Skip to content
Grade 12 · Nervous system · Extra lesson

Neurons, impulses and synapses

Label a neuron, explain how the myelin sheath speeds up impulses, and describe how a synapse passes the message on in one direction only.

Structure of a neuron

Type of neuronFunction
Sensory (afferent)Carries impulses from receptors to the central nervous system
Interneuron (connector)Links sensory and motor neurons inside the central nervous system
Motor (efferent)Carries impulses from the central nervous system to effectors (muscles, glands)

The nerve impulse and myelin

A nerve impulse is an electrochemical message: a change in charge across the membrane that travels along the axon. Myelin is made by Schwann cells in the peripheral nervous system and by oligodendrocytes in the brain and spinal cord.

Myelin insulates the axon, so the impulse cannot pass along the membrane where myelin is present. Instead it jumps from one node of Ranvier to the next — saltatory conduction. This makes impulses in myelinated neurons much faster than in unmyelinated ones of the same diameter.

Multiple sclerosis: the immune system damages myelin in the central nervous system, so impulses slow down or are blocked, causing weakness, numbness and poor coordination.

The synapse

A synapse is the small gap between the axon terminal of one neuron and the next cell. When an impulse arrives:

  1. vesicles in the axon terminal release a neurotransmitter (for example acetylcholine) into the gap;
  2. the neurotransmitter diffuses across the gap and binds to receptors on the next neuron;
  3. this starts a new impulse in the next neuron;
  4. the neurotransmitter is broken down by enzymes, so the signal stops.

Synapses make sure impulses travel in one direction only, because only the axon terminal releases neurotransmitter and only the next cell has receptors. Many drugs act at synapses, by copying, blocking or preventing the breakdown of neurotransmitters.

Worked example: tracing a reflex

✏️ Touching a hot plate
1Receptors in the skin detect heat and start an impulse in a sensory neuron.
2The sensory neuron carries it to the spinal cord, where it crosses a synapse to an interneuron.
3The interneuron passes it across a synapse to a motor neuron.
4The motor neuron carries it to the arm muscle (effector), which contracts and pulls the hand away — before the brain is aware of the pain.
0/8
Review the explanations above.
Answer in your exercise book, then open each memo to mark your work.
Question 1 · (4 marks)
State the function of each part of a neuron: (a) dendrite, (b) axon, (c) myelin sheath, (d) axon terminal.
Show memo
(a) Carries impulses towards the cell body ✓
(b) Carries impulses away from the cell body ✓
(c) Insulates the axon and speeds up impulse transmission ✓
(d) Releases neurotransmitters into the synapse ✓
Question 2 · (3 marks)
Explain why an impulse travels faster along a myelinated axon than along an unmyelinated axon.
Show memo
Myelin insulates the axon so the impulse cannot travel along the covered membrane ✓. The impulse jumps from one node of Ranvier to the next ✓ (saltatory conduction), covering the distance in fewer steps, so it is faster ✓.
Question 3 · (4 marks)
Describe how an impulse is passed from one neuron to the next across a synapse.
Show memo
The impulse reaches the axon terminal and vesicles release a neurotransmitter ✓. It diffuses across the synaptic gap ✓, binds to receptors on the next neuron ✓, and starts an impulse in that neuron ✓.
Question 4 · (3 marks)
Give TWO reasons why synapses are important, and explain how multiple sclerosis affects the nervous system.
Show memo
Synapses ensure impulses travel in one direction ✓ and allow impulses to be filtered, combined or blocked ✓ (accept: allow many neurons to connect).
In MS, myelin is damaged so impulses slow down or are blocked ✓.