Nerve impulses (A-level only) Flashcards – AQA A-Level Biology - (7402)

What is the role of a motor neurone?

To carry nerve impulses from the central nervous system towards an effector.

What structure is contained in a motor neurone’s cell body?

The nucleus.

What is the function of the branched dendrites arising from a motor neurone’s cell body?

They receive signals.

What is the role of a motor neurone’s long axon?

It carries impulses away from the cell body to the axon terminals.

What do a motor neurone’s branched axon terminals connect with?

An effector.

Which cells form the myelin sheath around axons in the peripheral nervous system?

Schwann cells.

What is the function of the myelin sheath around an axon?

It electrically insulates the axon membrane.

What are nodes of Ranvier?

Gaps between successive sections of myelin sheath, where regions of axon membrane are exposed.

What is the resting potential of a neurone?

The potential difference across its membrane when it is not transmitting an action potential; the inside is negative relative to the outside.

How does the sodium–potassium pump move ions across a neurone’s membrane?

It uses ATP to move three sodium ions out for every two potassium ions in.

What sodium and potassium concentration differences does the sodium–potassium pump maintain across a neurone’s membrane?

Sodium concentration is higher outside; potassium concentration is higher inside.

At rest, how does a neurone membrane’s permeability to potassium compare with its permeability to sodium?

It is more permeable to potassium ions than to sodium ions.

Why do potassium ions diffuse out of a resting neurone through leak channels?

They move down their concentration gradient, from the higher potassium concentration inside.

How does a resting neurone’s negative interior affect potassium movement?

It attracts potassium ions back, opposing further net outward movement.

What influences together determine the resting potential?

The balance of electrical and concentration influences, together with the small sodium leak and continued action of the sodium–potassium pump.

What is an action potential?

A rapid, temporary change in membrane potential.

What happens to sodium permeability when depolarisation reaches threshold?

Voltage-gated sodium channels open, increasing sodium permeability.

How does sodium entry produce further depolarisation during an action potential?

Sodium ions enter down their electrochemical gradient, causing further depolarisation and opening more sodium channels.

What happens to the membrane potential at the peak of depolarisation?

The inside becomes positive relative to the outside.

How is the membrane repolarised during an action potential?

Sodium channels inactivate and voltage-gated potassium channels open. Potassium ions leave the neurone, repolarising the membrane.

How does hyperpolarisation occur after an action potential?

Continued potassium movement briefly makes the membrane potential more negative than its resting value, before the resting potential is restored.

What does the all-or-nothing principle say about the threshold for an action potential?

A stimulus that reaches threshold triggers an action potential; one that does not reach threshold does not.

How does a stronger stimulus affect the size of an action potential once triggered?

It does not make the action potential larger.

How does an action potential trigger an action potential in an adjacent region of axon membrane?

Local currents depolarise the adjacent region to threshold, opening voltage-gated sodium channels and generating a new action potential.

How does action potential propagation differ in non-myelinated and myelinated axons?

In a non-myelinated axon, successive membrane regions are depolarised. In a myelinated axon, action potentials are regenerated at successive nodes of Ranvier; this is saltatory conduction.

How do local currents travel between nodes of Ranvier in a myelinated axon?

Myelin insulates the membrane between nodes, and local currents spread beneath the myelin.

What is the refractory period following an action potential?

An interval when another action potential cannot be generated or requires a stronger-than-usual stimulus.

Why can voltage-gated sodium channels not immediately reopen after an action potential?

They are initially inactivated.

Why can the membrane remain less excitable as sodium channels recover after an action potential?

Continued potassium permeability and hyperpolarisation reduce excitability until the membrane returns towards its resting state.

How does the refractory period affect the separation of action potentials?

It separates successive action potentials into discrete impulses.

How does the refractory period affect the frequency of action potentials?

It limits how frequently they can be generated.

Why does the refractory period help an impulse travel forwards?

The membrane just behind the advancing action potential is temporarily unable to generate another.

Which three factors affect the speed of impulse conduction along an axon?

Myelination, axon diameter and temperature.

How does myelin increase impulse conduction speed along an axon?

It insulates the membrane between nodes of Ranvier, so action potentials are regenerated at the nodes rather than along every part of the membrane.

Why does a larger axon diameter increase impulse conduction speed?

It reduces resistance to local currents inside the axon, allowing adjacent regions to reach threshold more quickly.

How does temperature affect impulse conduction speed within a suitable physiological range?

Higher temperatures generally speed conduction by increasing the rate of ion movement and channel-related processes; cooling slows them.