How do nerve cells transmit signals and pass them between cells?
The structure of neurones, the resting potential and action potential, the transmission of impulses along axons, and synaptic transmission across a cholinergic synapse.
A CCEA A-Level Biology answer on the structure of neurones, the resting and action potentials, the transmission of impulses along axons, and synaptic transmission across a cholinergic synapse.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
CCEA wants you to describe the structure of neurones, explain the resting potential and the action potential, describe how impulses travel along an axon, and explain transmission across a cholinergic synapse.
Neurones and the resting potential
A myelin sheath made by Schwann cells insulates many axons, so the impulse cannot pass through the membrane there and instead jumps between the gaps (nodes of Ranvier) in saltatory conduction, greatly increasing the speed of transmission. Conduction is also faster in wider axons and at higher temperatures.
The action potential
The action potential is all-or-nothing: a stimulus either reaches the threshold and produces a full action potential of the same size, or it does not fire at all. A stronger stimulus does not make a bigger action potential; instead it increases the frequency of action potentials, which is how the brain detects stimulus intensity.
Synaptic transmission
At a cholinergic synapse, the arriving action potential opens voltage-gated calcium channels in the presynaptic membrane. Calcium entry causes vesicles of acetylcholine to fuse with the membrane and release the neurotransmitter into the synaptic cleft by exocytosis. Acetylcholine diffuses across and binds to receptors on the postsynaptic membrane, opening sodium channels and depolarising it; if the threshold is reached, a new action potential is generated. The enzyme acetylcholinesterase then breaks down the acetylcholine so the synapse can reset, preventing continuous stimulation.
Examples in context
Example 1. How nerve gases and insecticides kill. Organophosphate nerve agents and some insecticides inhibit acetylcholinesterase, so acetylcholine is not broken down. The postsynaptic membrane is stimulated continuously, muscles (including the diaphragm) go into uncontrolled contraction, and the victim cannot breathe. This shows the importance of acetylcholinesterase in resetting the synapse, and is a striking applied example of synapse biology.
Example 2. Multiple sclerosis and lost myelin. In multiple sclerosis the immune system damages the myelin sheath. Without myelin, saltatory conduction is lost and impulses travel slowly or not at all, causing muscle weakness, numbness and coordination problems. This directly demonstrates why the myelin sheath matters for fast conduction, and links the structure of neurones to a real disease.
Try this
Q1. Explain how the resting potential is maintained. [3 marks]
- Cue. The sodium-potassium pump moves 3 sodium out for 2 potassium in, and the membrane is more permeable to potassium, leaving the inside negative.
Q2. Explain why an action potential travels in only one direction. [2 marks]
- Cue. The region just behind is in its refractory period and cannot be re-stimulated immediately, so the impulse can only move forward.
Q3. Explain how a synapse ensures an impulse passes in one direction only. [2 marks]
- Cue. Only the presynaptic neurone releases neurotransmitter and only the postsynaptic membrane has the receptors, so transmission can only occur from pre- to postsynaptic.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20196 marksDescribe the changes in membrane permeability and ion movement that produce an action potential in an axon.Show worked answer →
A 6-mark answer should sequence the resting state, depolarisation, repolarisation and recovery with the ions involved.
Resting potential: the membrane is at about minus 70 millivolts, maintained by the sodium-potassium pump (3 sodium out for 2 potassium in) and the membrane being more permeable to potassium, leaving the inside negative.
Depolarisation: a stimulus opens voltage-gated sodium channels, so sodium ions diffuse in down their gradient, making the inside less negative; if the threshold is reached, more sodium channels open and the potential rises rapidly to about plus 40 millivolts.
Repolarisation: sodium channels close and voltage-gated potassium channels open, so potassium ions diffuse out, restoring the negative inside.
Hyperpolarisation and recovery: a brief overshoot occurs as potassium channels close slowly, then the sodium-potassium pump and ion movements restore the resting potential.
Refractory period: during recovery the membrane cannot fire again, ensuring one-way travel.
Markers reward the resting potential and its maintenance, sodium in for depolarisation, potassium out for repolarisation, and the refractory period.
CCEA 20215 marksExplain how a nerve impulse is transmitted across a cholinergic synapse from one neurone to the next.Show worked answer →
A 5-mark answer needs the ordered steps from arrival of the impulse to resetting.
The action potential arrives at the presynaptic membrane and causes voltage-gated calcium channels to open, so calcium ions diffuse into the presynaptic neurone.
Calcium causes synaptic vesicles to move to and fuse with the presynaptic membrane, releasing the neurotransmitter acetylcholine into the synaptic cleft by exocytosis.
Acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane, opening sodium channels so sodium ions enter and depolarise it; if threshold is reached a new action potential is generated.
The enzyme acetylcholinesterase breaks down the acetylcholine into choline and ethanoic acid, so the postsynaptic neurone is not continuously stimulated, and the products are recycled.
Markers reward calcium entry, vesicle fusion and acetylcholine release, binding and depolarisation of the postsynaptic membrane, and breakdown by acetylcholinesterase.
Related dot points
- The principle of homeostasis and negative feedback, the structure of the kidney and nephron, ultrafiltration and selective reabsorption, and osmoregulation by ADH.
A CCEA A-Level Biology answer on the principle of homeostasis and negative feedback, the structure of the kidney and nephron, ultrafiltration and selective reabsorption, and osmoregulation by ADH.
- The endocrine system and how hormones act, the control of blood glucose by insulin and glucagon, the difference between nervous and hormonal control, and the basis of diabetes.
A CCEA A-Level Biology answer on the endocrine system and how hormones act, the control of blood glucose by insulin and glucagon, the differences between nervous and hormonal control, and the basis of diabetes.
- Plant growth responses (tropisms), the role of auxin (IAA) in phototropism and gravitropism, and the commercial uses of plant growth substances.
A CCEA A-Level Biology answer on plant growth responses (tropisms), the role of auxin in phototropism and gravitropism, and the commercial uses of plant growth substances.
- The light-dependent and light-independent stages of photosynthesis, the stages of aerobic respiration, anaerobic respiration, and the role of ATP and electron carriers.
A CCEA A-Level Biology answer on the light-dependent and light-independent stages of photosynthesis, the stages of aerobic respiration, anaerobic respiration, and the role of ATP and electron carriers.
- Diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis, and the factors that affect the rate of movement across membranes.
A CCEA A-Level Biology answer on diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis, and the factors that affect the rate of movement across cell membranes.
Sources & how we know this
- CCEA GCE Biology specification — CCEA (2016)