5.5.4 Velocities of Nerve Impulse

The velocity of a nerve impulse varies depending on the type of neuron fibre. Myelinated fibres conduct impulses much faster than non-myelinated fibres. The velocity also depends on the diameter of the neuron fibre. The short journey across the synapse takes about a millisecond, which is longer than a nerve impulse takes to travel the same distance.

Definition

Velocity of Nerve Impulse: The velocity of a nerve impulse is the speed at which the impulse travels along a neuron fibre. It varies depending on the type of fibre (myelinated or non-myelinated) and the diameter of the fibre.

Synaptic Delay: The short journey across the synapse takes about a millisecond, longer than a nerve impulse takes to travel the same distance. This time is therefore called synaptic delay.

Main Idea

• Velocities of nerve impulse in the axon membrane and in the synaptic cleft are variable.
• In human non-myelinated fibres, nerve impulses travel at 1 to 3 metres per second.
• Myelinated fibres conduct at speeds of up to 120 meters per second.
• The velocity of nerve impulse is faster in myelinated neuron fibre due to saltatory conduction.
• Saltatory conduction is the rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another.
• It is up to 50 times faster than conduction through the fastest unmyelinated axons.
• Another reason myelinated fibres conduct faster is that the myelin sheath acts as an insulating sheath and prevents loss of energy.
• Velocity also depends upon the diameter of neuron fibres.
• Thick neuron fibres conduct faster impulse than thin fibres.
• Resistance to electrical current flow is inversely proportional to the cross-sectional area of the conductor.
• With the increase in thickness of neuron fibres, there is a decrease in resistance of fibre to nerve impulse.
• The short journey across the synapse takes about a millisecond, longer than a nerve impulse takes to travel the same distance. This time is called synaptic delay.

Core Concept

• Velocity of nerve impulse = speed of impulse
• **Non-myelinated fibres** = 1 to 3 m/s
• **Myelinated fibres** = up to 120 m/s
• Saltatory conduction = jumping from node to node
• 50 times faster = myelinated vs unmyelinated
• Myelin sheath = insulating sheath; prevents loss of energy
• Diameter = thicker fibres conduct faster
• Resistance = inversely proportional to cross-sectional area
• Synaptic delay = about 1 millisecond

Detailed Explanation

Velocities of Nerve Impulse

Velocities of nerve impulse in the axon membrane and in the synaptic cleft are variable.

Speed in Non-Myelinated Fibres

- In human non-myelinated fibres, nerve impulses travel at 1 to 3 metres per second.

Speed in Myelinated Fibres

• Myelinated fibres conduct at speeds of up to 120 meters per second.
• The velocity of nerve impulse is faster in myelinated neuron fibre due to saltatory conduction.
• Saltatory conduction is the rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelinated regions of membrane.
• It is up to 50 times faster than conduction through the fastest unmyelinated axons because they don't have to travel throughout every single space before moving to the next.
• Another reason that myelinated fibres conduct faster impulse is that myelin sheath acts as an insulating sheath and prevents loss of energy, so myelinated neuron fibres require less energy.

Effect of Diameter

• Velocity of nerve impulse also depends upon diameter of neuron fibres.
• Thick neuron fibres conduct faster impulse than thin fibres.
• Resistance to electrical current flow is inversely proportional to the cross-sectional area of the conductor.
• So with the increase in thickness of neuron fibres, there is a decrease in resistance of fibre to nerve impulse.

Synaptic Delay

• The short journey across the synapse takes about a millisecond, longer than a nerve impulse takes to travel the same distance.
• This time is therefore called synaptic delay.

Summary Table of Velocities

Factors Affecting Velocity

Intermediate Level

Velocities of Nerve Impulse

Factors Affecting Velocity

Saltatory Conduction

Synaptic Delay

Comparison of Myelinated and Non-Myelinated Fibres

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Velocities of nerve impulse are variable.
• 2. Non-myelinated fibres: 1 to 3 m/s.
• 3. Myelinated fibres: up to 120 m/s.
• 4. Saltatory conduction increases velocity.
• 5. Saltatory conduction: impulse jumps from node to node.
• 6. Up to 50 times faster than unmyelinated.
• 7. Myelin sheath acts as an insulating sheath.
• 8. Myelin prevents loss of energy.
• 9. Velocity depends on diameter of neuron fibres.
• 10. Thick fibres conduct faster.
• 11. Resistance is inversely proportional to cross-sectional area.
• 12. Increase in thickness decreases resistance.
• 13. Synaptic delay is about 1 millisecond.
• 14. Synaptic delay is longer than nerve impulse travel.
• 15. Saltatory conduction is faster than continuous conduction.
• 16. Myelinated fibres require less energy.
• 17. Non-myelinated fibres require more energy.
• 18. Velocity is important for quick responses.
• 19. Myelination is essential for rapid communication.
• 20. Demyelination slows impulse transmission.

Remember

• - Velocity of nerve impulse = speed of impulse
• - **Non-myelinated fibres** = 1 to 3 m/s
• - **Myelinated fibres** = up to 120 m/s
• - Saltatory conduction = jumping from node to node
• - 50 times faster = myelinated vs unmyelinated
• - Myelin sheath = insulating sheath; prevents loss of energy
• - Diameter = thicker fibres conduct faster
• - Resistance = inversely proportional to cross-sectional area
• - Synaptic delay = about 1 millisecond

Common Mistakes

• 1. Thinking all nerve fibres conduct at the same speed — Velocity varies.
• 2. Confusing myelinated and non-myelinated speeds — Myelinated = fast; non-myelinated = slow.
• 3. Forgetting the role of saltatory conduction — It increases velocity.
• 4. Mixing up the diameter effect — Thicker fibres = faster.
• 5. Forgetting the role of myelin — Myelin insulates and prevents energy loss.
• 6. Confusing synaptic delay with nerve impulse velocity — Synaptic delay is the time across the synapse.
• 7. Thinking resistance increases with thickness — Resistance decreases with thickness.
• 8. Believing all fibres are myelinated — Some fibres are non-myelinated.
• 9. Mixing up the speeds — Non-myelinated: 1–3 m/s; myelinated: up to 120 m/s.
• 10. Forgetting the 50 times faster fact — Saltatory conduction is up to 50 times faster.

Quick Revision

Velocities of nerve impulse in the axon membrane and in the synaptic cleft are variable. In human non-myelinated fibres, nerve impulses travel at 1 to 3 metres per second. Myelinated fibres conduct at speeds of up to 120 meters per second. The velocity of nerve impulse is faster in myelinated neuron fibre due to saltatory conduction. Saltatory conduction is the rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelinated regions of membrane. It is up to 50 times faster than conduction through the fastest unmyelinated axons because they don't have to travel throughout every single space before moving to the next. Another reason that myelinated fibres conduct faster impulse is that myelin sheath acts as an insulating sheath and prevents loss of energy, so myelinated neuron fibres require less energy. Velocity of nerve impulse also depends upon diameter of neuron fibres. Thick neuron fibres conduct faster impulse than thin fibres because resistance to electrical current flow is inversely proportional to the cross sectional area of the conductor, so with the increase in thickness of neuron fibres there is decrease in resistance of fibre to nerve impulse. The short journey across the synapse takes about a millisecond, longer than a nerve impulse takes to travel the same distance. This time is therefore called synaptic delay.

60-Second Revision

• Velocity of nerve impulse = speed of impulse
• Non-myelinated fibres = 1 to 3 m/s
• Myelinated fibres = up to 120 m/s
• Saltatory conduction = jumping from node to node
• 50 times faster = myelinated vs unmyelinated
• Myelin sheath = insulating sheath; prevents loss of energy
• Diameter = thicker fibres conduct faster
• Resistance = inversely proportional to cross-sectional area
• Synaptic delay = about 1 millisecond
• Warning: Myelinated fibres conduct faster than non-myelinated fibres

NEXT TOPIC

• Chapter:
• 5. Nervous System of Man

• Topic:
• 5.5.5 ROLE OF LOCAL CIRCUITS IN SALTATORY CONDUCTION

Source: