4: Neural Conduction and Synaptic Transmission Flashcards
Why isn’t dopamine an effective treatment for Parkinson’s disease?
Because it doesn’t penetrate the blood-brain barrier effectively.
What is the membrane potential?
The difference in electrical charge between the inside and the outside of the cell.
What do microelectrodes do?
They record the membrane potential of neurons.
What is the average resting potential of a neuron?
-70mV
What is the term for a neuron that has a resting membrane potential?
Polarised.
What is the ionic explanation for the resting potential?
There are more negative than positive charges outside the neuron than inside it. Unequal ratio.
What are the 4 ions that contribute to resting potential?
- Sodium/Na+
- Potassium/K+
- Chloride/Cl-
- Protein
What are the passive and active factors that contribute to the unequal distribution of ions in the resting neuron?
- Passive factors continuously drive K+ ions out of the resting neuron and Na+ ions in.
- ACTIVE pumping required to do OPPOSITE of passive factors.
What is the role of sodium-potassium pumps?
- To take Na+ ions out of the neuron, and to bring K+ ions inside the neuron.
- It is a type of ‘transporter’.
What does depolarization of the neuron do?
- Decreases resting membrane potential from -70mV to -67mV
- Causes an excitatory postsynaptic potential (EPSP).
What does hyperpolarization of the neuron do?
- Increases the resting membrane potential from -70mV to -72mV.
- Causes an inhibitory postsynaptic potential (IPSP).
What are graded responses?
- EPSPs and IPSPs
- A weak signal fires weak PSPs, strong signal fires strong PSP.
What is meant by PSPs being decremental?
Their signal fades over distance.
Where are action potentials generated?
NEXT to the axon hillock on the axon.
What mV is the threshold of excitation?
-65mV
What type of response is an action potential?
An all-or-none response.
What is integration in an action potential?
The adding up of all the EPSPs and IPSPs, then decided whether it exceeds the threshold of excitation or not.
What do voltage-activated ion channels do (2)?
- Responsible for the activation and conduction of action potentials.
- Open or close in responses to membrane potential level changes.
What is the absolute refractory period?
A brief period once an action potential has been fired in which a second one cannot be fired.
What is the relative refractory period?
A brief period after the absolute refractory period whereby the firing of a second action potential is possible, providing more-than-normal levels of stimulation are provided.
Why are APs nondecremental, but PSPs are decremental?
Because the former is an active conduction, whereas the conduction of the latter is passive.
What is antidromic conduction?
Conduction going from the terminal end towards to cell body.
What is orthodromic conduction?
Conduction going from the cell body to the terminal buttons.
What are the nodes of Ranvier?
- The gaps between adjacent myelin sections.
- Where the sodium channels are based in myelinated axons.
What is saltatory conduction?
The conduction of action potentials in myelinated axons.
What is the average conduction speed of myelinated mammalian neurons?
100m per second.
What is the average conduction speed of unmyelinated mammalian neurons?
1m per second - HUGE difference from myelinated axons!
Average conduction speed in human motor neurons…
60m per second.
Where do many axodendritic synapses terminate?
On dendritic spines.
What is the advantage of presynaptic facilitation and inhibition (as opposed to PSPs)?
They can selectively influence a particular synapse rather than the whole neuron.
What is the difference between directed and nondirected synapses?
The sites of neurotransmitter release and reception are:
- Close to each other in directed synapses
- Distant from each other in nondirected synapses.
What are neuropeptides?
- Large neurotransmitters
- Short amino-acid chains comprising between 3-36 amino acids.
Where and how are small-molecule neurotransmitters synthesized?
In the cytoplasm of the terminal button, where they are packed into the synaptic vesicles by the button’s Golgi complex.
Where and how are neuropeptides synthesized?
In the cytoplasm of the cell body on ribosomes, and are then packaged by vesicles in the cell body’s Golgi complex and transported by microtubules.
What is coexistence?
When neurons contain 2, not 1, neurotransmitters.
What is exocytosis?
The process of neurotransmitter release.
Briefly describe the exocytosis of small-molecule neurotransmitters.
- A temporary influx of Ca+ ions from the voltage-activated calcium channels near the presynaptic membrane causes an action potential.
- In turn, this releases the small-molecule neurotransmitters in a pulse-like manner.
How do released neurotransmitters produce signals in postsynaptic neurons?
By binding to receptors.in the postsynaptic membrane.
What is a ligand?
A molecule that binds to another.
A neurotransmitter is a ligand of its receptor.
What are receptor subtypes?
The different types of receptors to which a particular neurotransmitter can bind.
What influences postsynaptic neurons by the binding of a neurotransmitter to a receptor subtype?
Whether the receptor is ionotropic or metabotropic.
What are ionotropic receptors?
Receptors that are associated with ligand-activated ion channels.
What are metabotropic receptors?
Receptors associated with signal proteins and G proteins.
The opening of sodium channels, increasing the flow of Na+ ions into the neuron causes what?
An EPSP (depolarization).
The opening of potassium or chloride channels, increasing the flow of K+ ions or Cl- ions causes what?
An IPSP (hyperpolarization).
What does a second messenger do?
It diffuses throughout the cytoplasm, potentially influencing neuronal activity.
Can influence genetic expression by binding to DNA.
What are autoreceptors?
- A type of metabotropic receptor with unusual features.
What are the 2 unusual features of autoreceptors?
- They bind to their own molecules
- Located on the presynaptic, rather than the postsynaptic, membrane.
The difference in how neuropeptides and small-molecule neurotransmitters are released is directly reflected by the types of effects they have. What are these?
- Neuropeptides: Slow, diffuse, and long-lasting signals.
- Small-molecule neurotransmitters: Rapid, brief, excitatory or inhibitory signals to nearby cells.
How does reuptake work?
Transporters immediately bring back the released neurotransmitters into the presynaptic buttons.
How does enzymatic degradation work?
Enzymes break down the released neurotransmitters in the synapse.
What enzyme breaks down acetylcholine?
Acetylcholinesterase.
How does the the recycling of neurotransmitters and vesicles work?
The terminal button draws in the released and broken down vesicles and neurotransmitters and recycles them.
What 4 remarkable findings are there for astrocytes?
- Release chemical transmitters
- Contain receptors for neurotransmitters
- Conduct signals
- Involved in neurotransmitter reuptake
What are gap junctions?
- A.K.A electrical synapses
- Narrow spaces between neurons bridged by connexins.
How are electrical signals and small molecules able to pass through the gap junctions to adjacent neurons?
By going through the connecting cytoplasm contained by the connexins.
What are the 4 classes of small-molecule neurotransmitters?
- Amino acids
- Monoamines
- Acetylcholine
- Unconventional neurotransmitters
What are the 4 most widely studied amino acid neurotransmitters?
- Glutamate
- Aspartate
- Glycine
- Gamma-aminobutyric acid (GABA)
What is GABA a synthesis of?
Glutamate.
Which amino acid is the most prevalent excitatory transmitter in the CNS?
Glutamate.
Which amino acid is the most prevalent inhibitory transmitter in the CNS?
GABA.
What are the 4 monoamine neurotransmitters?
- Dopamine
- Epinephrine
- Norepinephrine
- Serotonin
What are the two divisions of the monoamine neurotransmitters?
- Catecholamines
- Indolamines
Which 3 monoamine neurotransmitters are catecholamines, and what are they synthesized from?
- Dopamine
- Norepinephrine
- Epinephrine
They are synthesized from tyrosine:
Tyrosine -> L-dopa -> Dopamine -> Norepinephrine -> Epinephrine
Which monoamine neurotransmitter is an idolamine, and what is its synthesis?
- Serotonin (a.k.a 5HT)
- Synthesized from tryptophan.
What is one class of unconventional neurotransmitters, and what two chemical compounds does it contain?
- Class: Soluble-gas neurotransmitters.
Contains: - Nitric oxide
- Carbon monoxide
What are the endocannabinoids?
A class of unconventional neurotransmitters.
What psychoactive constituent of marijuana are endocannabinoids similar to?
THC (delta-9-tetrahydrocannabinol).
What is the name of the most widely studied endocannabinoid?
Anandamide
What are the 5 (loose) categories of neuropeptides?
- Pituitary peptides
- Hypothalamic peptides
- Brain-gut peptides
- Opioid peptides
- Miscellaneous peptides
What are agonists?
Drugs that facilitate effects of a certain neurotransmitter.
What are antagonists?
Drugs that inhibit effects of a certain neurotransmitter.
How do receptor blockers/antagonists work?
They bind to postsynaptic receptors without activating them, thus the access of the neurotransmitter.
How does atropine exert its antagonistic effect?
It binds to muscarinic receptors, which in turn blocks the effects of Ach (these bind to muscarine).
How does curare exert its antagonistic effects?
It binds to nicotinic receptors. Serious! It blocks transmissions at neuromuscular junctions, so can cause asphyxiation.
How is botox deadly?
It is a nicotinic antagonist that blocks the release of Ach at neuromuscular junctions!
Which area in the brain contains a high concentration of opioid receptors?
The periaqueductal gray.
What are the two major families of endogenous opioids?
- Enkephalins
- Endorphins
All endogenous opioids are….
Neuropeptides with metabotropic receptors.
