Topic 8 Grey Matter model answers

Model answers from all the past papers (June 2013)

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UNIT 5 TOPIC 8: Grey Matter
Nerve Impulses and Neurones
Suggest one neurotransmitter substance that might bind to receptors of cardiac muscle cells in
the SAN (1) noradrenaline, acetylcholine, norepinephrine.
Describe the events that begin the depolarisation of the membrane of a neurone. (2)
First there is *depolarisation of the adjacent membrane which leads to *changes in potential
difference across the membrane. These changes leads to the *opening of the sodium gates so
*sodium ions move into the neurone.
What ions can pass across the membrane at
A? Sodium ions
What ions can pass across the membrane at
B? Potassium ions
Give an explanation for the movement of
ions at position C on the diagram. (3)
*The voltage dependent sodium channels close
and the voltage dependent *potassium
channels open, so there is *increased
permeability of the membrane to potassium ions. Potassium ions flow out of the axon down the
electrochemical gradient (attracted by the negative charge outside the cell membrane) and they also
diffuse down the concentration gradient.
Explain how the potential difference across the membrane is returned to the resting level in the
time between 1.5 ms and 4.0 ms on the diagram. (3)
The *PD becomes less negative because the *membrane remains permeable to potassium ions
which *move into the nerve cell/neurone/axon because of the *charge difference. The *potassium
ion is removing a positive charge from the outside so that *equilibrium is established e.g. diffusion
gradient balanced by potential difference.
Eugenol can be used to reduce pain. Suggest an explanation for how eugenol affects the
movement of sodium ions and reduces pain. (6)
There will be a *higher concentration of Na+ outside of neurone with *sodium ions moving in to cause
depolarisation and the pain. *Eugenol may affect Na+/voltage-dependent gates so that the *Na+
influx is reduced, so *depolarisation is less likely to occur. *No impulse is transmitted along the
neurone so there is *no transmission of an impulse to the next neurone so the *pain is not being
sensed as the impulse is stopped before is enters the CNS.
Suggest an explanation for how eugenol affects the movement of calcium ions to reduce
pain.(6)
* Ca2+ does not enter the presynaptic membrane due to *Ca2+ channels not opening/decreased
sensitivity of membrane to Ca2+, this means *fewer vesicles move towards/fuse with the
presynaptic membrane and so *less neurotransmitter is released into/less diffuses across the
synaptic cleft. This means *less neurotransmitter can bind to receptors on the post-synaptic
membrane *reducing depolarisation/less Na+ or cation channels open so the *threshold intensity/

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Pain is not sensed as the impulse stopped before entering the
CNS/ leaving the sensory neurone.
Explain how the structure of the axon cell membrane is related to the conduction of nerve
impulses. (3)
*The phospholipid membrane restricts ion movement so that they can only move in/out by the
*proteins that span the membrane. *The protein gates/channels allow diffusion/movement of ions,
e.g. *the sodium potassium pump moves ions
Explain how myelination increases the speed of transmission of nerve impulses in a sensory
neurone.…read more

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There is no myelin sheath at the nodes of Ranvier so that
*depolarisation can occur there. At the nodes the impulse will jump from node to node (salutatory
conduction).
Part A is a: SENSORY NEURONE
Part B is a: RELAY NEURONE
Part C is a: MOTOR NEURONE
Part D is a: SYNAPSE
Describe and explain the sequence of events that occurs at the synapse, after a
neurotransmitter has been released.…read more

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The Eye
Suggest why the pupil appears black (1)
*Pigment at back of eye absorbs light/no light is reflected out (from the choroid)
There are two sets of iris muscles, the radial muscles and the circular muscles. They work
antagonistically to alter the size of the pupil. Explain why these two sets of muscles need to be
antagonistic.…read more

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Because *lack of visual stimulation during the *critical period limits brain development *due to a lack
of connections *within the visual cortex. *This means that the brain cannot interpret this visual
information correctly. *Mice are also different from humans.
Suggest why this type of treatment for blindness in humans could be regarded as
controversial. (2)
*The embryo supplies cells to the new host, but *some people may have ethical objections to the
use of embryonic cells because it has a right to life.…read more

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Suggest why the environment should be kept constant in this investigation. (2)
To * reduce the number of variables/to keep all variables constant and to ensure that *only the
effect of the treatment is measured.
When light reaches a rod cell the voltage across the cell surface membrane can change. This
can lead to the formation of an action potential in an optic neurone. Describe how light causes
a change in the voltage across the cell surface membrane of a rod cell.…read more

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Plants
How can the active form of phytochrome (Pfr) be converted back to the inactive form (Pr), other
than by exposing it to far red light? Darkness
Compare plant coordination with hormonal coordination in animals. (4)
*Both involve chemicals that are *transported away from the production site, however the
mechanism of transport is different e.g. diffusion in plants, blood system in animals.…read more

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Acts as a control and can be used in *comparison e.g. to show that flowering would not happen
without the cover.
Using your own knowledge of photoreceptors, explain the results of these investigations.
What do they suggest about the control of flowering in cocklebur plants? (4)
*Six hours too short to cause flowering in plant E but *eight hours is long enough to
cause flowering.…read more

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Acetylcholine is a chemical which acts as a neurotransmitter. The drug MDMA (ecstasy)
changes behaviour by increasing the concentration of serotonin in brain synapses.
Compare the use of computed tomography (CT) with magnetic resonance imaging (MRI) for
studying brain structure. (3)
*MRI produces a clearer image than CT scans, as CT *can only identify larger/main structures whilst
MRI can identify smaller structures, both will *identify soft tissue of the brain though. *MRI uses radio
waves whilst CT uses X-rays.…read more

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Computed tomography (CT) and functional magnetic resonance imaging (fMRI) are used to
investigate brain structure and function. The CT scans below show two different human brains
with abnormal areas. These areas are indicated by arrows.
Describe how these images could help a doctor to determine appropriate treatment of the
abnormalities. (2)
From these images you would know the *nature of abnormality e.g. caused by bleeding and its
density. You would also know the *location and *size of the abnormality.…read more

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