3.6.2 Nervous coordination for A-Level Biology (AQA)

In a motor neurone, what is the long extension that carries electrical impulses away from the cell body towards an effector?

Axon

Motor neurones transmit impulses from the CNS to muscles or glands. The axon conducts the action potential along the neurone.

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What insulating structure surrounds many motor neurone axons and increases the speed of impulse transmission?

Myelin sheath

The myelin sheath is formed by Schwann cells and allows impulses to travel faster via saltatory conduction.

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Fill in the blanks:

The electrical charge difference across the membrane of an unstimulated neurone is known as the ______ ______.

resting potential

Typically about −70 mV. It exists because of ion distribution and membrane permeability differences.

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During the resting state, which ion has a higher concentration outside the neurone than inside?

Sodium ions

Na⁺ concentration is higher outside the axon, while K⁺ concentration is higher inside.

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Which ion is at a higher concentration inside the neurone compared with outside during the resting state?

Potassium ions

Maintained by selective permeability and the sodium–potassium pump.

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True or False:

At resting potential, the neurone membrane is more permeable to potassium ions than to sodium ions.

True

K⁺ leak channels allow potassium ions to diffuse out more readily than Na⁺ diffuses in.

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What is the rapid reversal of membrane potential caused by increased sodium ion permeability called?

Depolarisation

Voltage-gated Na⁺ channels open, allowing Na⁺ to diffuse into the neurone.

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What name is given to the electrical change that travels along the axon of a neurone?

Action potential

The action potential propagates along the axon and forms the basis of nerve impulses.

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True or False:

The strength of an action potential varies depending on stimulus intensity.

False

Action potentials follow the all-or-nothing principle; they occur fully or not at all.

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What principle states that once threshold is reached, an action potential occurs at full size?

All-or-nothing principle

A stimulus below threshold produces no action potential; above threshold produces a full response.

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Fill in the blank:

The jumping movement of impulses between nodes of Ranvier in myelinated neurones is called ______ conduction.

saltatory

Saltatory conduction greatly increases conduction speed because depolarisation only occurs at nodes.

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What term describes the short period after an action potential when another cannot be generated?

Refractory period

Ensures impulses are discrete and limits the maximum frequency of nerve impulse transmission.

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True or False:

Increasing axon diameter generally increases the speed of nerve impulse conduction.

True

Larger diameters reduce internal resistance to ion flow.

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What physical factor, related to kinetic energy of ions, can increase the speed of nerve impulse transmission when it rises?

Temperature

Higher temperatures increase ion movement, speeding up depolarisation and conduction.

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What narrow gap separates the presynaptic neurone from the postsynaptic cell at a synapse?

Synaptic cleft

The synaptic cleft is the small space across which neurotransmitter diffuses.

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Which part of the presynaptic neurone contains synaptic vesicles filled with neurotransmitter?

Synaptic knob

The synaptic knob is the swollen ending of the presynaptic neurone.

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What structure in the postsynaptic membrane contains receptors that bind acetylcholine?

Ligand-gated sodium ion channels

Acetylcholine binds to receptors associated with sodium ion channels in a cholinergic synapse.

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Which ion entering the presynaptic knob triggers synaptic vesicles to move to and fuse with the presynaptic membrane?

Calcium ions

Arrival of an action potential opens voltage-gated calcium ion channels.

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What happens first when an action potential reaches the presynaptic membrane?

Depolarisation of the presynaptic membrane

This depolarisation opens voltage-gated calcium ion channels.

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After calcium ions enter the synaptic knob, what process releases acetylcholine into the synaptic cleft?

Exocytosis

Synaptic vesicles fuse with the presynaptic membrane and release neurotransmitter.

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Once released, how does acetylcholine cross the synaptic cleft?

Diffusion

Acetylcholine moves down its concentration gradient to the postsynaptic membrane.

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What immediate effect does acetylcholine binding have on the postsynaptic membrane?

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Sodium ion channels open

Sodium ions diffuse in, causing depolarisation of the postsynaptic membrane.

What name is given to the depolarisation produced in the postsynaptic membrane when neurotransmitter binds?

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Excitatory postsynaptic potential

An excitatory postsynaptic potential makes the postsynaptic neurone more likely to fire.

What enzyme rapidly breaks down acetylcholine in the synaptic cleft?

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Acetylcholinesterase

This prevents continuous stimulation of the postsynaptic membrane.

Why is transmission across a **cholinergic synapse** **unidirectional**?

Receptors are only on the postsynaptic membrane. ## Footnote Vesicles release acetylcholine from the presynaptic side only, so impulses pass one way.

Why does **breakdown of acetylcholine** matter for **repeated signalling**?

It prevents continuous depolarisation. ## Footnote The membrane can repolarise and be ready for a new signal.

# Fill in the blank: In a **cholinergic synapse**, acetylcholine is hydrolysed into acetate and _\_\_\_\_.

choline ## Footnote Choline is reabsorbed into the presynaptic neurone and reused to make acetylcholine.

What happens to **choline** after acetylcholine is broken down in the **synaptic cleft**?

It is reabsorbed into the presynaptic neurone. ## Footnote It is recycled to synthesise more acetylcholine.

What is **temporal summation** at a synapse?

Addition of successive impulses from one neurone. ## Footnote Rapid, repeated release of neurotransmitter can build depolarisation to threshold.

What is **spatial summation** at a synapse?

Addition of impulses from several neurones at once. ## Footnote Neurotransmitter released from different presynaptic neurones combines at one postsynaptic neurone.

Why might one **excitatory impulse** fail to trigger an **action potential** in the postsynaptic neurone?

Depolarisation does not reach threshold. ## Footnote A single excitatory postsynaptic potential may be too small on its own.

# True or False: **Temporal summation** depends on impulses arriving from different **presynaptic neurones** at the same time.

False ## Footnote That describes spatial summation; temporal summation is repeated impulses from one neurone.

# True or False: **Spatial summation** can allow a postsynaptic neurone to reach **threshold** when several excitatory synapses are active together.

True ## Footnote The combined depolarisations may be enough to trigger an action potential.

How do **inhibitory synapses** reduce the chance of an **action potential** being generated?

They hyperpolarise the postsynaptic membrane. ## Footnote Inhibitory neurotransmitters often cause chloride ions to enter or potassium ions to leave.

What is the effect of **chloride ions** entering the **postsynaptic neurone** at an inhibitory synapse?

hyperpolarisation ## Footnote The membrane potential becomes more negative and further from threshold.

How can an **inhibitory synapse** counteract an **excitatory synapse** on the same postsynaptic neurone?

By cancelling depolarisation ## Footnote The combined effect may prevent threshold from being reached.

# True or False: **Inhibitory synapses** make the inside of the postsynaptic neurone **less negative**.

False ## Footnote They usually make it more negative, moving the membrane potential away from threshold.

What specialised region of a **muscle fibre** acts as the **postsynaptic membrane** at a neuromuscular junction?

Sarcolemma ## Footnote The synapse is between a motor neurone and a muscle fibre rather than another neurone.

What is the usual effect when an **action potential** reaches a **neuromuscular junction**?

Contraction of the muscle fibre ## Footnote Neuromuscular junctions are designed to ensure a reliable response in the muscle.

Give one structural feature that makes a **neuromuscular junction** well adapted for **rapid transmission** to a muscle fibre.

Many folds in the postsynaptic membrane. ## Footnote These folds increase surface area for receptors and help transmission.

Compared with a typical **cholinergic synapse** between neurones, how does the amount of **acetylcholine released** at a neuromuscular junction differ?

More acetylcholine is released. ## Footnote This helps ensure the muscle fibre reaches threshold.

Compared with many neurone-to-neurone synapses, how is the **postsynaptic membrane** at a **neuromuscular junction** adapted?

More receptors ## Footnote The high receptor density increases the chance of depolarising the muscle fibre.

# True or False: Both **cholinergic synapses** and **neuromuscular junctions** use acetylcholine as the neurotransmitter.

True ## Footnote The difference is mainly in the type of postsynaptic cell and reliability of transmission.

# True or False: A **neuromuscular junction** links one neurone directly to another neurone.

False ## Footnote It links a motor neurone to a muscle fibre.

Why is transmission at a **neuromuscular junction** less likely to fail than at a **cholinergic synapse** between neurones?

It produces a larger end-plate potential. ## Footnote More neurotransmitter, more receptors and membrane folds make transmission highly reliable.

# Fill in the blank: The small depolarisation produced in the **muscle fibre membrane** at a neuromuscular junction is called an end-plate _\_\_\_\_.

potential ## Footnote If this reaches threshold, an action potential spreads across the sarcolemma.

What would be the effect of a drug that blocks **voltage-gated calcium ion channels** in the **presynaptic membrane**?

Less acetylcholine released. ## Footnote Without calcium ion influx, vesicles are less likely to fuse with the membrane.

What effect would a drug that **inhibits acetylcholinesterase** have on the **postsynaptic membrane**?

Prolonged depolarisation ## Footnote Acetylcholine would remain in the cleft longer and continue to stimulate receptors.

What would happen if **receptor proteins** on the **postsynaptic membrane** were blocked by a drug?

Sodium ion channels would not open. ## Footnote Acetylcholine could not trigger the usual depolarisation.

Predict the effect of a drug that prevents **synaptic vesicles** fusing with the **presynaptic membrane**.

Neurotransmitter would not be released. ## Footnote No acetylcholine in the cleft means no postsynaptic depolarisation.

What would be the likely effect of a drug that increases **chloride ion movement** into the **postsynaptic neurone**?

Inhibition would increase. ## Footnote Greater hyperpolarisation makes an action potential less likely.

Why can **low-frequency impulses** fail to produce a response, while **high-frequency impulses** can succeed?

Temporal summation ## Footnote Repeated impulses arriving close together build the depolarisation to threshold.

Why can several **weak inputs** arriving together trigger an **action potential** when each alone cannot?

Spatial summation ## Footnote Their combined excitatory effects can bring the postsynaptic membrane to threshold.

3.6.2 Nervous coordination Flashcards

Explain how neurons transmit impulses and how synapses coordinate nervous responses. (53 cards)