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Lecture 4

HMB200H1 Lecture Notes - Lecture 4: Voltage Clamp, Tandem Pore Domain Potassium Channel, Threshold Voltage


Department
Human Biology
Course Code
HMB200H1
Professor
Franco Taverna
Lecture
4

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HMB200H1S: Intro Neuroscience
Lecture 4, Wed Jan 29 2020
when more than one channel opens, can break down the problem to individual channels (ions
seem to move independently)
since concentration gradients don’t change, only variable will be electrical gradients
permeabilities of different ion channels are the same, new Em is sum of E for the 2 ions
using voltage clamp, you can change voltage
Hodgkin and Huxley
small K+ leak channels vary linearly with voltage changes through simple V=IR (Ohm’s
Law)
when voltage changed to -9mV, huge non-linear change in current that first depolarized
the membrane then repolarized it
transient rise in Na+ permeability (Na channels opening then closing, voltage moves
through ENa)
transient rise in K+ permeability (way above leak current levels, K+ channels opening
then closing voltage returns towards Ek)
both sodium (1st) and potassium (2nd) voltage-sensitive channels are attuned to the
threshold voltage of about -50 mV
if cell membrane changes to reach voltage, both types of channels open to allow ion
flow across the membrane
but they have different characteristics, dynamics, and timing, which results in
characteristics of a typical action potential (large, transient influx of Na+ first then
followed by large, transient outflux of K+)
= rapid depolarization followed by repolarization
voltage change opens central pore of voltage-sensitive ion channel
probability of sodium channel opening is dependent on voltage
conductance = sum of all open channels
more depolarized, more channels open
large scale opening of voltage dependent ion channel, overwhelms small K+ leak channel
action potential
Na+ channels open first, large inward current
K+ channels open second, large outward current
convert current voltage
absolute refractory period ©
state of an axon in the repolarizing period during which a new action potential cannot
be elicited because sodium channels are inactivated
relative refractory period (B)
state of an axon in the later phase of an action potential during which increased
electrical current is required to produce another action potential
summary
action potential: rise in Na+ permeability, followed by rise in K+ permeability
channel dynamics results in AP waveform
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electrical activity of membrane passive to active signaling
e.g. sensory stimuli depolarize neurons by activation of Na+ channels
voltage changes passively move and may reach integration zone
integration zone contains large number of voltage dependent ion channels
if voltage reaches threshold, channels open and action potential happens
action potential passively diffuses, may being next group of channels to threshold
enough depolarization opens enough Na+ channels for the wave of charge change to
continue
passive diffusion of charge
action potentials move as a wave with myelination
faster longer passive phases
saltatory conduction
propagation of an action potential at a successive nodes of Ranvier
rate law
intensity in all-or-none signals
intensity of stimulus being transmitted in an axon is represented by the rate at which
that axon fires
higher intensity signal = higher rate of firing (more AP/s)
lower intensity signal = lower rate of firing (fewer AP/s)
signaling is remarkably consistent (given same stimulus)
a consistent stimulus = same frequency of APs
= same amount of neurotransmitter released
= same response (e.g. amplitude)
but signaling may change very rapidly and profoundly
plasticity is a foundational feature of the nervous system
plasticity comes from
dynamic channel functions
changes to channel function through signaling
different types of channel subunits can be expressed
different types of channels can be expressed
evolutionary processes
hyperpolarizing afterpotentials (aftershoot phase)
some neurons have 2 different K+ channels
one closes fast, one closes slow
summary
signals activate neurons typically by simple linear depolarization
graded voltage changes waves travel passively
if voltage changes reach threshold at integration zone and action potential is generated
rate law - intensity of a signal is related to the action potential frequency (rate law)
action potentials passively move down axon but can be actively renewed to rapidly
move down axons
refractory periods create directionality and limit frequency of action potentials
neurotransmission
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