Nervous Coordination (A Level Only)

邊玩邊學

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課程筆記

Motor Neurone Structure

Nerve impulses are electrical signals that pass along neurones; a bundle of neurones is called a nerve.
Sensory neurones carry impulses from receptors to the CNS; relay neurones connect sensory and motor neurones within the CNS; motor neurones carry impulses from the CNS to effectors.
Motor neurones have a cell body at one end (within the CNS), a long axon, and axon terminal endings located within effectors.
Some motor neurones have a myelin sheath, a fatty substance made by Schwann cells that wrap around the axon.
The presence of myelin speeds up the conduction of nerve impulses.
The long axon allows impulses to be transmitted over long distances, and dendrites allow connections with other neurones.

Resting Potential

Neurones not actively transmitting impulses have a resting potential of about -70 mV across their cell surface membrane.
This means the inside of the axon is 70 mV more negative than the outside.
Resting potential is established and maintained by sodium-potassium pumps and the outward flow of potassium ions.
Sodium-potassium pumps use ATP to actively transport 3 Na⁺ out for every 2 K⁺ in, creating a larger concentration of positive ions outside the axon.
Potassium ions diffuse out of the axon by facilitated diffusion through open potassium ion channels; sodium ion channels are closed at this point.
The difference in permeability to sodium and potassium is called differential membrane permeability.
The open potassium channels involved in resting potential are not voltage-gated and should not be confused with voltage-gated potassium channels.

Action Potentials

When a neurone is stimulated, sodium ion channels open, sodium ions enter the axon, and the inside becomes less negative; this is depolarisation (the generator potential).
If the membrane potential reaches about -50 mV (the threshold potential), voltage-gated sodium ion channels open and more sodium ions enter.
Enough sodium ions enter for the membrane potential to reach about +30 mV; this is an action potential.
During repolarisation, voltage-gated sodium channels close and voltage-gated potassium channels open, allowing potassium ions to diffuse out.
Hyperpolarisation occurs when potassium ions continue to leave until the inside is more negative than resting potential; this is the refractory period.
The all-or-nothing principle: if threshold is not reached, no action potential occurs; if threshold is reached, an action potential of the same size (+30 mV) always occurs.
A stronger stimulus produces a higher frequency of action potentials; a weaker stimulus produces a lower frequency.
The refractory period ensures new action potentials are generated ahead of the original, so impulses travel in one direction, and keeps impulses as separate events.

Nerve Impulse Transmission

Action potentials occur at the point of stimulation and are transmitted along the axon as nerve impulses.
In non-myelinated neurones, sodium ions diffuse along the axon, depolarising the next section; voltage-gated sodium channels open if threshold is reached.
The impulse does not travel backwards because the membrane behind the action potential is in a hyperpolarised state.
In myelinated neurones, the myelin sheath stops diffusion of sodium and potassium ions, so depolarisation cannot occur in myelinated sections.
Sodium ions diffuse from one node of Ranvier to the next, setting up local currents and initiating action potentials at each node.
Action potentials appear to jump from node to node; this is saltatory conduction, which is much faster than in unmyelinated axons of the same diameter.

Speed of Impulse Conduction

The speed of impulse conduction depends on myelination, axon diameter, and temperature.
Myelinated neurones conduct impulses faster because depolarisation occurs only at the nodes of Ranvier, relying on diffusion of sodium ions (saltatory conduction).
In unmyelinated neurones, depolarisation must occur along the whole membrane, which is relatively slow.
Wider axons conduct impulses faster: larger diameter gives a higher volume of cytoplasm, reducing resistance to ion flow, and fewer ions leak across the membrane.
Higher temperatures increase the speed of conduction because molecules have more kinetic energy, so diffusion and respiration (providing ATP) are faster.
In mammals, stable body temperature limits the effect of environmental temperature; in reptiles, nerve impulse transmission is affected by external temperature.

Calculating Maximum Impulse Frequency

After an action potential, the axon enters a refractory period during which it cannot be stimulated again.
The duration of the refractory period determines the maximum frequency of impulse conduction.
Maximum frequency = time ÷ duration of the refractory period; for 1 second, it is 1 ÷ duration of the refractory period.
Units may be impulses sec⁻¹, action potentials sec⁻¹, or Hz; 1 Hz = one impulse per second.
Convert milliseconds to seconds before calculating (divide by 1000).
Example: a refractory period of 2.75 ms = 0.00275 s, so maximum frequency = 1 ÷ 0.00275 = 364 action potentials sec⁻¹.

Synapse Structure and Synaptic Transmission

Synapses are junctions between cells in the nervous system, consisting of a presynaptic cell, a postsynaptic cell, and the synaptic cleft between them.
The postsynaptic cell can be another neurone or an effector cell (e.g. a muscle cell).
Nerve signals cross synapses as neurotransmitters, which are released from vesicles, diffuse across the cleft, and bind to receptors on the postsynaptic cell.
At a cholinergic synapse, an action potential causes voltage-gated calcium channels to open and calcium ions to enter the presynaptic neurone.
Calcium ions cause vesicles containing acetylcholine (ACh) to fuse with the presynaptic membrane and release ACh into the synaptic cleft.
ACh diffuses across the cleft and binds to receptor proteins on the postsynaptic membrane, opening sodium ion channels and depolarising the postsynaptic membrane.
If threshold is reached, a new action potential is generated in the postsynaptic neurone.
Acetylcholinesterase hydrolyses ACh in the synaptic cleft; the products are absorbed by the presynaptic cell to produce more ACh.

Summation and Inhibitory Synapses

Synapses ensure unidirectional transmission because calcium channels and neurotransmitter vesicles are only in the presynaptic cell, and receptors are only on the postsynaptic membrane.
A single impulse may not release enough neurotransmitter to reach threshold; this allows the nervous system to filter out low-level stimuli.
Summation occurs when multiple impulses arrive together, increasing the chance of reaching threshold.
Temporal summation: rapid, repeated release of neurotransmitter from one neurone; neurotransmitter builds up until threshold is reached.
Spatial summation: multiple impulses arrive at the same time from several presynaptic cells; their combined neurotransmitter reaches threshold.
Excitatory synapses initiate a new action potential by causing an influx of positive ions.
Inhibitory synapses prevent a new action potential by causing hyperpolarisation, e.g. by opening potassium channels (K⁺ outflow) or chloride channels (Cl⁻ inflow).
A neurone may receive both excitatory and inhibitory input, allowing complex information processing.

Drugs and Synapses

Drugs can affect synaptic transmission by stimulating neurotransmitter release, providing chemicals to synthesise neurotransmitters, imitating neurotransmitters by binding to receptors, or preventing reuptake.
Dopamine agonists bind to dopamine receptors and produce the same effect as dopamine; dopamine precursors are converted into dopamine inside neurones.
These treatments are used in Parkinson's disease, where not enough dopamine is produced in specific parts of the brain.
Morphine mimics endorphins, binds to endorphin receptors, and stimulates dopamine release, leading to pain relief and pleasure.
Cocaine binds to dopamine transporter proteins on the presynaptic membrane, blocking reabsorption and causing dopamine to build up in the synapse, overstimulating the postsynaptic neurone.
Cannabinoids bind to receptors on presynaptic membranes of neuromuscular junctions, causing calcium ion channels to close, reducing neurotransmitter release and weakening muscle contraction.
MDMA stimulates the release of several neurotransmitters, most notably serotonin, altering mood.

Neuromuscular Junctions

Neuromuscular junctions are specialised synapses between a motor neurone (presynaptic cell) and a muscle cell (postsynaptic cell).
When an impulse arrives, calcium ion channels open, calcium ions enter the neurone, and vesicles of ACh fuse with the presynaptic membrane.
ACh diffuses across the cleft and binds to receptors on the sarcolemma, opening sodium ion channels and depolarising the muscle cell.
If threshold is reached, an action potential is generated and transmitted into the muscle cell via T-tubules.
This causes voltage-gated calcium channels in the sarcoplasmic reticulum to open, releasing calcium ions into the sarcoplasm and triggering muscle contraction.
Acetylcholinesterase breaks down ACh when stimulation ends, and the products are reabsorbed by the presynaptic cell.
Unlike cholinergic synapses, neuromuscular junctions have folded postsynaptic membranes (storing AChE), T-tubules, are always excitatory, and represent the end of a nerve pathway.

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1.Which type of neurone carries nerve impulses from receptors to the central nervous system (CNS)?
Easy
- ASensory neurone
- BRelay neurone
- CMotor neurone
- DInterneuron
2.Which of the following correctly describes the resting potential of a neurone?
Easy
- AThe inside of the axon is 70 mV more negative than the outside
- BThe inside of the axon is 70 mV more positive than the outside
- CThe inside of the axon is 30 mV more positive than the outside
- DThe inside of the axon is 30 mV more negative than the outside
3.Myelin is a fatty substance produced by Schwann cells that wraps around the axon.
Easy
True or false?
4.During an action potential, what is the threshold potential that must be reached for voltage-gated sodium ion channels to open?
Medium
- A-70 mV
- B-50 mV
- C+30 mV
- D+40 mV
5.Which of the following is the correct sequence of events during an action potential?
Easy
- ADepolarisation → Repolarisation → Hyperpolarisation
- BRepolarisation → Depolarisation → Hyperpolarisation
- CHyperpolarisation → Depolarisation → Repolarisation
- DDepolarisation → Hyperpolarisation → Repolarisation
6.During the transmission of an action potential along a non-myelinated axon, why does the nerve impulse not travel backwards?
Medium
- AThe membrane behind the action potential is hyperpolarised and cannot be stimulated
- BSodium ions diffuse only forwards along the axon
- CVoltage-gated sodium channels are permanently closed behind the action potential
- DThe myelin sheath prevents backward transmission
7.Which of the following statements about the refractory period are true? (Select all that apply.)
Medium
- AIt ensures nerve impulses are transmitted in one direction
- BIt allows nerve impulses to merge together
- CIt is the time when the membrane is hyperpolarised
- DIt limits the maximum frequency of action potentials
- EIt occurs during depolarisation
8.Match each structure of a cholinergic synapse with its correct description.
Medium
- Presynaptic membrane
- Synaptic cleft
- Postsynaptic membrane
- Vesicle
- Membrane of the neurone that carries the impulse towards the synapse
- Gap between the two cells
- Membrane of the cell that receives the signal
- Structure containing neurotransmitter

更多題目

9.Place the events of synaptic transmission at a cholinergic synapse in the correct order.
Medium
- Action potential arrives at presynaptic membrane
- Calcium ion channels open and calcium ions enter
- Vesicles fuse with presynaptic membrane and release ACh
- ACh diffuses across synaptic cleft and binds to receptors
- Sodium ion channels open and postsynaptic membrane depolarises
10.Match each neurone type with the direction in which it carries nerve impulses.
Medium
- Sensory neurone
- Relay neurone
- Motor neurone
- From receptors to the CNS
- Connects sensory and motor neurones within the CNS
- From the CNS to effectors
11.Place the stages of an action potential in the correct order, starting from the resting potential.
Medium
- Depolarisation
- Repolarisation
- Hyperpolarisation (refractory period)
- Return to resting potential
12.Which of the following statements about the sodium-potassium pump in a neurone are correct? (Select all that apply.)
Medium
- AIt actively transports 3 sodium ions out of the axon for every 2 potassium ions transported in.
- BIt uses ATP to transport ions against their concentration gradients.
- CIt transports sodium ions into the axon and potassium ions out of the axon.
- DIt contributes to the resting potential by making the inside of the axon more negative than the outside.
- EIt is not required once the resting potential has been established.
13.During the transmission of an action potential along a myelinated axon, why does the impulse appear to jump from one node of Ranvier to the next?
Medium
- AAction potentials can only be generated at the nodes of Ranvier because myelin insulates the axon and prevents ion movement except at the nodes.
- BThe myelin sheath conducts the action potential directly along its length, bypassing the axon membrane.
- CSodium ions diffuse through the myelin sheath, causing depolarisation along the entire axon.
- DVoltage-gated sodium channels are present all along the axon, but they open only at the nodes due to the myelin.
14.Which of the following factors affect the speed of impulse conduction along an axon? (Select all that apply.)
Medium
- AThe presence or absence of a myelin sheath
- BThe diameter of the axon
- CThe temperature
- DThe length of the axon
- EThe number of synapses
15.Match each part of a cholinergic synapse with its correct function.
Medium
- Presynaptic membrane
- Synaptic cleft
- Postsynaptic membrane
- Site of release of acetylcholine from vesicles
- Gap across which neurotransmitters diffuse
- Contains receptors that bind acetylcholine
16.Which of the following are differences between a neuromuscular junction and a cholinergic synapse? (Select all that apply.)
Medium
- AThe postsynaptic membrane in a neuromuscular junction has folds that store acetylcholinesterase.
- BT-tubules are present in a neuromuscular junction but not in a cholinergic synapse.
- CA neuromuscular junction is always excitatory, whereas a cholinergic synapse can be excitatory or inhibitory.
- DA neuromuscular junction is between two neurones, while a cholinergic synapse is between a neurone and a muscle cell.
- EAcetylcholine is the neurotransmitter in both.
17.Cocaine binds to dopamine transporter proteins on the presynaptic membrane. What is the effect of this on the postsynaptic neurone?
Medium
- ADopamine builds up in the synapse, causing overstimulation of the postsynaptic neurone.
- BDopamine is broken down more rapidly, reducing stimulation of the postsynaptic neurone.
- CDopamine cannot be released from the presynaptic neurone, preventing any signal transmission.
- DDopamine binds to the transporter proteins and is taken back into the presynaptic neurone more quickly.
18.Which of the following statements about spatial and temporal summation are correct? (Select all that apply.)
Hard
- ASpatial summation involves multiple impulses arriving at the same time from several presynaptic cells.
- BTemporal summation involves rapid, repeated release of neurotransmitter from one neurone.
- CBoth types of summation can lead to threshold potential being reached in the postsynaptic neurone.
- DTemporal summation involves impulses from different presynaptic cells arriving simultaneously.
- ESpatial summation involves impulses arriving one after the other from a single presynaptic cell.
19.In an unmyelinated neurone, depolarisation must occur along the whole membrane of the axon, which makes conduction relatively slow.
Easy
True or false?
20.In an experiment, a neurone is stimulated with a weak stimulus that does not reach threshold potential. What is observed?
Hard
- ANo action potential is generated.
- BA smaller action potential is generated.
- CA normal-sized action potential is generated.
- DMultiple action potentials are generated.
21.Which of the following are features of a motor neurone? (Select all that apply.)
Medium
- AA cell body located within the CNS
- BA long axon
- CAxon terminal endings located within effectors
- DA myelin sheath made by Schwann cells in all motor neurones
- EDendrites that allow connections with other neurones
22.Place the events of an action potential in the correct order, starting from the resting potential.
Hard
- Sodium ion channels open and sodium ions enter the axon
- Membrane potential reaches threshold (-50 mV)
- Voltage-gated sodium ion channels open and more sodium ions enter
- Membrane potential reaches +30 mV
- Voltage-gated sodium channels close and voltage-gated potassium channels open
- Potassium ions diffuse out, causing repolarisation
23.Match each term with its correct description.
Medium
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Membrane potential becomes less negative due to sodium ion influx
- Membrane potential becomes more negative due to potassium ion efflux
- Membrane potential becomes more negative than resting potential
24.During an action potential, the membrane potential reaches +40 mV and then falls steeply. Use the information from Figure 1 to explain this fall.
Medium
- AVoltage-gated sodium ion channels close and voltage-gated potassium ion channels open, so potassium ions diffuse out of the axon.
- BVoltage-gated potassium ion channels close and sodium ion channels remain open, so sodium ions continue to enter.
- CSodium-potassium pumps actively transport sodium ions out and potassium ions in, restoring the resting potential.
- DCalcium ion channels open and calcium ions diffuse into the axon, making the inside more negative.
25.Researchers investigated the effect of cocaine on the movement of rats. Group A were not given cocaine, group B were given cocaine, and group C were mutant rats with reduced dopamine transporter function given cocaine. Figure 1 shows the results. Which conclusion is best supported by the data?
Medium
- ACocaine increases movement by blocking dopamine transporters, leading to dopamine build-up in the synapse.
- BCocaine decreases movement by inhibiting dopamine release from the presynaptic neurone.
- CCocaine has no effect on movement in rats with normal dopamine transporter function.
- DCocaine affects movement by binding to dopamine receptors on the postsynaptic membrane.
26.Cocaine affects the reuptake of noradrenaline, causing increased concentration in synapses/neuromuscular junctions. Which of the following could explain how this leads to a heart attack? (Select all that apply.)
Hard
- AIncreased noradrenaline increases heart rate.
- BIncreased noradrenaline increases force of heart contraction.
- CIncreased noradrenaline causes vasodilation of coronary arteries.
- DIncreased noradrenaline increases blood pressure, increasing risk of plaque rupture.
- EIncreased noradrenaline decreases oxygen demand of the heart.
27.A neurone has a refractory period of 4 ms. What is the maximum frequency of action potentials per second along this neurone?
Medium
- A250 Hz
- B400 Hz
- C40 Hz
- D0.25 Hz
28.In a myelinated neurone, action potentials occur only at the nodes of Ranvier, which speeds up conduction by saltatory conduction.
Easy
True or false?
29.Match each factor affecting the speed of impulse conduction with its correct effect.
Medium
- Presence of myelin
- Increased axon diameter
- Increased temperature
- Increases speed by allowing saltatory conduction
- Increases speed by reducing resistance to ion flow
- Increases speed by increasing kinetic energy of ions
30.Place the events of transmission at a cholinergic synapse in the correct order.
Medium
- Action potential arrives at presynaptic membrane
- Calcium ion channels open and calcium ions enter presynaptic neurone
- Vesicles fuse with presynaptic membrane and release acetylcholine
- Acetylcholine diffuses across synaptic cleft and binds to receptors
- Sodium ion channels open and postsynaptic membrane depolarises
31.A student sets up an experiment using electrodes to measure the membrane potential of a single axon. They find that the resting potential is -70 mV. Which of the following best explains why the inside of the axon is negatively charged relative to the outside?
Medium
- AThree sodium ions are actively transported out for every two potassium ions transported in, and potassium ions diffuse out through open channels
- BThree potassium ions are actively transported out for every two sodium ions transported in, and sodium ions diffuse in through open channels
- CSodium and potassium ions are transported in equal numbers, but sodium ions diffuse out faster than potassium ions diffuse in
- DThe sodium-potassium pump transports sodium ions in and potassium ions out, both against their concentration gradients
32.A scientist investigates the effect of temperature on the speed of impulse conduction in a mammal and a reptile. The mammal maintains a constant body temperature of 37 °C, while the reptile's body temperature varies with its environment. The reptile is placed in an environment at 15 °C. Which of the following best predicts and explains the results?
Medium
- AThe reptile's impulse conduction will be slower than normal because lower temperatures reduce the kinetic energy of ions, slowing diffusion and reducing ATP production for active transport
- BThe reptile's impulse conduction will be faster than normal because lower temperatures cause ion channels to open more frequently
- CThe mammal's impulse conduction will also slow down significantly because its body temperature will drop to match the environment
- DBoth organisms will show identical conduction speeds because temperature affects all neurones equally regardless of body temperature regulation
33.A researcher is investigating the effect of a drug on synaptic transmission. The drug is found to bind to and block calcium ion channels in the presynaptic membrane of a cholinergic synapse. Which of the following would be observed? (Select all that apply.)
Medium
- AVesicles containing acetylcholine would not fuse with the presynaptic membrane
- BAcetylcholine would not be released into the synaptic cleft
- CSodium ion channels in the postsynaptic membrane would not open
- DAn action potential would still be generated in the postsynaptic neurone at the same frequency
- EAcetylcholinesterase would be unable to break down acetylcholine in the synaptic cleft
34.In an experiment, a neurone is stimulated with a series of impulses. The first impulse does not generate an action potential in the postsynaptic neurone. However, when a second impulse arrives at the same presynaptic neurone very shortly after the first, an action potential is generated in the postsynaptic neurone. Which type of summation does this demonstrate?
Hard
- ATemporal summation, because multiple impulses arrive at the same presynaptic neurone in rapid succession
- BSpatial summation, because impulses arrive from different presynaptic neurones at the same time
- CTemporal summation, because the impulses arrive at different presynaptic neurones at different times
- DSpatial summation, because the impulses arrive at the same presynaptic neurone at different times
35.In an unmyelinated neurone, the action potential is transmitted along the whole length of the axon membrane, whereas in a myelinated neurone, action potentials only occur at the nodes of Ranvier.
Easy
True or false?
36.Match each feature of a neurone or synapse with its correct effect on the speed of impulse conduction.
Medium
- Myelination
- Larger axon diameter
- Higher temperature
- Smaller axon diameter
- Increases speed because depolarisation only occurs at the nodes of Ranvier
- Increases speed because there is less resistance to ion flow in the cytoplasm
- Increases speed because ions have more kinetic energy and respiration provides more ATP
- Decreases speed because there is more resistance to ion flow and more ion leakage
37.Place the following events that occur at a cholinergic synapse in the correct order, starting from the arrival of an action potential at the presynaptic neurone.
Medium
- An action potential arrives at the presynaptic neurone and depolarises the membrane
- Voltage-gated calcium ion channels open and calcium ions diffuse into the presynaptic neurone
- Vesicles containing acetylcholine fuse with the presynaptic membrane and release ACh into the synaptic cleft
- ACh diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic membrane
- Sodium ion channels open in the postsynaptic membrane and sodium ions diffuse in
- Acetylcholinesterase breaks down ACh in the synaptic cleft
38.A drug is found to have a very similar molecular structure to GABA, an inhibitory neurotransmitter. GABA normally binds to receptors on the postsynaptic membrane and causes chloride ion channels to open. In a patient with epilepsy caused by insufficient GABA, the drug is given as a treatment. Which of the following best explains how the drug reduces neuronal activity?
Hard
- AThe drug binds to GABA receptors, causing chloride ions to enter the postsynaptic neurone, making the membrane potential more negative and making it less likely that threshold is reached
- BThe drug binds to GABA receptors, causing potassium ions to enter the postsynaptic neurone, making the membrane potential more positive and triggering an action potential
- CThe drug blocks GABA receptors, preventing inhibitory signals and reducing neuronal activity
- DThe drug mimics GABA by causing sodium ions to enter the postsynaptic neurone, increasing the likelihood of an action potential
39.Which of the following is the correct definition of resting potential?
Easy
- AThe difference in charge across a neurone cell surface membrane when it is not actively transmitting a nerve impulse
- BThe brief reversal of charge across a neurone membrane when a stimulus reaches threshold
- CThe maximum membrane potential reached at the peak of an action potential
- DThe movement of sodium ions out of the axon by active transport
40.In the sodium-potassium pump, how many sodium ions are transported out of the axon for every two potassium ions transported in?
Easy
- A2
- B3
- C1
- D4
41.Complete the sentence about the sodium-potassium pump.
Easy
The sodium-potassium pump actively transports ____ sodium ions out of the axon for every ____ potassium ions that it transports in.
42.An unmyelinated neurone conducts impulses at 0.5 m s⁻¹ while a myelinated neurone conducts at 150 m s⁻¹. Calculate how much faster the myelinated neurone is, in m s⁻¹.
Medium
43.An unmyelinated neurone conducts impulses at 0.5 m s⁻¹, while a myelinated neurone conducts at 150 m s⁻¹. Calculate how many times faster the myelinated neurone is compared to the unmyelinated neurone. Give your answer to the nearest whole number.
Medium
44.Complete the sentence about the resting potential.
Medium
The resting potential of a neurone is approximately ____ mV, and it is maintained by the action of the ____ pump and the outward diffusion of ____ ions.
45.A graph shows that the refractory period of an axon is 2.75 milliseconds. Calculate the maximum frequency of action potentials per second along this axon. Give your answer to the nearest whole number.
Medium
46.Complete the sentence about the all-or-nothing principle.
Medium
According to the all-or-nothing principle, if a stimulus is strong enough to reach the threshold potential, an action potential will always be generated, and all action potentials are the same size at around ____ mV. A stronger stimulus produces a higher ____ of action potentials, not a larger one.
47.Assuming the frequency remains constant, calculate how many complete action potentials will occur in 1 second.
Medium
48.The average transmission speed of an impulse through the motor axon is approximately 45 m s⁻¹. Using the information from Figure 1, calculate the maximum distance travelled during a single action potential. Give your answer in metres.
Medium
49.An axon has a refractory period of 3.2 milliseconds. Calculate the maximum frequency of action potentials per second. Give your answer to the nearest whole number.
Medium
50.Complete the sentence about the refractory period.
Medium
The refractory period ensures that nerve impulses travel in one direction because the membrane behind the action potential is in a ____ state.
51.Name the enzyme that breaks down acetylcholine in the synaptic cleft.
Easy
52.A neurone has a refractory period of 4 milliseconds. Calculate the maximum frequency of action potentials per second along this neurone. Give your answer to the nearest whole number.
Medium
53.Explain how the presence of tubocurarine chloride in a synapse prevents the generation of an action potential in the postsynaptic neurone.
Hard
54.Explain how the refractory period ensures the unidirectional flow of a nervous impulse in a neurone.
Hard
55.Explain how the structure of a myelinated motor neurone allows rapid transmission of nerve impulses over long distances.
Hard
56.Explain the steep increase in sodium ion permeability seen between 0.5 ms and 0.7 ms.
Hard
57.Explain the steep increase in sodium ion permeability seen between 0.5 ms and 0.7 ms during an action potential.
Hard
58.An unmyelinated neurone conducts impulses at 0.5 m s⁻¹ while a myelinated neurone conducts at 150 m s⁻¹. Calculate how much faster the myelinated neurone is, in m s⁻¹.
Medium
- A164.5 m s⁻¹
- B134.5 m s⁻¹
- C179.5 m s⁻¹
- D149.5 m s⁻¹
59.An unmyelinated neurone conducts impulses at 0.5 m s⁻¹, while a myelinated neurone conducts at 150 m s⁻¹. Calculate how many times faster the myelinated neurone is compared to the unmyelinated neurone. Give your answer to the nearest whole number.
Medium
- A330
- B300
- C270
- D360
60.A graph shows that the refractory period of an axon is 2.75 milliseconds. Calculate the maximum frequency of action potentials per second along this axon. Give your answer to the nearest whole number.
Medium
- A436 action potentials s⁻¹
- B400 action potentials s⁻¹
- C364 action potentials s⁻¹
- D328 action potentials s⁻¹
61.Assuming the frequency remains constant, calculate how many complete action potentials will occur in 1 second.
Medium
- A328 action potentials
- B364 action potentials
- C400 action potentials
- D436 action potentials
62.The average transmission speed of an impulse through the motor axon is approximately 45 m s⁻¹. Using the information from Figure 1, calculate the maximum distance travelled during a single action potential. Give your answer in metres.
Medium
- A2.124 m
- B0.124 m
- C-0.876 m
- D1.124 m
63.An axon has a refractory period of 3.2 milliseconds. Calculate the maximum frequency of action potentials per second. Give your answer to the nearest whole number.
Medium
- A375 Hz
- B344 Hz
- C282 Hz
- D313 Hz
64.A neurone has a refractory period of 4 milliseconds. Calculate the maximum frequency of action potentials per second along this neurone. Give your answer to the nearest whole number.
Medium
- A275 action potentials per second
- B250 action potentials per second
- C300 action potentials per second
- D225 action potentials per second

複習筆記

## Motor Neurone Structure - **Sensory neurones** carry impulses from receptors to the CNS; **relay neurones** connect sensory and motor neurones within the CNS; **motor neurones** carry impulses from the CNS to effectors. - Motor neurones have a **cell body** at one end (within the CNS), a **long axon**, and **axon terminal endings** in effectors. - **Myelin sheath** is a fatty substance made by **Schwann cells** wrapping around the axon; it speeds up conduction. ## Resting Potential - Resting potential is the potential difference across the membrane when not transmitting impulses: about **-70 mV** (inside more negative than outside). - Established and maintained by: - **Sodium-potassium pumps** using ATP to actively transport **3 Na⁺ out for every 2 K⁺ in**. - **Outward flow of K⁺** by facilitated diffusion through open potassium ion channels. - Sodium ion channels are closed at rest; this is **differential membrane permeability**. ## Action Potentials - **Depolarisation**: sodium ion channels open, Na⁺ enters down electrochemical gradient; if threshold (-50 mV) reached, voltage-gated Na⁺ channels open; membrane potential reaches about **+30 mV** (action potential). - **Repolarisation**: voltage-gated Na⁺ channels close; voltage-gated K⁺ channels open, K⁺ diffuses out; inside becomes more negative. - **Hyperpolarisation**: K⁺ channels remain open longer than needed, membrane becomes more negative than resting potential; this is the **refractory period**. - **All-or-nothing principle**: if stimulus too weak, threshold not reached, no action potential; if threshold reached, action potential always same size (~+30 mV). Stronger stimulus = higher frequency of action potentials. - **Refractory period** ensures impulses travel in one direction (new action potentials generated ahead) and are separate events. ## Nerve Impulse Transmission - **Non-myelinated**: Na⁺ diffuses along axon, depolarising next section; voltage-gated Na⁺ channels open if threshold reached. - **Myelinated**: myelin stops ion diffusion; Na⁺ diffuses from one **node of Ranvier** to the next, setting up local currents; action potentials appear to jump from node to node — **saltatory conduction**. ## Speed of Impulse Conduction - **Myelination**: faster (depolarisation only at nodes). - **Axon diameter**: wider = faster (higher volume of cytoplasm reduces resistance; fewer ions leak). - **Temperature**: higher = faster (more kinetic energy, faster diffusion and respiration). ## Calculating Maximum Impulse Frequency - Maximum frequency = **time ÷ duration of refractory period**. - For 1 second: **1 ÷ duration of refractory period** (convert ms to s). - Units: impulses sec⁻¹, action potentials sec⁻¹, Hz. ## Synapse Structure - **Presynaptic cell** (neurone), **postsynaptic cell** (neurone or effector), **synaptic cleft**. - Neurotransmitters released from vesicles, diffuse across cleft, bind to receptors on postsynaptic cell. ## Synaptic Transmission (Cholinergic) 1. Action potential arrives, depolarises presynaptic membrane. 2. Voltage-gated Ca²⁺ channels open; Ca²⁺ diffuses in. 3. Vesicles fuse with presynaptic membrane; **acetylcholine (ACh)** released into cleft. 4. ACh diffuses and binds to receptors on postsynaptic membrane. 5. Na⁺ channels open; Na⁺ diffuses in; postsynaptic membrane depolarised; if threshold reached, new action potential. 6. **Acetylcholinesterase** hydrolyses ACh; products reabsorbed by presynaptic cell. - **Unidirectionality**: Ca²⁺ channels and vesicles only in presynaptic cell; receptors only on postsynaptic cell. - **Summation**: temporal (rapid repeated release from one neurone) and spatial (multiple impulses from several presynaptic cells at same time). - **Inhibitory synapses**: lower membrane potential by K⁺ outflow or Cl⁻ inflow, preventing action potential. ## Drugs & Synapses - Drugs may stimulate release, provide precursors, imitate neurotransmitter, or prevent reuptake. - **Cocaine** blocks dopamine transporters, preventing reabsorption; dopamine builds up, overstimulating postsynaptic neurone. ## Neuromuscular Junctions - Specialised synapse between motor neurone and muscle cell. - ACh binds to receptors on **sarcolemma**; Na⁺ enters, depolarisation; action potential transmitted via **T-tubules**; Ca²⁺ released from sarcoplasmic reticulum; muscle contraction. - **Differences from cholinergic synapses**: postsynaptic membrane has folds with AChE; T-tubules present; always excitatory; end of nerve pathway.

Resting potential: The potential difference across the cell surface membrane of a neurone when not actively transmitting impulses, approximately -70 mV (inside more negative than outside).
Sodium-potassium pump: Carrier protein that uses ATP to actively transport 3 sodium ions out of the axon for every 2 potassium ions transported in.
Depolarisation: The process during an action potential where sodium ion channels open, sodium ions enter the axon, and the inside of the axon becomes less negative.
Threshold potential: 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.
Repolarisation: The stage of an action potential where voltage-gated sodium channels close and voltage-gated potassium channels open, allowing potassium ions to diffuse out and the inside of the membrane to become more negative.
Hyperpolarisation: When the inside of the membrane becomes more negative than the resting potential due to continued outward movement of potassium ions; the membrane is in the refractory period.
Refractory period: The time during which the membrane is hyperpolarised and cannot be stimulated again; ensures unidirectional transmission and separate nerve impulses.
All-or-nothing principle: If a stimulus is too weak, threshold potential is not reached and no action potential occurs; if threshold is reached, an action potential of the same size (~+30 mV) is always generated.
Saltatory conduction: The apparent jumping of action potentials from one node of Ranvier to the next in myelinated neurones, allowing faster impulse transmission.
Synaptic cleft: The gap between the presynaptic and postsynaptic cells at a synapse.
Cholinergic synapse: A synapse that uses the neurotransmitter acetylcholine (ACh).
Acetylcholinesterase: Enzyme that catalyses the hydrolysis of acetylcholine in the synaptic cleft, breaking it down and allowing products to be reabsorbed by the presynaptic cell.
Temporal summation: Rapid, repeated release of neurotransmitters from one neurone, building up neurotransmitter in the cleft until threshold is reached.
Spatial summation: Multiple impulses arriving at the same time from several different presynaptic cells, releasing enough neurotransmitter to reach threshold.
Neuromuscular junction: A specialised synapse between a motor neurone and a muscle cell, where acetylcholine binds to receptors on the sarcolemma, leading to muscle contraction.

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