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Synaptic Transmission . Chapter 4 Pages 95-122. 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.

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synaptic transmission

Synaptic Transmission

Chapter 4

Pages 95-122

chemical synapses
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.
how a synapse works
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.
ion channels
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
control of gating
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.
kinds of receptors
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
effects of drugs
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)
three kinds of neurotransmitters
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
amino acids
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.
monoamines
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.
peptides
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.