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Coordination and Response: Question 6

Syllabus 14.1

Structured Extended 6 marks

A researcher records electrical activity along a sensory neurone as it carries an impulse towards the spinal cord. Just before the impulse reaches a relay neurone, the researcher notes that the sensory neurone does not actually touch the relay neurone, a very narrow gap separates the two cells.

(a) Name this junction between two neurones, and name the narrow gap found within it. [2]

(b) Describe how an impulse arriving at the end of the sensory neurone is passed across this junction to the relay neurone. [3]

(c) Explain why this junction allows impulses to travel across it in one direction only, from the sensory neurone to the relay neurone. [1]

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Worked solution

Part (a): Naming the junction and the gap

Where two neurones meet, but do not physically touch, the junction is called a synapse. The very narrow space between the two neurone membranes at this junction is the synaptic cleft.

Part (b): Crossing the synapse

An electrical impulse cannot jump across the synaptic cleft directly, so the signal must be converted into a chemical form to cross the gap:

  1. The impulse arrives at the end of the sensory neurone, where small sacs called vesicles contain a neurotransmitter chemical.
  2. This causes the vesicles to release the neurotransmitter into the synaptic cleft.
  3. The neurotransmitter diffuses across the narrow gap.
  4. It binds to specific receptor molecules on the membrane of the relay neurone, triggering a new electrical impulse in that neurone.

Part (c): Why transmission is one-way

The neurotransmitter is only ever released from the sensory neurone side of the synapse, and the receptor molecules that respond to it are only found on the membrane of the relay neurone. Because the chemical signal can only be produced on one side and detected on the other, the synapse only allows the impulse to be passed in a single direction, from the sensory neurone to the relay neurone.

Final answers

  • (a) Synapse; the gap is the synaptic cleft.
  • (b) Neurotransmitter is released from vesicles in the sensory neurone, diffuses across the synaptic cleft, and binds to receptors on the relay neurone, triggering a new impulse.
  • (c) Neurotransmitter release and its receptors are only present on one side each, so transmission across the synapse can only happen in one direction.