Neurones, action potentials and reflex pathways

What you’ll learn

• Relate neurone structure to signal transmission.
• Explain depolarisation, repolarisation and saltatory conduction.
• Trace a spinal reflex through the central and peripheral nervous systems.

Understand the idea

The central nervous system is the brain and spinal cord; the peripheral nervous system connects it to receptors and effectors. Sensory neurones carry information towards the CNS, relay neurones connect cells within it, and motor neurones carry signals to muscles or glands. In a spinal reflex, sensory input enters the cord and communicates through the appropriate circuit with a motor neurone. Grey matter contains many cell bodies and synapses; white matter is rich in myelinated axons.

At rest, selective membrane permeability and ion gradients maintain a negative potential inside an axon. The sodium–potassium pump maintains the gradients over time. Reaching threshold opens voltage-gated sodium channels: sodium entry depolarises the membrane. Sodium channels then inactivate and potassium channels open, allowing potassium exit and repolarisation, sometimes followed by a brief hyperpolarisation. The refractory period helps limit firing frequency and supports one-way propagation along an activated axon.

Local currents bring the next region to threshold, regenerating the action potential without a progressive decrease in its amplitude. In the peripheral nervous system, Schwann cells form myelin around many axons. Myelin reduces current leakage; action potentials are regenerated mainly at nodes of Ranvier, producing faster saltatory conduction. Stronger stimuli are generally encoded by higher impulse frequency and recruitment of more neurones, rather than larger action potentials.

Key formulae

$$\text{conduction speed}=\frac{\text{distance travelled}}{\text{time taken}}$$

Worked example

Explain how myelination allows a withdrawal signal to travel quickly without changing the size of each action potential.

1. The myelin sheath electrically insulates internodal membrane and reduces current leakage.
2. Local current reaches the next node, where voltage-gated channels regenerate an action potential.
3. Regeneration at separated nodes reduces the membrane area that must undergo the full channel cycle, while each action potential remains all-or-nothing.

Answer: Saltatory conduction increases transmission speed; stimulus information remains encoded mainly in impulse frequency rather than impulse amplitude.