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29 Cards in this Set

  • Front
  • Back
What are the two types of synapses?
Chemical and Electrical.
What characterizes electrical synapses?
Direct intercellular electrical connections through the ion channels in gap junctions.
Give an example of electrical synapses in the heart.
Intercalated discs.
How do APs use electrical synapses?
They travel through the gap junctions between cells and can spread quickly throughout an organ or tissue. This enables it to act like a syncytium.
What is a syncytium?
A multinucleate cell.
What characterizes chemical synapses?
The release of chemical neurotransmitters which either open postsynaptic channels or activate G-proteins to generate second messengers to open channels.
What happens to a neurotransmitter when it is released into the synaptic cleft?
It diffuses away, it is degraded by extracellular enzymes, or it is retrieved by the cell. Sometimes, it is broken down and parts of it are retrieved.
What mechanisms are used to retrieve neurotransmitters from extracellular space?
Endocytosis or special carrier transport mechanisms.
Why do neurotransmitters have to be retrieved, degraded, etc.?
The body must ensure that the action of the transmitter doesn't go on forever or that desensitization is not excessive.
What is desensitization to a neurotransmitter?
The process by which some ligand-gated ion channels become less responsive to the ligand with continued exposure.
What's the classic example of a chemical synapse?
The neuromuscular junction.
When an AP invading the terminal of a motorneuron depolarizes the terminal membrane, ____ open.
Voltage-gated calcium channels.
What causes release of ACh into the synaptic cleft?
Ca ions that rush into the pre-synaptic motorneuron through V-gated channels and cause fusion of synaptic vesicles (containing ACh) with the terminal membane.
How does ACh get to the postsynaptic (muscle) membrane?
It diffuses.
What does ACh do once it reaches the postsynaptic membrane?
It opens non-selective cation channels on the muscle membrane (ACh receptor channels), whose ionic current depolarizes the muscle membrane to threshold (endplate potential or EPP).
What is end plate potential (EPP)?
A local depolarization in the post-synaptic membrane caused when ACh opens non-selective cation channels on the muscle membrane. EPP is generally suprathreshold and the resulting AP stimulates muscle contraction.
What happens in myasthenia gravis?
EPPs are subthreshold. This is an autoimmune disease in which ACh receptor channels are destroyed, leading to muscle weakness. This is an example of postsynaptic disruption of chemical synaptic transmission.
What is used to treat myasthenia gravis?
Neostigmine, an inhibitor of acetylcholinesterase. The idea is to allow ACh to bind to the few channels that are left. More ACh=more chance to bind.
What is alpha-bungarotoxin?
A snake toxin that disrupts postsynaptic chemical transmission by irreversibly blocking ACh receptor channels.
What is curare?
A plant toxin that disrupts postsynaptic chemical transmission by reversibly blocking ACh receptor channels.
What's an example of pre-synaptic disruption in chemical synaptic transmission?
The use of Ca channel blockers, and botox injections. Botox has botulinum toxin A, which breaks down a SNARE needed for fusion of the synaptic vesicles.
How are nerve-nerve synapses different from nerve-muscle synapses?
1) Nerve cell postsynaptic potentials are usually subthreshold and quite small.
2) Nerve cells have IPSPs as well as EPSPs.
3) Nerve cells must integrate numerous inputs and decide whether to fire an AP.
What usually causes an EPSP?
Opening of non-selective cation channels.
What AAs are usually used to cause EPSPs in the brain?
Glutamate and aspartate.
What causes IPSPs?
Opening of either K selective or Cl selective ion channels. Most common IPSP neurotransmitter in the brain is GABA. These tend to hyperpolarize.
What's the reversal potential for an EPSP?
Non selective cation channel. Vrev=0.
What's the reversal potential for an IPSP?
Near the Nernst for K or Cl.
Why does myelin work in impulse transmission?
Very few ions leak, so there is abundant positive charge to depolarize the axon at nodes of Ranvier.
What are the 3 functional regions of a nerve cell?
1) Input w/ convergence-dendrites, soma
2) Impulse initiation-axon hillock
3) Impulse conduction-axon