30 September 2011

Section B: Membrane PotentialsSection 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 class1. Why doesn’t an action potential reach + 60 mV?

a) Voltage- gated Na+ channels open and spontaneously close quicklyb) Voltage-gated K+ channels open a little later than the Na+ channelsc) Na+ K+ ATPase quickly pumps out the Na+ that enters during an APd) As the membrane approaches +60 mV, the driving force for Na+ entry is

weakere) 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 class1. Why doesn’t an action potential reach + 60 mV?

a) Voltage- gated Na+ channels would be forced “shut” at + 60 mVb) Voltage-gated K+ channels open a little sooner than the Na+ channels.c) Na+ K+ ATPase quickly pumps out the Na+ that enters during an APd) As the membrane approaches +60 mV, the driving force for Na+ entry is

weakere) 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.

Axon Hillock ofinterneuron or efferent neuron

Axon

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?

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Trigger Zone of Sensory Neuron

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.

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Figure 6.23

AP CV (up to 100 m/s)Location of channelsEnergy RequirementsAxon diameterClustering of V-gated channels at Nodes of Ranvier

Reminder: influx of Na+ is very quickly followed by efflux of K+ (not shown above)

Saltatory ConductionS 3 What’s at theend of an axon?

Figure 6.24Section C: Synapses and Synaptic Transmission

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Anatomy of an Electrical Synapse (aka Gap Junction)

Comparison to Chemical Synapses•Directionality •Response time•Sign inversion?

Uncommon in human CNS.Common in cardiac muscleand some smooth muscle.

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Anatomy of a Chemical Synapse

Presynaptic cell

Postsynaptic cell

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Figure 6.27

Vesicle release proportional to Ca++ influx (High f AP leads to residual Ca++ in terminal)

Fates of neurotransmitters:1)Bind to receptor on Post-synaptic cell2)Diffusion away from synapse3)Enzymatic degradation e.g. Acetylcholinesterase (AChE) and Monoamine Oxidase (MAO)4)Uptake by astrocytes5)Reuptake into presynaptic terminal (e.g. SSR)

S 7Most 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.

Tetanus toxin & Botulinum toxin disrupt SNARE function.

Figure 6.33Presynaptic FacilitationPresynaptic Inhibition

Who Cares?

Mechanism: vary Ca++ entry in presynaptic terminal B.

Size of PSP is Variable!

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