Topic 8: Grey Matter

Question 1

Resting Membrane Potential of a neuron

Answer 1

• The outside of the membrane is positively charged compared to the inside because there are more sodium ions outside the cell than there are on the inside.
• This means the membrane is polarised (has a potential difference/voltage). The voltage of the membrane in about -70.
• The resting membrane potential is primarily established by a sodium-potassium pump which works by active transport (uses ATP) to move three sodium ions (3Na+) outside of the neuron for every 2 potassium ions (2K+) which move into the neurons.

This process sets up 2 concentration gradients:
1. There is an inward concentration gradient for sodium - high conc. outside, low conc. inside. Sodium ion channels are primarily shut at this stage, which maintains this gradient.
2. There is an outward concentration gradient for potassium - high con. on inside, low conc. on outside. Potassium ion channels are mainly shut a this stage, which maintains this gradient.

Question 2

Describe what happens when an action potential reaches a neuron

Answer 2

1. A stimulus to the neuron triggers the opening of sodium ion channels. If the stimulus is above the threshold for that neuron then it will trigger more sodium ion channels to open (positive feedback).

Depolarisation:
When the sodium ion channels open, sodium ions diffuse into the cell down their electrochemical gradient (+ve to -ve). and concentration (high to low) gradients. This makes the inside of the neuron less negative.

Repolarisation:
At a potential difference of around +40mV*, the voltage-gated sodium ion channels close and voltage-gated potassium channels open. Potassium will now diffuse out of the neuron down the electrochemical ion concentration gradient. The inside of the cell becomes more negative again.

Hyperpolarisation
Potassium ion channels are slow to close so there’s a slight ‘overshoot’, in which the neuron becomes more negative than the resting potential because too many K+ ions leave the cell.

Refractory Stage
During repolaristaion and hyperpolarisaton the neuron cannot receive another stimulus (sodium ion channels cannot open again until the resting potential has been re-established)

The resting membrane potential & concentration gradients of Na+ & K+ are re-established using the sodium-potassium pump.

Question 3

What is an action potential

Answer 3

An action potential is a wave of positive charge which moves along the axon. It is an all-or-nothing response because it is only one size (+40mV). If the body wants to communicate a stronger message then it needs to either:
- Stimulate more neurons
- Send the message more frequently

Question 4

Explain how an action potential moves along an axon

Answer 4

The action potential travels along the neuron as a wave of depolarisation - some sodium ions that enter the neuron diffuse sideways, which causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part changing the potential differnece, thus creating local currents - thus stimulating another action potenial.

Question 5

Why is the refractory stage important

Answer 5

• Allows time for the neurone to reset.
• Ensures that action potentials don’t overlap.
• Ensures that action potentials are unidirectional (can pass in one direction only).

Question 6

Motor Neurones

Answer 6

Structure:
- Many short dendrites. Cell body at one end. One long axon carries nerve impulses from the cell body to effector cells.

Function
- Involved in transmitting electrical impulses from the CNS to muscles & glands in the body

Question 7

Sensory Neurones

Answer 7

Structure:
- One long dendron carries nerve impulses from receptor cells to the cell body, which is in the middle of the neurone. One short axon carries nerve impulses for the cell body to the CNS.

Function:
- Transmits impulses from receptors to the CNS

Question 8

Relay neurones

Answer 8

Structure:
- Many short dendrites carry impulses from sesnory neurones to the cell body. An axon carries nerve impulses from the cell body to motor neurones.
No myelin sheath

Function:
- Located within the CNS, involved in transmitting the electrical impulses from sensory neurones to motor neurons.

Question 9

General structure & function of neurones

Answer 9

Neurones help co-ordinate communication within the nervous system
They all have a cell body composed of a nucleus as well as organelles such as mitochondria in the cytoplasm. These provide energy (in the form of ATP) needed for active transport of ions into & out of the cell in an impulse. They also contain dendrites (extensions involved into conducting imulses towards the cell body), and axons (which conduct impulsess away from the cell body).

Question 10

Detail the role of myelination?

Answer 10

Myelination is a layer of fatty substance/lipid formed from schwann cells wrapped around the neurone - can increase the speed of impulses by acting as an electrical insulator, and allowing impulses to travel by saltatory conduction.

Question 11

What is tho role of myelination in saltatory conduction

Answer 11

Myelination plays a key role in saltatory conduction byinsulating axons and allowing action potentials to “jump” from one node of Ranvier to the next, which speeds up the transmission of nerve impulses.

Question 12

Describe the process of synaptic transmission

Answer 12

1. An action potential triggers a calcium influx:
   When an AP arrives at the presynaptic membrane the depolarisation stimulates voltage-gated calcium ion channels to open. Calcium ions enter the neurone
2. Calcium influx causes Neurotransmitter Release:
   The influx of calcium ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic memberane. They then fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft via exocytosis.
3. The Neurotransmitters trigger an Action potential in the postsynaptic neurone:
   The neurotransmitters diffuse across the synaptic cleft and binds to specific receptors of the postsynaptic membrane, stimulating the opening of sodium ion channels in the postsynaotic neurone. The influx of sodium ions into the postsynaptic membrane causes depolarisation and an action poetential is genereted on the postsynaptic membrane if the threshold is reached.

Synaptic transmission is contolled with the help of digestive enzymes in the synaptic cleft, which break down excess neurotransmitters to prevent overstimulation on the postsynaptic membrane. These are then taken up by the pre-synaptic membrane and reused. Re-uptake of NTs, the presence of receptors on one side of the synapse only & the refractory period serves to ensure that the AP is unidirectional.

Question 13

Acetylcholine

Answer 13

A neurotransmitter involved in muscle contraction & the control of heart rate

Question 14

The role of neurotransmitters: excitatory vs inhibitory

Answer 14

Excitatory neurones stimulate/the production of an action potential in the postsynaptic membrane by stimulating theopening sodium ion channels.
Inhibitory neurones cause chloride ions to open in the postsynaptic membrane, so negative chloride ions enter, thus causing hyperopolarisation of the postsynaptic membrane. Therefore triggering a new action potential becomes difficult.

Whether or not an action potential is triggered in the post-synaptic membrane depends on the summation of excitatory & inhibitory neurones and if voltage reaches the threshold for that neurone.

Question 15

What is Synaptic divergence?

Answer 15

Impulse diverge in when one neurone connects to many neurones alllowing info to be dispersed to different parts of the body.

Question 16

What is synaptic convergence?

Answer 16

When many neurones connect to one neurone so info is amplified (made stronger).

Question 17

8.5) Describe how light travels through the eye

Answer 17

Light enters the eye through the pupil and the amount of light entering is controlled by the muscles of the iris. The lens (the shape of which is controlled by ciliary muscles) of the eye focuses light on the retina where the photoreceptors are located, specifically in the fovea. Subsequently, the nerve impulses recieved by the photoreceptors cells are then carried via the optic nerve to the brain. The point where the optic nerve leaves the eye is known as the blind spot so there are no photoreceptor cells there.

Question 18

Photoreceptors

Answer 18

Photoreceptors are light receptors in the eye. The retina contains two types of photoreceptors:

Rod cells:
- Located around the outer retina
- Sensitive to light intensity (detects presence and brightness of light)
- Only produce monochromatic vision (black and white)

Cone cells:
- Grouped together in the fovea
- Sensitive to different wavelengths of visible light
- generates coloured images

Question 19

What are receptors?

Answer 19

Receptors are specialised cells that can generate an electrical impulse in a sensory neurone when stimulated by a particular stimulus.

Question 20

Rod and cone cells differ due to the type of vision they provide.
Explain how else to the two photoreceptors differ.

Answer 20

They differ by their level of sensitivity - Cones can only work in bright conditions whereas rods are much more sensitive and dim light in sufficient for them to work. This is brought about as a result of convergence - three rods converge onto a single bipolar cell (connecting photoreceptors to the optic nerve) - this acts to amplify the signals triggered by light. Whereas, only one cone cell synapses with each bipolar neurone - so there is no amplification of the signal - making these photoreceptors less sensitive.

Question 21

The action of rod cells in the dark

Answer 21

In the dark, the rods aren’t stimulated: Sodium ions diffuse into the cell through open sodium ion channels whilst also being actively pumped out of the cell. As a result the inside of the cell is only slightly more negative compared to the outside, thus causing the membrane to be slighlty depolarised. Consequently calcium ion channels open in the presynaptic membrane and Ca2+ ions diffuse into the rod cell causing the release of a neurotransmitter called glutamate. Glutamate serves to inhibit the bipolar neurone by preventing sodium ion channels from opening in the post synaptic memebrane, so depolarisation doesn’t occur and an action potential isn’t triggered. It creates an IPSP (a temporary hyperpolarisation in the post-synaptic membrane) and as a result no information is transmitted to the brain.

Question 22

The action of rod cells in the light

Answer 22

Rods contain a light-sensitive pigment called rhodopsin which absorbs light energy. In the presence of enough light intensity, rhodopsin splits into retinal & opsin. Retinal changes structure from cis retinal to trans retinal, which activates opsin. Opsin binds to the membrane of the rod cell thus causing sodium ion channels to close without affecting their active removal. Because sodium ions cannot diffuse back in, the membrane become hyperpolarised. As a result, Calcium ion channels don’t open in the pre-synaptic celft and no neurotransmitter in released into the synaptic cleft. Consequently sodium ion channels remain open in the post-synaptic cleft and sodium ions can diffuse in. Once a threshold is reached more sodium ion channels open and an action potential is generated in the bipolar neurone.
Opsin is reconverted to rhodopsin afterwards using enzymes, energy and vitamin A. Trans retinal&raquo_space; cis retinal.

Question 23

Why do rod cells contain lots of mitochondria?

Answer 23

To provide ATP for resynthesis of rhodopsin and active transport of Na+ ions.

Question 24

8.8) Cerebrum

Answer 24

Largest part of the brain composed of two halves - cerebral hemispheres. Involved in controlling vision, thinking, learning & emotion as well as voluntary control of the body. Different parts of the cerebrum have different functions:
- Frontal Lobe: Thinking & decision making
- parietal lobe: controls sensory info (touch, spatial awareness)
- Occiptial Lobe: processes visual info
- Temporal Lobe: processes auditory info

Question 25

8.8) Cerebellum

Answer 25

Located underneath the cerebrum & plays and important role in co-ordinating muscle movements as well as balance.

Question 26

8.8. Corpus callosum

Answer 26

A band which connects the two cerebral hemispheres

Question 27

Hypothalamus

Answer 27

Found just beneath the middle part of the brain is involved in thermoregulation as well as production of hormones involved in the control of the pituitary gland.

Question 28

Medulla Oblangata

Answer 28

Located at the base of the brain controls many vital body processes such as breathing, heart rate & blood pressure.

Question 29

CT scans

Answer 29

Use X-rays to produce cross-section images of the brain. Denser structures absorb more radiation than less dense structures and therefore they show up lighter on the scan. Produces 2D & 3D images.

Strengths:
- Can be used for medical diagnosis which show up damaged/abnormal areas.
- Generally cheaper than MRI scanners.

Limitations:
- Static images - shows structure but not function, which have to be worked out through behavioural changes.
- Lower quality images & weaker resolution than MRI scanners.

Question 30

MRI

Answer 30

Magnetic Resonance Imaging scanners use magnetic field and radio waves for imaging soft tissue.

Strengths:
- Can be used for diagnosis as diseased tissue can be seen.
- Can determine the exact size and location of tumours as they show up a lighter colour.
- Gives 2D/3D images
- Higher quality images for soft tissue than CT. Better resolution.

Limitations:
- Gives static image - only shows structure, function has to be worked out.
- MRI scanners cost a lot more than a CT scanner.
- Not good for people with metallic implants or pace makers.

Question 31

FMRIs

Answer 31

Uses magnetic fields & monitors oxygen uptake to show brain activity.

Strengths:
- Shows abnormal activity in the brain, which can be used for medical diagnosis.
- Shows live activity - can investigate structure and function.
- Detailed, high resolution images

Weakness:
- Expensive
- Safe, painless and non-invasive
- May not be appropriate for people with metal or electronic implants.

Question 32

PET scans

Answer 32

Positron emission tomography scans used radioactive isotopes with a short half-life to monitor areas of activity in the brain. Follows the flow of blood (and thus radioisotopes) through the brain and can be used to monitor activity whilst performing different activities. it is detected by the emission of positrons as the radioisotopes decay.

Strengths:
- Very detailed
- Investigates structure & function in real-time (live image).

Limitations:
- Invasive

Question 33

Hubel and Wiesel conducted experiments on animals to investigate the structure and development of the visual cortex.
i) Describe Hubel and Wiesel’s experiment on kittens (3 Marks)
ii) Explain what their experiment showed about the development of the human visual system. (1 Mark)
iii) Do their experiments give evidence for a critical period in the development of the human visual system? Explain your answer, with reference to what is meant by a critical period (2 Marks)

Answer 33

i) Hubel and Wiesel stitched shut one eye of very young kittens for several months (1 mark). When they unstitched the eyes, they found that the kitten’s eye that had been stitched up was blind (1 mark). They also found the ocular dominance columns that were stimulated by the open eye had become become bigger and taken over the ocular dominance columns that weren’t visually stimulated for the shut eye - switched dominance (1 mark).

ii) Hubel and Wiesel’s experiment showed that the visual cortex only develops properly if both eyes are visually stimulated in the very early stages of life. (1 mark)

iii) Yes, their experiments give evidence for a critical period in the development of the human visual system because our visual cortex is also made up of ocular dominance columns (1 mark). The critical period is the period of time early in life when it’s critical that you’re exposed to the right visual stimuli for the visual system to develop properly (1 mark).

Question 34

Where in the brain are ocular dominance columns found? What are they and how are they arranged?

Answer 34

Ocular dominance columns are found in the visual cortex (an area of the cerebral cortex at the back of the brain). They are groups of neurons, which receive visual info from either the left or the right eye. They are arranged in an alternating pattern (left,right,left,right) across the visual cortex.

Question 35

How have scientists investigated visual development in humans?

Answer 35

Cataracts

Question 36

Describe the role of a critical period in the development of the visual system in cats and other mammals.

Answer 36

Critical period is the period in early life of animals (including humans) when, for the
proper development of visual cortex, exposure to visual stimuli is critical.
● Baby mammals have lots of neurons (synapses) in the visual cortex, which needs to be
properly organised to process visual information.
● Synapses that do not receive the visual signal during the critical period are destroyed
and the rest retained (strengthened) developing visual cortex properly.
This means if eyes are not stimulated with visual info during this critical period of development, the visual cortex will not develop properly as many of the synapses will be destroyed.

Question 37

Discuss the use of animals in medical research

Answer 37

Arguments AGAINST using animals:
- Animals are often different from humans, so drugs tested on animals may have different effects on humans.
- Experiments can cause pain & distress to animals
- There are alternatives to using animals in research e.g. using cultures of human cells.
- Some people think that animals have the right to not be experimented on. They cannot consent.

Arguments FOR using animals
- Utilitarianism argument - the benefits (saving many human lives) may outweigh the costs (harm to animals).
- Animals are similar to humans in certain aspects, so research has led to lots of medical breakthroughs.
- Animal experiments are only done when its absolutely necessary & sceintists follow strict rules e.g. animals must be properly looked after, painkiller & anaesthetics must be used to minimise pain.
- Using animals is currently the only way to the study how a drug affects the whole body. e.g. cell cultures can’t be used to sudy behavioural effects.
- Some people think that humans have a greater right to life than animals because we have more complex brain (i.e. compared to rats, fish & fruit flies).

Question 38

Describe the role of visual stimulation to the development of the visual cortex during the critical period

Answer 38

• Oscular dominance columns (develop in visual cortex) (1)
• Neurones form synapses with these (cells/columns) (1)
• (Stimuli/action potentials/impulses) along neurones required to strengthen connections (with cells of ocular dominance columns)
• Stimulation during the critical period is needed to form (effective) connections in the visual cortex.

Question 39

Absolutism

Answer 39

The belief that either animals always should or should not be used.

Question 40

Utilitarianism

Answer 40

The belief that animals should be used in research only when the overall expected benefits of the study outweigh the harm done to the animals.

Question 41

Nature

Answer 41

Genes

Question 42

Nurture

Answer 42

Environment

Question 43

Explain why it is difficult to determine whether a characteristic is influenced more by nature or nurture

Answer 43

• Genetic & environmental factors interact therefore they are hard to seperate in order to investigate the role of one in isolation.
• There are lots of different genes and lots of different environmental factors to investigate.

Question 44

State the 5 different methods used to investigate the role of nature and nurture in brain development

Answer 44

1. Animal experiments
2. Twin studies
3. Cross-cultural studies
4. Newborn Studies
5. Brain Damage studies

Question 45

Explain how animal experiments are used to investigate the role of nature/nurture in brain development

Answer 45

• Animals of the same species are likely to be genetically similar, therefore when placed in different environments any difference in brain development are likely to be due to nurture.
• Animals can be genetically engineered to lack a particular gene and then raised with individuals who have the gene in a similar environment. Any differences are likely to be due to nature.

Question 46

Explain how twin studies are used to investigate the role of nature/nurture in brain development

Answer 46

Identical twins are genetically identical. If identical twins are raised seperately then they’ll have identical genes but different environments.
- Any similarities are likely due to nature.
- Any differences are likely due to nurture.

Question 47

Explain how cross-cultural studies are used to investigate the role of nature/nurture in brain development

Answer 47

Children brought up in different cultures have different environmental influences, e.g. beliefs and education. Scientists can study the effects of different upbringings on brain development by comparing large groups of children who are the same age but from different culutres.
- Major differences in characteristics are likely due to nurture
- Any similarities in brain development between dif. cultures are likely due to nature

Question 48

Explain how newborn studies are used to investigate the role of nature/nurture in brain development

Answer 48

A newborn babies brain hasn’t really been affected by the environment. This means scientists can see which aspects of brain development are more likely to be due to nature than nurture.
- Functions that babies are born with are likely to be due to nature.
- Areas of the brain/functions that take time to develop (i.e speech) are more likely to be due to nurture.

Question 49

Explain how brain damage studies are used to investigate the role of nature/nurture in brain development

Answer 49

Aa children brains are still developing, scientists can study the effects of brain damage on their development by comparing the development of a chosen function in children with and without brain damage.
- If the characteristic still develops in children with brain damage, then brain development is more likely to be due to nurture.
- If the characteristic doesn’t develop in children with brain damage, then it is more likely due to nature for that characteristic (because nurture isn’t having an effect).

Question 50

Tropism

Answer 50

Tropism is a plants growth response to external stimulus

Question 51

Phototrpoism

Answer 51

The growth of a plant in response to light
Shoots = positvely phototropic (grow towards light)
Roots = Negatively phototropic (grow away from light)

Question 52

Geotropism

Answer 52

The growth of a plant in response to gravity
Shoots = Negatively geotropic and grow upwards
Roots = positively geotropic and grow downwards

Question 53

Indoleacetic Acid (IAA)

Answer 53

IAA is an important auxin (growth factor) in plants
- produced in the tips of shoots
- regulates the trancription of genes related to cell elongation and growth
- controls tropism
- moves by diffusion and active transport over short distances, and via the phloem over longerr distances
- They promote cell elongation in the shoots and inhibit growth in the roots.

In phototropism:
IAA moves to the more shaded parts of the shoots and roots causing uneven growth. They cause cells to elongate in the shoots (by increasing the transcription of genes related to cell elongation) so they bend towards light. They inhibit growth in the roots (by inhibiting the transcription of genes related to growth) so they bend away from light.

In geotropism:
IAA moves to the underside of shoots and roots so there’s uneven growth. Cause shoots to grow upwards and roots to grow downwards

Question 54

Cell elongation

Answer 54

IAA can cause cell elongation - as opposed to a physical growth - CE is brought about by the ‘loosening’ of cell walls, which go on to take in water by osmosis & stretch (elongate). Protons pumped into the cell wall from the cell cytoplasm disrupt bonds between cellulose molecules, allowing the cell wall to be stretched.

Question 55

Explain the role of phytochromes in plant flowering and germination in short day plants compared to long day plants

Answer 55

Photochromes are a type of photoreceptor. They are blue-green pigment molecules which absorb light. They regulate the transcription of genes involved in flowering.
Two types:
Pfr = Active; absorbs far red light (which is present at night)
Pr = Inactive; absorbs red light; red light is more predominant (day light)

Pr is converted into Pfr when it’s exposed to red light (in the day time) and vice versa.

Summer = long days, short nights
winter = short days, long nights

short day plants: flower in the winter, inhibited by Pfr
Summer: Short nights mean Pfr doesn’t have enough time to be degenerated into Pr. Pfr accumulates inhibting flowering (by inhibiting transcription of genes responsible for flowering).
Winter: Long nights means that Pfr has enough time to be degenerated back into Pr. No accumulation of Pfr means that flowering is no longer inhibited (genes responsible for flowering are transcribes).

Long Day plants: Flower in summer, promoted by Pfr
Summer: Short nights means that pfr doesn’t have enough time to degenerate into pr. Pfr accumulates and promotes flowering.
Winter: Long nights means that pfr is fully regenerated into pr. Not enough pfr to promote flower growth.

Question 56

i) What is mean by ‘habituation’?
ii) What are the advantages of habituation?

Answer 56

i) Habituation is a phenomenon where an organism becomes insensitive to repeated stimuli over time which does not threaten their survival or does not benefit them in any way. It is a type of learned behaviour where animals learn to ignore unimportant stimuli.

ii) Habituation means that animals don’t waste energy responding to unimportant stimuli. It also means that they can spend more time doing other activities for their survival, such as feeding.

Question 57

Describe the process of habituation

Answer 57

Habituation means fewer electrical impulses are sent to the effectors.
1) Repeated exposure to a stimulus causes calcium channels in the presynaptic cell to become less responsive, decreasing the amount of calcium ions that enter the presynaptic cell
2) The decrease in influce of Ca+ ions means that less neurotransmitter is released from vesicles into the synaptic cleft, so fewer neurotransmitters can bind to receptors in the postsynaptic membrane.
3) Fewer sodium ion channels on the postsynaptic membrane open - as a result lest depolarisation occurs, so there is a reduced chance of the threshold for an action potential being reached on the post-synaptic membrane.
4) As a result, fewer signals are sent to the effector to carry out the response.

Question 58

CORE PRACTICAL 18: Investigating habituation to a stimulus

Answer 58

IV: Number of touches (1-10)
DV: Time taken for the snail to re-emerge from it’s shell (seconds)
CVs: Use the same snail, touch in the same place, keep in stable environment

Method:
1. With a damp cotton wool bud touch snail between the eyebuds.
2: Time the length of time it takes for the snail to emerge fully from its shell again.
3: Repeat for a total of 10 touches.

Graph: plot a graph of number of touches against time to re-emerge from shell and use spearman’s Rank correlation co-efficient to determine whether the correlation between the two varaibles are statistically significant.

Conclusions:
- Negative correlation: As number of touches increases, time taken for the snail to re-emerge decreases because the snail becomes habituated to the stimuli. This is because channels in the presynaptic membrane become less responsive to the stimuli over time. So less neurotransmitter is released into the synaptic cleft. This is because there is evolutionary advantages not to use resourced on responding to a benign stimulus.

Notes:
It’s important to treat snails ethically and re-release them.
Safety: Biohazard - wash hands & use disinfectant.

Question 59

What is parkinson’s disease?

Answer 59

• A brain disorder that affects motor skills
• Due to a loss of dopaminergic neurones in the cerebral cortex of brain. Neurones in the part of the brain that control movement are destroyed resulting in a lack of dopamine.
• Less dopamine is released into the synaptic clefts, so less dopamine is available to bind to receptors on the post-synaptic membranes.
• Fewer sodium ion channels on the postsynaptic membrane open, so the postsynaptic cell is less likely to depolarise.
• This means that fewer action potentials are produced, leading to symptoms like tremors and slow movement.
• Scientists have developed drugs to increase the level of dopamine in the brain.

Question 60

L-Dopa

Answer 60

L-Dopa is a dopamine precursor used to treat parkinson’s disease. It has a similar structure to dopamine & is able to cross the blood-brain barrier & pass into the brain, where it is converted into dopamine by an enzyme. This increases dopamine levels in the brain to replace the neurotransmitter lost by death of neurons. Therefore more nerve impulses occur in along the neurones in the brain regions involved in movement.

Question 61

Why can’t dopamine be given directly to patients suffering from parkinson’s disease?

Answer 61

Dopamine is too large to cross through the blood brain barrier.

Question 62

Describe the link between serotonin and depression

Answer 62

Correlation between low serotonin levels and depression. Serotonin transmits nerve impulses across synapses in parts of the brain that control mood. Depression may be caused by low serotonin levels due to a problem with post-synaptic receptors (i.e. not enough) or other linked variables.

Evidence shows that drugs, which increase serotonin levels in synaptic cleft e.g. selective serotonin re-uptake inhibitors (SSRIs) are effective treatments.

Question 63

How do SSRIs work?

Answer 63

SSRIs work by binding to serotonin re-uptake proteins on the presynaptic membrane, preventing them from re-absorbing serotonin. More serotonin is available in the synaptic cleft to bind to receptors on the post-synaptic cell.

Question 64

MDMA

Answer 64

Chemical in ecstasy. MDMA increases the level of serotonin (a neurotransmitter that regulates mood) by inhibiting the reuptake of serotonin into presynaptic neurones - it binds to and blocks the reuptake proteins on the presynaptic membrane. MDMA also triggers the release of serotonin from presynaptic nuerones. This means that serotonin levels stay high in the synaptic cleft and cause depolarisation of the postsynaptic neurone in parts of the brain that control mood. The effect of MDMA is mood elevation.

Question 65

Outline how the nervous system responds to a stimuli

Answer 65

1. Receptor cells detect changs in the enivronment or stimuli.
2. Nerve impulses travel from receptor cells along sensory neurones to the CNS.
3. The CNS (containing relay neurones) acts as a co-ordinating centre for the impulse that arrives from the receptors, determining which part of the body needs to respond and sends out a new set of impulses along motor neurones.
4. Motor neurones send impulses to the effectors (muscles or glands) to bring about a response.

Question 66

Describe what happens to the pupil in bright light

Answer 66

1. Photoreceptors detect change in environment (bright)
2. Radial muscles relax
3. Circular muscles contract.
4. Pupil constricts (diameter of pupil narrows).
5. Less light enters the eye.

Question 67

Describe what happens to the pupil in dim light

Answer 67

1. Photoreceptors detect change in environment (dark).
2. Radial muscles contract
3. circular muscles relax
4. pupil dilates (diameter of pupil widnes)
5. More light enters the eye