Bio-electricity (RMP- Action Potential) give me MPT Level 10 marks answer of this question

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Bio-electricity: Resting Membrane Potential (RMP) and Action Potential

Source: Costanzo Physiology 7th Ed. | Ganong's Review of Medical Physiology 26th Ed.

1. RESTING MEMBRANE POTENTIAL (RMP)

Definition

The resting membrane potential is the potential difference that exists across the membrane of excitable cells (nerve, skeletal muscle, cardiac muscle) between action potentials. By convention, it is expressed as the intracellular potential relative to the extracellular potential.

Typical value: -70 to -80 mV (cell interior is negative relative to exterior)

Ionic Basis

Ion	Intracellular	Extracellular	Equilibrium Potential
K⁺	140 mEq/L	4 mEq/L	-85 mV
Na⁺	14 mEq/L	140 mEq/L	+65 mV
Cl⁻	4 mEq/L	114 mEq/L	-90 mV

At rest:

High permeability to K⁺ and Cl⁻ → K⁺ diffuses out down its concentration gradient → creates a negative interior (K⁺ diffusion potential)
Low permeability to Na⁺ → minimal inward Na⁺ current
Because K⁺ conductance dominates at rest, RMP (~~-70 mV) is close to the K⁺ equilibrium potential (~~-85 mV) but far from the Na⁺ equilibrium potential (+65 mV)

Governing Equations

Nernst Equation (for a single ion X):

E_x = (61/z) × log([X]o / [X]i)

Goldman Equation (for multiple ions - considers relative permeability):

Em = (RT/F) × ln [(PK[K⁺]o + PNa[Na⁺]o + PCl[Cl⁻]i) / (PK[K⁺]i + PNa[Na⁺]i + PCl[Cl⁻]o)]

Chord Conductance Equation (weights each ion by its conductance):

Em = (GK/GT)EK + (GNa/GT)ENa + (GCl/GT)ECl + (GCa/GT)ECa

Role of Na⁺/K⁺ ATPase Pump

Pumps 3 Na⁺ out for every 2 K⁺ pumped in (electrogenic)
Maintains the concentration gradients that generate RMP
Directly contributes a small negative charge (net outward current)

Factors Affecting RMP

Factor	Effect on RMP
↑ Extracellular K⁺	Depolarization (less negative)
↓ Extracellular K⁺	Hyperpolarization (more negative)
Blocking Na⁺/K⁺ pump	Gradual depolarization
↑ Na⁺ permeability	Depolarization toward ENa

2. ACTION POTENTIAL (AP)

Definition

An action potential is a rapid, transient reversal of membrane potential in an excitable cell, initiated when the membrane is depolarized to threshold.

Threshold

Approximately -60 mV in nerve/skeletal muscle
Depolarizing the cell from RMP (-70 mV) to threshold triggers the AP

Phases of the Action Potential

$Action potential of nerve - voltage, Na⁺ conductance, K⁺ conductance with absolute and relative refractory periods$

Fig. 1.13 - Costanzo Physiology: Voltage and conductance changes during the action potential of nerve

Phase 1 - Resting State

Membrane at -70 mV
Voltage-gated Na⁺ channels: closed (activation gate closed, inactivation gate open)
K⁺ channels: mostly open (high K⁺ conductance)

Phase 2 - Upstroke (Depolarization)

Stimulus depolarizes membrane to threshold (~-60 mV)
Rapid opening of activation gates on voltage-gated Na⁺ channels
Na⁺ conductance rapidly exceeds K⁺ conductance
Na⁺ rushes in down its electrochemical gradient (inward current)
Membrane potential shoots toward ENa (+65 mV) but does not quite reach it
Peak AP ≈ +30 to +40 mV
Blocked by: tetrodotoxin (TTX) from puffer fish, lidocaine (local anesthetic)

Phase 3 - Repolarization

Caused by two simultaneous events:

Na⁺ channel inactivation - Inactivation gates (h-gates) close (slower than activation gate opening) → Na⁺ conductance falls to zero
Delayed K⁺ channel opening - Voltage-gated K⁺ channels open → K⁺ flows outward → membrane repolarizes toward EK (-85 mV)

Phase 4 - After-Hyperpolarization (Undershoot)

Membrane transiently goes below -70 mV (closer to -80 to -85 mV)
Caused by persistently elevated K⁺ conductance after Na⁺ channels have closed
As K⁺ channels gradually close, membrane returns to RMP

3. IONIC MECHANISM - VOLTAGE-GATED Na⁺ CHANNEL

The Na⁺ channel has two gates:

Activation gate (m-gate): Closed at rest; opens rapidly upon depolarization
Inactivation gate (h-gate): Open at rest; closes slowly upon depolarization

State	Activation Gate	Inactivation Gate	Channel
Resting	Closed	Open	Not conducting
Depolarized (upstroke)	Open	Open	Conducting (Na⁺ enters)
Inactivated	Open	Closed	Not conducting
After repolarization	Closed	Opens again	Returns to resting state

This gating mechanism explains why the AP is all-or-none and why Na⁺ channels must fully reset before another AP fires.

4. CHARACTERISTICS OF THE ACTION POTENTIAL

1. All-or-None Law

An AP either fires fully or not at all. Once threshold is reached, the AP is inevitable and always has the same shape/amplitude for that cell type. Subthreshold stimuli cause only local, graded potentials.

2. Stereotypical Shape

Every AP for a given cell type depolarizes to the same peak and repolarizes to the same resting level.

3. Propagation (Non-decremental)

APs propagate without loss of amplitude along the entire length of the axon.

5. REFRACTORY PERIODS

Absolute Refractory Period (ARP)

Occurs during the upstroke and the early repolarization phase
Na⁺ channels are inactivated (inactivation gates closed) - impossible to open regardless of stimulus strength
No AP can be generated no matter how strong the stimulus
Ensures unidirectional propagation and limits firing frequency

Relative Refractory Period (RRP)

Occurs during late repolarization and the after-hyperpolarization
Some Na⁺ channels have recovered; K⁺ conductance still elevated
An AP can be generated but only with a stronger-than-normal stimulus
Threshold is elevated during this period

6. PROPAGATION OF ACTION POTENTIALS

Propagation of AP by local currents along a nerve fiber: A - active region at initial segment, B - current flow toward adjacent inactive region, C - adjacent region fires AP and propagates forward

Fig. 1.15 - Costanzo Physiology: Spread of depolarization down a nerve fiber by local currents

Propagation occurs by spread of local currents:

Active region (depolarized): interior becomes positive
Adjacent inactive region: interior is negative
Positive charges flow intracellularly from active → inactive region
This depolarizes the adjacent region to threshold → fires its own AP
The original active region repolarizes (is in ARP, so cannot re-fire)
Result: AP moves in ONE direction only (forward propagation)

7. CONDUCTION VELOCITY

Factors increasing conduction velocity:

Factor	Mechanism	Example
Myelination	Saltatory conduction (AP jumps between nodes of Ranvier)	Aα fibers: 70-120 m/s
Larger axon diameter	Less internal resistance to current flow	Motor neurons
Temperature ↑	Faster ion channel kinetics	-

Saltatory conduction: In myelinated nerves, the myelin sheath insulates the internodal regions. Current flows only at nodes of Ranvier (gaps in myelin every 1-2 mm), so the AP "jumps" from node to node - much faster than continuous conduction.

8. CLINICAL CORRELATES

Condition	Mechanism	Effect
Hyperkalemia	↑ [K⁺]o → less negative RMP → inactivation gates on Na⁺ channels close → no APs	Muscle weakness, cardiac arrhythmia
Hypokalemia	↑ K⁺ gradient → hyperpolarization → harder to reach threshold	Muscle weakness
Local anesthetics (lidocaine, bupivacaine)	Block voltage-gated Na⁺ channels	Loss of pain sensation
Tetrodotoxin (TTX)	Blocks Na⁺ channels externally	Paralysis (puffer fish poisoning)
Multiple Sclerosis	Demyelination → loss of saltatory conduction	Slowed nerve conduction
Lambert-Eaton syndrome	Antibodies against voltage-gated Ca²⁺ channels	Reduced neurotransmitter release

SUMMARY TABLE

Parameter	Value
Resting Membrane Potential	-70 mV
Threshold	~-60 mV
Peak of AP	~+30 to +40 mV
Na⁺ Equilibrium Potential (ENa)	+65 mV
K⁺ Equilibrium Potential (EK)	-85 mV
Duration of AP (nerve)	~1-2 ms
Absolute Refractory Period	~1 ms
Relative Refractory Period	~1-2 ms after ARP

Key Memory Points for 10-mark answer:

RMP = -70 mV, set by K⁺ dominance and Na⁺/K⁺ pump
AP = all-or-none; upstroke by Na⁺ in, repolarization by K⁺ out + Na⁺ inactivation
ARP = Na⁺ inactivation gates closed; RRP = partial recovery + elevated K⁺ conductance
Propagation = local currents, unidirectional because of ARP
Myelination → saltatory conduction → faster velocity

- Costanzo Physiology 7th Ed., Chapter 1 (Cell Physiology), pp. 26-32 - Ganong's Review of Medical Physiology 26th Ed., Chapter 4

Teach me in simple language

Bio-electricity in Simple Language 🧠⚡

Let me teach you this like telling a story.

The Story of a Nerve Cell

Think of a nerve cell like a house with a security system.

The inside of the house = inside of the cell
The outside = outside the cell
The walls = cell membrane
The doors and windows = ion channels

PART 1: Resting Membrane Potential (RMP)

"The House is Locked and Quiet"

When a nerve cell is just sitting there doing nothing (at rest), it has a charge difference across its membrane.

The inside is NEGATIVE (-70 mV) compared to outside.

Why is the inside negative? - The Simple Story

Imagine you have two types of people:

🟡 Potassium (K⁺) - lives mostly INSIDE the cell (140 units inside vs 4 outside)
🔵 Sodium (Na⁺) - lives mostly OUTSIDE the cell (140 units outside vs 14 inside)

What happens at rest?

The membrane has K⁺ doors (channels) that are open at rest.

K⁺ says: "There are too many of us inside! Let me go out!" - so K⁺ leaks out.

But wait - K⁺ carries a positive charge. When it leaves, the inside becomes more and more NEGATIVE.

K⁺ keeps going out until... the negativity inside pulls it back. It reaches a balance point at -70 mV. This balance is the Resting Membrane Potential.

Simple rule: K⁺ leaks out → inside becomes negative → that's your RMP of -70 mV

The Security Guard - Na⁺/K⁺ Pump

There's a pump in the membrane that works like a security guard:

Throws 3 Na⁺ OUT for every 2 K⁺ it brings IN
This keeps Na⁺ concentrated outside and K⁺ concentrated inside
It uses ATP (energy) to do this
Because it pumps more positive charges OUT than IN, it makes the inside slightly more negative (electrogenic)

PART 2: Action Potential (AP)

"The Alarm Goes Off!"

Now imagine someone throws a rock at the house (a stimulus arrives). If the rock is big enough, the alarm goes off - this is the Action Potential.

The "big enough" threshold = -60 mV

If the stimulus depolarizes (makes less negative) the membrane from -70 mV to -60 mV → BOOM - action potential fires!

If the stimulus is too weak and doesn't reach -60 mV → nothing happens (all-or-none law).

The 4 Steps of the Action Potential - Like a Drama in 4 Acts 🎭

ACT 1: AT REST (-70 mV)

Na⁺ channels: LOCKED (closed, but ready)
K⁺ channels: slightly open (leaking quietly)
Inside: -70 mV (negative and calm)

ACT 2: THE UPSTROKE - "Flood of Sodium!" ⬆️

A stimulus arrives → membrane depolarizes to -60 mV (threshold)

The Na⁺ channel doors FLY OPEN!

Na⁺ says: "Finally! I've been waiting outside for so long! I'm rushing IN!"

Na⁺ floods into the cell because:

There's more Na⁺ outside (concentration gradient pulls it in)
Inside is negative (electrical gradient pulls it in)

The inside rapidly becomes POSITIVE - shoots up to +30 to +40 mV

This is the UPSTROKE - the spike of the action potential.

Na⁺ rushes IN = cell goes from -70 mV to +40 mV in less than 1 millisecond!

ACT 3: REPOLARIZATION - "Doors Slam Shut + K⁺ Rushes Out" ⬇️

Two things happen together:

Thing 1 - Na⁺ channels INACTIVATE: The Na⁺ channel has two doors (gates):

Front door (activation gate): opened when stimulus hit
Back door (inactivation gate): was open at rest, now slowly CLOSES

Once the back door closes → Na⁺ can no longer enter → upstroke stops.

Thing 2 - K⁺ channels OPEN WIDE: Depolarization slowly opens more K⁺ channels.

K⁺ says: "Inside is now positive?! That's unusual! I'm getting out!"

K⁺ rushes OUT → inside becomes negative again → repolarization

K⁺ rushes OUT + Na⁺ channels close = cell goes back negative

ACT 4: AFTER-HYPERPOLARIZATION - "Overshoot of negativity" ⬇️⬇️

The K⁺ channels are still open a little too long.

Too much K⁺ leaves → inside goes MORE negative than normal → briefly reaches -80 to -85 mV

Then K⁺ channels slowly close → membrane returns to -70 mV (resting state).

This dip below normal is called the after-hyperpolarization or undershoot.

The Complete Picture

+40 mV  |        /\
        |       /  \
        |      /    \
  0 mV  |     /      \
        |    /        \
        |   /          \
-60 mV  |--/ threshold  \
-70 mV  |/  RMP          \____/  ← after-hyperpolarization
-85 mV  |                       ← K⁺ equilibrium potential
        |________________________
              Time (milliseconds)

PART 3: Refractory Periods

"Don't Bother Me Right Now!"

After an AP fires, the cell cannot immediately fire again. This is called the Refractory Period.

Absolute Refractory Period (ARP) - "COMPLETELY UNAVAILABLE"

Happens during the upstroke + early repolarization
Na⁺ back doors (inactivation gates) are CLOSED
NO stimulus, no matter how strong, can fire another AP
Like a person who just sprinted 100m - they physically cannot sprint again for a moment
Why important: ensures the AP travels in ONE direction only (can't go backwards into the ARP zone)

Relative Refractory Period (RRP) - "POSSIBLE BUT HARDER"

Happens during late repolarization + after-hyperpolarization
Some Na⁺ channels have recovered, but K⁺ conductance is still high
A stronger-than-normal stimulus CAN fire an AP
Like someone who is tired but could still run if you really pushed them

PART 4: Propagation - "How Does the Signal Travel?"

Propagation of AP by local currents along a nerve fiber

Think of a line of dominoes 🁣🁣🁣🁣🁣

When the first domino (active region) falls (fires AP), it pushes the next one (adjacent region) which pushes the next one, and so on.

The mechanism:

Active region: inside becomes positive (+40 mV)
Next region: still negative inside (-70 mV)
Positive charges flow from active → next region (local current)
Next region gets depolarized to threshold → fires its own AP
Original region is now in ARP → cannot fire again → signal only goes FORWARD

PART 5: Myelination - "The Express Highway"

Unmyelinated nerve = a dirt road

AP fires at every single point along the axon - slow!

Myelinated nerve = a highway with toll booths (nodes of Ranvier)

Myelin sheath covers most of the axon (acts as insulator)
Gaps in myelin every 1-2 mm = Nodes of Ranvier (the toll booths)
AP only fires at the nodes → "jumps" from node to node
This is called Saltatory Conduction (saltare = to jump in Latin)
Much, much faster! (70-120 m/s vs 0.5-2 m/s in unmyelinated)

Quick Summary Table

What?	Simple Version	Value
RMP	Cell at rest, inside negative	-70 mV
Threshold	Minimum to trigger AP	-60 mV
Upstroke	Na⁺ floods IN	-70 → +40 mV
Repolarization	K⁺ floods OUT	+40 → -70 mV
After-hyperpolarization	K⁺ lingers	-70 → -85 mV
ARP	Cannot fire at all	During upstroke
RRP	Can fire with strong stimulus	During repolarization
Saltatory conduction	AP jumps between nodes	Fast!

The ONE sentence to remember everything:

"At rest, K⁺ leaks out making inside -70 mV. A stimulus opens Na⁺ channels (rush in, +40 mV = upstroke), then Na⁺ channels inactivate and K⁺ rushes out (repolarization), the signal travels forward because the used region is refractory and can't re-fire."

That's the whole story! 🎉

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