Synaptic Transmission

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Synaptic Transmission

• How a neuron communicates with another neuron and
  the effects of drugs on this process.
• Types of Neurotransmitters

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Neuron to Neuron

• Axons branch out and end near dendrites of
  neighboring cells
• Axon terminals are the tips of the axons
  branches
• A gap separates the axon terminals from dendrites
• Gap is the Synapse

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Steps to Synaptic Transmission

• Action Potential causes synaptic vesicle to open

• Neurotransmitter (NT) released into synapse

• NT locks onto receptor molecule in postsynaptic
  membrane (on receiving dendrite)
• Receptor site opens and allows positive sodium
  ions to enter the dendrite triggering action
  potential.

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Excitatory and Inhibitory Messages

• Excitatory messageincreases the likelihood that
  the postsynaptic neuron will activate
• Inhibitory messagedecreases the likelihood that
  the postsynaptic neuron will activate.

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Locks and Keys

• Neurotransmitter molecules have specific shapes

• Receptor molecules have binding sites

• When NT binds to receptor, ions enter

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Neurotransmitters Bonding at a Receptor Site
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Drugs Impact on Synaptic Transmission
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Some Drugs Work on Receptor Sites

• Some Drugs are shaped extremely similar to a NT
  and like a copy of a key unlock the receptor site
• Agonists fit receptor well and mimic the NT
  causing Action Potential
• e.g., nicotine

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Some Drugs Work on Receptor Sites

• Some drugs are shaped like neurotransmitters but
  do NOT unlock the receptor site thus blocking it
  from receiving natural NT.
• Antagonists fit the receptor but poorly and
  block the NT stopping the action potential
  message
• e.g., beta blockers

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Natural Neurotransmitters are like a key to a
lock. They bond to the receptor site unlocking
it
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Antagonist Drugs block the receptor site. Like a
key in the wrong lock, they wont open the
door/receptor site.
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Agonist drugs mimic neurotransmitters and open
the receptor sitejust like a copied key will
unlock a door.
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Types of Neurotransmitters

• Acetylcholine
• Serotonin
• Norepinephrine
• Dopamine
• Endorphins
• GABA
• Glutamate

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Acetylcholine (Ach)

• Excitory neurotransmitter found in neuromuscular
  junctions
• Involved in muscle contractions
• Involved in learning and memory

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Disruption of Acetylcholine Functioning

• Curareblocks ACh receptors
• paralysis results
• Nerve gases and Black Widow spider venom mimics
  Ach too much ACh leads to severe muscle spasms
  and possible death

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Disruptions in ACh Functioning

• Cigarettesnicotine works on ACh receptors by
  mimicking ACh
• can artificially stimulate skeletal muscles,
  leading to slight trembling movements

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Alzheimers Disease

• Deterioration of memory, reasoning, and language
  skills
• Low levels of Ach found in those with Alzheimers
  disease
• Symptoms may be due to loss of ACh neurons

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Serotonin

• Involved in mood, hunger, sleep
• Low levels involved in depression
• Prozac works by keeping serotonin in the synapse
  longer, giving it more time to exert an effect
  (re-uptake inhibitor)

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Norepinephrine

• Arousal, learning memory
• Fight or flight response
• Low levels found in those with depression.

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Dopamine

• Involved in movement, attention and learning.
  Also pleasure rewarding sensations.
• Too much Dopamine involved in schizophrenia.
  Thorazine blocks it.
• Drugs like cocaine and nicotine mimic dopamine
• Loss of dopamine-producing neurons is cause of
  Parkinsons disease. Give L-Dopa (converts to
  dopamine) to combat this.

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Parkinsons Disease

• Results from loss of dopamine-producing neurons
  in the substantia nigra
• Symptoms include
• difficulty starting and stopping voluntary
  movements
• tremors at rest
• stooped posture
• rigidity
• poor balance

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Parkinsons Disease

• Treatments
• L-dopa
• transplants of fetal dopamine-producing
  substantia nigra cells
• adrenal gland transplants
• electrical stimulation of the thalamus has been
  used to stop tremors

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GABA

• Inhibition (slows down) of brain activity
• Influences anxiety when in low supply.
• Antianxiety medicationss Valium Xanax increase
  GABA activity slowing down the brain.
• Alcohol mimics GABA
• Plays a dual role in sleep day excites the
  brain, night slows down the brain.
• Huntingtons disease involves loss of neurons in
  striatum that utilize GABA
• Symptoms
• jerky involuntary movements
• mental deterioration

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Glutamate

• Major excitatory neurotransmitter
• Too much glutamate (and too little GABA)
  associated with epileptic seizures

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Endorphins

• Control pain and pleasure by blocking the
  neurotransmitter Substance P which transmits pain
  messages to the brain.
• Released in response to pain
• Morphine and codeine work on endorphin receptors
  involved in healing effects of acupuncture
• Runners high feeling of pleasure after a long
  run is due to heavy endorphin release

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Summary

• Neuron structure
• Action potentials
• Synapse
• Neurotransmitters
• Receptors and ions
• Agonists and antagonists