Nerve Impulse Transmission

• Continuous Conduction: In neurones lacking a myelin sheath, the impulse must travel through every single point along the axon membrane. This process is known as continuous conduction and is relatively slow because every section must undergo the full cycle of depolarisation and repolarisation.

• Sequential Activation: The influx of $Na^+$ at one point depolarises the adjacent membrane area to its threshold potential (roughly $-55\text{ mV}$). This causes voltage-gated sodium channels in that new section to open, generating a new action potential.

• Unidirectional Flow: Although sodium ions diffuse in both directions internally, the impulse only moves forward. This is because the membrane section immediately behind the impulse is in a refractory period, meaning its sodium channels are inactivated and cannot respond to the local current.

• Myelin Insulation: The myelin sheath, formed by Schwann cells, acts as an electrical insulator that prevents the movement of ions across the axon membrane. This insulation is interrupted at regular intervals by gaps called Nodes of Ranvier.

• Saltatory Mechanism: Because ion exchange can only occur at the nodes, the action potential 'jumps' from one node to the next. This process, called saltatory conduction, is significantly faster than continuous conduction because only a small fraction of the membrane needs to be depolarised.

• Energy Efficiency: Saltatory conduction is more energy-efficient for the neurone. Since depolarisation only occurs at the nodes, fewer sodium and potassium ions need to be pumped back across the membrane by the $Na^+/K^+$ pump to restore the resting potential.

• Myelination: The presence of a myelin sheath is the most significant factor, increasing speed by allowing saltatory conduction. In humans, myelinated fibres can conduct at speeds up to $120\text{ m/s}$, while non-myelinated fibres may only reach $0.5\text{ m/s}$.

• Axon Diameter: Axons with a larger diameter conduct impulses faster. This is because a wider axon has a larger volume of cytoplasm, which reduces the electrical resistance to the flow of ions (local currents).

• Temperature: Higher temperatures increase the kinetic energy of ions, leading to faster diffusion across the membrane and along the axon. Additionally, enzymes involved in ATP production for active transport work more efficiently at higher temperatures, though extreme heat can denature these proteins.

Comparison of Conduction Types

• Speed vs. Frequency: It is vital to distinguish between the speed of a single impulse (how fast it travels) and the frequency of impulses (how many occur per second). Speed is determined by physical axon properties, while frequency is limited by the refractory period.

• Calculation Precision: When calculating maximum impulse frequency, always use the formula $f = \frac{1}{\text{refractory period}}$. Ensure the refractory period is converted from milliseconds (ms) to seconds (s) before dividing.

• Terminology Accuracy: Use the term 'local currents' when explaining how one action potential triggers the next. Avoid saying the impulse 'is' electricity; describe it as a 'wave of depolarisation'.

• Link Structure to Function: If asked why a specific neurone is fast, mention all three factors: myelination (saltatory conduction), large diameter (low resistance), and optimal temperature (kinetic energy).

• The Refractory Rule: Always remember that the refractory period determines the maximum frequency of impulses. If the refractory period is $2\text{ ms}$, the neurone can never fire more than $500$ times per second.