AQA A-Level Biology: Structure and Function of Neurons in Nerve Impulse Transmission — mark scheme explained

The question

Explain how a myelinated motor neurone differs from a non-myelinated motor neurone in terms of structure and function. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 1: Identify the key structural differences between myelinated and non-myelinated motor neurones.

• S2

  Myelinated motor neurones have a myelin sheath, which is an insulating layer of fatty tissue wrapped around the axon in segments by Schwann cells. Non-myelinated motor neurones lack this myelin sheath.

• S3

  Step 2: Describe how these structural differences affect function.

• S4

  The myelin sheath in myelinated motor neurones allows for saltatory conduction, where action potentials jump from one node of Ranvier to the next. This significantly increases the speed of nerve impulse transmission. In non-myelinated motor neurones, action potentials propagate continuously along the entire length of the axon, which is slower and requires more energy.

• S5

  Step 3: Summarize the key points.

• S6

  Myelinated motor neurones have a myelin sheath that enables faster conduction through saltatory conduction. Non-myelinated motor neurones lack this myelin sheath and rely on continuous conduction, which is slower.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Identify the key structural differences between myelinated and non-myelinated motor neurones.

2. S2

   Myelinated motor neurones have a myelin sheath, which is an insulating layer of fatty tissue wrapped around the axon in segments by Schwann cells. Non-myelinated motor neurones lack this myelin sheath.

3. S3

   Step 2: Describe how these structural differences affect function.

4. S4

   The myelin sheath in myelinated motor neurones allows for saltatory conduction, where action potentials jump from one node of Ranvier to the next. This significantly increases the speed of nerve impulse transmission. In non-myelinated motor neurones, action potentials propagate continuously along the entire length of the axon, which is slower and requires more energy.

5. S5

   Step 3: Summarize the key points.

6. S6

   Myelinated motor neurones have a myelin sheath that enables faster conduction through saltatory conduction. Non-myelinated motor neurones lack this myelin sheath and rely on continuous conduction, which is slower.

7. Final answer: Myelinated motor neurones have a myelin sheath that allows for faster nerve impulse transmission through saltatory conduction. Non-myelinated motor neurones lack this myelin sheath and rely on continuous conduction, which is slower.

Common mistakes

• Confusing the roles of Na + and K + ions in resting potential and action potentials. — Always remember that the cell membrane is more permeable to K + than Na + , leading to a negative resting potential. During depolarization, Na + channels open, and during repolarization, K + channels open.
• Forgetting the all-or-nothing principle in action potentials. — Emphasize that the all-or-nothing principle means that if the threshold potential is reached, the action potential will occur with a consistent amplitude and duration, regardless of the strength of the initial stimulus.
• Misunderstanding the role of myelination in nerve impulse speed. — Practice explaining that myelin sheaths allow action potentials to jump from one node of Ranvier to the next, significantly increasing the speed of nerve impulse transmission compared to continuous conduction in non-myelinated axons.
• Confusing absolute and relative refractory periods. — Remember that the absolute refractory period is when Na + channels are inactivated, preventing any new action potentials. The relative refractory period is when the membrane is less excitable and requires a stronger stimulus to reach the threshold potential.
• Failing to explain how axon diameter affects conduction speed. — Practice explaining that larger axons have less resistance to ion flow, leading to faster conduction speeds. Smaller axons have more resistance, making conduction slower.
• Not considering the impact of temperature on nerve impulse transmission. — Remember that higher temperatures generally increase the rate of molecular movement, leading to faster nerve impulse transmission. Lower temperatures have the opposite effect.

Where the marks go

• Full worked solution (all marking points)4 marks

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