Topic 8 Flashcards
How does Responding to their environment help organisms survive
- Animals increase their chances of survival by responding to changes in their external environment, e.g. by avoiding harmful environments such as places that are too hot or too cold.
- They also respond to changes in their internal environment to make sure that the conditions are always optimal for their metabolism (all the chemical reactions that go on inside them).
- Plants also increase their chances of survival by responding to changes in their environment (see p. 184).
- Any change in the internal or external environment is called a stimulus.
How do Receptors detect stimuli and effectors produce a response
- Receptors detect stimuli — they can be cells or proteins on cell surface membranes. There are loads of different types of receptors that detect different stimuli.
- Effectors are cells that bring about a response to a stimulus, to produce an effect. Effectors include muscle cells and cells found in glands, e.g. the pancreas.
- Receptors communicate with effectors via the nervous system (see below) or the hormonal system (see the next page), or sometimes using both.
How does the nervous system send information as electrical impulses
1) The nervous system is made up of a complex network of cells called neurones.
There are three main types of neurone:
* Sensory neurones transmit electrical impulses from receptors to the
central nervous system (CNS) — the brain and spinal cord.
* Motor neurones transmit electrical impulses from the CNS to effectors.
* Relay neurones transmit electrical impulses between sensory neurones and motor neurones.
- A stimulus is detected by receptor cells and an electrical impulse is sent
along a sensory neurone. - When an electrical impulse reaches the end of a neurone chemicals called neurotransmitters take the information across to the next neurone,
which then sends an electrical impulse - The CNS processes the information and sends impulses along motor neurones to effectors.
- You need to know how your eyes (and the eyes of other mammals) respond to dim light
(to help you see better) or bright light (to protect them):
How does the hormonal system send information as chemical signals?
1) The hormonal system is made up of glands and hormones
2. Hormones are secreted when a gland is stimulated
3) Hormones diffuse directly into the blood, then they’re taken around the body by the circulatory system.
4. They diffuse out of the blood all over the body but each hormone will only bind to specific receptors for that hormone, found on the membranes of some cells (called target cells).
5. The hormones trigger a response in the target cells (the effectors).
What is a gland?
A gland is a group of cells that are specialised to secrete a useful substance, such as a hormone. E.g. the pancreas secretes insulin.
What are hormones?
Hormones are ‘chemical messengers’. Many hormones are proteins or peptides, e.g. insulin. Some hormones are steroids, e.g. progesterone.
What can glands be stimulated by?
Glands can be stimulated by a change in concentration of a specific substance (sometimes another hormone).
* They can also be stimulated by electrical impulses.
Nervous vs Hormonal Communication
Nervous:
- Uses electrical impulses.
- Faster response — electrical impulses are
really fast
- Localised response — neurones carry
electrical impulses to specific cells.
- Short-lived response — neurotransmitters
are removed quickly.
Hormonal:
- Uses chemicals
- Slower response — hormones travel at the
‘speed of blood’.
- Widespread response — target cells
can be all over the body.
- Long-lived response — hormones
aren’t broken down very quickly.
How and why are receptors specific to one kind of stimulus?
- Receptors are specific — they only detect one particular stimulus, e.g. light, pressure or glucose concentration.
- There are many different types of receptor that each detect a different type of stimulus.
Some receptors are cells, e.g. photoreceptors are receptor cells that connect to the nervous system. - Some receptors are proteins on cell surface membranes, e.g. glucose receptors are proteins found in the cell membranes of some pancreatic cells.
- When a nervous system receptor is in its resting state (not being stimulated), there’s a difference in charge between the inside and the outside of the cell. This means there’s a voltage across the membrane.
- The membrane is said to be polarised. The voltage across the membrane is called the potential difference.
- It is generated by ion pumps and ion channels
- When a stimulus is detected, the permeability of the cell membrane to ions changes (ions are stopped from moving, or more move in and out of the cell). This changes the potential difference.
- If the change in potential difference is big enough it’ll trigger an action potential — an electrical impulse along a neurone. An action potential is only triggered if the potential difference reaches a certain level called the threshold level.
How do Photoreceptors work as Light Receptors in Your Eye?
- Light enters the eye through the pupil. The amount of light that enters is controlled by the muscles of the iris.
- Light rays are focused by the lens onto the retina, which lines the inside of the eye. The retina contains photoreceptor cells — these detect light.
- The fovea is an area of the retina where there are lots of photoreceptors. Nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve, which is a bundle of neurones.
- Where the optic nerve leaves the eye is called the blind spot — there aren’t any photoreceptor cells, so it’s not sensitive to light.
How do photoreceptors convert light into an electrical impulse?
- Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive pigments.
- Light bleaches the pigments, causing a chemical change.
- This triggers a nerve impulse along a bipolar neurone.
- Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.
- The human eye has two types of photoreceptor — rods and cones.
- Rods are mainly found in the peripheral parts of the retina, and cones are found packed together in the fovea
- Rods only give information in black and white (monochromatic vision), but cones give information in colour (trichromatic vision). There are three types of cones — red-sensitive, green-sensitive and blue-sensitive. They’re stimulated in different proportions so you see different colours.
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How do rod cells react when its dark?
- Sodium ions (Na+) are pumped out of the cell using active transport.
- But sodium ions diffuse back in to the cell through open sodium channels.
- This makes the inside of the cell only slightly negative compared to the outside — the cell membrane is said to be depolarised.
- This triggers the release of neurotransmitters.
- But the neurotransmitters inhibit the bipolar neurone — the bipolar neurone can’t fire an action potential
so no information goes to the brain.
How do rod cells react to bright light?
- Light energy causes rhodopsin to break apart into retinal and opsin — this process is called bleaching.
- The bleaching of rhodopsin causes the sodium ion channels to close.
- So sodium ions are actively transported out of the cell, but they can’t diffuse back in.
- This means sodium ions build up on the outside of the cell, making the inside of the membrane much more negative
than the outside — the cell membrane is hyper-polarised. - When the rod cell is hyper-polarised it stops releasing neurotransmitters. This means there’s no inhibition of the bipolar neurone.
- Because the bipolar neurone is no longer inhibited, it depolarises. If the change in potential difference reaches the threshold, an action potential is transmitted to the brain via the optic nerve.
Structure and function of neurones?
- All neurones have a cell body with a nucleus (plus cytoplasm and all the other organelles you usually get in a cell).
- The cell body has extensions that connect to other neurones — dendrites and dendrons carry nerve impulses towards the cell body, and axons carry nerve impulses away from the cell body.
Motor Neurones
- Many short dendrites carry nerve
impulses from the central nervous
system (CNS) to the cell body. - One long axon carries nerve impulses
from the cell body to effector cells.
Sensory Neurones
- One long dendron carries nerve impulses from receptor cells to the cell body, which is located in the middle of the neurone.
- One short axon carries nerve impulses
from the cell body to the CNS.
Relay Neurones
- Many short dendrites carry nerve impulses from sensory neurones to the cell body.
- An axon carries nerve impulses from the cell body to motor neurones.
How are neurone cell membranes polarised at rest?
- In a neurone’s resting state (when it’s not being stimulated), the outside of the membrane is positively charged compared to the inside. This is because there are more positive ions outside the cell than inside.
- So the membrane is polarised — there’s a difference in charge.
- The voltage across the membrane when it’s at rest is called the resting potential — it’s about –70 mV.
- The resting potential is created and maintained by sodium-potassium pumps and potassium ion channels in a neurone’s membrane:
What is a sodium-potassium pump?
These pumps use active transport
to move three sodium ions (Na+)
out of the neurone for every two potassium ions (K+) moved in. ATP is needed to do this.
What is a potassium ion channel?
- These channels allow facilitated diffusion of potassium ions (K+) out of the neurone, down their concentration gradient.
How does a sodium-potassium pump work?
- The sodium-potassium pumps move sodium ions out of the neurone, but the membrane isn’t permeable to sodium ions, so they can’t diffuse back in. This creates a sodium ion electrochemical gradient (a concentration gradient of ions) because there are more positive sodium ions outside the cell than inside.
- The sodium-potassium pumps also move potassium ions in to the neurone, but the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels.
- This makes the outside of the cell
positively charged compared to the
inside.
How does an action potential work?
- Stimulus — this excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative.
- Depolarisation — if the potential difference reaches the threshold (around –55 mV), more sodium ion channels open. More sodium ions diffuse into the neurone.
- Repolarisation — at a potential difference of around +30 mV the sodium ion channels close and potassium ion channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential.
- Hyperpolarisation — potassium ion channels are slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (i.e. less than –70 mV).
- Resting potential — the ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential and maintains it until the membrane’s excited by another stimulus.
What happens after an action potential?
After an action potential, the neurone cell membrane can’t be excited again straight away. This is because the ion channels are recovering and they can’t be made to open — sodium ion channels are closed during repolarisation and potassium ion channels are closed during hyperpolarisation.
- This period of recovery is called the refractory period.
How does a wave of depolarisation occur?
- When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways.
- This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part.
- This causes a wave of depolarisation to travel along the neurone.
- The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an action potential.
How does the refractory period produce discrete impulses?
- During the refractory period, ion
channels are recovering and can’t be
opened. - So the refractory period acts as a time
delay between one action potential
and the next. - This makes sure that action potentials
don’t overlap but pass along as
discrete (separate) impulses. - The refractory period also makes sure
action potentials are unidirectional
(they only travel in one direction).
How does a bigger stimulus lead to more frequent impulses?
- Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is.
- If threshold isn’t reached, an action potential won’t fire.
- A bigger stimulus won’t cause a bigger action potential, but it will cause them to fire more frequently.
How does the prevention of the movement of sodium ions stop action potentials from occurring?
- Local anaesthetics are drugs that stop you from feeling pain in a localised area of your body.
- They work by binding to sodium ion channels in the membrane of neurones.
- This stops sodium ions from moving into the neurones, so their membranes will not depolarise.
- This prevents action potentials from being conducted along the neurones and stops information about pain reaching the brain.
Why do action potentials go faster in myelinated neurones?
1) Some neurones are myelinated — they have a myelin sheath.
2) The myelin sheath is an electrical insulator.
3) It’s made of a type of cell called a Schwann cell.
4) Between the Schwann cells are tiny patches of bare membrane called the nodes of Ranvier. Sodium ion channels are concentrated at the nodes.
5) In a myelinated neurone, depolarisation only happens at the nodes of Ranvier (where sodium ions can get through the membrane).
6) The neurone’s cytoplasm conducts enough electrical charge to depolarise
the next node, so the impulse ‘jumps’ from node to node.
7) This is called saltatory conduction and it’s really fast.
8) In a non-myelinated neurone, the impulse travels as a wave along the whole length of the axon membrane.
9) This is slower than saltatory conduction (although it’s still pretty quick).
10) The speed at which an impulse moves along a neurone is known as the conduction velocity. A high conduction velocity means that the impulse is travelling quickly.
What is a synapse + how does it work?
- A synapse is the junction between a neurone and another neurone, or between a neurone and an effector cell, e.g. a muscle or gland cell.
- The tiny gap between the cells at a synapse is called the synaptic cleft.
- The presynaptic neurone (the one before the synapse) has a swelling called a synaptic knob. This contains synaptic vesicles filled with chemicals called neurotransmitters.
- When an action potential) reaches the end of a neurone it causes neurotransmitters to be released into the synaptic cleft. They diffuse across to the postsynaptic membrane (the one after the synapse) and bind to specific receptors.
- When neurotransmitters bind to receptors they might trigger an action potential (in a neurone), cause muscle contraction (in a muscle cell), or cause a hormone to be secreted (from a gland cell).
- Because the receptors are only on the postsynaptic membranes, synapses make
sure impulses are unidirectional — the impulse can only travel in one direction. - Neurotransmitters are removed from the cleft so the response doesn’t keep happening, e.g. they’re taken back into the presynaptic neurone or they’re broken down by enzymes (and the products are taken into the neurone).
- There are many different neurotransmitters, e.g. acetylcholine and dopamine. Acetylcholine is involved in muscle contraction and the control of heart rate
How do neurotransmitters transmit nerve impulses between neurones?
- An action potential triggers calcium influx
- An action potential arrives at the synaptic
knob of the presynaptic neurone.
- The action potential stimulates
voltage-gated calcium ion channels
in the presynaptic neurone to open.
- Calcium ions diffuse into the synaptic knob.
(They’re pumped out afterwards by active transport.) - Calcium influx causes neurotransmitter release
- The influx of calcium ions into the
synaptic knob causes the synaptic
vesicles to move to the presynaptic
membrane. They then fuse with the
presynaptic membrane.
- The vesicles release the neurotransmitter
into the synaptic cleft — this is called
exocytosis. - The neurotransmitter triggers an action potential in the postsynaptic neurone
- The neurotransmitter diffuses across the synaptic cleft and binds to specific
receptors on the postsynaptic membrane.
- This causes sodium ion channels in
the postsynaptic neurone to open. - The influx of sodium ions into the postsynaptic membrane causes depolarisation. An action potential on the postsynaptic membrane is generated if the threshold is reached.
- The neurotransmitter is removed from the synaptic cleft so the response doesn’t keep happening
How do synapses allow information to be dispersed or amplified?
- When one neurone connects to many neurones information can be dispersed to different parts of the body. This is called synaptic divergence.
- When many neurones connect to one neurone information can be amplified (made stronger). This is called synaptic convergence.
How does summation at synapses tune the nervous response?
- If a stimulus is weak, only a small amount of neurotransmitter will be released from a neurone into the synaptic cleft. This might not be enough to excite the postsynaptic membrane to the threshold level and stimulate an action potential.
- Summation is where the effect of neurotransmitter released from many neurones (or one neurone that’s stimulated a lot in a short period of time) is added together.
What is a tropism?
A tropism is the response of a plant to a directional stimulus (a stimulus coming from a particular direction).
- Plants respond to directional stimuli by
regulating their growth.
- A positive tropism is growth towards the
stimulus.
- A negative tropism is growth away from the
stimulus.
What is phototropism?
- Phototropism is the growth of a plant
in response to light. - Shoots are positively phototropic
and grow towards light. - Roots are negatively phototropic
and grow away from light.
What is geotropism?
- Geotropism is the growth of a plant in
response to gravity. - Shoots are negatively geotropic and grow
upwards. - Roots are positively geotropic and grow
downwards.
What are growth factors and how do they bring about responses?
- Plants don’t have a nervous system so they can’t respond using neurones, and they don’t have a circulatory system so they can’t respond using hormones either.
- Plants respond to stimuli using growth factors — these are chemicals that speed up or slow down plant growth.
- Growth factors are produced in the growing
regions of the plant (e.g. shoot tips, leaves) and they move to where they’re needed in the other parts of the plant. - Growth factors called auxins stimulate the growth of shoots by cell elongation — this is where cell walls become loose and stretchy, so the cells get longer.
- High concentrations of auxins inhibit growth in roots though.
- There are many other plant growth factors such as:
* Gibberellins — stimulate flowering and seed germination.
* Cytokinins — stimulate cell division and cell differentiation.
* Ethene — stimulates fruit ripening and flowering.
* Abscisic acid (ABA) — involved in leaf fall.
IAA
- Indoleacetic acid (IAA) is an important auxin that’s produced in the tips of shoots in flowering plants. When it enters the nucleus of a cell, it’s able to regulate the transcription of genes related to cell elongation and growth.
- IAA is moved around the plant to control tropisms — it moves by diffusion and
active transport over short distances, and via the phloem over longer distances. - This results in different parts of the plants having different amounts of IAA. The uneven distribution of IAA means there’s uneven growth of the plant.
How does IAA work in phototropism?
Phototropism — IAA moves to the more shaded parts of the shoots and roots, so there’s uneven growth.
How does IAA work in geotropism?
Geotropism — IAA moves to the underside of shoots and roots, so there’s uneven growth.
How do plants detect light?
- Plants detect light using photoreceptors called phytochromes.
- They’re found in many parts of a plant including the leaves, seeds, roots and stem.
- Phytochromes control a range of responses. For example, plants flower in different seasons depending on how much daylight there is at that time of year, e.g. some plants flower during summer when there are long days.
- Phytochromes are molecules that absorb light. They exist in two states — the PR state absorbs red light at a wavelength of 660 nm, and the PFR state absorbs far-red light at a wavelength of 730 nm.
- Phytochromes are converted from one state to another when exposed to light:
* PR is quickly converted into PFR when it’s exposed to red light.
* PFR is quickly converted into PR when it’s exposed to far-red light.
* PFR is slowly converted into PR when it’s in darkness. - Daylight contains more red light than far-red light, so more PR is converted into PFR than PFR is converted to PR.
- So the amount of PR and PFR changes depending on the amount of light,
e.g. whether it’s day or night, or summer or winter. - The differing amounts of PR and PFR control the responses to light by regulating the transcription of genes involved in these responses. E.g. flowering — in some plants, high levels of PFR stimulates flowering.
- When nights are short in the summer, there’s not much time for PFR to be converted back into PR, so PFR builds up and genes involved in flowering are transcribed. This means the plants flower in summer.
The cerebrum
- The cerebrum is the largest part of the brain.
- It’s divided into two halves called cerebral
hemispheres. - The cerebrum has a thin outer layer called
the cerebral cortex. The cortex has a large
surface area so it’s highly folded to fit into
the skull. - The cerebrum is involved in vision, learning,
thinking, emotions and movement. - Different parts of the cerebrum are involved
in different functions, e.g. the back of the
cortex is involved in vision and the front is
involved in thinking.
The hypothalamus
- The hypothalamus is found just beneath
the middle part of the brain. - The hypothalamus automatically
maintains body temperature at the
normal level (thermoregulation) - The hypothalamus produces hormones
that control the pituitary gland — a
gland just below the hypothalamus.
The medulla oblongata
- The medulla oblongata is at the base of the brain, at the top of the spinal cord.
- It automatically controls breathing rate and heart rate.
The cerebellum
- The cerebellum is underneath the cerebrum and it also has a folded cortex.
- It’s important for coordinating movement and balance.
Computed Tomography (CT)
- CT scanners use radiation (X-rays) to produce cross-section images of the brain.
- Dense structures in the brain absorb more radiation than less dense structures so show up as a lighter colour on the scan.
- CT scans are potentially dangerous because they use X-rays — X-rays can cause mutations in DNA, which may lead to cancer. The risk of developing cancer as a result of having a CT scan is very low
How does a CT scan investigate brain structure and function?
A CT scan shows the major structures in the brain — but it doesn’t show the functions of these structures. However, if a CT scan shows a diseased or damaged brain structure and the patient has lost some function, the function of that part of the brain can be worked out. E.g. if an area of the brain is damaged and the patient can’t see, then that area is involved in vision.
How does a CT scan help with medical diagnosis?
CT scans can be used to diagnose medical problems because they show damaged or diseased areas of the brain, e.g. bleeding in the brain after a stroke:
- Blood has a different density from brain tissue so it shows up as a lighter colour on a CT scan.
- A scan will show the extent of the bleeding and its location in the brain.
- You can then work out which blood vessels have been damaged and what brain functions are likely to be affected by the bleeding.
Magnetic Resonance Imaging (MRI)
MRI scanners use a really strong magnetic field and radio waves to produce cross-section images of the brain.
How do MRI scans investigate brain structure and function?
Compared to CT scans, MRI gives higher quality images for soft tissue types (such as the brain), and better resolution between tissue types for an overall better resolution
final picture.
MRIs allow you to clearly see
the difference between normal and abnormal (diseased or damaged) brain tissue. For example, a scan can show diseased tissue caused by multiple sclerosis (a disease of the central nervous system).
However, as with CT scanning, brain function can only be worked out by looking at damaged areas.
Functional Magnetic Resonance Imaging (fMRI)
fMRI scanners are like MRI scanners (see previous page), but they show changes in brain activity as they happen:
More oxygenated blood flows to active areas of the brain (to supply the neurones with oxygen and glucose).
Molecules in oxygenated blood respond differently to a magnetic field than those in deoxygenated blood — the signal returned to the scanner is stronger from the oxygenated blood, which allows more active areas of
the brain to be identified.
How do MRI scans help with medical diagnosis?
MRI scans can also be used to diagnose medical problems because they show
damaged or diseased areas of the brain, e.g. a brain tumour (an abnormal mass
of cells in the brain):
- Tumour cells respond differently to a magnetic field than healthy cells, so they show up as a lighter colour on an MRI scan.
- A scan will show the exact size of a tumour and its location in the brain. Doctors can then use this information to decide the most effective treatment.
- You can also work out what brain functions may be affected by the tumour.
How do fMRI scans help investigate brain function and structure
An fMRI scan gives a detailed, high resolution picture of the brain’s structure, similar to an MRI scan — but they can also be used to
research the function of the brain.
If a function is carried out whilst in the scanner, the part of the brain that’s involved with that function will be more active. E.g. a patient might be asked to move their left hand when in the fMRI scanner.
The areas of the brain involved in moving the hand will be highlighted on the fMRI scan
How do fMRI scans help with medical diagnosis?
fMRI scans show damaged or diseased areas of the brain and allow you to study conditions caused by abnormal activity in the brain (some conditions don’t have an obvious structural cause).
E.g. an fMRI scan can be taken of a patient’s brain before and during a seizure.
This can help to pinpoint which part of the brain’s not working properly and find the cause of the seizure.
Then the patient can receive the most effective treatment for the seizures.
Positron Emission Tomography (PET)
PET scanners can show how active different areas of the brain are.
1. A radioactive tracer is introduced into the body and is absorbed into the tissues.
- The scanner detects the radioactivity of the tracer — building up a map of radioactivity in the body.
- Different tracers can be used — e.g. radioactively labelled glucose can be used to look at glucose metabolism.
How do PET scans investigate brain structure and function?
PET scans, like fMRI scans, are very detailed and can be used to investigate both the structure and the function of the brain in real time.
How do PET scans help with medical diagnosis?
PET scans can show if areas in the brain are
unusually inactive or active, so they are particularly useful for studying disorders that change the brain’s activity.
E.g. in Alzheimer’s disease, metabolism in
certain areas of the brain is reduced — PET scans show this reduction when compared to a normal brain.
What is habituation?
- Animals (including humans) increase their chance of survival by responding to stimuli
- But if the stimulus is unimportant (if it’s not threatening or rewarding), there’s no point in responding to it.
- If an unimportant stimulus is repeated over a period of time, an animal learns to ignore it.
This reduced response to an unimportant stimulus after repeated exposure over time is called habituation. - Habituation means 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. E.g. prairie dogs use alarm calls to warn others of a predator but they’ve habituated to humans because we’re not a threat.
- They no longer make alarm calls when they see humans, so they don’t waste time or energy.
- Animals still remain alert to important stimuli (stimuli which might threaten their survival).
E.g. you can become habituated to the sound of traffic at night and get to sleep, but if you hear an unfamiliar noise, like a burglar, you’ll wake up.
How can you investigate habituation (SNAILS)
- Gently brush something soft, like a blade of grass, across the surface of the snail’s
skin — close to its tentacles. The snail should withdraw them back into its head. - Using a stopwatch, time how long it takes for the snail to fully extend its tentacles again after you touched it.
- Repeat this process at regular intervals and record the time it takes for
the tentacles to fully extend every time you touch the snail.
If habituation has taken place the snail should re-extend it’s tentacles quicker the more you repeat the stimulus(or it might not withdraw at all eventually).
If habituation hasn’t occurred the snail will take the same length of time to re-extend its tentacles each time. The snail should still remain alert to an unfamiliar stimulus, e.g. if you cast a shadow over the snail, it should still withdraw its tentacles or even its entire head.
Why does habituation cause fewer electrical impulses being sent to effectors?
- Repeated exposure to a stimulus decreases the amount of calcium ions that enter the presynaptic neurone.
- This decrease in the influx of calcium ions means that less neurotransmitter is released from vesicles into the synaptic cleft, so fewer neurotransmitters can bind to receptors in the postsynaptic membrane.
- Fewer sodium ion channels on the postsynaptic membrane open — so there is a reduced chance of the threshold for an action potential being reached on the postsynaptic membrane.
4.As a result, fewer signals are sent to the effector to carry out the response.
The visual cortex
- Made up of ocular dominance columns
- The visual cortex is an area of the cerebral cortex at the back of your brain.
- The role of the visual cortex is to receive and process visual information.
- Neurones in the visual cortex receive information from either your left or right eye.
- Neurones are grouped together in columns called ocular dominance columns. If they receive information from the right eye they’re called visual cortex right ocular dominance columns, and if they receive information from
the left eye they’re called left ocular dominance columns. - The columns are the same size and they’re arranged in an alternating pattern (left, right, left, right) across the visual cortex.
Hubel and Wiesel experiment
- Some animals have fairly similar brains to humans. This means scientists can do experiments on these animals (that would be unethical to do in humans) to investigate brain development.
- The structure of the visual cortex was discovered by two scientists called Hubel and Wiesel.
- They used animal models to study the electrical activity of neurones in the visual cortex.
- They found that the left ocular dominance columns were stimulated when an animal used its left eye, and the right ocular dominance columns were stimulated when it used its right eye.
- Hubel and Wiesel (1963) investigated how the visual cortex develops by experimenting on very young kittens
How did Hubel and Wiesel use animal models to study the visual cortex?
- They stitched shut one eye of each kitten so they could only see out of their other eye.
- The kittens were kept like this for several months before their eyes were unstitched.
- Hubel and Wiesel found that the kitten’s eye that had been stitched up was blind.
- They also found that ocular dominance columns for the stitched up eye were a lot smaller than normal, and the ocular dominance columns for the open eye were a lot bigger than normal.
- The ocular dominance columns for the open eye had expanded to take over
the other columns that weren’t being stimulated — when this happens, the
neurones in the visual cortex are said to have switched dominance.
- THEY THEN investigated if the same thing happened in an adult cat’s brain:
- They stitched shut one eye of each cat, who
were kept like this for several months. - When their eyes were unstitched, Hubel and
Wiesel found that these eyes hadn’t gone
blind. - The cats fully recovered their vision and their
ocular dominance columns remained the
same.
What were the results of Hubel and Wiesel?
They repeated the experiments on young and adult monkeys and saw the same results.
Hubel and Wiesel’s experiments showed that the visual cortex only develops into normal left and right ocular dominance columns if both eyes are visually stimulated in the very early stages of life.
What did Hubel and Wiesel’s Experiments provide evidence for?
- Hubel and Wiesel’s experiments on cats show there’s a period in early life when it’s critical that a kitten is exposed to visual stimuli for its visual cortex to develop properly. This is called the critical period.
- The human visual cortex is similar to a cat’s visual cortex (the human visual cortex has ocular dominance columns too) so Hubel and Wiesel’s experiments provide evidence for a critical period in humans.
- Scientists have also investigated visual development in humans, e.g. by looking at cataracts in the eye:
- A cataract makes the lens in the eye go
cloudy, causing blurry vision. - If a baby has a cataract, it’s important to
remove the cataract within the first few
months of the baby’s life — otherwise their
visual system won’t develop properly and
their vision will be damaged for life. - If an adult has a cataract then it’s not so
serious — when the cataract is removed,
normal vision comes back straight away. This
is because the visual system is already
developed in an adult.
How does visual stimulation organise the neurones during the critical period?
- Baby mammals (including humans) are born
with lots of neurones in their visual cortex.
These neurones need visual stimulation to
become properly organised. - Proper organisation of the visual cortex
involves the elimination of unnecessary
synapses to leave behind those that are
needed in processing visual information.
- During the critical period of development, synapses that receive visual stimulation and pass nerve impulses into the visual cortex are retained.
- Synapses that don’t receive any visual stimulation and don’t pass on any nerve impulses to the visual cortex are removed.
- This means that if the eyes are not stimulated with visual information during this critical period of development, the visual cortex will not develop properly as many of the synapses will be destroyed.
Arguments AGAINST using animals in medical research?
- Animals are different from humans, so drugs tested on animals may have different effects in humans.
- Experiments can cause pain and distress to animals.
- There are alternatives to using animals in research, e.g. using cultures of human cells or using computer models to predict the effects of experiments.
- Some people think that animals have the right to not be experimented on, e.g. animal rights activists.
Arguments FOR using animals in medical research?
- Animals are similar to humans, so research has led to loads of medical breakthroughs, e.g. antibiotics, insulin for diabetics and organ transplants.
- Animal experiments are only done when it’s absolutely necessary and scientists follow strict rules, e.g. animals must be properly looked after, painkillers and anaesthetics must be used to minimise pain.
- Using animals is currently the only way to study how a drug affects the whole body — cell cultures and computers aren’t a true representation of how cells may react when surrounded by other body tissues. It’s also the only way to study behaviour.
- Other people think that humans have a greater right to life than animals because we have more complex brains, e.g. compared to rats, fish, fruit flies (which are commonly used in experiments).
Investigating the role of nature and nurture in Brain Development
1.Brain development is how the brain grows and how neurones connect together.
- Measures of brain development include the size of the brain, the number of neurones it has and the level of brain function (e.g. speech, intelligence) a person has.
- Your brain develops the way it does because of both your genes (nature) and your environment (nurture) — your brain would develop differently if you had different genes or were brought up in a different environment.
- Scientists disagree about whether nature or nurture influences brain development the most — this argument is called the nature-nurture debate
Why is it hard to investigate the effects of nature and nurture?
- Genetic and environmental factors interact, so it’s difficult to know which one is the main influence.
- There are lots of different genes and lots of different environmental factors to investigate.
- To do an accurate experiment, you need to cancel out one factor to be able to investigate the other.
- This is really difficult to do — you’d need to cut out all environmental influences to investigate the role of a genetics, and vice versa.
Investigating the effects of nature and nature on brain development: Animal Experiments
- Scientists study the effects of different environments on the brain development of animals of the same species. Individuals of the same species will be genetically similar, so any differences in their brain development are more likely to be due to nurture than nature.
- To study the effects of different genes, scientists can genetically engineer mice to lack a particular gene and then raise mice with and without the gene in similar environments.
- Differences between the brain development of the genetically engineered mice and normal mice are more likely to be due to nature than nurture.
Investigating the effects of nature and nature on brain development: Twin Studies
1) If identical twins are raised separately then they’ll have identical genes but different environments.
2) Scientists can compare the brain development of separated identical twins — any differences between them are due to nurture not nature, and any similarities between them are due to nature.
3) For example: Identical twins have very similar IQ scores — suggesting nature plays a big role in intelligence.
4) Scientists can use this comparison to show the relative contribution of environmental and
genetic factors to brain development.
5) However, even if they’ve been raised separately, twins will still have shared the same environment in the womb — so environmental and genetic factors are not completely separated.
6) Identical twins raised together are genetically identical and have similar environments — this means it’s hard to tell if any differences between them are due to nature or nurture. So scientists compare them to non-identical twins (who are genetically different but have similar environments) — they act like a control to cancel out the influence of the environment. Any difference in brain development between identical and non-identical twins is more likely to be due to nature than nurture.
Investigating the effects of nature and nature on brain development: Cross-cultural studies
1) Children brought up in different cultures have different environmental influences, e.g. beliefs and education.
2) Scientists can study the effects of a different upbringing on brain development by comparing large groups of children who are the same age but from different cultures.
3) Scientists look for major differences in characteristics. Any differences in brain development between different cultures are more likely to be due to nurture than nature. Any similarities in brain development between different cultures are more likely to be due to nature than nurture.
Investigating the effects of nature and nature on brain development: Newborn studies
1) The brain of a newborn baby hasn’t really been affected by the environment.
2) Scientists study the brains of newborn babies to see what functions they’re born with and how developed different parts of the brain are — what they’re born with is more likely to be due to nature than nurture.
Investigating the effects of nature and nature on brain development: Brain damage studies
- Damage to an adult’s brain can lead to the loss of brain function, e.g. a stroke may cause loss of vision.
- If an adult’s brain is damaged, it can’t repair itself so well because it’s already fully developed. But a child’s brain is still developing — so scientists can study the effects of brain damage on their development.
- Scientists compare the development of a chosen function in children with and without brain damage.
- If the characteristic still develops in children who have brain damage, then brain development is more likely to be due to nurture than nature for that characteristic.
- If it doesn’t develop in children who have brain damage, then brain development is more likely to be due to nature than nurture for that characteristic (because nurture isn’t having an effect).
Imbalances in some neurotransmitters can contribute to disorders: Parkinson’s
- Parkinson’s disease is a brain disorder that affects the motor skills (the movement) of people.
- In Parkinson’s disease the neurones in the parts of the brain that control movement are destroyed.
- These normally produce the neurotransmitter
dopamine, so losing them causes a lack of dopamine. - This means that less dopamine is released into the synaptic clefts, so less dopamine is available to bind to the receptors on the postsynaptic membranes.
- Fewer sodium ion channels on the postsynaptic membrane open, so the postsynaptic cell is less likely to depolarise.
- This means fewer action potentials are produced, leading to symptoms like tremors (shaking) and slow movement.
- Scientists know that the symptoms are caused by a lack of dopamine so they’ve developed drugs (e.g. L-dopa) to increase the level of dopamine in the brain.
Imbalances in some neurotransmitters can contribute to disorders: Depression
- Scientists think there’s a link between a low level of the neurotransmitter serotonin and depression.
- Serotonin transmits nerve impulses across synapses in the parts of the brain that
control mood. - Scientists know that depression is linked to a low level of serotonin so they’ve developed drugs (antidepressants) to increase the level of serotonin in the brain.
- Some drugs that are used to treat depression (called selective serotonin reuptake inhibitors — SSRIs) increase serotonin levels by preventing its reuptake at synapses - more serotonin remains cleft, so more impulses travel along axon
How does L-dopa work?
- L-dopa is a drug that’s used to treat the symptoms of Parkinson’s disease.
- Its structure is very similar to dopamine.
- When L-dopa is given, it’s absorbed into the brain and converted into dopamine by the enzyme dopa-decarboxylase (dopamine can’t be given to treat Parkinson’s disease because it can’t enter the brain). This increases the level of dopamine in the brain.
- A higher level of dopamine means that more nerve impulses are transmitted across synapses in the parts of the brain that control movement.
- This gives sufferers of Parkinson’s disease more control over their movement.
How does MDMA work?
- MDMA increases the level of serotonin in the brain.
- Usually, serotonin is taken back into a presynaptic neurone after triggering an action potential, to be used again.
- MDMA increases the level of serotonin 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 neurones.
- This means that serotonin levels stay high in the synapse and cause depolarisation of the postsynaptic neurones in parts of the brain that control mood.
- So the effect of MDMA is mood elevation
How is information from Genome Sequencing Projects being used to create new drugs?
- The Human Genome Project (HGP) was a 13 year long project that identified all of the genes found in human DNA (the human genome).
- The information obtained from the HGP is stored in databases.
- Scientists use the databases to identify genes, and so proteins, that are involved in disease.
- Scientists are using this information to create new drugs that target the identified proteins, e.g. scientists have identified an enzyme that helps cancer cells to spread around the body — a drug that inhibits this enzyme is being developed.
- The HGP has also highlighted common genetic variations between people.
- It’s known that some of these variations make some drugs less effective, e.g. some asthma drugs are less effective for people with a particular mutation.
- Drug companies can use this knowledge to design new drugs that are tailored to people with these variations — these are called personalised medicines.
- Doctors can also personalise a patient’s treatment by using their genetic information to predict how well they will respond to different drugs and only prescribe the ones that will be most effective.
What are the social, moral and ethical issues with using Genome sequencing projects to create new drugs?
- Creating drugs for specific genetic variations will increase research costs for drugs companies. These new drugs will be more expensive, which could lead to a two-tier health service — only wealthier people could afford these new drugs.
- Some people might be refused an expensive drug because their genetic make-up indicates that it won’t be that effective for them — it may be the only drug available though.
- The information held within a person’s genome could be used by others, e.g. employers or insurance companies, to unfairly discriminate against them. For example, if a person is unlikely to respond to any drug treatments for a certain disease an insurance company might increase their life insurance premium.
- Revealing that a drug might not work for a person could be psychologically damaging to them, e.g. it could be their only hope to treat a disease.
Drugs can be produced using genetically modified organisms: Genetically Modified Organisms
- The gene for the protein (drug) is isolated
using enzymes called restriction enzymes. - The gene is copied using PCR
- Copies are spliced into plasmids (small circular molecules of DNA), through ligase
- The plasmids are transferred into microorganisms.
- The modified microorganisms are grown in large containers so that they divide and produce lots of the useful protein, from the inserted gene.
- The protein can then be purified and used as a drug.
Drugs can be produced using genetically modified organisms: Genetically Modified Plants
- The gene for the protein is inserted through the Ti plasmid into a bacterium
- The bacterium infects a plant cell.
- The bacterium inserts the gene into the plant cell DNA — the plant cell is now genetically modified.
- The plant cell is grown into an adult plant — the whole plant contains a copy of the gene in every cell.
- The protein produced from the gene can be purified from the plant tissues, or the protein (drug) could be delivered by eating the plant.
Drugs can be produced using genetically modified organisms: Genetically Modified Animals
- The gene for the protein (drug) is injected into the nucleus of a fertilised animal egg cell.
- The egg cell is then implanted into an adult animal — it grows into a whole animal that contains a copy of the gene in every cell.
- The protein produced from the gene is normally purified from the milk of the animal.
Benefits with using GMO’s
- Agricultural crops can be modified so that they give higher yields or are more nutritious. This means these plants can be used to reduce the risk of famine and malnutrition.
- Crops can also be modified to have pest resistance, so that fewer pesticides are needed. This reduces costs (making food cheaper) and reduces any environmental problems associated with using pesticides.
- Industrial processes often use enzymes. These enzymes can be produced from genetically modified organisms in large quantities for less money, which reduces costs.
- Some disorders can now be treated with human proteins from genetically engineered
organisms instead of with animal proteins. Human proteins are safer and more effective. For example, Type 1 diabetes used to be treated with cow insulin but some people had an allergic reaction to it. Human insulin, produced from genetically modified bacteria, is more effective and doesn’t cause an allergic reaction in humans. - Vaccines produced in plant tissues don’t need to be refrigerated. This could make
vaccines available to more people, e.g. in areas where refrigeration (usually needed for
storing vaccines) isn’t available. - Producing drugs using plants and animals would be very cheap because once the plants
or animals are genetically modified they can be reproduced using conventional farming methods. This could make some drugs affordable for more people, especially those in
poor countries.
Risks with using GMO’s
- Some people are concerned about the transmission of genetic material. For example,
if herbicide-resistant crops interbreed with wild plants it could create ‘superweeds’ — weeds that are resistant to herbicides, and if drug crops interbreed with other crops people might end up eating drugs they don’t need (which could be harmful). - Some people are worried about the long-term impacts of using GMOs.
- There may be unforeseen consequences.
Some people think it’s wrong to genetically modify animals purely for human benefit.
Treatments of Parkinson’s
- Slowing the loss of dopamine
- Treating the symptoms
- Use of dopamine agonists
Why can dopamine itself not be used as a drug?
- Cannot cross blood-brain barrier
Serotonin
- Neurones secreting serotonin located in the
brain stem - Axons extended in the cortex, cerebellum
and spinal cord - therefore a huge area of
the brain - Dopamine and noradrenaline also are
thought to play a part
Eukaryotic genes in prokaryotic organisms
- Eukaryotic genes have introns, so bacteria
cannot splice - SO mRNA —–> cDNA by reverse
transcriptase - cDNA (no introns) used to transform bacteria
Benefits: Eukaryotic genes in prokaryotic organisms
- Bacteria can be grown in an aseptic
fermenter - Not released into environment
- Low cost of production
Negatives: Eukaryotic genes in prokaryotic organisms
- Complex or membrane bound proteins cannot be made
Genome
All the DNA of an organism
Genomics
The science of working out the order of bases in the strands of DNA which make up the genome
