The structure and function of sensory, relay and motor neurons. The…

The idea

A neuron carries information electrically along its own length and chemically to the next cell. Sensory neurons run from receptors to the CNS and have long dendrites and short axons; relay neurons lie within the CNS connecting the other two and have short dendrites and short axons; motor neurons run from the CNS to muscles and glands, with short dendrites and one long axon. At rest the inside of the cell is negatively charged, roughly -70 millivolts; a strong enough stimulus depolarises it and an action potential travels down the axon. At the terminal, vesicles fuse and release neurotransmitter into the synaptic cleft, where it diffuses across and binds to receptors on the postsynaptic dendrite. Excitatory transmitters such as glutamate make firing more likely; inhibitory ones such as GABA and serotonin make it less likely, and summation decides whether the threshold is crossed.

What earns the marks

• Sensory, relay and motor neurons each given their location, direction of travel and dendrite/axon length.
• Action potential described: resting negative charge, depolarisation by a stimulus, impulse along the axon to the terminal.
• Vesicles release neurotransmitter into the synaptic cleft and it diffuses across.
• Neurotransmitter binds to specific postsynaptic receptor sites and the chemical message becomes an electrical one again.
• Excitation defined as making the postsynaptic neuron more likely to fire, inhibition as making it less likely.
• Summation: the net effect of excitatory and inhibitory input determines whether the neuron fires.
• Reuptake or enzyme breakdown clears the cleft, and transmission is one-way because only the presynaptic terminal holds vesicles.

Common mistakes and how to put them right

• Writing that the impulse 'jumps' or travels electrically across the synapse.

  Put it right: Across the cleft the signal is chemical and moves by diffusion.

• Describing inhibition as 'stopping the message'.

  Put it right: An inhibitory transmitter makes the postsynaptic membrane more negative, so firing is less likely, not impossible.

• Giving relay neurons long dendrites, which belongs to the sensory neuron.

• Naming transmitters without saying what they do.

  Put it right: GABA earns nothing unless it is labelled inhibitory and linked to reduced firing.

Exam tips

• Write synaptic transmission as an ordered sequence and keep the order: action potential, vesicles, cleft, receptors, new potential, reuptake. Examiners credit the chain, not a bag of terms.
• An 'explain' question on excitation and inhibition wants the consequence for firing, so finish every sentence with 'more likely to fire' or 'less likely to fire'.
• If the stem involves a drug, state which stage it acts on (release, binding or reuptake) and whether firing rises or falls as a result.