30 September 2011
Section B: Membrane Potentials
Section C: Synapses
Test Monday:
Turn in Take-Home Portion at beginning.
All personal materials on floor at front of room.
At least one vacant seat to nearest neighbor.
Coverage up to and including Ch 6 B
Check your Multiple Choice Grade on Moodle Site Tuesday
Wednesday 1QQ on Synapses.
1QQ # 11 for 8:30 class
1. Why doesn’t an action potential reach + 60 mV?
a)
b)
c)
d)
Voltage- gated Na+ channels open and spontaneously close quickly
Voltage-gated K+ channels open a little later than the Na+ channels
Na+ K+ ATPase quickly pumps out the Na+ that enters during an AP
As the membrane approaches +60 mV, the driving force for Na+ entry is
weaker
e) Na+ is not the only permeable ion.
2. Which are the accurate statements regarding V-gated K+
channels?
a)
The more the membrane is depolarized, the more K+ channels will open,
and the membrane will depolarize even more, generating a positive
feedback cycle.
b) These channels inactive after a short open time and can only reopen if
the membrane potential returns to negative values.
c) These channels are “blocked” by lidocaine, xylocaine, and novocaine.
d) These channels close as the membrane repolarizes.
1QQ # 11 for 9:30 class
1. Why doesn’t an action potential reach + 60 mV?
a)
b)
c)
d)
Voltage- gated Na+ channels would be forced “shut” at + 60 mV
Voltage-gated K+ channels open a little sooner than the Na+ channels.
Na+ K+ ATPase quickly pumps out the Na+ that enters during an AP
As the membrane approaches +60 mV, the driving force for Na+ entry is
weaker
e) Na+ channels open only briefly and then quickly inactivate.
2. Which are the accurate statements regarding V-gated K+
channels?
a)
The more the membrane is depolarized, the more K+ channels will open,
and K+ will leave the cell, contributing to repolarization.
b) These channels inactive after a short open time and can only reopen if
the membrane potential returns to negative values.
c) These channels are “blocked” by lidocaine, xylocaine, and novocaine.
d) These channels open shortly after the V-gated Na+ channels open.
S1
The Questions:
How does an action potential move along the axon?
Why doesn’t the amplitude get smaller with distance?
Why is the conduction of an action potential unidirectional?
Axon Hillock of
interneuron or
efferent neuron
Axon
Trigger Zone of
Sensory Neuron
S2
In unmyelinated axons, action potential must be generated at each
point along the membrane, a relatively slow process that involves influx
of Na+ which sets up positive feedback cycle.
In myelinated axons, action potential must be generated only at the
nodes of Ranvier, which allows AP to be conducted much faster and
with fewer ions moving, and thus less energetically expensive.
S3
Saltatory Conduction
Figure 6.23
AP CV (up to 100 m/s)
Reminder: influx of Na+ is
very quickly followed by efflux
Location of channels
of K+ (not shown above)
Energy Requirements
Axon diameter
Clustering of V-gated channels at Nodes of Ranvier
What’s at the
end of an axon?
S4
Section C: Synapses and Synaptic Transmission
Figure 6.24
S5
Anatomy of an Electrical S 8
Synapse (aka Gap Junction)
Comparison to Chemical Synapses
• Directionality
• Response time
• Sign inversion?
Uncommon in human CNS.
Common in cardiac muscle
and some smooth muscle.
S6
Anatomy of a Chemical Synapse
Presynaptic cell
Postsynaptic cell
S7
Most neurotransmitters are synthesized
in the axon terminal.
Exceptions: Peptide NTs originate in cell
body, move in vesicles by fast orthograde
axonal transport to axon terminal.
Figure 6.27
Vesicle release proportional to
Ca++ influx (High f AP leads to
residual Ca++ in terminal)
Tetanus toxin & Botulinum
toxin disrupt SNARE function.
Fates of neurotransmitters:
1) Bind to receptor on Postsynaptic cell
2) Diffusion away from synapse
3) Enzymatic degradation
e.g. Acetylcholinesterase (AChE)
and Monoamine Oxidase (MAO)
4) Uptake by astrocytes
5) Reuptake into presynaptic
terminal (e.g. SSR)
S8
Presynaptic Facilitation
Presynaptic Inhibition
Mechanism: vary Ca++
entry in presynaptic
terminal B.
Size of PSP is
Variable!
Figure 6.33
Who Cares?