Pearson Edexcel International A Level Biology: Neurones, Reflexes and Synapses (YBI11)

This guide follows the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology (YBI11) specification, Issue 2, and covers statements 8.1 to 8.7 of Topic 8 (Coordination, Response and Gene Technology). Topics 1 to 4 are International AS (IAS) content and Topics 5 to 8 are International A2 (IA2) content, so this material belongs to the IA2 stage. There are no tiers. Statement 8.6 has two parts, (i) and (ii), and both are covered. Test yourself with the practice questions, keep the revision notes for the final weeks, and use the course hub, the checklist and the free diagnostics to plan. For muscle as an effector, see skeletal muscle, cardiac control and adrenaline.

What the statements ask for

Statement	You must be able to
8.1	Describe the structure and function of sensory, relay and motor neurones, including Schwann cells and myelination
8.2	Explain how the nervous system makes effectors respond to a stimulus
8.3	Describe a spinal reflex arc, including grey matter and white matter of the spinal cord
8.4	Explain conduction of an action potential along an axon, with changes in membrane permeability to sodium and potassium ions
8.5	Explain the role of myelination in saltatory conduction
8.6 (i)	Describe synapses, including neurotransmitters and acetylcholine
8.6 (ii)	Explain how the pupil dilates and contracts
8.7	Explain drug effects on impulse transmission: nicotine, lidocaine, cobra venom alpha toxin, L-DOPA in Parkinson’s disease, and MDMA (ecstasy)

8.1 Neurones

A neurone is a cell specialised to carry electrical impulses. Three types matter.

• Sensory neurone. Carries impulses from a receptor towards the central nervous system (CNS). Its cell body sits off to one side of the long fibre, in a swelling just outside the spinal cord. A long fibre runs from the receptor to the cell body and on into the spinal cord.
• Relay neurone. Lies wholly inside the CNS and connects sensory neurones to motor neurones.
• Motor neurone. Carries impulses from the CNS to an effector. Its cell body lies inside the CNS and has many short projections (dendrites). One long axon runs out to the effector and ends in branched terminals.

Schwann cells wrap around axons outside the CNS. Each Schwann cell winds its surface membrane many times round the axon. This lipid-rich layer is the myelin sheath. It is an electrical insulator, because ions cannot cross it. Between neighbouring Schwann cells are short gaps, the nodes of Ranvier, where the axon membrane is exposed to the fluid around it.

Feature	Sensory	Relay	Motor
Direction	Receptor to CNS	Within the CNS	CNS to effector
Cell body	Off to the side of the fibre	In the CNS	At the CNS end, with dendrites
Length of fibre	Long	Short	Long

8.2 How the nervous system produces a response

Every response follows one chain: stimulus, receptor, sensory neurone, coordinator (CNS), motor neurone, effector, response. A receptor converts the energy of the stimulus into an electrical change in its membrane. If this change is large enough it triggers action potentials in the sensory neurone. The CNS processes the information. Motor neurones then carry impulses to an effector. Muscles contract and glands secrete. At each junction between neurones, or between a neurone and an effector, the impulse crosses a synapse.

8.3 The spinal reflex arc

A reflex is a rapid, automatic response that does not need conscious thought. It protects the body, for example by pulling a hand from a hot object.

The spinal cord has grey matter in the centre, roughly H-shaped, and white matter around it. Grey matter contains the cell bodies of relay and motor neurones, and the synapses between neurones. White matter contains axons running up and down the cord to and from the brain, and these axons are myelinated, which gives the pale colour. Sensory neurones enter the cord by the dorsal root, and the swelling on that root holds their cell bodies. Motor neurones leave by the ventral root.

Pathway for a withdrawal reflex:

1. A pain receptor in the skin is stimulated and action potentials pass along the sensory neurone.
2. The sensory neurone enters the cord by the dorsal root and synapses with a relay neurone in the grey matter.
3. The relay neurone synapses with a motor neurone.
4. The motor neurone leaves by the ventral root and carries impulses to a muscle (the effector).
5. The muscle contracts and the limb is withdrawn.

Information also passes up the white matter to the brain, so you become aware of the pain after the reflex has started. The reflex is fast because the pathway is short and has few synapses.

Worked example: timing a reflex

A reflex pathway has a sensory neurone 0.50 m long and a motor neurone 0.45 m long. Both conduct at 50 m/s. The total reflex time (stimulus to start of muscle response) is 21 ms. Ignore conduction in the relay neurone and assume two synapses. Find the delay at each synapse.

• Sensory time = distance / speed = 0.50 / 50 = 0.010 s = 10 ms.
• Motor time = 0.45 / 50 = 0.009 s = 9 ms.
• Time in axons = 19 ms. Time at synapses = 21 − 19 = 2 ms.
• Delay at each synapse = 2 / 2 = 1.0 ms.

8.4 The action potential

An axon membrane has a resting potential of about −70 mV (inside negative). It exists because:

• The sodium-potassium pump uses ATP to move 3 sodium ions (Na⁺) out for every 2 potassium ions (K⁺) in.
• At rest the membrane is far more permeable to K⁺ than to Na⁺, because K⁺ leaves through open channels and few Na⁺ channels are open. K⁺ diffuses out down its concentration gradient and leaves the inside negative.

An action potential is a brief reversal of this potential. The stages:

1. Stimulus. Some voltage-gated Na⁺ channels open and Na⁺ diffuses in, making the inside less negative.
2. Threshold. If the potential reaches about −55 mV, many more voltage-gated Na⁺ channels open. This positive feedback causes rapid depolarisation.
3. Peak. The inside reaches about +40 mV. The Na⁺ channels close and cannot reopen straight away.
4. Repolarisation. Voltage-gated K⁺ channels open, K⁺ diffuses out and the inside becomes negative again.
5. Hyperpolarisation. The K⁺ channels close slowly, so the potential briefly falls below −70 mV. The pump and K⁺ channels then restore the resting potential.

The response is all-or-nothing: a stimulus below threshold gives no action potential, and larger stimuli do not give larger ones. Stimulus strength is coded by the frequency of action potentials and by how many neurones fire.

The refractory period follows each action potential. The Na⁺ channels are inactive, so another action potential cannot be started during this time. This ensures impulses travel one way only, keeps them separate, and limits their maximum frequency.

Conduction along the axon

An action potential at one point lets Na⁺ flow into the axon. These ions spread sideways inside the axon and depolarise the next region of membrane to threshold. That region fires, and the process repeats along the axon. The region just behind is refractory, so the impulse moves forwards.

Worked example: reading a trace

A recording shows a resting potential of −70 mV, a peak of +40 mV, and a recovery to resting after 4.0 ms. Assume the axon cannot fire again for 4.0 ms after an action potential begins.

• Change from rest to peak = 40 − (−70) = 110 mV.
• Maximum frequency = 1 / 0.0040 s = 250 per second.
• A neurone recorded firing 12 times in 0.40 s has a frequency of 12 / 0.40 = 30 per second, well below the maximum.

8.5 Myelination and saltatory conduction

Myelin stops ions crossing the membrane, so action potentials can occur only at the nodes of Ranvier. Local currents carry the depolarisation inside the axon from one node to the next, and the impulse appears to jump. This is saltatory conduction. It is faster because:

• The action potential does not have to be regenerated along the whole length, only at nodes.
• Fewer ions cross the membrane, so less ATP is needed for the pump to restore the resting potential.

Worked example: comparing speeds

Electrodes 45 mm apart record an impulse on two axons. On an unmyelinated axon the delay between electrodes is 30 ms. On a myelinated axon it is 0.60 ms.

• Unmyelinated: 0.045 / 0.030 = 1.5 m/s.
• Myelinated: 0.045 / 0.00060 = 75 m/s.
• Ratio = 75 / 1.5 = 50 times faster.

8.6 (i) Synapses and acetylcholine

A synapse is the junction between two neurones. The two cells are separated by a narrow synaptic cleft. The presynaptic knob contains many mitochondria and vesicles of neurotransmitter. The postsynaptic membrane has receptors for the transmitter. In a cholinergic synapse the transmitter is acetylcholine (ACh).

1. An impulse reaches the presynaptic knob.
2. This opens voltage-gated calcium channels, and Ca²⁺ diffuses in.
3. Ca²⁺ makes vesicles move to the presynaptic membrane and fuse with it. ACh is released by exocytosis.
4. The ACh spreads across the cleft by diffusion and attaches to receptors on sodium channels in the postsynaptic membrane.
5. The channels open, Na⁺ enters and the postsynaptic membrane depolarises. If it reaches threshold, an action potential starts.
6. The enzyme acetylcholinesterase breaks ACh into choline and ethanoate. The products are taken back into the presynaptic knob and ACh is made again using ATP from the mitochondria.

Synapses make transmission unidirectional, because only the presynaptic side releases transmitter. Transmission across the cleft causes a short synaptic delay. Several presynaptic neurones can release transmitter onto one postsynaptic neurone, so their effects add together: summation. Some neurotransmitters inhibit instead, by making the postsynaptic membrane hyperpolarised, so that an action potential is less likely.

8.6 (ii) The pupil

The iris contains two sets of muscle. Circular muscle forms rings around the pupil. Radial muscle runs outwards like the spokes of a wheel. They are antagonistic.

• Bright light. Receptors in the retina send impulses to the brain. Motor neurones release acetylcholine onto the circular muscle, which contracts. The radial muscle relaxes. The pupil constricts, reducing the light that reaches the retina and protecting it.
• Dim light. The radial muscle contracts and the circular muscle relaxes. The pupil dilates and more light enters.

Worked example: pupil area

In dim light the pupil diameter is 7.0 mm. In bright light it is 2.5 mm.

• Area = π r². Dilated: π × 3.5² = 38.5 mm². Constricted: π × 1.25² = 4.91 mm².
• Percentage decrease in diameter = (7.0 − 2.5) / 7.0 × 100 = 64.3%.
• Percentage decrease in area = (38.48 − 4.91) / 38.48 × 100 = 87.2%, a much bigger change, because area depends on the square of the diameter.

8.7 Drugs and impulse transmission

Drug	Where it acts	Effect
Nicotine	Postsynaptic ACh receptors	Mimics ACh and opens the channels. It is not broken down by acetylcholinesterase, so stimulation continues
Lidocaine	Voltage-gated Na⁺ channels in axons	Blocks the channels, so action potentials are not generated or conducted and sensation is lost (local anaesthetic)
Cobra venom alpha toxin	Postsynaptic ACh receptors	Binds and blocks them, so ACh cannot depolarise the membrane. Muscles are paralysed
L-DOPA	Brain	Converted to dopamine, replacing the dopamine lost in Parkinson’s disease
MDMA (ecstasy)	Serotonin synapses	Causes release of serotonin and blocks its reuptake, so serotonin stays active in the cleft

Parkinson’s disease involves loss of dopamine-releasing neurones in the brain. Dopamine itself does not cross from blood into the brain well, but L-DOPA does. Once in the brain it is converted to dopamine, which relieves symptoms. It does not cure the disease.

Common errors

• Saying Na⁺ “pumps” in during depolarisation. It diffuses through channels. The pump is separate.
• Writing that the action potential “jumps”. The depolarisation spreads by local currents and the action potential is regenerated at nodes.
• Forgetting calcium ions in synaptic transmission.
• Confusing grey matter (cell bodies, synapses) with white matter (myelinated axons).
• Saying a bigger stimulus gives a bigger action potential.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification, Issue 2, Pearson Education Limited. Statements 8.1 to 8.7 of Topic 8.