Resting potential. Graded Potentials. Action Potential. Neurotransmission. Neurotransmitters: many Circuits: create behaviors and sensations 1

Transcription

1 Neurophysiology: Key Concepts to Understand Resting potential All cells in our body maintain a transmembrane potential at rest referred to the resting potential. All neural activity begins with a change in the resting potential. ( Graded Potentials neurons receive chemical signals from other cells and translate the chemical message into electrical signals that have variable amplitudes and sign (depolarizing and hyperpolarizing) Action Potential large graded potentials trigger action potentials that get sent from the dendrites to the synaptic terminals. Action potentials are all-ornone (have the same shape and size once triggered). Neurotransmission Action potentials gets translated into chemical signals that are sent from one cell to the next Neurotransmitters: many Circuits: create behaviors and sensations 1

2 Resting Potential is due to 2 factors: concentration gradients and selective permeability of the membrane to ions Ion IN (mm) OUT(mM) Na K Ca Mg Cl Big Anions High Low 2 Figure 3.15

3 Resting Membrane Potential? 3 Figure 11.7

4 Observation: All cells have maintain a membrane potential with the inside negative relative to the outside. This is measured by placing a measuring device (electrode) inside the cell and one outside connected to a display meter. 4

5 Observation: Model cell with 2 compartments. Both filled with water to start. Left side inside cell, right side outside cell. Membrane barrier separates the 2 compartments. Since the 2 compartments are isolated, there is no difference in potential between the 2 and the voltmeter reads 0 mv. 5

6 Observation: Add high concentration of KCl to the inside (100 mm) and low concentration to the outside (10 mm). KCL is salt and dissociates in water. Concentration gradient (diffusive force) favors K + and Cl - leaving from In to Out but there is no pathway. There is no potential difference between In and Out. 6

7 Observation: There is no path for Cl - to move across the membrane, but there is a K + channel in the membrane so K + is free to move down its concentration gradient. 7

8 What happens? K + diffuses down its concentration gradient out of the cell and leaves an excess of negative charge behind (Cl - ). Eventually the concentration gradient favoring K + exit from the cell is exactly balanced by the electrical gradient (Cl - ) pulling K + back in to the cell. When these 2 forces (the chemical and electrical gradients) exactly balance, the cell is at the resting potential. 8

9 Chemical and Electrical gradients determine the resting potential just like in model 1. Chemical (concentration) gradient: caused by different concentrations of a single ion on either side of the plasma membrane e.g. high potassium concentration inside the cell tends to make K + ions leave the cell (movement down concentration gradient). 2. Electrical gradient: caused by different total numbers of positively and negatively charged particles on either side of membrane Because the membrane is permeable to K + at rest, as K + leaves the cell moving down its concentration gradient, excess negative charge accumulates inside cells. Note that K + is still more concentrated inside than outside, but it is the TOTAL balance of charges that matters (there are more total + charges outside than inside). *An important point: Only a few ions need to move across the membrane to generate the membrane potential so the concentration gradient does not change with K ion flow by very much (you can calculate how many about.0001% of K + ions move to generate resting potential). 9

10 Membrane Potential In real cells, the membrane is selectively permeable to K + ions at rest and no other ions*. Therefore the concentration gradient for K + sets the resting potential of the neuron just like in the model cell. K + diffuses down its concentration gradient through K + channels, leaving behind a slight excess negative charge inside, generating an electrical gradient that eventually opposes K exit from the cell *(this is a slightly simplified perspective: In real cells, there is a small permeability to Na at rest (a few leaky Na channels are open). This means over time, Na enters, K exits and after along time, the concentration gradients would change. The Na/K pump maintains the gradients moving 3 Na out and 2 K back in. The pump has little effect on membrane potential other than to maintain the important ion gradients. 10

11 Review: Electrical and Chemical 1. K+ starts to leave Cell Forces 2. Opposite electrical force develops due to charge separation 3. Enough K+ leaves to create charge separation equal and opposite to concentration. gradient Red = chemical gradient force Black = electrical gradient force Electrochemical Equilibrium Vm= -60mV 11

12 Why -60 mv? The membrane potential of the cell will exactly balance the force due to the concentration gradient. If you know the concentration gradient of the permeant ion you can determine the cell resting potential (the electrical force that exactly balances the concentration force). The Nernst equation expresses the energy of the concentration gradient in terms of the electrical gradient: For any monovalent ion (like K + ), a 10 fold concentration gradient contains exactly the same energy as a 60mV voltage difference (at 37C). This means that if [K] is 10 mm out and 100 mm in, the membrane potential of the cell is -60 mv. In this case the energy of the concentration gradient is exactly balanced by -60 mv (electrical gradient) due to separation of charge. The resting potential of the cell = -60 mv. 12

13 Why do we care? At rest, leaky K channels are open and set resting potential. But cells have other ion channels in their membranes that open and close in response to stimulation. When these channels open, they shift resting potential. EX: What would happen to the membrane potential of our model cell if a stimulus opened a leaky Na + channel (e.g. acetylcholine at the neuromuscular junction)? 13

14 Experiment: Close the leaky K channel, open a leaky Na channel with acetylcholine. Questions to ask: What is the chemical (concentration) gradient for permeant ion? Favors Na entry What is the electrical gradient for permeant ion? Favors Na entry Does the cell get more + or more when the Na channel opens? Positive By how much? Simplified situation IN (cytoplasm) OUT (blood, interstitial fluid) Ion IN (mm) OUT(mM) Na K Ca [Na+] = low [K+] = high [Ca2+] = very low [Cl-] = low [Na+] = high [K+] = low [Ca2+] = low [Cl-] = high 14

15 Nernst Equation can be used to predict equilibrium potential The Nernst Equation (to right) can be used to predict the voltage or electrical gradient that exactly equals balances a given concentration gradient. It is a mathematical expression for equating these two forces and tells you what the resting potential (the equilibrium potential) will be in the case of a single permeant ion (only leaky Na channels open in the cell membrane). EX: If [Na + ] in = 1mM and [Na + ] out = 100mM, E= 120mV since 60log(10) = 60(2) = 120 mv. This means that if the inside of the cell is 120 mv with respect to the outside of the cell, there is no net movement of Na + into the cell, even though there is an open Na + channel. The cell resting potential in this case would be +120 mv. Can you draw this on a plot of membrane potential vs. time? Simplified equation: where z=valence of ion (charge: +1, - 1, +2), Erest = membrane potential, 60 = simplification of constants (R gas constant, T temperature, F Faraday s constant), ln = Log 10 x) E rest 60 z log 10 Log 10 1 = 0 Log = 1 Log = 2 Log 10.1 = -1 Log = -2 [ ion] [ ion] out in 15

16 FYI : Recording membrane potential patch clamp technique 16

17 Why are these equilibrium potential values slightly different than our model cell?? K+? Na+ 17

18 Real Ion Concentrations for mammalian cell solutions Mammalian Intracellular and Extracellular Solution compositions Ion IN (mm) OUT(mM) Na K Ca Mg Cl DNA/RNA High Low/0 Big Anions High Low IN (cytoplasm) [Na+] = low [K+] = high [Ca2+] = very low [Cl-] = low OUT (blood, interstitia fluid) [Na+] = high [K+] = low [Ca2+] = low [Cl-] = high Nernst Potential practice and great tutorial on membrane potential: Heitler website 18

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20 Key Concept All cells have negative resting membrane potentials defined by two factors: Permeability of the cell membrane to ions (what ion channel is open) Concentration gradient for the permeant ion. Any change in membrane potential, reflects a change in one of these two factors. In real cells, concentration gradients change very little. Therefore, changes in cell membrane potential (e.g. the action potential) are caused by changes in membrane permeability to ions (opening and closing of ion channels). Review: 20

21 FYI: Electricity Definitions Voltage (V) measure of potential energy generated by charge separation Potential difference voltage measured between two points (e.g. resting membrane potential) Current (I) the flow of electrical charge between two points (the movement of ions through ion channels) Resistance (R) hindrance to charge flow (ion channels, membrane, cytoplasm) Insulator substance with high electrical resistance to current flow (the cell membrane, myelin sheath) Conductor substance with low electrical resistance to current flow (the cytoplasm or interstitial fluid) 21

22 Functional Classification Ion Channels Passive or leak channels: non-gated channels that are always open (e.g. K leak channel that sets rest) Active or gated channels that open and close to mediate shifts in membrane potential (e.g. action potentials) Chemically-gated channels that open or close in response to chemical binding (receptors like the acetylcholine receptor) Voltage-gated channels that open/close in response to changes in membrane potential (e.g. voltage-gated Na channel of the action potential) Mechanically-gated channels that open/close in response to mechanical stimulation (touch receptors, base of hair shaft) Permeant ion classification: what ion can fit through the channel (e.g. Na + vs. K + vs. Ca + + ) 22

23 Active or Gated Channels Figure

24 Membrane Potential in most neurons is dynamic 24

25 Changes in Membrane Potential Membrane potential can shift in response to changes in membrane permeability caused by opening or closing of gated ion channels Changes in membrane potential are used by neurons for communication - receiving, integrating, and sending information. Two types of shifts in membrane potential graded potentials action potentials 25

26 Graded potentials are caused by the opening of gated channels usually in response to chemical stimulus (transmitter binding). Response shifts membrane potential positive or negative to rest Depolarization membrane potential becomes less negative (shifts towards 0mV) if Na channel opens Hyperpolarization membrane potential becomes more negative than the resting potential if Cl channel opens Repolarization the membrane returns to its resting membrane potential (as chemically gated channel closes) 26

27 Graded Synaptic Potentials EX: Resting membrane exposed to stimulus (ligand binding to receptors, Mechanical stimulation, Temperature changes, Spontaneous change in permeability) Gated sodium channel opens Sodium ions move into cell through channel down electrochemical gradient Cell depolarizes>local current spreads and depolarizes nearby regions of cell membrane (graded potential)>signal decays with distance (leak across membrane and resistance of cytoplasm) Change in potential is proportional to the stimulus 27

28 Graded Potentials (aka local potentials) - Summary Short-lived, local changes in membrane potential Amplitude varies directly with the strength of the stimulus therefore amplitude is a code for stimulus strength Can be depolarizing (excitatory) or hyperpolarizing (inhibitory) Can summate (add) Decrease in amplitude with distance from source (not good for long distance transfer of info) Sufficiently strong graded potentials can initiate action potentials (threshold) 28

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30 Action Potentials Action potentials, or nerve impulses, are the underlying functional feature of the nervous system. Regenerative change in cell membrane potential Propagated along the length of the axon. All or none: always the same amplitude, the same shape regardless of stimulus Travel from cell body down axon to terminal Tutorials: swf and t/action_potential.html and here Epilepsy Movie: How Neurons Work GOOD GENERAL SUMMARY 30

31 Initiating Action Potential Action potentials are only generated by excitable cells (e.g. muscle cells, neurons), cells that contain voltage-gated channels (even oocytes have voltage-gated channels). All or None: If a stimulus exceeds threshold, the action potential is the same shape and amplitude no matter how large the stimulus for a given cell. The AP is NOT proportional to the stimulus. The action potential is either triggered, or not Weak (sub-threshold) stimuli are not relayed into action potentials Strong (threshold) stimuli are relayed into action potentials Threshold = voltage that, if reached will trigger an action potential where enough voltage-gated sodium channels open to cause net inward current to be greater than outward current and a regenerative cycle to * begin. Na+ coming in through gated channels depolarizes the membrane causing more voltage-gated Na+ channels to open. Since the Axon Hillock has the highest concentration of voltage-gated sodium channels it is the site where threshold is lowest and therefore where APs are initiated 31

32 Action Potential AP sequence: 1 resting state 2 depolarization phase regenerative opening of Na channels (Why does this cause depolarization?) 3 repolarization phase Na channels close to inactivated state, K channels open slowly increasing permeability to K (Why does this cause repolarization?) 4 hyperpolarization undershoot, big permeability to K (leak channels plus voltagegated K channels). (Would you see a hyperpolarization in a cell with rest set by K channels?) 1 - Return to rest 32

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34 AP Details 1. REST: Voltage-gated Na + and K + channels are closed, K leak channels open at rest and set resting potential V-gated Na + channel Activation gates closed in the resting state Inactivation gates open in the resting state 2. Depolarizing Phase: v-gated Na + activation gates are opened; v-gated K + activation gates are closed Threshold a critical level of depolarization (-55 to -50 mv) where depolarization becomes self-generating. Na entry depolarizes cell>opens more v-gated Na channels. Na + permeability increases; membrane potential is more positive than 0 mv (thus reverses) 3. Repolarizing Phase: V-gated Na inactivation gates close thus channel closes and membrane permeability to Na + declines to resting levels V-gated K + gates open>k leaves cell down electrochemical gradient>cell repolarizes 4. Repolarization/Hyperpolarization: K + exits the cell through leak and v-gated K channels. K equilibrium potential is reached = rest. V-gated K channels close with repolarization. 34

35 Action Potential A different view of AP channels same story Figure (Navigator) 35

36 Details: Refractory Periods or how frequently can APs be fired? Absolute Refractory Period: No AP can be fired if cell is stimulated again. Defined as the time from the opening of the Na + activation gates until the closing of inactivation gates. There are not enough available Na channels to generate depolarization to threshold. The absolute refractory period: Ensures that each action potential is separate Enforces one-way transmission of nerve impulses (Na channels are closed to inactivated state and cannot be reopened during this phase). Relative Refractory Period: A stronger-thanthreshold stimulus can initiate another action potential. The cell is hyperpolarized. Some of the inactivation gates of the Na channel are open but activation gates are closed. Many voltage-gated K channels open keeping the membrane near Ek. A larger stimulus is required to reach threshold. 36

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38 2 Methods of Propagating Action Potentials Continuous propagation: unmyelinated axons Slow (1 m/s) Local currents depolarize each patch of membrane to threshold After firing AP, membrane is in refractory period therefore AP travels in 1 direction Saltatory propagation: myelinated axons Fast (up to 150 m/s) Local currents depolarize each patch of unwrapped membrane called a node of Ranvier to threshold. The local currents travel fast between nodes down the center of the axon and there is no leak across the membrane due to the glial cell insulation (high resistance). This step speeds up AP propagation. After firing AP, membrane is in refractory period therefore AP travels in 1 direction Myelinated axons are only found in vertebrates Tutorial on AP propagation: 38

39 Slow AP Propagation: Continuous Properties: An new action potential occurs in each little patch of membrane Speed: 1 meter/second 39

40 Faster AP Propagation: Saltatory Speed = meters/second 40

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42 How Fast do Axons Conduct APs? Factors that determine speed: Diameter - the larger the diameter, the faster the AP travels Myelination - myelination dramatically increases impulse speed Classification of Axons: Group A: Fast, large diameter myelinated motor axons (Group A) can conduct up to 150 meters/sec (300 mph) Group B (Intermediate): 15 m/s (40 mph) Group C (small, unmyelinated): 1 m/s (2 mph): pain fibers 42

43 Information Coding: What Pathway is firing Aps? How Frequent are the Aps? All action potentials are alike in terms of shape. To interpret the APs, the nervous system pays attention to 2 variables: Pathway/Wiring: Pathways are wired to provide specific information to parts of the brain for interpretation EX: Info from the eyes travels to different parts of the brain than auditory info from ear. AP frequency: The intensity of the stimulus is coded by AP frequency. Strong stimuli can generate an action potential more often than weaker stimuli therefore the frequency of impulse transmission codes the intensity of the stimulus. Example: Boiling hot water poured on your hand stimulates a temperature and pain receptors in your hand to trigger more action potentials than a warm water wash of your hands. The information coming into your brain as action potentials would be integrated so that you respond appropriately. Warm water? Boiling water? 43

44 Key Concept: Coding for Stimulus Intensity AP Frequency 44

45 Multiple Sclerosis (MS) effects AP propagation: MS is an autoimmune disease Glial cells that form myelin in the CNS and PNS are attacked by the immune system and killed AP propagation is disrupted > many APs fail to reach axon terminal therefore communication is disrupted Symptoms: Vision and hearing problems, muscle weakness, and urinary incontinence Treatments: Drugs that block immune system (like interferon beta-1a and -1b, Avonex, Betaseran, and Copazone) Web Info: Multiple Sclerosis: Montel Williams on Oprah: NYT Patient Stories: