Neurophysiology NEUROTRANSMISSION

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Chapter 12

• Neurophysiology NEUROTRANSMISSION

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Chemical Synapse Vocabulary

• Presynaptic neuron
• Postsynaptic neuron
• Synapse means of communication between each
  neuron and the next cell space
• Excitatory neurotransmitters chemicals that
  cause nerve impulses (e.g. ACh, glutamate)
• Inhibitory neurotransmitters chemicals that
  inhibit nerve impulses (e.g.gama aminobutyric
  acid GABA)

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Steps of Synaptic Transmission

• Step 1 Action potential arrives at the axon
  terminal
• Step 2 Release of neurotransmitter from a
  vesicle
• Step 3 Neurotransmitter binds to receptor site
  on ion channel on postsynaptic neuron
• Step 4 Ions cross the membrane through open
  channels
• Step 5 The influx of ions causes action
  potential in postsynaptic neuron (details are
  coming up - ?)
• Step 6 Removal of neurotransmitter
• www.mind.ilstu.edu/flash/synapse_1.swf

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Major Neurotransmitters in the Body

• Acetylcholine regulates muscles and memory
  mostly excitatory
• Dopamine produces feelings of pleasure mostly
  inhibitory
• GABA major inhibitory neurotransmitter in the
  brain
• Glutamate - major excitatory neurotransmitter in
  the brain
• Seratonin involved in many functions including
  mood, appetite, and sensory perception
  inhibitory in pain pathways
• Norephinephrine regulates normal brain
  processes and is a part of the fight-or-flight
  response usually excitatory

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Resting Membrane Potential (RMP)
The outside of the cell is more positive (Na)
than the inside (K)

• Electrical charge gradient associated with the
  cell membrane typically -70 millivolts

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Protein Channels

• Ion protein channels are chemically, mechanically
  or voltage regulated
• Ion channels open in response to the particular
  stimulus and allow ions to flow in or out the
  cell
• Flow of ions changes the membrane
  potential/voltage
• Sudden change in membrane potential that
  accompanies activity action potential (nerve
  impulse)

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A Special Ion Channel Na/K ATPase

• When a neuron is at rest, there is a slow leakage
  of Na into the cell K out of the cell (along
  concentration gradient)
• Na/K ATPase pumps 3 Na out and 2 K in per ATP
  hydrolysis thus prevents reaching equilibrium
  of Na K ions

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Steps of Action Potential

• Step 1 Resting
• Step 2 Depolarization
• Step 3 Repolarization
• Step 4 Return to normal permeability

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• Step 1 Resting (-70 mV)
• Step 2 Depolarization
• Neurotransmitters bind to their receptors on a
  postsynaptic neuron, chemically gated Na
  channels open, Na flows in, and local potential
  reaches a threshold limit (-55mV)
• Then, voltage-gated Na channels open and Na
  ions rush into the cell
• Membrane potential reaches 30 mV

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• Step 3 Repolarization
• Na channels close when the inside of the axon
  becomes sufficiently positive (30 mV)
• Voltage-regulated K channels open K flows out

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• Step 4 Return to resting potential
• Ion movements drive the membrane potential back
  toward resting membrane potential value
• Na/K ATPase continues pumping ions, adjusting
  levels back to resting equilibrium levels
• Hyperpolarization - briefly the exterior of the
  membrane is more negative than resting potential
  voltage level
• Refractory period - the time during which a nerve
  cell cannot generate another action potential
  despite stimulation

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And Another Look

• www.blackwellpublishing.com/matthews/channel.html
• www.blackwellpublishing.com/matthews/actionp.html

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• Closer Look at Action Potential
• http//itc.gsw.edu/faculty/gfisk/anim/actionpotent
  ial.swf