WINDSOR UNIVERSITYSCHOOL OF MEDICINE

Bioelectrical Phenomena of the Cell-RMP and Action Potential.

Dr.Vishal Surender.MD.

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Basic Concepts

Volt

A charge difference between two points in space

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Basic Concepts

Ions – charged particles

Anions – Negatively charged particles

Cations – Positively charged particles

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Basic ConceptsForces that determine ionic movement

Electrostatic forcesOpposite charges attractIdentical charges repel

Concentration forcesDiffusion – movement of ions through

semipermeable membraneOsmosis – movement of water from region of

high concentration to low

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I. Membrane Potential

Separation of opposite charges across the membrane leads to a constant potential difference across the resting cell membrane

Cell’s ability to fire an action potential is due to the cell’s ability to maintain the cellular resting potential at approximately –70 mV (-.07 volt)

The basic signaling properties of neurons are determined by changes in the resting potential

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Membrane Resting Potential

Every cell has a separation of electrical charge across its cell membrane.

The membrane potential results from a separation of positive and negative charges across the cell membrane.

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Membrane Resting Potential

excess of positive charges outside and negative charges inside the membrane

maintained because the lipid bilayer acts as a barrier to the diffusion of ions

gives rise to an electrical potential difference, which ranges from about 60 to 70 mV. (Microelectrode)

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Concept of Resting Potential (RP)

A potential difference across the cell membrane at the resting stage or when the cell is not stimulated.

Property: It is constant or stable It is negative inside relative to the outside Resting potentials are different in different

cells.

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Resting Membrane Potential

Na+ and Cl- are more concentrated outside the cell

K+ and organic anions (organic acids and proteins) are more concentrated inside.

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Intracellular vs extracellular ion concentrations

Ion Intracellular ExtracellularNa+ 5-15 mM 145 mMK+ 140 mM 5 mMMg2+ 0.5 mM 1-2 mMCa2+ 10-7 mM 1-2 mMA- 65nM/L 0

Cl- 5-15 mM 110 mM

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Effect of The Sodium-Potassium Pump on membrane potential(20%)

• Dissipation of ionic gradients is ultimately prevented by Na+-K+ pumps

extrudes 3Na+ from the cell while taking in 2 K +

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Effect of movement of potassium alone on Membrane Potential

Potassium ions, concentrated inside the cell tend to move outward down their concentration gradient through non gated potassium channels/K+ leak channels

But the relative excess of negative charge inside the membrane tend to push potassium ions back in to the cell

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Potassium equilibrium-90 mV

Equilibrium Potentials• The two major forces acting on ion movement are the concentration and electrical gradients across the membrane.

• When these forces are equal but acting in opposite directions across a cell membrane for a particular ion the membrane potential is also the equilibrium potential for that ion.

Membrane potential that would exactly balance the diffusion gradient and prevent the net movement of a particular ion.

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Calculating equilibrium potentialNernst Equation

Allows theoretical membrane potential to be calculated for particular ion.

Value depends on the ratio of [ion] on the 2 sides of the membrane.

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Nernst equation

Equilibrium potential (mV) , Eion = ln RTzF

[C]o

[C]i

where,[C]o and [C]i = extra and intracellular [ion] R = Universal gas constant (8.3 joules.K-1.mol-1)T = Absolute temperature (°K)F = Faraday constant (96,500 coulombs.mol-1)z = Charge of ion (Na+ = +1, Ca2+ = +2, Cl- = -1)

For K+, with [K+]o = 4 mmol.l-1 and [K+]i = 150 mmol.l-1

At 37°C, EK = -97mV ENa = +60mv

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Resting Membrane Potential

• Electrostatic and Chemical forces act together on Na+ ions to drive them into the cell

• The Na+ channel close during the resting state

Na+ is more concentrated outside than inside and therefore tends to flow into the cell down its concentration gradient

Na+ is driven into the cell by the electrical potential difference across the membrane.

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Resting Membrane PotentialResting membrane potential is less than Ek because

some Na+ can also enter the cell.The slow rate of Na+ influx is accompanied by slow

rate of K+ outflux.Depends upon 2 factors:

Ratio of the concentrations of each ion on the 2 sides of the plasma membrane.

Specific permeability of membrane to each different ion.

Resting membrane potential of most cells ranges from - 65 to – 85 mV.

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Factors that affect resting potential Difference of K+ ion concentration across the

membrane Due to the fact that the cell membrane at rest is

much more permeable to K+ ions than Na+ ions the movement of K+ ions has a greater influence on the resting membrane potential. The resting membrane potential of the cell at rest in this case is -70mV.

Permeability of the membrane to Na+ and K+. Action of Na+ pump

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Basic Electrophysiological Terms I:

Polarization: a state in which membrane is polarized at rest, negative inside and positive outside.

Depolarization: the membrane potential becomes less negative than the resting potential (close to zero).

Hyperpolarization: the membrane potential is more negative than the resting level.

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Basic Electrophysiological Terms I:

Reverspolarization: a reversal of membrane potential polarity.

The inside of a cell becomes positive relative to the outside.

Repolarization: restoration of normal polarization state of membrane.

a process in which the membrane potential returns toward from depolarized level to the normal resting membrane value.

WINDSOR UNIVERSITYSCHOOL OF MEDICINE

Action Potential.

Dr.Vishal Surender.MD.

Objectives-• To understand that rapid changes in permeability of the neuronal membrane produce the action potential. • To recognize that altering voltage-gated ion channels changes membrane permeability. • To understand the movement of sodium and potassium ions during the action potential. • To examine refractory periods. • To learn about conduction velocity.

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Basic Electrophysiological Terms I:

Polarization: a state in which membrane is polarized at rest, negative inside and positive outside.

Depolarization: the membrane potential becomes less negative than the resting potential (close to zero).

Hyperpolarization: the membrane potential is more negative than the resting level.

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Basic Electrophysiological Terms I:

Reverspolarization: a reversal of membrane potential polarity.

The inside of a cell becomes positive relative to the outside.

Repolarization: restoration of normal polarization state of membrane.

a process in which the membrane potential returns toward from depolarized level to the normal resting membrane value.

The Action Potential Neurons communicate over long distances by generating and sending an electrical signal called a nerve impulse, or action potential. • The action potential is a large change in membrane potential from a resting value of about -70 millivolts to a peak of about +30 millivolts, and back to -70 millivolts again. • The action potential results from a rapid change in the permeability of the neuronal membrane to sodium and potassium based on a triggering event. The permeability changes as voltage-gated ion channels open and close.

The Action Potential Begins at the Axon Hillock

• The action potential is generated at the axon hillock, where the density of voltage-gated sodium channels is greatest. • The action potential begins when signals from the dendrites and cell body reach the axon hillock and cause the membrane potential there to become more positive, a process called depolarization.

During Depolarization Sodium Moves into the Neuron

• As the axon hillock depolarizes, voltage-gated channels for sodium open rapidly, increasing membrane permeability to sodium. • Sodium moves down its electrochemical gradient into the cell.

voltage-gated sodium channels

Threshold • If the stimulus to the axon hillock is great enough, the neuron depolarizes by about 15 millivolts and reaches a trigger point called threshold. • At threshold, an action potential is generated. Weak stimuli that do not reach threshold do not produce an action potential. Thus we say that the action potential is an all-or-none event. • Action potentials always have the same amplitude and the same duration.

• At -55 millivolts the membrane is depolarized to threshold, and an action potential is generated. • Threshold is a special membrane potential where the process of depolarization becomes regenerative, that is, where a positive feedback loop is established.

An Action Potential is Generated When a Positive Feedback Loop is Established

• When, and only when, a neuron reaches threshold, a positive feedback loop is established. • At threshold, depolarization opens more voltage-gated sodium channels. • This causes more sodium to flow into the cell, which in turn causes the cell to depolarize further and opens still more voltage-gated sodium channels. • This positive feedback loop produces the rising phase of the action potential.

depolarization

Openvoltage-gated Na channels

Inward flow of Narising phase

Interrupting the Positive Feedback Loop: Voltage-Gated Sodium Channels Inactivate • The rising phase of the action potential ends when the positive feedback loop is interrupted. • Two processes break the loop: 1. the inactivation of the voltage-gated sodium channels. 2. the opening of the voltage-gated potassium channels. • The voltage-gated sodium channels have two gates: 1. A voltage-sensitive gate opens as the cell is depolarized. 2. A second, time-sensitive inactivation gate stops the movement of sodium through the channel after the channel has been open for a certain time. • At the resting membrane potential, the voltage sensitive gate is closed. • As the neuron is depolarized, the voltage-sensitive gate opens. • At a certain time after the channel opens, it inactivates. • At the peak of the action potential, voltage-gated sodium channels begin to inactivate. As they inactivate, the inward flow of sodium decreases, and the positive feedback loop is interrupted.

voltage-sensitive gate

time-sensitive gate

resting

depolarized

inactive

Interrupting the Positive Feedback Loop: Voltage-Gated Potassium Channels Open

• The voltage-gated potassium channels respond slowly to depolarization. They begin to open as the membrane depolarizes, but responds so slowly that they become fully activated only after the action potential reaches its peak.

• potassium moves out of the cell as voltage-gated potassium channels open. As potassium moves out, depolarization ends, and the positive feedback loop is broken.

• Both the inactivation of sodium channels and the opening of potassium channels interrupt the positive feedback loop. This ends the rising phase of the action potential.

Repolarization

•We have seen potassium leaving the cell as voltage-gated potassium channels opened. • With less sodium moving into the cell and more potassium moving out, the membrane potential becomes more negative, moving toward its resting value. • This process is called repolarization.

Repolarization

Hyperpolarization • In many neurons, the slow voltage-gated potassium channels remain open after the cell has repolarized. Potassium continues to move out of the cell, causing the membrane potential to become more negative than the resting membrane potential. • This process is called hyperpolarization. • By the end of the hyperpolarization, all the potassium channels are closed.

Hyperpolarization

• The rapid increase in sodium permeability is responsible for the rising phase/Depolarization of the action potential. • The rapid decrease in sodium permeability and simultaneous increase in potassium permeability is responsible for the repolarization of the cell. • The slow decline in potassium permeability is responsible for the hyperpolarization.

•Rest. •Voltage-gated sodium and potassium channels are closed when the neuron is at rest. • Depolarization. Voltage-gated sodium channels open rapidly, resulting in movement of sodium into the cell. This causes depolarization. • Repolarization. Voltage-gated sodium channels continue to inactivate, then reset to the closed state. Potassium channels continue to open. This results in a net movement of positive charge out of the cell, repolarizing the cell. • Hyperpolarization. Some voltage-gated potassium channels remain open, resulting in movement of potassium out of the cell. This hyperpolarizes the cell.

Ion Channel Activity During the Action Potential: Summary

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The Relative Refractory Period • Immediately after the absolute refractory period, the cell can generate another action potential, but only if it is depolarized to a value more positive than normal threshold. This is true because some sodium channels are still inactive and some potassium channels are still open. This is called the relative refractory period. • The cell has to be depolarized to a more positive membrane potential than normal threshold to open enough sodium channels to begin the positive feedback loop.

The Action Potential is Propagated Along the Axon • After an action potential is generated at the axon hillock, it is propagated down the axon. • Positive charge flows along the axon, depolarizing adjacent areas of membrane, which reach threshold and generate an action potential. The action potential thus moves along the axon as a wave of depolarization traveling away from the cell body.

Conduction Velocity Depends on Diameter and Myelination of the Axon • Conduction velocity is the speed with which an action potential is propagated. • Conduction velocity depends on two things:

1. The diameter of the axon. • As the axon diameter increases, the internal resistance to the flow of charge decreases and the action potential travels faster.

2. How well the axon is insulated with myelin. • myelinated axons have areas of insulation interrupted by areas of bare axon called nodes of Ranvier. • In a myelinated axon, charge flows across the membrane only at the nodes, so an action potential is generated only at the nodes. The action potential appears to jump along the axon. This type of propagation is called saltatory conduction. • A myelinated axon typically conducts action potentials faster than an unmyelinated axon of the same diameter.

• More speed is gained by insulating an axon with myelin than by increasing the axon diameter.

λ = √R/RiRm Membrane resistance Ri Internal resistance

Table 4–1 Nerve Fiber Types in Mammalian Nerve. a

Fiber Type Function Fiber Diameter (m)

Conduction Velocity (m/s)

A alpha Proprioception; somatic motor 12–20 70–120

beta Touch, pressure 5–12 30–70Gamma Motor to muscle spindles 3–6 15–30Delata Pain, cold, touch 2–5 12–30B Preganglionic autonomic <3 3–15C Dorsal root Pain, temperature, some

mechano-reception0.4–1.2 0.5–2

SympatheticPostganglionic sympathetic 0.3–1.3 0.7–2.3

Nerve Fiber Types in Mammalian Nerve

For a television game show, 16 contestants volunteer to be stranded on a deserted island in the middle of the South China Sea. They must rely on their own survival instincts and skills. During one of the challenges, one team wins a fishing spear. They catch a puffer fish and cook it over the open flames of their barbecue. None of them are very skilled in cooking, but they enjoy the fish anyway. One of the contestants, a worldwide traveler, comments that it tastes like Fugu. After dinner, they all develop a strange tingling around their lips and tongue. They all become weak, and their frailty progresses to paralysis. They all die. What was the cause of death?

A Tetrodotoxin B Botulism C Bacillus cereus food poisoning D Tetanus E Ciguatoxin

A well-meaning third year medical student accidentally pushes an unknown quantity of KCl IV to a patient. If the concentration of potassium outside a neuron were to increase from 4 mEq/L to 8 mEq/L, what would you expect to happen to the minimal stimulus required for initiation of an action potential? A The minimal stimulus required for initiation of an action potential would remain the same B The minimal stimulus required for initiation of an action potential would increase C The minimal stimulus required for initiation of an action potential would decrease D The minimal stimulus required for initiation of an action potential would stay the same, but the amplitude of the peak of the action potential would increase

E The minimal stimulus required for initiation of an action potential would stay the same, but the conduction velocity of the action potential down an axon would slow

Clinical Focus-Channelopathies•Voltage-gated channels for sodium, potassium, calcium, and chloride are intimately associated with excitability in neurons and muscle cells and in synaptic transmission.•Channelopathies affecting neurons include episodic and spinocerebellar ataxias, some forms of epilepsy, and familial hemiplegic migraine. •Ataxias are a disruption in gait mediated by abnormalities in the cerebellum and spinal motor neurons. One specific ataxia associated with an abnormal potassium channel is episodic ataxia with myokymia. In this disease, which is autosomal-dominant, cerebellar neurons have abnormal excitability and motor neurons are chronically hyperexcitable. This hyperexcitability causes indiscriminant firing of motor neurons, observed as the twitching of small groups of muscle fibers, akin to worms crawling under the skin (myokymia).•One of the best-known sets of channelopathies is a group of channel mutations that lead to the Long Q-T (LQT) syndrome in the heart. The QT interval on the electrocardiogram is the time between the beginning of ventricular depolarization and the end of ventricular repolarization. In patients with LQT, the QT interval is abnormally long because of defective membrane repolarization, which can lead to ventricular arrhythmia and sudden death

Local Anesthetics. Among the most important stabilizersare the many substances used clinically as local anesthetics, including procaine and tetracaine. Most of these act directly on the activation gates of the sodium channels, making it much more difficult for these gates to open, thereby reducing membrane excitability.