Transcript neuro3

S8
Electrical and
concentration gradient
driving forces for
Sodium and Potassium
Size and
Direction
of Arrows
show
driving
forces!
What would
happen to
membrane
potential if
suddenly PNa
became very
great?
The G-H-K Equation!
Why is resting
membrane
potential closer
to EK than ENa?
How does the membrane potential
change if
1) permeability to sodium increases
2) Permeability to potassium increases
S9
The Goldman Hodgkin Katz
Equation
• If you know the concentrations of ALL
permeable ions and their relative
permeabilities, you can calculate the
membrane potential using the GHK
Equation.
S 14
Which ion moving in
which direction (into or
out of cell) is responsible
for depolarization and
overshoot?
Which ion moving in
which direction (into or
out of cell) is responsible
for repolarization and
hyperpolarization?
Increase
PNa+
Can the membrane
potential go more
negative than -90 mV?
Increase
PK+
Increase
PK+
How do ions get across the membrane? Ion channels!
S3
Types and locations of Ion Channels
Sensory neuron
Leak Channels
Gated Channels
….. Ligand-gated
….. Mechanically-gated
….. Voltage-gated
w/ LGCs and MGCs
Intracellular
Recording
Electrode or
Stimulating
Electrode
Interneurons & Motoneurons
w/ LGCs
w/ VGCs
S4
Expanded on next slide
What happens when the
membrane is
depolarized by more
than about 15 mV?
Action potentials are all or nothing.
Analogy of
shutter release
pressure on a
camera, either
trips shutter or
not.
How is the intensity of a
stimulus encoded by action
potential if all action potentials
have the same size (amplitude)?
S5
Relevance of the GHK equation
Changes in membrane
permeability produce changes in
membrane potential via the opening
and closing of ion channels!
S6
To reset from inactivated state
to closed state, membrane must
repolarize.
Open at -55 mV
Membrane must repolarize to
“reset” Na+ Channels to be
capable of opening again.
Compare and contrast voltagegated Na and K channels based on
time to open and duration of
open time.
S7
Voltage-gated Na+ channel
Tetrodotoxin from ovary of
Puffer fish, used in
Japanese sushi (fugu)
scienceblogs.com/.../upload/2006/03/channel.jpg
S8
What types of ion-channels are
labeled in this neuron in red?
TTX with red fluorescent marker
S9
Rising
Phase
Falling
Phase
Relative permeabilities
Duration of AP
Refractory periods
absolute RP
relative RP
Why does the
peak of the
action potential
not reach ENa?
Properties of V-gated
Na+ and K+ channels
account for the shape
of the action potential
and the refractory
periods.
S 10
S 11
Natural ways to Initate an
Action Potential
Graded depolarization in
cell body reach threshold
at axon hillock
Graded depolarization in
in receptive membranes of
sensory neurons reach
threshold for AP at trigger
zone. i.e. nociceptors and
stretch receptors.
Unstable membrane
potential cycles: pacemaker
potentials in pacemaker cells
of heart, smooth muscles of
gut, and medullary neurons
for respiratory rhythm.
S 12
Who
Cares?
Novacaine, lydocaine, xylocaine,
All block voltage-gated Na+ channels
Prevent action potentials, so stimulus
does not result in an action potential
in sensory neurons which would
convey that information to the brain
where person would be conscious of
the stimulus!
S 13
Questions About Action Potential Conduction:
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?
What is the absolute refractory period and what is going on with voltage gated sodium
channels that accounts for the absolute refractory period?
What is the relative refractory period and what is going on with voltage gated sodium
channels that accounts for the relative refractory period?
Axon
Hillock
Axon
S 14
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.
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
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?