Post on 30-Dec-2015
description
transcript
What is an excitable membrane?
• Any plasma membrane that can hold a charge and propagate electrical signals.
Two types of Excitable Membranes
1. Muscle Cells – excite and then contract.
2. Neurons – transmit electrical impulses
Excitable Membrane Function: Outline
1. Resting Membrane Potential
2. Graded Potentials
3. Action Potentials
Resting Membrane Potential
• All excitable membranes maintain a non-0 resting membrane potential
Neurons = -70 mV
Muscle Cells: -85 mV
Simple DiffusionSimple Diffusion
molecules molecules across across membranesmembranes
Net movement from an area of high concentration to low concentration
Simple diffusion is ONLY ONLY ONLY efficient over short distances!!!!!!!!!!!!!!!!!!!!!
GradientsGradients
e.g. e.g. Pressure, concentration, temperature, energyPressure, concentration, temperature, energy
Molecules move “down” gradients Molecules move “down” gradients from from “Hi” to “Lo”,“Hi” to “Lo”, spontaneously spontaneously
A A GRADIENTGRADIENT is a difference in any parameter over distance is a difference in any parameter over distance
Simple Diffusion Across a MembraneSimple Diffusion Across a Membrane
Outside Inside
Net flux (JNet flux (Jnet net ) occurs from high to low concentration) occurs from high to low concentration
and will continue until concentration gradient disappearsand will continue until concentration gradient disappears
Cell MembraneCell Membrane
CCoo > C > Cii
JJnetnet == P P xx AA xx (Co – Ci)(Co – Ci)
Fick’s First Law of DiffusionFick’s First Law of Diffusion
JJnet net = net = net rate of diffusionrate of diffusion
PP = permeability constant = permeability constantA A = membrane surface area = membrane surface areaCo - CiCo - Ci = concentration gradient= concentration gradient
P and A = biological components!!P and A = biological components!!
PermeabilityPermeabilityAnd And Surface Area Surface Area
varies betweenvaries between 1) cell types 1) cell types 2) organ systems2) organ systems
P and A = biological components!!P and A = biological components!!
PermeabilityPermeabilityAnd And Surface Area Surface Area
varies betweenvaries between 1) cell types 1) cell types 2) organ systems2) organ systems
Systems differ due to differences in Exchange across cell membranes
Cell Membranes are Cell Membranes are selectively permeableselectively permeable
Protein Channel
Transporter Protein
ATP-ase PumpProtein
Resting Membrane Potential: Membrane Channels
1) LOTS OF K+ Leaks out by Diffusion
2) Na+ cannot leak in
3) Cl– Leaks out electrical repulsion due to Proteins
1
2
3
K+Na+
Cl -
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
0
-100
Vo
ltag
e
Time
2) Chloride leak ensures stabilization of resting potential
Neg. ions moving out make membrane a little more positive
1
2
Resting Membrane Potential: Maintenance of Conc. Gradients
How can a cell maintain [ions] different from diffusion equilibrium?
For resting potentials to be maintained excitable cells must maintain [ions] different from equilibrium
K+Na+
Cl -
Active Transport Active Transport Active Transport Active Transport
The net movement of molecules against a chemical or electrical gradientThe net movement of molecules against a chemical or electrical gradient
Active Transport Active Transport
Net flux (JNet flux (Jnet net ) occurred from low to high concentration) occurred from low to high concentration
Outside InsideCCoo less than C less than Cii
drmunro
Cell MembraneCell Membrane
Active transportActive transport
CoCo ConcConcinsideinside
(mmol/L)(mmol/L)
timetime
Steady StateSteady StateCi = CoCi = Co
jjii = j = jee
jjnetnet = 0 = 0
((requires the use of ATPrequires the use of ATP))
ATP useATP usemaintains the maintains the conc. differenceconc. difference
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!
Excitement of the Excitable Membrane
• Excitable membranes will deviate from resting potential when a Stimulus is applied
The resulting small amplitude fluctuations are called
Graded Potentials
Stimulus is any external factor that causes a change in membrane voltage
Examples: Electricity Pressure Light
Graded Potentials: Characteristics
2) Amplitude (voltage) is equal to stimulus strength
Stimuli
Membrane Voltage
Graded Potentials: Characteristics
3) Degrade over then length of a membrane
Stimulus applied
Length of Excitable Membrane
Loss of Graded Potential
Graded Potentials: Summation
4) Summation: The closer successive STIMULI, the greater amplitude the graded potential
Action PotentialDefinition: Depolarization of an excitable membrane in response to a threshold stimulus
Sub-threshold stimuli Threshold stimulus
Graded Potentials
Two ways to reach THRESHOLD
1) Single, Large Amplitude Stimulus = directly reach membrane threshold voltage
2) Many subthreshold stimuli close together = SUMMATION of graded potentials
Threshold Voltage
Characteristics of Action Potentials
1) All-or-None: when they happen they are ALWAYS exactly the same
Action Potential: All-or-None Principle
Threshold Stimulus Supra-Threshold Stimulus
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!
Action Potential: All-or-None Principle
Threshold Stimulus Sub-threshold Stimulus
NONE: If the stimulus is not strong enough to reach threshold voltage, no action potential will occur
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.
Action Potential: All-or-None Principle
Characteristics of the Action Potential: 2) 5 stages
(1) Stimulus to Threshold
(2) (3)
(4)
(5) Return to Resting Potential
Action Potential: 1) Stimulus to Threshold
Every stimulus causes some Na+ Channels to OPEN
Resulting in Graded Potentials
[Na+]
[Na+]
Activation gate opens
(1) Stimulus to Threshold
When the stimulus is strong enough, enough Na+ channels open
to bring the membrane to threshold voltage
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
Action Potential: 2) Depolarization
Once threshold voltage is achieved:
1) ALL activation gates on Na+ Voltage Gated Channels open 2) Na+ RUSHES into Cell
3) Cell Membrane DEPOLARIZES
Action Potential: 3) Repolarization
After a set amount of TIME the INACTIVATION GATE of the Na+ channels CLOSE
This stops Na+ Influx!
Simultaneously, Voltage Gated K+ activation gates OPEN
K+ then leaves the cell by diffusing DOWN its concentration gradient
K+ efflux causes the cell membrane to REPOLARIZE
Action Potential: 4) Hyperpolarization
K+ channels close VERY VERY slowly…..
Thus, a lot of K+ leaves the cell
Membrane potential OVERSHOOOTS
resting to ~ -100 mV
Action Potential: 5) Return to Resting Potential
All activation gates are CLOSED
But, membrane is HYPERPOLARIZED….so how does it reset to -70 mV?But, membrane is HYPERPOLARIZED….so how does it reset to -70 mV?
Na+-K+ ATPase Pump Restores Ion Concentrations….thus, K+ & Cl- can leak……thus membrane re-stabilizes to -70 mV
Characteristics of Action Potentials
1) All-or-None: when they happen they are ALWAYS exactly the same
2) 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
Action Potential: Refractory Periods
Na+ activation gates open K+ activation gates OPEN
No stimulus can produce 2nd AP
SupraThreshold Stimulus can
produce 2nd AP
Guarantee that each AP can undergo its Depolarization/Repolarization Phase
Characteristics of Action Potentials
1) All-or-None: when they happen they are ALWAYS exactly the same
2) 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
Action Potentials: Do not DIMINISHStimulus Applied
Once started, an Action Potential will maintain it strength down the length of a neuron or muscle cell!
Characteristics of Action Potentials
1) All-or-None: when they happen they are ALWAYS exactly the same
2) 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
AP are frequency modulated!
Poked with a finger
Low frequency of AP
High frequency of AP
Weak threshold stimulus
Strong threshold stimulus
Abnormal Membrane Potentials
• Hyperkalemia: HIGH K+ in ECF (ISF)
– Consequences: More excitable membranesCELLS ALWAYS IN REFRACTORY PERIOD, Heart stops!
Normokalemia Hyperkalemia
Given during Lethal Injection!