5.5 Nerve Impulse | Nervous System Of Man | Biology 12

A nerve impulse is a signal transmitted along a nerve fiber. It consists of a wave of electrochemical changes that travel from one end of a neuron to the other, carrying information from receptors to the Central Nervous System (CNS) and from the CNS to effectors.

Neuron Structure→

Membrane Potential

The electrical potential (a capacity to do electrical work) across the neuron's membrane. It exists in two primary states:

Resting Membrane Potential (RMP)
Action Potential (AP)

Resting Membrane Potential (RMP)

The RMP is the membrane potential of a neuron that is not currently sending an impulse. It is in a polarized state.

Characteristics:
- The outer surface of the membrane is more positive than the inner surface.
- The potential difference is approximately -70 mV (the inside is negative relative to the outside).
Factors Maintaining RMP:
1. Sodium-Potassium (Na⁺/K⁺) Pump:
  - Actively transports ions against their concentration gradients, using energy (ATP).
  - Pumps 3 Na⁺ ions out for every 2 K⁺ ions pumped in.
  - This results in a net positive charge moving out, making the inside more negative.
  - Maintains high [K⁺] inside and high [Na⁺] outside the neuron.
2. Negative Organic Ions:
  - Large, negatively charged molecules (proteins, amino acids, RNA) are trapped inside the neuron, contributing to the negative internal charge.
3. Potassium (K⁺) Leak Channels:
  - The membrane is more permeable to K⁺ than to Na⁺ at rest due to "leaky" K⁺ channels.
  - K⁺ ions continuously leak out of the cell down their concentration gradient, further increasing the positive charge on the outside.

Fig. 5.11: Ionic movement across neuron membrane during the resting state, maintained by the Na⁺/K⁺ pump and leak channels.

Action Potential (AP)

An action potential is a rapid, temporary reversal of the membrane potential that occurs when a neuron is stimulated by a threshold stimulus. It is a wave of depolarization followed by repolarization.

Fig. 5.12: Conduction of a nerve impulse as a wave of depolarization and repolarization.

Phases of an Action Potential

Stimulus and Depolarization:
- A threshold stimulus (a stimulus strong enough to trigger an AP) is received.
- Voltage-gated Na⁺ channels open rapidly.
- Na⁺ ions rush into the neuron, down their electrochemical gradient.
- The membrane potential changes from -70 mV to a positive value (up to +50 mV). This reversal of charge is called depolarization.
Repolarization:
- After a brief period (~1 ms), the Na⁺ channels close.
- Voltage-gated K⁺ channels open.
- K⁺ ions rush out of the neuron, down their electrochemical gradient.
- The exit of positive K⁺ ions makes the inside of the membrane negative again, restoring the polarized state.
Hyperpolarization:
- The K⁺ channels are slow to close, causing a brief "overshoot" where the membrane becomes more negative than the resting potential.
Refractory Period:
- A brief recovery period after an AP during which the neuron cannot generate another impulse.
- The sodium-potassium pump actively works to restore the original ion concentrations (high K⁺ in, high Na⁺ out).
- This period ensures the one-way travel of the nerve impulse and limits the frequency of firing (to about 250 impulses/second).

Action potential graph — Fig. 5.13: Graph showing the change in membrane potential during an action potential.

Phase	Membrane State	Key Event	Ion Channel Status	Membrane Potential (mV)
Resting	Polarized	Na⁺/K⁺ pump active	Leaky K⁺ channels open	-70 mV
Depolarization	Depolarized	Influx of Na⁺ ions	Voltage-gated Na⁺ channels open	Rises to +50 mV
Repolarization	Repolarizing	Efflux of K⁺ ions	Na⁺ channels close, K⁺ channels open	Falls from +50 mV
Hyperpolarization	Hyperpolarized	K⁺ channels slow to close	K⁺ channels closing	Dips below -70 mV

Velocity of Nerve Impulse

The speed at which a nerve impulse travels is variable and depends on two main factors:

Myelination:
- Myelinated fibers are covered by an insulating myelin sheath. The impulse "jumps" from one gap (Node of Ranvier) to the next in a process called saltatory conduction. This is extremely fast (up to 120 m/s).
- Non-myelinated fibers conduct the impulse continuously along the entire axon membrane, which is slower (1-3 m/s). Myelination also conserves energy.
Axon Diameter:
- Thicker axons have less resistance to electrical current flow and therefore conduct impulses faster than thin axons.

Synaptic Delay: The transmission of a signal across a synapse takes approximately one millisecond, which is significantly slower than impulse travel along an axon.

Possible Questions/Answers

Q: What is a nerve impulse? A: A wave of electrochemical changes (an action potential) that travels along the length of a neuron to transmit a signal.
Q: What is the difference between Resting Membrane Potential (RMP) and Action Potential (AP)? A: RMP is the stable, negative charge (-70 mV) of an unstimulated neuron (polarized state). AP is a rapid, temporary reversal of this charge (depolarization to +50 mV) that propagates along the axon when the neuron is stimulated.
Q: How is the Resting Membrane Potential maintained? A: It is maintained by three factors: the sodium-potassium pump (3 Na⁺ out, 2 K⁺ in), the presence of large negative organic ions inside the cell, and the outward leakage of K⁺ ions through leak channels.
Q: What causes depolarization during an action potential? A: The rapid opening of voltage-gated sodium (Na⁺) channels and the subsequent influx of Na⁺ ions into the neuron.
Q: What is saltatory conduction? A: It is the rapid transmission of a nerve impulse along a myelinated axon, where the action potential "jumps" from one Node of Ranvier to the next, which is much faster than continuous conduction in unmyelinated axons.

Membrane Potential

The electrical potential (a capacity to do electrical work) across the neuron's membrane. It exists in two primary states:

Resting Membrane Potential (RMP)

Action Potential (AP)

The RMP is the membrane potential of a neuron that is not currently sending an impulse. It is in a polarized state.

Characteristics:

The outer surface of the membrane is more positive than the inner surface.
The potential difference is approximately -70 mV (the inside is negative relative to the outside).

Factors Maintaining RMP:

Sodium-Potassium (Na⁺/K⁺) Pump:
- Actively transports ions against their concentration gradients, using energy (ATP).
- Pumps 3 Na⁺ ions out for every 2 K⁺ ions pumped in.
- This results in a net positive charge moving out, making the inside more negative.
- Maintains high [K⁺] inside and high [Na⁺] outside the neuron.
Negative Organic Ions:
- Large, negatively charged molecules (proteins, amino acids, RNA) are trapped inside the neuron, contributing to the negative internal charge.
Potassium (K⁺) Leak Channels:
- The membrane is more permeable to K⁺ than to Na⁺ at rest due to "leaky" K⁺ channels.
- K⁺ ions continuously leak out of the cell down their concentration gradient, further increasing the positive charge on the outside.

Fig. 5.11: Ionic movement across neuron membrane during the resting state, maintained by the Na⁺/K⁺ pump and leak channels.

Fig. 5.12: Conduction of a nerve impulse as a wave of depolarization and repolarization.

Stimulus and Depolarization:

A threshold stimulus (a stimulus strong enough to trigger an AP) is received.
Voltage-gated Na⁺ channels open rapidly.
Na⁺ ions rush into the neuron, down their electrochemical gradient.
The membrane potential changes from -70 mV to a positive value (up to +50 mV). This reversal of charge is called depolarization.

Repolarization:

After a brief period (~1 ms), the Na⁺ channels close.
Voltage-gated K⁺ channels open.
K⁺ ions rush out of the neuron, down their electrochemical gradient.
The exit of positive K⁺ ions makes the inside of the membrane negative again, restoring the polarized state.

Hyperpolarization:

The K⁺ channels are slow to close, causing a brief "overshoot" where the membrane becomes more negative than the resting potential.

Refractory Period:

A brief recovery period after an AP during which the neuron cannot generate another impulse.
The sodium-potassium pump actively works to restore the original ion concentrations (high K⁺ in, high Na⁺ out).
This period ensures the one-way travel of the nerve impulse and limits the frequency of firing (to about 250 impulses/second).

Fig. 5.13: Graph showing the change in membrane potential during an action potential.

Phase	Membrane State	Key Event	Ion Channel Status	Membrane Potential (mV)
Resting	Polarized	Na⁺/K⁺ pump active	Leaky K⁺ channels open	-70 mV
Depolarization	Depolarized	Influx of Na⁺ ions	Voltage-gated Na⁺ channels open	Rises to +50 mV
Repolarization	Repolarizing	Efflux of K⁺ ions	Na⁺ channels close, K⁺ channels open	Falls from +50 mV
Hyperpolarization	Hyperpolarized	K⁺ channels slow to close	K⁺ channels closing	Dips below -70 mV

The speed at which a nerve impulse travels is variable and depends on two main factors:

Myelination:

Myelinated fibers are covered by an insulating myelin sheath. The impulse "jumps" from one gap (Node of Ranvier) to the next in a process called saltatory conduction. This is extremely fast (up to 120 m/s).
Non-myelinated fibers conduct the impulse continuously along the entire axon membrane, which is slower (1-3 m/s). Myelination also conserves energy.

Axon Diameter:

Thicker axons have less resistance to electrical current flow and therefore conduct impulses faster than thin axons.

Synaptic Delay: The transmission of a signal across a synapse takes approximately one millisecond, which is significantly slower than impulse travel along an axon.

Possible Questions/Answers

Q: What is a nerve impulse? A: A wave of electrochemical changes (an action potential) that travels along the length of a neuron to transmit a signal.

Q: What is the difference between Resting Membrane Potential (RMP) and Action Potential (AP)? A: RMP is the stable, negative charge (-70 mV) of an unstimulated neuron (polarized state). AP is a rapid, temporary reversal of this charge (depolarization to +50 mV) that propagates along the axon when the neuron is stimulated.

Q: How is the Resting Membrane Potential maintained? A: It is maintained by three factors: the sodium-potassium pump (3 Na⁺ out, 2 K⁺ in), the presence of large negative organic ions inside the cell, and the outward leakage of K⁺ ions through leak channels.

Q: What causes depolarization during an action potential? A: The rapid opening of voltage-gated sodium (Na⁺) channels and the subsequent influx of Na⁺ ions into the neuron.

Q: What is saltatory conduction? A: It is the rapid transmission of a nerve impulse along a myelinated axon, where the action potential "jumps" from one Node of Ranvier to the next, which is much faster than continuous conduction in unmyelinated axons.