The structure of a neurone and the transmission of a nerve impulse, the events at a synapse, the action of hormones, and the differences between nervous and hormonal coordination.

What this dot point is asking

Edexcel wants you to describe the structure of a neurone, explain how a nerve impulse is transmitted, describe the events at a synapse, explain how hormones act, and contrast nervous and hormonal coordination. The ionic basis of the action potential and the synapse sequence are heavily examined, often with a voltage-time graph to interpret.

The neurone and the action potential

When a neurone is stimulated past a threshold, voltage-gated sodium ion channels open and sodium ions rush in, making the inside positive (about $+40 \text{ mV}$): this is depolarisation, producing an action potential. The sodium channels then close and potassium channels open, so potassium ions leave to restore the resting potential (repolarisation), often with a brief overshoot (hyperpolarisation) before the resting potential is re-established. The action potential is all-or-nothing: a stronger stimulus does not make a bigger action potential, it makes them more frequent.

During the refractory period the sodium channels are inactivated, so that region cannot fire again immediately. This ensures the action potential travels in one direction only and sets a limit on impulse frequency.

The synapse

At a synapse, the action potential reaching the synaptic knob opens voltage-gated calcium channels, so calcium ions enter. This causes vesicles to fuse with the presynaptic membrane and release a neurotransmitter (such as acetylcholine) by exocytosis. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane, opening sodium channels so the postsynaptic neurone depolarises and may reach threshold for a new action potential. The neurotransmitter is then broken down (acetylcholine by acetylcholinesterase) or reabsorbed, so the response is brief and the synapse is ready for the next impulse. Synapses also ensure one-way transmission (only the presynaptic side has vesicles) and allow summation, where several weak inputs add up to trigger a response.

Hormonal coordination

Hormones are chemical messengers made by endocrine glands and carried in the blood to target cells that have the correct receptor. For example, adrenaline prepares the body for action, and insulin lowers blood glucose.

• Nervous coordination: fast, very short-lived, precise (specific target cells), uses electrical impulses and neurotransmitters.
• Hormonal coordination: slower to act, longer-lasting, more widespread, uses chemicals carried in the blood to any cell with the right receptor.

Examples in context

Example 1. Nerve agents and acetylcholinesterase. Some pesticides and nerve agents inhibit acetylcholinesterase, the enzyme that breaks down acetylcholine at synapses. Acetylcholine then stays bound to receptors, so the postsynaptic neurone (or muscle) is continuously stimulated, causing uncontrolled muscle contraction and paralysis. This shows why removing the neurotransmitter is essential for normal, brief signalling.

Example 2. Adrenaline and the fight or flight response. A frightening stimulus triggers nervous signals to the adrenal glands, which release adrenaline into the blood. Adrenaline reaches many tissues at once, raising heart rate, dilating airways and releasing glucose from the liver. This is a clear contrast of the two systems working together: a fast nervous trigger producing a slower, widespread, longer-lasting hormonal response.

Try this

Q1. Explain why myelinated neurones transmit impulses faster than unmyelinated ones. [2 marks]

• Cue. The impulse jumps between nodes of Ranvier (saltatory conduction), so it travels faster.

Q2. Describe how a neurotransmitter crosses a synapse. [3 marks]

• Cue. Calcium ions trigger vesicles to release neurotransmitter, which diffuses across the gap and binds to receptors on the next neurone.