Synaptic Transmission

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Synaptic Transmission

• Chapter 4
• Pages 95-122

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Chemical Synapses

• Most synapses in the brain are chemical.
• Electronically coupled gap junction synapses
  occur in special circumstances (embryos, cardiac
  muscle) where close coordination needed.
• Chemical synapses release a chemical into the
  synaptic cleft which transfers information (a
  neural signal) from one neuron to another.

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How a Synapse Works

• The presynaptic neuron experiences an action
  potential.
• Presence of the action potential at the terminal
  button permits calcium (Ca) to enter the neuron
  via voltage-gated calcium channels.
• Calcium triggers the release of neurotransmitter
  from synaptic vesicles.
• Neurotransmitter crosses the synaptic cleft and
  opens ion channels in the post-synaptic neuron.

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Ion Channels

• Found in all cells throughout the body.
• Open and close in response to signals.
• Selectively permeable to specific ions
• High rate of flow (conductance)
• Resting channels usually open
• Gated channels open and close
• Refractory period temporarily cannot be opened

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Control of Gating

• Binding of neurotransmitters, hormones, or second
  messengers from within the cell.
• Voltage-gated responds to a change in the
  membrane potential.
• Stretch or pressure gated mechanical forces.
• Phosphorylation energy comes from a phosphate
  that binds with the channel.
• Dephosphorylation removal of the phosphate.

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Kinds of Receptors

• All neurotransmitters bind and act at more than
  one kind of receptor.
• Two main kinds of receptors
• Ion channel receptors
• G-protein-coupled receptors

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Effects of Drugs

• Exogenous ligands drugs that come from outside
  the body.
• Endogenous ligands naturally occurring
• Agonist binds with and opens a channel.
• Endogenous or exogenous (e.g., drug)
• Receptors are often named for their agonists
• Antagonist binds with and closes a channel.
• Reversible (curare) or irreversible (snake venom)

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Three Kinds of Neurotransmitters

• Amino acids released from synaptic vesicles
• Monoamines released from synaptic vesicles
• Peptides larger molecules released from
  secretory granules.
• There may be more substances that act as
  neurotransmitters

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Amino Acids

• Glutamate (Glu) excitatory, widespread.
• GABA inhibitory, widespread.
• Glycine (Gly) inhibitory at some locations.
• The poison Strychnine blocks GABA receptors
  interfering with inhibition so excitations
  overwhelm the brain.

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Monoamines

• Cholinergic Acetylcholine (ACh), used by
  muscles
• Catecholaminergic regulate thinking, mood,
  synethesized from tyrosine
• Dopamine (DA)
• Norepinephrine (Noradrenaline) (NE)
• Epinephrine (Adrenaline) widespread
• Serotonin (5-HT) broken down by MAO
• LSD binds at 5-HT receptors.

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Peptides

• Formed by chains of amino acids.
• Oxytocin released during childbirth to regulate
  maternal behavior.
• Vasopressin regulates proper blood volume and
  salt concentration in body.
• Opioids (endorphins) naturally occurring
  painkillers
• Morphine binds with opioid receptors.