Speed of the Nerve Impulse (AQA A-Level Biology): Revision Notes
Speed of the Nerve Impulse
The speed at which action potentials travel along neurones varies significantly depending on several key factors. Understanding these factors helps explain why different types of neurones conduct nerve impulses at different rates, from slow pain signals to rapid motor responses.
Three key factors affecting conduction speed
Myelination
Myelination is the most significant factor determining nerve impulse speed. Many neurones possess a myelin sheath, which acts as an electrical insulator surrounding the axon.
The myelin sheath consists of Schwann cells that wrap around the axon in the peripheral nervous system. These cells create a fatty, insulating layer that prevents electrical current from leaking out of the axon. However, small gaps exist between adjacent Schwann cells, called nodes of Ranvier.
The myelin sheath acts like the plastic coating around electrical wires, preventing electrical current from leaking out and ensuring efficient transmission of the nerve impulse.
In myelinated neurones, depolarisation occurs only at the nodes of Ranvier, where sodium ion channels are concentrated. The electrical charge effectively 'jumps' from one node to the next, bypassing the insulated sections. This process is called saltatory conduction (from the Latin 'saltare', meaning to jump), and it dramatically increases transmission speed.
In contrast, non-myelinated neurones must depolarise along their entire length. The nerve impulse travels as a continuous wave, with depolarisation occurring at every point along the axon membrane. This continuous conduction is considerably slower than saltatory conduction, though still relatively fast.
Key Difference: Saltatory conduction allows electrical signals to "jump" between nodes of Ranvier, while continuous conduction requires depolarisation at every point along the axon. This fundamental difference explains why myelinated neurones conduct impulses much faster than non-myelinated ones.
Axon diameter
The diameter of the axon directly influences conduction speed. Larger diameter axons conduct action potentials faster than smaller ones because they offer less resistance to the flow of electrical current.
This relationship exists because electrical current flows through the cytoplasm inside the axon. A wider axon contains more cytoplasm, providing more pathways for ion movement and reducing overall resistance. With reduced resistance, depolarisation spreads more rapidly to adjacent regions of the axon membrane.
Think of this like water flowing through pipes - a wider pipe allows more water to flow with less resistance. Similarly, a wider axon allows electrical current to flow more easily, resulting in faster nerve conduction.
Temperature
Temperature affects nerve impulse speed through its influence on ion movement and protein function. As temperature increases, ions diffuse faster through the cytoplasm and across membranes, speeding up the depolarisation process.
However, this relationship has limits. Nerve impulse speed increases with temperature up to approximately 40°C. Beyond this temperature, the proteins that form ion channels and pumps begin to denature, losing their functional shape. This protein denaturation actually decreases conduction speed and can ultimately prevent nerve function entirely.
Critical Temperature Limit: While higher temperatures generally speed up nerve conduction, temperatures above 40°C cause protein denaturation, which impairs nerve function. This is why high fevers can affect nervous system function.
Comparing conduction types
The difference between myelinated and non-myelinated conduction is striking. Saltatory conduction in myelinated axons can achieve speeds of over 100 metres per second, while continuous conduction in non-myelinated axons typically reaches only 1-2 metres per second.
Speed Comparison Example:
Myelinated neurone (saltatory conduction):
- Speed: >100 m/s
- Mechanism: Jumps between nodes of Ranvier
- Example: Motor neurones controlling rapid movements
Non-myelinated neurone (continuous conduction):
- Speed: 1-2 m/s
- Mechanism: Continuous wave along entire axon
- Example: Some pain sensory neurones
This speed difference explains why motor neurones controlling rapid movements are heavily myelinated, while some sensory neurones carrying less urgent information (such as dull pain signals) may lack myelination.
Clinical significance
Understanding nerve conduction speed has important medical applications. Damage to myelin sheaths, as occurs in multiple sclerosis, dramatically slows nerve conduction and impairs nervous system function. Similarly, measuring nerve conduction velocity helps diagnose various neurological conditions.
Medical Application: Nerve conduction velocity tests are commonly used diagnostic tools. By measuring how fast electrical signals travel along nerves, doctors can identify damage to myelin sheaths, nerve compression, or other neurological problems.
Key Points to Remember:
- Myelination creates saltatory conduction, dramatically increasing impulse speed by allowing electrical signals to 'jump' between nodes of Ranvier
- Larger axon diameters reduce electrical resistance, permitting faster conduction of action potentials
- Higher temperatures speed up ion movement and nerve conduction, but only up to about 40°C before proteins denature
- Saltatory conduction is much faster than continuous conduction because depolarisation only occurs at nodes rather than along the entire axon length
- These factors work together to determine the overall speed of nerve impulse transmission in different types of neurones