University:
Kentucky State University
Course:
BIO 301 | Human Anatomy
Academic year:

2023
Views:

34

Pages:

33
Author:

Nicole Wang

Main Topics (L11) • Resting Membrane Potential (RMP): • Chord Conductance Equation: to calculate membrane potential • Neuron action potential • Voltage Gated Sodium and Potassium channels: Activation and inactivation • Characteristics of the Action potential: ALL-or-none Law. Coding for stimulus intensity. Refractory periods: (a) Absolute refractory period, (b) Relative refractory period. • Grading potential: (local potential): Graded potentials vs. action potentials Resting Membrane Potential (RMP) 1. Membrane potential of a cell not producing any impulses a. Because the membrane is most permeable to K+, it has the greatest influence. b. A change in the permeability of the membrane for any ion will change the resting potential. c. A change in the concentration of any ion inside or outside the cell will change the resting potential. d.Key to how neurons work  (they are at rest till a Stimulus changes the membrane potential  action potential) 2. K+, Na+, Ca2+ and Cl− contribute to the resting Sheet Note 1 • At rest:‫جهد الراحة‬ • Rest: No changing in membrane voltage.(but there is a movement of ions across the membrane. • All living cells have membrane potential, dead cells have no membrane Potential. • Rest membrane potential is associated with excitable tissues (neurons and Muscles) and all other tissues are always at rest (because they don’t produce action potential). • Neurons and muscles are either between rest state or at action potential. • All cell types have different permeability to ions. So, have different membrane potentials (Neurons -70 mv, Cardiac cells -85mv). • K+ Is the main contributor to the resting potential. Resting Membrane Potential (RMP) • 3. In most cells, the resting potential is between 65mV and -85mV. a. Neurons are usually at −70mV. b. Close to K+ equilibrium potential *But we must calculate the other ions influence 4. When a neuron sends an impulse, it changes the permeability of Na+, driving the membrane potential closer to the equilibrium potential for Na+. 5. Na/K pump, counter K+ leaking out, keeps both the resting potential and the concentration differences stable (electrogenic effect) To +keep K+ inside and Na outside Sheet note #2 Cardiac cells usually at -85 mV If we want to calculate resting membrane potential to any cell we should look at equilibrium potential+ permeability for each ion Processes that influence the resting membrane potential Large negative charged proteins Fixed Anions + Less permeable Unequal permeabilities to Na+ of plasma membrane to diffusible ions More permeable to K+ *cell K+ higher on inside Na+ higher on outside Na+/K+ pump Na+/K+ pump Uneven distribution of ions across the plasma membrane EK = –90 mV ENa = +66 mV Higher voltage magnitude because it’s more permeable. Resting membrane potential (RMP) = –70 mV The concentration gradient moves the ions, and this pump to recover them Sheet Note3 • Resting membrane potential (RMP) has a close value to EK but aren’t equal, because it isn’t the only ion the moves across the membrane (other ions: Na+, Ca2+ and Cl− ) • Resting membrane potential (RMP) depends on ion concentration inside and outside AND the permeability. Chord Conductance Equation • It is used to determine the membrane potential based on the conductance values as well as equilibrium potential values of the ions contributing to the membrane potential. • • The conductance for each ion is used as a major determinant of that ion's contribution to the membrane potential. The larger the conductance of a given ion, the larger the contribution of that ion will be in setting the membrane potential. Vm : membrane potential G: is conductance for the ion V k, Na, Cl : Equilibrium potential of K, Na, Cl respectively Sheet Note 4 • In Goldman-Hodgkin-Katz (GHK) Equation . )permeablity( • But in Chord Conductance Equation Summary • The Nernst equation is used to calculate Equilibrium potentials if there is only one type of channels at the plasma membrane ‫كل ايون لحال‬ • The Goldman equation is used to calculate the Diffusion Potential when there are more than one type of channels Every channel and membrane permeability to it • The Chord conductance equation is used to calculate the membrane potential, it depends on the magnitude of the conductance for each ion (related to total number of open channels for that ion) Neuron action potential (Nerve impulse) • Neurons have a resting potential of −70mV. Not stimulated yet • Neurons and muscle cells can change their membrane potentials (Excitable tissue) • Caused by changes in the permeability to certain ions • Ions will follow their electrochemical gradient • Flow of ions are called ion currents which occur in limited areas where ion channels are located Change in membrane potential from NEGATIVE  POSITIVE indicates that there is action potential. Changes in Membrane Potential • Changes can be recorded on an Oscilloscope by recording the voltage inside and outside the cell. Axon Recording electrodes . mV +60 +40 0 –40 . –60 –80 - rmp Depolarization (stimulation) Hyperpolarization (inhibition) Action Potential Polarized cell • At rest, a neuron is considered polarized when the inside is more negative than the outside. +30 Action potential 0 1 Membrane potential (millivolts) Na+ 2 K+ in out Threshold –50 –70 Stimulus Resting membrane potential 0 Milliseconds, so its very fast 1 2 3 Time (msec) 4 5 6 7 Action Potential Depolarization 1. At threshold membrane potential (−55mV), voltagegated Na+ channels open, and Na+ rushes in. As the cell depolarizes, more Na+ channels are open, and the cell becomes more and more permeable to Na+. a. This is a positive feedback loop. b. Causes an overshoot of the membrane potential c. Membrane potential reaches +30mV. d. This is called depolarization Voltage gated ion channels are exclusive for Excitable tissues +30 Action potential 0 1 Membrane potential (millivolts) 2 Na+ in K+ out Threshold –50 –70 Stimulus Resting membrane potential 0 1 2 3 Time (msec) 4 5 6 7 Sheet Note 5 • If the stimuli was strong enough to change membrane potential of neuron to -55  action potential • If not, it will remain and stay at rest. -First of all sharp increase of membrane potential will happen, it’s called “depolarization”, it will jump from -55mv to +30mv. -How does the depolarization happen? Cause the opening of voltage gated sodium channels(positive feedback occurs opening of channel will stimulate the other channels to open) -flow of Na+ ions to inside the cell, which is big enough to change and inverse the voltage to (positive charge +). -when the membrane potential reaches +30mv the channels are inactivated (not only closed but also inactivated). -voltage gated sodium channels OPEN:-55mv CLOSE:+30mv. Action Potential Repolarization 2. At +30mV, Na+ channels close, and K+ channels open. a. Results in repolarization of membrane potential b. This is a negative feedback loop c. A return to resting potential is called repolarization. +30 Action potential 0 1 Membrane potential (millivolts) Na+ 2 K+ in out Threshold –50 –70 Stimulus Resting membrane potential 0 1 2 3 Time (msec) 4 5 6 7 Sheet Note 6 • voltage gated sodium channels close and potassium voltage gated channels open at +30mv. • The amount of K+ ions that leaves the cells are more, because there is more Potassium channels than sodium. -It will cause a steep and sharp decrease. -the normal (rest potential ) is by the (Na+-K+) ATPase pump. **K+ voltage gated channels close slowly, so more ions are leaving the cell, this is the cause why the membrane potential reaches less than -70mv. . - Action Potential Hyperpolarization • When the membrane potential inside the cell decreases (becomes more negative), this is called hyperpolarization. Occurs when positive ions leave the cell (usually K+) or negative ions (Cl−) enter the cell. +30 Action potential 0 1 Membrane potential (millivolts) 2 Na+ in K+ out Threshold –50 –70 Stimulus Resting membrane potential 0 1 2 3 Time (msec) 4 5 6 7 Action Potential After-Hyperpolarization a. Repolarization actually overshoots resting potential and gets down to −85mV. b. This does not reach potassium equilibrium potential because voltagegated K+ channels are inactivated as the membrane potential falls. c. Na+/K+ pumps quickly reestablish resting potential. +30 Action potential 0 1 Membrane potential (millivolts) 2 Na+ in K+ out Threshold –50 –70 Stimulus Resting membrane potential 0 1 2 3 Time (msec) 4 5 6 7 Voltage Gated Sodium and Potassium channels: Activation and inactivation • Changes in membrane potential are controlled by changes in the flow of ions through channels. – 1. Na+ has only voltage-gated channels that are closed at rest; the membrane is less permeable to Na+ at rest. – 2. K+ has two types of channels: 1) Not gated (always open); sometimes called K+ leakage channels 2) Voltage-gated K+ channels; open when a particular membrane potential is reached; closed at resting potential 1. Voltage-Gated Na+ Channels Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. These channels open if the membrane potential depolarizes to −55mV (threshold) b. Sodium rushes in due to the electrochemical gradient. c. Membrane potential climbs toward sodium equilibrium potential. d. These channels are deactivated at +30mV. Channel closed at resting membrane potential Channel open by depolarization (action potential) Channel inactivated during refractory period Sheet Note 7 The voltage-gated sodium channel in three separate states A. Resting membrane potential (at -70mV): Closed but capable of opening Activation gate is closed Inactivation gate is opened No Na passes Depolarization (from – 50mV to+35mV): This means that the highest number of Na is opened at threshold potential (means highest number of Na ions) and as we approaches the +35mV the number decrease but when we reaches +35mV no Na channel opened (means no further Na ions enters) Open, or activated Activation gate is opened Inactivation gate is opened Na passes C. Repolarization (from +30mV to-70mV) Closed and not capable of opening Activation gate is opened Inactivation gate is closed No Na passes 2. Voltage-Gated K+ Channels a. At around +30mV, voltage-gated K+ channels open, and K+ rushes out of the cell following the electrochemical gradient. b. This makes the cell repolarize back toward the potassium equilibrium potential. Characteristics of the Action potential: ALL-or-none Law a. Once threshold has been reached, an action potential will happen. b. The size of the stimulus will not affect –70 mV the size of the action RMP potential; it will always reach +30mV. c. The size of the stimulus will not affect action potential (-70duration. mV  +30 mV  -70 mV) = 3 mSec Action potentials (all have same amplitude) Weakest Strongest Stimuli (single, quick shocks) - Sheet Note 8 • All of none means that if the stimulation made a change in the membrane potential that reaches -55mv(threshold) the action potential happens(all) if not, there is no action potential (none). • First arrow indicates a weak stimulus, doesn’t reach the threshold, so no action potential (NONE) • The other stimuluses: all of them reach the threshold and differs in their strength but all have the same amplitude and same duration. • The duration is 3ms and amplitude is +30mv.(always the same regarding the strength of stimuli). Characteristics of the Action potential: Coding for Stimulus Intensity - • Action potentials A stronger stimulus will make identical action potentials occur more frequently (frequency modulated) –70 mV RMP Strength • On Off On Stimulus On Off Stimulus A stronger Stimulus stimulus may also Stimuli Time activate more (sustained for indicated times) axons in a nerve (recruitment) Stimulus strength is coded by the frequency rather than the amplitude of action potentials Off Sheet Note 9 -the difference between the stimuluses is the frequency. -frequency is the duration between the impulses ( the previous and next action potential that occurs). -less duration between the impulses the more it gets stronger. Strong frequency . • Action potentials can only increase in frequency to a certain point  There is a refractory period after an action potential when the neuron cannot become excited again. • 1. The absolute refractory period: occurs during the action potential. Na+ channels are inactive (not just closed). Membrane potential (millivolts) Characteristics of the Action potential: Refractory periods +30 Absolute refractory period (due to inactivated Na+ channels) Relative refractory period (due to continued outward diffusion of K+) 1 2 3 Time (milliseconds) 0 –55 –70 0 4 5 Characteristics of the Action potential: Refractory periods refractory period: when K+ channels are still open. Only a very strong stimulus can overcome this. Stereotypical size and shape of nerve: Each normal action potential for a given cell type look identical, depolarizes to the same potential, and repolarizes back to the same resting potential Membrane potential (millivolts) • 2. The relative +30 Absolute refractory period (due to inactivated Na+ channels) Relative refractory period (due to continued outward diffusion of K+) 1 2 3 Time (milliseconds) 0 –55 –70 0 4 5 Sheet Note 10 • in Depolarization and Repolarization refectory period is absolute • • In hyperpolarization it’s relative • Why it is relative? action potential • • So, in hyperpolarization the neuron can respond, but in Depolarization and Repolarization it’s impossible to respond. 10 Graded potential: (local potential): • Graded potentials are changes in membrane potential that vary in size and strength. • They are summative, and decremental • Their magnitude is determined by the strength of the stimulus. But they will never reach the Examples: threshold postsynaptic potentials receptor potentials end-plate potentials pacemaker potentials slow-wave potentials Sheet Note 11 -Stimulus  graded potential (local) at dendrites and cell body -Action potential occurs at axon hillock. axon hillock -graded potential can be inhibitory causes hyperpolarization. axon hillock action potential . axon hillock threshold - excitatory action potential it will fade out -graded potential is before the action potential and it must reach the axon hillock and change the membrane potential to reach the threshold to produce action potential. . Graded potential vs. action potential

Nerve and Synapses Physiology

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