OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What function does the fatty sheath surrounding an axon serve?

1. It provides nutrients to the neuron

2. It increases the speed of nerve impulses

3. It connects the neuron to the brain

4. It produces neurotransmitters

The correct answer is: It increases the speed of nerve impulses

The fatty sheath surrounding an axon, known as the myelin sheath, plays a crucial role in increasing the speed of nerve impulses. This sheath is made up of glial cells, which wrap around the axon in segments. The presence of myelin creates insulating layers that allow electrical signals, or action potentials, to propagate more rapidly along the axon. This occurs through a process called saltatory conduction, where the nerve impulse jumps from one node of Ranvier (the spaces between the myelin segments) to another, significantly speeding up transmission compared to non-myelinated axons. In contrast, the other options do not accurately describe the primary function of the myelin sheath. While one might think of nutrients in relation to neuron health, the myelin sheath does not provide nutrients directly; instead, these are supplied by other types of supporting cells. Connecting neurons to the brain is a function related more to the overall structure and organization of the nervous system rather than the myelin sheath itself. Lastly, neurotransmitter production occurs in the neuron’s cell body and axon terminals, not in the myelin sheath, which focuses solely on insulating the axon to enhance impulse transmission.