Nervous Coordination (A Level only) (AQA A Level Biology): Flashcards

The difference in charge across the cell surface membrane of a neurone that is not transmitting a nerve impulse, approximately -70 mV.

Approximately -70 mV, meaning the inside of the axon is 70 mV more negative than the outside.

The sodium-potassium pumps in the cell surface membrane.

The outward flow of potassium ions.

3 sodium ions (Na^+^) are actively transported out of the axon.

2 potassium ions (K^+^) are actively transported in to the axon.

ATP, as the ions are moved by active transport.

By facilitated diffusion, through open potassium ion channels in the cell surface membrane, down their concentration gradient.

True.

Potassium ion channels are open while sodium ion channels are closed; this differential membrane permeability allows potassium ions to diffuse out.

It increases the concentration of positive ions outside the axon, making the inside more negative and helping to achieve -70 mV.

The membrane is more permeable to potassium ions (channels open) than to sodium ions (channels closed).

The sodium-potassium pump actively transports three sodium ions out of the axon for every two potassium ions moved in.

Depolarisation.

Repolarisation.

Hyperpolarisation (the refractory period).

Sodium ion channels in the axon membrane open.

Sodium ions diffuse into the axon down an electrochemical gradient, so the inside becomes less negative.

If the membrane reaches the threshold potential (~-50 mV), voltage-gated sodium ion channels open and more sodium ions enter, reaching about +30 mV.

The membrane potential (around -50 mV) that must be reached for voltage-gated sodium ion channels to open and an action potential to be generated.

The voltage-gated sodium ion channels close, stopping any further sodium ion influx.

Voltage-gated potassium ion channels open, so potassium ions diffuse out of the axon down their concentration gradient.

The inside of the membrane becomes more negative.

When the continued outward movement of potassium ions makes the inside of the membrane more negative than the resting potential.

The voltage-gated potassium ion channels close, and the action of the sodium-potassium pumps restores the membrane to resting potential.

If the threshold is not reached there is no action potential; if it is reached an action potential always results and is always the same size (~+30 mV).

By the frequency of action potentials.

A stronger stimulus produces a higher frequency of action potentials.

A weaker stimulus produces a lower frequency of action potentials.

At rest the membrane potential is about -70 mV; during an action potential the membrane depolarises to about +30 mV.

The time during which the axon membrane is hyperpolarised and cannot be stimulated to produce another action potential.

It ensures new action potentials are generated ahead of, not behind, the original, so nerve impulses travel in one direction only.

It ensures nerve impulses remain separate events rather than merging together.

The initial influx of sodium ions increases the sodium ion concentration inside the axon.

Sodium ions diffuse along the axon, depolarising the membrane in the next section.

Voltage-gated sodium ion channels open in the new section, causing a new influx of sodium ions.

The membrane behind the action potential is in a hyperpolarised (refractory) state, so it cannot be depolarised again immediately.

A fatty layer surrounding some axons, formed from Schwann cells.

The regions of the axon in between each Schwann cell, where the membrane is not covered by myelin.

The myelin sheath stops the diffusion of sodium and potassium ions across the membrane.

Sodium ions diffuse along the axon from one node of Ranvier to the next, setting up local currents.

An action potential is generated at each node, so the impulse appears to jump between them.

The process in myelinated axons where action potentials appear to jump from one node of Ranvier to the next, allowing much faster transmission.

True.

Because depolarisation only occurs at the nodes of Ranvier, the impulse travels much faster than in an unmyelinated axon of the same diameter.

In myelinated axons action potentials only occur at the nodes of Ranvier, so the impulse appears to jump between them in a process called saltatory conduction.

The presence or absence of myelin.

Axon diameter.

Temperature.

In unmyelinated neurones depolarisation must occur along the whole membrane of the axon, which is slow.

In myelinated neurones depolarisation only occurs at the nodes of Ranvier, so the impulse undergoes fast saltatory conduction.

A wider axon conducts impulses more quickly.

A larger volume of cytoplasm reduces resistance to the flow of ions.

Fewer ions are lost by leakage, so membrane potential is maintained more easily.

Impulses are conducted more quickly because molecules have more kinetic energy.

Diffusion of ions across membranes and along axons is faster.

Respiration is faster, providing more ATP for active transport of ions.

True.

Mammals maintain a stable body temperature, so external temperatures have a limited effect, unlike in reptiles whose body temperature fluctuates with the environment.

Conduction in which action potentials appear to jump between the nodes of Ranvier of a myelinated axon, increasing the speed of transmission.

A larger diameter axon has a higher volume of cytoplasm, which reduces the resistance to the flow of ions.

A reptile's body temperature fluctuates with its environment, so the speed of its nerve impulse transmission is affected by external temperatures.

Nerve impulses are conducted more quickly in axons with a wider diameter and at higher temperatures.

The period of recovery after an action potential during which the axon is unresponsive; it sets a minimum time between action potentials, determining the maximum frequency of impulses.

time / duration of the refractory period

1 / duration of the refractory period

Convert to seconds: 2.75 / 1000 = 0.00275 s.

Apply the equation: 1 / 0.00275 = 364 action potentials s^-1^ (to 3 s.f.).

impulses s^-1^

action potentials s^-1^

Hz

One impulse per second.

So that every part of the equation uses the same units, giving an answer in impulses per second.

True.

Because maximum frequency = 1 / duration of the refractory period, a shorter refractory period gives a higher maximum frequency.

The maximum frequency of impulses per second is calculated as 1 divided by the duration of the refractory period.

A junction between cells in the nervous system.

The presynaptic cell.

The postsynaptic cell.

The synaptic cleft (the gap between the two cells).

The neurone that carries a nerve impulse towards the synapse.

The cell that receives the nerve signal at the synapse; it may be another neurone or an effector cell (e.g. a muscle cell).

The gap between the presynaptic and postsynaptic cells.

Chemical signals that transmit a nerve signal across a synapse.

It is released from vesicles in the presynaptic cell.

It diffuses across the synaptic cleft.

It binds to receptors on the postsynaptic cell.

False.

The postsynaptic cell may be another neurone or an effector cell, such as a muscle cell.

At a synapse, neurotransmitters are released from vesicles in the presynaptic cell and diffuse across the synaptic cleft to bind to receptors on the postsynaptic cell.

A synapse that uses the neurotransmitter acetylcholine (ACh).

An action potential arrives at the presynaptic cell, causing depolarisation of the membrane.

Voltage-gated calcium ion channels open and calcium ions diffuse in.

Vesicles containing ACh fuse with the presynaptic membrane, releasing ACh into the synaptic cleft.

ACh diffuses across the cleft and binds to receptors on the postsynaptic membrane.

Associated sodium ion channels open and sodium ions diffuse in.

The membrane depolarises, and if threshold is reached a new action potential is generated.

It catalyses the hydrolysis of ACh in the synaptic cleft; the products are absorbed by the presynaptic cell and used to make more ACh.

Calcium ion channels and neurotransmitter vesicles are only present in the presynaptic cell.

Receptors for the neurotransmitter are only present on the postsynaptic cell.

When multiple impulses together release enough neurotransmitter to reach threshold and trigger an action potential, which a single impulse might not.

Temporal summation: rapid, repeated release of neurotransmitter from one presynaptic neurone.

Spatial summation: neurotransmitter released at the same time from several presynaptic cells.

They cause hyperpolarisation by lowering the membrane potential.

This can occur by an outflow of positive ions (e.g. K⁺ leaving) or an inflow of negative ions (e.g. Cl^- entering).

False.

That describes spatial summation. Temporal summation is the rapid, repeated release of neurotransmitter from one presynaptic neurone.

At a cholinergic synapse, the arrival of an action potential opens voltage-gated calcium ion channels, triggering the release of acetylcholine from vesicles.

Stimulating the release of a neurotransmitter.

Providing the chemicals needed to synthesise neurotransmitters.

Imitating a neurotransmitter by binding to its specific receptor.

Preventing the reuptake of a neurotransmitter by the presynaptic neurone.

A drug that binds to dopamine receptors and produces the same effect as dopamine.

A substance that can be converted into dopamine inside neurones, increasing dopamine levels in the brain.

It binds to dopamine transporter proteins on the presynaptic membrane.

This blocks the reabsorption of dopamine, so dopamine builds up in the synapse.

The postsynaptic neurone is overstimulated, producing feelings of pleasure.

It mimics endorphins and binds to endorphin receptors, stimulating dopamine release, which leads to pain relief and a feeling of pleasure.

They bind to receptors on the presynaptic membranes of neuromuscular junctions, causing calcium ion channels to close.

This reduces neurotransmitter release, weakening muscle contraction.

Serotonin; this can alter mood.

True.

Preventing reuptake by the presynaptic neurone causes the neurotransmitter to build up in the synaptic cleft, prolonging its effect.

Cocaine blocks the dopamine transporter proteins, preventing the reabsorption of dopamine and causing it to build up in the synapse.

A specialised synapse located between a motor neurone and a muscle cell.

The presynaptic cell is a motor neurone.

The postsynaptic cell is a muscle cell.

Calcium ion channels open and calcium ions diffuse in to the neurone.

Vesicles of acetylcholine (ACh) fuse with the presynaptic membrane and release ACh, which diffuses across the cleft.

ACh binds to receptors on the sarcolemma, opening sodium ion channels so sodium ions diffuse in, depolarising the sarcolemma.

Acetylcholine (ACh).

When stimulation ends it breaks down ACh; the products are reabsorbed by the presynaptic cell.

False.

A neuromuscular junction is always excitatory; only cholinergic synapses between neurones can be excitatory or inhibitory.

In both, calcium ions trigger release of the neurotransmitter, which is acetylcholine.

In both, ACh diffuses across the cleft, binds to receptors to open sodium ion channels, and is broken down by AChE.

A cholinergic synapse is between two neurones, whereas a neuromuscular junction is between a motor neurone and a muscle cell.

A cholinergic synapse can be excitatory or inhibitory and may pass the impulse on, whereas a neuromuscular junction is always excitatory and is the end of a nerve pathway.

The folds store acetylcholinesterase (AChE).

At a neuromuscular junction the neurotransmitter acetylcholine binds to receptors on the sarcolemma, opening sodium ion channels in the muscle cell.