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45 Cards in this Set
- Front
- Back
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How many synaptic connections does the average neuron form? |
1000 |
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How many neurons does the brain contain?
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At least 85 billion
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What are the two signaling mechanisms? |
Axonal conduction and synaptic transmission |
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Describe electrical synapses. |
Rapid speed of transmission Stereotyped function Do not allow inhibitory actions or long-lasting changes in effectiveness |
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Describe chemical synapses. |
Amplify neuronal signals Allow a small presynaptic nerve terminal to alter the potential of a large postsynaptic cell |
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Do chemical and electrical synapses have the same morphologies? |
No |
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Describe the electrical properties of electrical synapses. |
Low-resistance, high-conductance channels |
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What two functions must the voltage-gated channels at an electrical synapse fulfill in the presynaptic cell?
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1) Depolarize the membrane of the presynaptic cell and initiate an action potential
2) Generate sufficient ionic current to produce a potential change in the postsynaptic cell, presynaptic terminal must be large and postsynaptic cell must be small |
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Are electrical synapses bidirectional or unidirectional/rectifying? |
Usually bidirectional, occassionally unidirectional |
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Why are electrical synapses unidirectional? |
Likely the voltage-sensitivity of the channel |
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Which circuit component do electrical synapses serve as? |
Mostly simple resistors |
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What is transmission at nonrectifying electrical synapses referred to as? |
Electrotonic transmission |
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Why have electrical synapses? |
Speed (important for escape responses) Threshold for generating an action potential becomes greater Resistance is decreased |
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What are two interesting features of behaviors mediated by electrical synapses? |
1) High threshold 2) Occur explosively in an all-or-none manner |
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What signals do electrical signals send? |
Developmental and regulatory |
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What is a unique benefit of electrical synapses? |
Allow compounds such as cAMP and other small peptides to pass from cell to cell |
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What is a gap junction? |
Zone of apposition between two neurons at the site of an electrical synapse |
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What is the major difference between the two classes (rectifying and nonrectifying) of electrical synapses? |
The extend to which channel gating is sensitive to voltage |
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Are gap junction channels always open? |
No |
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When do gap junctions close? |
Lowered pH or elevated Ca2+, some slightly sensitive to voltage |
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Can neurotransmitters alter gap junctions? |
Yes, through signal transduction pathways |
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What is the composition of gap junctions? |
A pair of cylinders (hemi-channels) one on each cell that connect by homophilic interactions |
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What is another name for a hemi-channel? |
Connexon |
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What is the composition of a connexon? |
Six identical subunits, called connexins |
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What are two functions of each connexin? |
1) Recognize the other 5 subunits to assemble a connexon 2) Recognize its counterpart hemi-channel in the apposed cell to form a complete conductive channel |
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How does a gap junction channel open? |
Each connexon rotates slightly with respect to the other |
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What is wider at the synaptic cleft, the synapse or the adjacent extracellular space? |
The synapse |
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What is the synaptic delay of a chemical synapse? |
0.3 ms |
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What are the two steps of chemical synaptic transmission?
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1) Presynaptic transmission: releases the chemical messenger
2) Postsynaptic receptive process: binding of the transmitter |
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What two advantages does neuronal signaling have over endocrine signaling? |
1) Faster 2) More directed (specific) |
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What are active zones? |
Specialized secretory machinery for focused release |
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List small molecules that can serve as transmitters. |
ACh GABA Glutamate Serotonin Dopamine Norepinephrine |
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What does the action of a transmitter depend on? |
Not the chemical nature of the transmitter, but instead on the properties of the receptors with which the transmitter binds |
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What are two common biochemical features of all receptors? |
1) Membrane-spanning proteins 2) Carry out an effector function, either gating an ion channel directly or indirectly, by initiating a second-messenger cascade |
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What are the two classes of receptors based on their gating of an ion channel? |
Direct and indirect |
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What is an ionophoric receptor?
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One that, when bound to a neurotransmitter, undergoes a conformational change that opens the channel
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Give an example of an ionophoric receptor. |
ACh at the neuromuscular junction |
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Give an example of an indirect receptor. |
Those for norepinephrine or serotonin at synapses in the cortex |
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How do indirect receptors communicate with ion channels? |
Through GTP-binding proteins (G-proteins) |
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What is the role of G-proteins? |
They couple the receptors to effector enzymes that produce one or another intracellular second messenger (or can act on channel directly) |
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Give two examples of intracellular second messengers. |
cAMP and diacylglycerol |
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What is the role of the second messenger? |
It acts on a channel directly, or more commonly activates one of a family of enzymes called protein kinases |
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What is the function of protein kinases? |
Modulate channels by phosphorylating either the channel protein or a regulatory protein that acts on the channel |
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How do the functions of direct and indirect receptors differ? |
Direct: fast synaptic actions, produces behavior Indirectly: slower (seconds to minutes), modulate behavior by altering excitability and strength of the synaptic connection |
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What two properties make electrical synapses suitable for fast stereotyped behaviors? |
Speed and synchrony |
