Excitable Membranes

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Excitable Membranes
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What is an excitable membrane?

• Any plasma membrane that can hold a charge and
  propagate electrical signals.

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Two types of Excitable Membranes
Both work in similar ways.

• Muscle Cells excite and then contract.
• Neurons transmit electrical impulses throughout
  the body (sensory and motor)

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Excitable Membrane Function Outline

• Resting Membrane Potential
• Graded Potentials
• Action Potentials

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Resting Membrane Potential

• All excitable membranes maintain a non-0 resting
  membrane potential

Neurons -70 mV Muscle Cells -85 mV
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How do we measure membrane potentials?

• ALWAYS REFER TO INSIDE relative to
  OUTSIDE!!!!!!!!!!

How is this resting potential achieved?
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Resting Membrane Potential Ionic Concentration
Gradients
K
Na Cl -
Proteins
This is an example of Physiological Steady State!
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Resting Membrane Potential Membrane Channels

• LOTS OF K Leaks out by Diffusion
• Na cannot leak in
• Cl Leaks out electrical repulsion due to Proteins

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K
Na Cl -
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Resting Membrane Potential

• 1) At rest, K leak results in a negative membrane

K
Na Cl -
Why? Positive Ions moving OUT of a cell result
in fewer positive ions inside the cell This
results in a MORE NEGATIVE ICF
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2) Chloride leak ensures stabilization of resting
potential Neg. ions moving out make membrane a
little more positive
Voltage
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-100
Time
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Resting Membrane Potential Maintenance of Conc.
Gradients
For resting potentials to be maintained
excitable cells must maintain ions different
from equilibrium
K
Na Cl -

• How can a cell maintain ions different from
  diffusion equilibrium?
• How is Steady State achieved?

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Na-K ATPase PUMP (Active Transport)
1) ATP binds to PUMP Na enters
2) ATP releases energy which pumps Na OUT
3) K enters PUMP
4) Return to original shape pumps K IN
The pump maintains Na OUT and K
IN. .thus, K can leak via channels
resulting in a negative resting potential!
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Excitement of the Excitable Membrane

• Excitable membranes will deviate from resting
  potential when a Stimulus is applied

Stimulus is any external factor that causes a
change in membrane voltage Examples
Electricity Pressure Light
The resulting small amplitude fluctuations are
called Graded Potentials
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Grade Potentials Characteristics

• Can result in hyper-polarization or
  depolarization

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Grade Potentials Characteristics

• 2) Amplitude (voltage) is equal to stimulus
  strength

Membrane Voltage
Stimuli
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Grade Potentials Characteristics

• 3) Degrade over then length of a membrane

Stimulus applied
Loss of Graded Potential
Length of Excitable Membrane
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Graded Potentials Summation
4) Summation The closer successive
STIMULI, the greater amplitude the graded
potential
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Action Potential
Definition Depolarization of an excitable
membrane in response to a threshold stimulus
Graded Potentials
Threshold stimulus
Sub-threshold stimuli
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Two ways to reach THRESHOLD

• Single, Large Amplitude Stimulus directly reach
  membrane threshold voltage
• 2) Many subthreshold stimuli close together
  SUMMATION of graded potentials

Threshold Voltage
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Characteristics of Action Potentials

• All-or-None when they happen they are ALWAYS
  exactly the same

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Action Potential All-or-None Principle
ALL As long as the stimulus is at or above
threshold, an action potential will occur and it
will always be the same magnitude and duration
The size of the stimulus has no effect on the
size of the action potential!
Threshold Stimulus
Supra-Threshold Stimulus
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Action Potential All-or-None Principle
NONE If the stimulus is not strong enough to
reach threshold voltage, no action potential will
occur
Threshold Stimulus
Sub-threshold Stimulus
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Action Potential All-or-None Principle

• Important Note
• The all-or-none principle ONLY applies to a
  particular membrane with certain ion
• Change the ion
• change in threshold stimulus, amplitude of
  AP,
• etc.

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Characteristics of Action Potentials

• All-or-None when they happen they are ALWAYS
  exactly the same
• They consist of 5 stages 1) Stimulus to
  Threshold
• 2) Depolarization
• 3) Repolarization
• 4) Hyperpolarization
• 5) Return to Resting Membrane
  Potential
• 3

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Action Potential 5 stages
(2)
(3)
(1) Stimulus to Threshold
(5) Return to Resting Potential
(4)
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Action Potential 1) Stimulus to Threshold
Na
Activation gate opens
Every stimulus causes some Na Channels to
OPEN Resulting in Graded Potentials
Na
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When the stimulus is strong enough, enough Na
channels open to bring the membrane to threshold
voltage
(1) Stimulus to Threshold
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Action Potential Ion channels on Plasma Membrane
Na and K are the VOLTAGE-GATED ION CHANNELS
responsible for action potentials
Note Na Voltage-Gated Channels have Activation
and Inactivation GATES K only have
Activation gates
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Action Potential 2) Depolarization
3) Cell Membrane DEPOLARIZES
Once threshold voltage is achieved 1) ALL
activation gates on Na Voltage Gated Channels
open 2) Na RUSHES into Cell
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Action Potential 3) Repolarization
After a set amount of TIME the INACTIVATION GATE
of the Na channels CLOSE This stops Na Influx!
K efflux causes the cell membrane to REPOLARIZE
Simultaneously, Voltage Gated K activation
gates OPEN K then leaves the cell by diffusing
DOWN its concentration gradient
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Action Potential 4) Hyperpolarization
Membrane potential OVERSHOOOTS resting to
-100 mV
K channels close VERY VERY slowly.. Thus, a
lot of K leaves the cell
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Action Potential 5) Return to Resting Potential
All activation gates are CLOSED
But, membrane is HYPERPOLARIZED.so how does it
reset to -70 mV?
Na-K ATPase Pump Restores Ion
Concentrations. thus, K Cl- can leakthus
membrane re-stabilizes to -70 mV
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Characteristics of Action Potentials

• All-or-None when they happen they are ALWAYS
  exactly the same
• They consist of 5 stages 1) Stimulus to
  Threshold
• 2) Depolarization
• 3) Repolarization
• 4) Hyperpolarization
• 5) Return to Resting Membrane
  Potential
• 3) Absolute Relative Refractory Periods

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Action Potential Refractory Periods
SupraThreshold Stimulus can produce 2nd AP
K activation gates OPEN
Na activation gates open
No stimulus can produce 2nd AP
Guarantee that each AP can undergo its
Depolarization/Repolarization Phase
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Characteristics of Action Potentials

• All-or-None when they happen they are ALWAYS
  exactly the same
• They consist of 5 stages 1) Stimulus to
  Threshold
• 2) Depolarization
• 3) Repolarization
• 4) Hyperpolarization
• 5) Return to Resting Membrane
  Potential
• 3) Absolute Relative Refractory Periods
• 4) Their strength DOES NOT diminish over distance

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Action Potentials Do not DIMINISH
Stimulus Applied
Once started, an Action Potential will maintain
it strength down the length of a neuron or
muscle cell!
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Characteristics of Action Potentials

• All-or-None when they happen they are ALWAYS
  exactly the same
• They consist of 5 stages 1) Stimulus to
  Threshold
• 2) Depolarization
• 3) Repolarization
• 4) Hyperpolarization
• 5) Return to Resting Membrane
  Potential
• 3) Absolute Relative Refractory Periods
• 4) Their strength DOES NOT diminish over distance
• 5) Stimulus strength determines the FREQUENCY of
  Action Potentials

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AP are frequency modulated!
Low frequency of AP
Weak threshold stimulus
Poked with a finger
High frequency of AP
Strong threshold stimulus
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Abnormal Membrane Potentials

• Hyperkalemia HIGH K in ECF (ISF)
• Consequences More excitable membranes
• CELLS ALWAYS IN REFRACTORY PERIOD, Heart stops!

Hyperkalemia
Normokalemia
Given during Lethal Injection!
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Abnormal Membrane Potentials

• Hypokalemia low K in ECF
• Consequences Hyperpolarization, less excitable
  membranes
• Muscles Neurons dont work

Hypokalemia
Normokalemia