Generation and Conduction of Nerve Impulse

A nerve impulse is an electrical signal that travels along the membrane of a neuron. It is produced because the neuron membrane can maintain a difference in ion concentration between the inside and outside.

Most Important Idea:

  • A neuron works because of unequal distribution of ions across its membrane.
  • Resting state = polarized
  • Impulse state = depolarized
  • Return to normal = repolarized

Resting Membrane Potential (The Ready State)

Neurons are excitable cells.A neuron that is not conducting an impulse is said to be in the resting state. In this state, its membrane is polarized. Think of it as a drawn bowstring, ready to fire.

Why is the membrane polarized?

Because ions are distributed unequally on the two sides of the membrane.

The Setup (Ion distribution)

Inside the axon (Axoplasm)

  • Packed with K+K^+ (Potassium) and negatively charged proteins. It has very little Na+Na^+.

Outside the axon (Extracellular fluid)

  • Packed with Na+Na^+ (Sodium) and has very little K+K^+.

The Rules of the Gates (Permeability)

In this resting state, the membrane's gates are more permeable to K+K^+ ions and nearly impermeable to Na+Na^+ ions. This means K+K^+ tends to diffuse outward more easily and Na+Na^+ cannot freely enter. It is also impermeable to the negatively charged proteins inside.

The Bouncer (Sodium-Potassium Pump)

The Na⁺–K⁺ pump actively maintains this unequal ion distribution. To keep this uneven distribution, an active pump uses ATP to constantly throw 3 Na+Na^+ OUTWARDS and bring 2 K+K^+ INWARDS.

The Result

Because more positive charge is being thrown out than brought in, the outer surface becomes positive, and the inner surface becomes negative. This electrical difference across the resting membrane is called the Resting Potential.

Key Points:

  • Resting neuron = polarized neuron
  • Inside: high K+K^+, negative proteins, low Na+Na^+
  • Outside: high Na+Na^+, low K+K^+
  • Na-K pump: 3 Na⁺ out, 2 K⁺ in
  • Outside positive, inside negative

Memory Trick:

  • Resting = Ready but reversed charges maintained

Action Potential (Depolarization - The Firing State)

When a suitable stimulus is applied to the neuron, the membrane at that point becomes activated and everything changes rapidly:

  1. Opening the Floodgates: The membrane at that specific site suddenly becomes freely permeable to Na+Na^+.
  2. The Influx: There is a massive, rapid rush of Na+Na^+ into the cell.
  3. Reversal of Polarity: Because so much positive charge just entered, the inside of the cell becomes positive, and the outside becomes negative. This is called Depolarization.
  4. The Spark: This sudden electrical potential difference is called the Action Potential, which is the actual nerve impulse.

Key Points:

  • Depolarization = Na⁺ enters
  • Inside becomes positive
  • Outside becomes negative
  • Action potential = nerve impulse

Repolarization (The Reset State)

The neuron cannot stay fired forever; it must reset to fire again.

  1. Closing the Gates: The sudden permeability to Na+Na^+ is extremely short-lived. The Na+Na^+ gates snap shut.
  2. The K+ Exodus: Immediately, the membrane becomes highly permeable to K+K^+. Within a fraction of a second, K+K^+ rushes OUT of the cell.
  3. Restoration: The exit of positive K+K^+ ions restores the resting state (outside positive, inside negative). The membrane is now repolarized and ready for a new stimulus.

Propagation of Impulse (The Domino Effect)

How does the impulse travel down the long axon?

  • Let's look at two adjacent spots on the axon: Site A (currently firing) and Site B (still resting).
  • At Site A (Depolarized), the inner surface is positive.
  • At Site B (Resting), the inner surface is negative.
  • The Current: Electricity flows from positive to negative.
    • On the inner surface, current flows from Site A →\rightarrow Site B.
    • On the outer surface, current flows from Site B →\rightarrow Site A to complete the circuit.
  • The Result: This local circuit triggers Site B to depolarize. Now Site B is firing, and the process repeats down the line. The action potential travels like a wave.

Saltatory Conduction (The Express Lane)

In myelinated nerve fibres, the thick myelin sheath acts as an electrical insulator. Ions cannot flow through it.

  • Therefore, the action potential can only occur at the bare gaps called the Nodes of Ranvier.
  • The impulse literally jumps from one node to the next.
  • This "jumping" is called Saltatory Conduction, and it makes the transmission of the impulse significantly faster and more energy-efficient than in non-myelinated fibres.

Generation and Conduction of Nerve Impulse

1. The Pump K IN Mnemonic

To remember the Sodium-Potassium pump ratio:

  • PUMP K IN: The pump brings 2 K+K^+ IN, meaning it must throw 3 Na+Na^+ OUT.
  • Or remember: NOKIA (Na+Na^+ Out, K+K^+ In, Active transport).

2. The State Tracker

State Primary Event Inside Charge Outside Charge
Resting Na/K pump running Negative Positive
Depolarization Na+Na^+ rushes IN Positive Negative
Repolarization K+K^+ rushes OUT Negative Positive

3. Saltatory = Speed:

  • Saltare means "to leap" in Latin. Myelinated axons use saltatory conduction to leap across Nodes of Ranvier, resulting in much faster impulses.

💡 Questions and Answers

Q1. What is the ratio of ions transported by the Sodium-Potassium pump?

A1: The sodium-potassium pump works actively to throw 3 Na+Na^+ OUT of the cell for every 2 K+K^+ it brings IN. This helps maintain the resting condition of the neuron.

Key points:

  • 3 Na+Na^+ out
  • 2 K+K^+ in
  • Maintains resting membrane potential

Q2: Which ion is responsible for depolarization of the membrane?

A2: Depolarization happens because sodium ions (Na+Na^+) rapidly enter the neuron when the membrane becomes permeable to them.

Key points:

  • Na⁺ enters
  • Inside becomes positive
  • Depolarization starts nerve impulse

Q3: Describe the charge distribution in a resting neuron.

A3: In a resting neuron, the outside of the membrane is positive and the inside is negative. This happens because of unequal distribution of ions on the two sides.

Key points:

  • Resting neuron = polarized
  • Outside = positive
  • Inside = negative

Q4: What happens during repolarization?

A4: During repolarization, sodium entry stops and potassium ions move out of the neuron. This brings the membrane back to its resting state.

Key points:

  • K+K^+ goes out
  • Resting polarity returns
  • Neuron becomes ready for next impulse

Q5: What is saltatory conduction?

A5: Saltatory conduction is the transmission of nerve impulse in which the impulse jumps from one Node of Ranvier to the next in a myelinated fibre. Because of this, conduction becomes very fast.

Key points:

  • Happens in myelinated axon
  • Impulse jumps node to node
  • Conduction becomes faster