1: Structure
2
2: Reflex arc
3
3: Transmission of Nerve Impulses
4
4: Synapse
6
5: Differences
7
6: Essays
8
6.1: Essays- Structure
9
6.2: Essays- Resting and Action Potential
11
7: Learning Notes
13
Colour Coding:
Key Words
Important Key Words
Key words in the Question
Paper 6 Questions
Additional Comments
Predicted Questions in Purple
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Regulation and Control:
Nervous System
Topic N
CIE Notes for Biology A2
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1: Structure
(M/J 11 41) Explain the importance of the myelin sheath in the functioning of a neurone. 3m
1. Myelin sheath speeds up the rate at which action potentials travel by insulating the axon membrane.
2. Depolarisation occurs only at nodes of Ranvier.
3. Local circuits are set up between nodes of Ranvier.
4. Action potential jumps from one node to the next; this form of impulse propagation known as saltatory
propagation, with conduction speeds of 100ms-1, being 50 times faster than in non-myelinated neurones.
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2: Reflex arc
(M/J 05) Explain briefly how the stimulus at the finger produces an impulse in the sensory neurone. 3m
1. A receptor potential is produced by sensory transduction in which receptor converts energy in a stimulus
into a change in the electrical potential across its membrane.
2. Sodium channels open, Na+ rush in and depolarise the membrane.
3. Receptor potential needs to exceed a certain threshold potential to produce action potential in sensory
neurone.
(M/J 05) Describe role of the motor neurone in the reflex arc. 3m
1. Motor neurone is originated from the CNS, in which it synapses with relay neurone in grey matter of spinal
cord
2. At the neuromuscular junction , neurotransmitters are released from the synaptic knob to bring about an
action potential that results in response by contraction of muscle
A neuromuscular junction is a specialised form of synapse found between motor neurone and skeletal muscle fibres.
It includes both the motor end-plate and the synaptic knob.
(M/J 05) Suggest why nerve impulses can only travel in one direction through the reflex arc. 2m
1. Refractory period will stop the impulse from going backward.
2. Vesicles containing transmitter substance are only found in pre-synaptic neurone
3. Receptors for transmitter substance are only found on post-synaptic membrane
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3: Transmission of Nerve Impulses
(O/N 04) Outline how the resting potential from A to B is maintained. 3m
1. Na+ is actively transported out and K+ is actively transported into the neurone using the Na+/K+ pump. The
pump removes 3 sodium ions from the axon for every 2 potassium ions it brings into the axon.
2. The concentration of K+ inside the axon is high and the concentration of the Na+ outside the axon is high.
3. Membrane slightly more leaky to K+, with K+ diffuses back out much faster than Na+ diffuses back in, causing
the axon more negative in the inside than the outside.
(O/N 04) Describe how the changes in the membrane bring about depolarisation from B to C. 3m
1. Na+ channels open, Na+ diffuses into the axon, bringing a change in potential difference across the membrane.
2. The inside of the axon becomes more positive relative to the outside. Action potential is triggered.
(O/N 04) Explain how the membrane is repolarised from C to D. 3m
1. At a point, depolarisation of the membrane causes Na+ channels to close.
2. K+ channels open and K+ diffuses out from the axon
3. The slight delay in closing all the K+ gates causes continued K+ outflow, causing hyperpolarisation.
4. K+ channels close.
5. As potassium ions continue to return to the insides of the axon through the Na+/ K+ pump, their positive
charges restores the normal resting potential.
(M/J 05) Suggest why nerve impulses can only travel in one direction through the reflex arc. 2m
1. Ref synapses
2. Vesicles containing transmitter only found in presynaptic neurone
3. Receptors for transmitter only found on post synaptic membrane
4. Ref to refractory period/ hyperpolarisation
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(O/N 07) Stimulus B resulted in an action potential. Describe what is occurring at C, D and E. 6m
C
1. Depolarisation/ inside (membrane) more positive
2. Sodium ions flow in
D
1. Repolarisation/ inside membrane more negative
2. Potassium ions flow out
E
1. Hyperpolarisation/ refractory period
2. More negative than resting potential
Suggest why stimulus A did not result in an action potential being produced whereas stimulus B did. 2m
1. A is a generator/ receptor potential difference
2. It does not overcome threshold potential to produce action potential
(O/N 11 43) Explain how the structure of a sensory neurone can enable the action potentials to reach the brain
quickly. 2m
1. Myelin sheath insulates axon
2. Idea of depolarisation/ action potentials only at nodes of Ranvier
3. Saltatory conduction
Refractory period: The time taken for the action potential to restore its resting potential after an action potential
An action potential is the change in the potential difference across an axon membrane which occurs during the
passage of the nerve impulse. There is a temporary reversal of the resting potential so that the inside is positive
compared to the outside.
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4: Synapse
(M/J 03) Describe the role of Calcium ions in synaptic transmission. 3m
1. Ref low Ca2+ in synaptic knob/ high Ca2+ outside knob
2. Action potential/ depolarization causes opening of Ca2+ channels
3. Ca2+ into synaptic knob
4. Causes vesicles to move towards presynaptic membrane
5. Vesicle contents/ transmistter/ exocytosis into synaptic cleft
(M/J 06) Outline the role of Ca2+ in synaptic transmission. 4m
1. Wave of depolarisation/ action potential, in pre-synaptic axon/ membrane
2. Ca2+ channels open
3. Ca2+ enter pre-synaptic neurone/ synaptic knob
4. Causes synaptic vesicles to move towards presynaptic membrane
5. Exocytosis of Ach
(M/J 06) Explain how a synapse one-way transmission of nerve impulses. 2m
1. Vesicles found only in pre-synaptic neurone
2. Receptors found only in post-synaptic membrane
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5: Differences
(O/N 04) State 3 difference between nervous and hormonal communication in mammals. 3m
1. Electrical vs chemical
2. Impulses along nerve cells vs hormones through blood
3. Rapid vs slow
4. Response immediate vs relatively slow
5. Responses short lived vs long lived.
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6: Essays
(O/N 09 41) Describe a reflex arc and explain why such reflex arcs are important. 7m
1. A strong stimulus is detected by the receptor.
2. An action potential is produced by sensory transduction and sent from the receptor along a sensory neurone
to the spinal cord where it enters by the dorsal root of spinal nerve.
3. Within the grey matter of the spinal cord, the sensory neurone forms a synapse with a relay neurone.
4. The nerve impulses are transmitted across the synapse.
5. From the relay neurone, the nerve impulses are transmitted across another synapse to a motor neurone.
6. The axon of the motor neurone leaves the spinal cord by the ventral root and conducts the impulses to
effector, resulting in a response.
8. Reflex actions are automatic, instinctive, non-voluntary, and fast.
9. Reflex arc is important to limit danger.
A reflex arc is the pathway along which impulses are carried from a receptor to an effector, without involving
conscious region of the brain.
Outline the functions of a sensory neurone and a motor neurone in a reflex arc. 6m
1. A sensory neurone has its cell body in the ganglion in the dorsal root of a spinal nerve. It has a very long
dendron that carries action potentials from a receptor towards its cell body and a shorter axon that carries
the action potentials into the spinal cord (or brain).
2. The ending of the dendron may be within a specialised receptor such as Pacinian corpuscle in the skin.
3. Pressure acting on the Pacinian corpuscle depolarises the membrane of the dendron and generates an action
potential. (In general, receptors transfer energy from a stimulus into energy in an action potential.)
4. The motor neurone has its cell body within the central nervous system (in the brain or the spinal cord). It has
many short dendrites and an axon much longer than dendrite. It will have many synapses, including several
with sensory neurones.
5. Thus the action potential from a sensory neurone can cross the synapse and set up an action potential in the
motor neurone, which will then transmit it to an effector such as a muscle or gland.
6. The action potential then causes the effector to respond, for example by contracting (if it is a muscle).
7. In a reflex arc, the impulses travel directly from the sensory to the motor neurone (or sometimes via an
intermediate neurone between them) without having to be processed in the brain.
8. This means the pathway from receptor to effector is as short as possible, so response can happen very quickly.
Acethycholinesterase splits each Ach molecule into acetate and choline.
The choline is taken back into the presynaptic neurone, where it is combined with acetyl co-enzyme A to form Ach
once more.
Ions enter neurone, not membrane.
(M/J 07) (O/N 09 42) Describe how a nerve impulse crosses a cholinergic synapse. 9m
1. When action potential reaches the presynaptic membrane, it causes Ca2+ channels to open in the presynaptic
membrane
2. Ca2+ flood into presynaptic, neurone/ knob R membrane
3. This causes vesicles of acetylcholine/ Ach to move towards presynaptic membrane to fuse with presynaptic
membrane
4. ACh released into synaptic cleft/ exocytosis of ACh
5. Ach diffuses across cleft
6. Ach binds to receptor (proteins) on postsynaptic membrane R neurone
7. This makes sodium ion channels in the postsynaptic membrane open
8. Sodium ions rush into postsynaptic neurone R membrane
9. This depolarises the postsynaptic membrane which sets up an action potential in the postsynaptic neurone
10. Acetylcholinesterase splits each Ach molecule into acetate and choline so that sodium channels will close.
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Describe the structure of a cholinergic synapse.
1. Synapses connect the axon of one neurone with the dendrite of another.
2. The two parts are separated by a gap called the synaptic cleft that is about 20 nm wide.
3. Synapses that use acethylcholine (ACh) as the transmitter substance are known as cholinergic synapses.
4. The pre-synaptic neurone has vesicles which contain ACh.
5. When action potential reaches the presynaptic membrane, it causes Ca2+ channels in the presynaptic
membrane to open
6. Ca2+ flood into presynaptic neurone/ knob R membrane
7. This causes vesicles of acetylcholine/ Ach to move towards presynaptic membrane to fuse with presynaptic
membrane
8. ACh released into synaptic cleft/ exocytosis of ACh
9. Ach diffuses across cleft
10. Ach binds to receptor (proteins) on postsynaptic membrane
(M/J 07) (O/N 09 42) Explain the roles of synapse in the nervous system. 6m
1. Ensure one-way transmission
2. Receptor proteins only in postsynaptic, membrane/ neurone
3. Vesicles only in presynaptic neurone
4. It prevents overstimulation which can damage an effector such as muscle. During intense stimulation, the rate
of release of neurotransmitters exceeds the rate of its synthesis. Here, the release of neurotransmitter stops.
5.
6.
7.
8.
Wide range of responses
Due to interconnection of many nerve pathways
Inhibitory synapses affect other synapses
It filters out low stimuli
9. Involved in memory and learning
10. Due to new synapses being formed
11. Synapse allows low frequency impulses to build up sufficient messenger substance over time that they trigger
a new impulse in the postsynaptic neurone- a process called summation.
6.1: Essays- Structure
(O/N 09 41) Describe the structure of a myelin sheath and explain its role in the speed of transmission of a nerve
impulse. 8m
1. Some nerve fibres are completely surrounded by myelin sheath formed by Schwann cells. Myelin sheath
consists mainly of lipid.
2. Myelin sheath speeds up the rate at which action potentials travel by insulating the axon membrane.
3. Ions cannot pass through the myelin sheath and depolarisation occurs only at nodes of Ranvier.
4. Local circuits are set up between nodes of Ranvier.
5. Action potential jumps from one node to the next; this form of impulse propagation known as saltatory
propagation, with conduction speeds of 100ms-1, being 50 times faster than in nonmyelinated neurones.
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(O/N 06) Describe how the structure of neurones speeds up the transmission of action potentials. 6m
1. Axon with large diameter/ large axon reduce resistance to nerve impulse and increase the speed of
transmission
2. Sensory neurone and motor neurone are wrapped by myelin sheath formed by Schwann cells. Myelin sheath
consists mainly of lipid.
3. Myelin sheath speeds up the rate at which action potentials travel by insulating the axon membrane.
4. Ions cannot pass through the myelin sheath and depolarisation occurs only at nodes of Ranvier.
5. Local circuits are set up between nodes of Ranvier.
6. Action potential jumps from one node to the next; this form of impulse propagation known as saltatory
propagation, with conduction speeds of 100ms-1, being 50 times faster than in nonmyelinated neurones.
Temperature affects the rate of diffusion of ions and therefore the higher the temperature, the faster the nerve
impulse. Above a certain temperature, the cell surface membrane proteins are denatured and impulses fail to be
conducted at all.
(M/J 10 42) Describe the structure of a myelinated sensory neurone. 7m
1. A sensory neurone has a cell body which contains a nucleus.
2. The cell body also contains many mitochondria and many RER/ nissl's granules.
3. It has a long dendron and a shorter axon
4. The terminal end of the axon is divided into a number of branches with swollen endings called synaptic knobs.
5. Each synaptic knob has many mitochondria and contains many synaptic vesicles. These vesicles contain
neurotransmitter substances such as acetylcholine.
6. Sensory neurone is wrapped by myelin sheath formed by Schwann cells.
7. Myelin sheaths consist mainly of lipids.
8. Between adjacent Schwann cells are small gaps called nodes of Ranvier.
(O/N 08) Describe the structure of a motor neurone. 7m
1. The motor neurone has a cell body which contains a nucleus, a nucleolus and other organelles such as
mitochondria, rough endoplasmic reticulum, and groups of ribosomes.
2. It has many short dendrites and an axon much longer than dendrite.
3. The terminal end of the axon is divided into a number of branches with swollen endings called synaptic knobs.
Each synaptic knob has many mitochondria and contains many synaptic vesicles. These vesicles contain
neurotransmitter substances such as acetylcholine.
4. The neuromuscular junction includes both the synaptic knob and the motor-end plate.
5. Motor neurone is wrapped by myelin sheath formed by Schwann cells.
6. Myelin sheaths consist mainly of lipids.
7. Between adjacent Schwann cells are small gaps called nodes of Ranvier.
Outline the functions of a sensory neurone and a motor neurone in a reflex arc. 6m
1. A sensory neurone has its cell body in the ganglion in the dorsal root of a spinal nerve. It has a very long
dendron that carries action potentials from a receptor towards its cell body and a shorter axon that carries the
action potentials into the spinal cord (or brain).
2. The ending of the dendron may be within a specialised receptor such as Pacinian corpuscle in the skin.
3. Pressure acting on the Pacinian corpuscle depolarises the membrane of the dendron and generates an action
potential. (In general, receptors transfer energy from a stimulus into energy in an action potential.)
4. The motor neurone has its cell body within the central nervous system (in the brain or the spinal cord). It has
many short dendrites and an axon much longer than dendrite. It will have many synapses, including several
with sensory neurones.
5. Thus the action potential from a sensory neurone can cross the synapse and set up an action potential in the
motor neurone, which will then transmit it to an effector such as a muscle or gland.
6. The action potential then causes the effector to respond, for example by contracting (if it is a muscle).
7. In a reflex arc, the impulses travel directly from the sensory to the motor neurone (or sometimes via an
intermediate neurone between them) without having to be process in the brain.
8. This means the pathway from receptor to effector is as short as possible, so response can happen very quickly.
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6.2: Essays- Resting and Action Potential
(O/N 11 41) How a resting potential is maintained in an axon. 9m
1. Axon phospholipid bilayer impermeable to K+/ Na+
2. The sodium- potassium pump, which is transmembrane and globular pumps, 3 Na+ out of the axon and 2 K+
into the axon using ATP
3. Na+ diffuse into the axon and K+ diffuse out of the axon down the electrochemical gradient
4. Through protein channels transport proteins
5. More K+ channels open than Na+ channels
6. Therefore membrane more permeable to K+/ more K+ leave than Na+ enter axon
7. Inside the neurone becomes relatively more negative than outside
8. - 65mV
9. Leaking of K+ responsible for resting potential
10. Voltage gated channels are closed.
(O/N 08) Explain how an action potential is transmitted along a motor neurone. 8m
1. Na+ channels open A sodium channels
2. Na+ enter cell R enter membrane
3. Inside becomes, less negative / positive / +40mV / depolarised
4. Na+ channels close A sodium channels
5. K+ channels open A potassium channels
6. K+ move out (of cell) R of membrane
7. Inside becomes, negative / repolarised A negative figure
5. Local circuit is set up between the depolarised region and the resting regions on either side of it
6. Myelin sheath insulate axon
7. Ions cannot pass through the myelin sheath and depolarisation occurs only at nodes of Ranvier
8. Action potential jumps from one node to the next; this form of impulse propagation known as saltatory
propagation
9. One-way transmission due to refractory period
(M/J 10 42) Explain how an action potential is transmitted along a sensory neurone. 8m
1. Na+ channels open, Na+ diffuses into the axon, bringing a change in potential difference across the
membrane.
2. The inside of the axon becomes more positive relative to the outside. Action potential is triggered.
3. At a point, depolarisation of the membrane causes Na+ channels to close
4. K+ channels open and K+ diffuses out from the axon and the membrane is repolarised
5. Local circuit is set up between the depolarised region and the resting regions on either side of it
6. Myelin sheath insulate axon
7. Ions cannot pass through the myelin sheath and depolarisation occurs only at nodes of Ranvier
8. Action potential jumps from one node to the next; this form of impulse propagation known as saltatory
propagation
9. One-way transmission due to refractory period
(O/N 06) Explain, using a named example, how sensory receptors in mammals convert energy into action potentials.
9m
1. When strong pressure is applied to the skin, the fibrous connective tissue of a Pacinian corpuscle is deformed.
The result is that the nerve ending is mechanically stretched.
2. This deformation causes Na+ channels to open, and Na+ flow in, resulting in depolarisation of the membrane.
3. K+ channels open
4. K+ leaves cell
5. The magnitude of receptor potential varies with the intensity of the stimulus
6. If receptor potential greater than threshold potential then action potential generated
7. If less than threshold only delocalised depolarisation
8. Increased stimulus strength leads to increased frequency of action potentials
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(O/N 11 41) Describe, using named examples, how sensory receptors in mammals generate action potentials. 6m
1. Sensory receptors detects stimuli
2. Some receptors are the ends of sensory neurones
3. More complex receptors known as are sense cells consist of epithelial cell able to detect stimuli.
4. Receptors are energy transducers. All receptors transform the energy of the stimulus into an electrical
response which initiates nerve impulses in the neurone leaving the receptor.
5. Stimuli cause sodium channels to open and sodium ions enter cell and depolarise the receptor
6. The magnitude of receptor potential varies with the intensity of the stimulus
7. If receptor potential greater than threshold potential then action potential generated
8. If less than threshold only delocalised depolarisation
9. Increased stimulus strength leads to increased frequency of action potentials
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7: Learning Notes
Refractory period: The time taken for the action potential to restore its resting potential after an action potential
Once an action potential has been created in any region of a neurone, there is a period afterwards when inward
movement of sodium ions is prevented because the sodium voltage-gated channels are closed. During this time it is
not possible for a further action potential to be generated. This is known as the refractory period.
The refractory period is made up of two portions:
 The absolute refractory period lasts for about 1ms, during which no new impulses can be passed, however
intense the stimulus.
 The relative refractory period lasts around 5 ms, during which a new impulse may be propagated provided the
stimulus exceeds the normal threshold value. The degree to which it needs to exceed the threshold value
diminishes over the period.
The refractory period serves two purposes:
1. The action potential cannot be propagated in the region that is refractory, i.e. it can only move in a forward
direction. This prevents the action potential from spreading out in both directions, which it would otherwise
do.
2. Because a new action potential cannot be formed immediately behind the first one, it ensures that action
potentials are separated from one another and therefore limits the number of potentials that can pass along a
neurone in given time.
Threshold value is a certain level of stimulus which triggers an impulse.
How can an organism determine the size of a stimulus? This is achieved in two ways:
1. By the number of impulses passing in a given time. This is known as frequency coding. The larger the stimulus,
the more impulses that are generated in a given time.
2. By having different neurones with different threshold values. The brain interprets the number and type of
neurones that pass impulses as a result of a given stimulus and thereby determine its size
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