduction

• Action potentials jump from node to node
• Saltatory conduction
• Insulation allows for passive spread of
  depolarization to cover greater distance
• Action potentials propagated more rapidly

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Action Potential Leads to Ca Influx At Axon
Terminal
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Ca Required For Release Of Neurotransmitter
Into Synaptic Cleft
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Neurotransmitter Triggers Depolarization of
Post-Synaptic Neuron
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Neurotransmitters

• Signaling molecules synthesized by neurons
• Packaged into neurosecretory vesicles
• Vesicles are targeted to axon terminals, where
  they are held in place by
• cytoskeletal elements underneath the PM
• Can be either excitatory or inhibitory

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Neurotransmitters

• Excitatory
• Excite the post-synaptic neuron
• Trigger influx of Na and subsequent
  depolarization
• Inhibitory
• Inhibit the post-synaptic neuron
• Bind to ligand-gated channels that allow Cl-
  to enter cell
• Hyperpolarization

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Ca And The Release Of Neurotransmitter

• Ca releases neurosecretory vesicles from the
  cytoskeleton in the axon terminal
• Ca is involved in docking and fusion of vesicle
  with membrane