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Edexcel Categories Archives: A-Level

5.8.16 – Some characteristics are controlled by alleles at many loci and how this can give rise to phenotypes which show continuous variation

Continuous variation: there is a wide range of phenotypes (e.g. height)

 

Discontinuous variation: phenotypes fall into discrete categories (e.g. blood type)

 

Discontinuous variation tends to be coded for by one gene with a few different alleles. However, continuous variation is more complex. This is usually coded for by many genes (polygenes), with many alleles, which produces the much greater range of possible phenotypes.

Polygenes can give rise to susceptibility to disease, usually with an environmental trigger. Diseases that are both genetic and environmental are called multifactorial.

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5.8.15 – The effects of drugs on synaptic transmissions

Drugs that affect synapses can drastically alter the functioning of the brain;

 

MDMA:

 

Active ingredient in ecstasy. This binds to protein pumps on the pre-synaptic membrane of nerves that secrete serotonin. The pumps would normally take serotonin up after it had been released, therefore reducing firing in post-synaptic nerves. BUT, when these channels are blocked, serotonin builds up in the cleft,  giving greater post-synaptic activation and a sense of euphoria.

L-Dopa:

 

This is a precursor of dopamine. When given to Parkinson’s sufferers it is turned into dopamine, which helps alleviate some of the symptoms of the disease.

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5.8.14 – How imbalances in, naturally occurring brain chemicals can contribute to health consequences and the development of new drugs

In Parkinson’s disease neurons in the brain die. All these neurons secrete dopamine

neurotransmitter, which causes difficulty in movement and limb shaking.

 

In depression neurons in the brain that secrete serotonin neurotransmitter stop working properly and serotonin levels fall.

In both cases treatments that increase the levels of neurotransmitter might prove successful in relieving the symptoms of these diseases

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5.8.12 – The role animal models have played in understanding human brain development and function

Pavlov’s Dogs

 

Pavlov had observed that an unconditioned stimulus causes an unconditioned response, i.e. food causes salivation. This is not learned and is, therefore, unconditioned.

What Pavlov discovered was that if a neutral stimulus, such as a bell is rung just before the food is given for a few occasions, the dog will salivate every time the bell is rung, even if no food is presented. In this case, the dog has learned that the bell signals food. The food is, therefore, a conditioned stimulus and it prompts a conditioned response.

US -> UR

US + CS -> UR

Eventually, CS -> CR

Hubel & Wiesel

 

  • Hubel & Wiesel investigated the critical
  • They used monkeys and kittens in their studies
  • Their work permanently blinded some animals and can be argued to be unethical.

 

 

 

 

Hubel & Wiesel’s Method:

 

  1. Raise monkeys from birth in three groups for 6 months

 

  1. Group 1 are the control (no blindfold), Group 2 are blindfolded  in both eyes, Group 3 are blindfolded in one eye (monocular deprivation)
  2. Test the monkeys to see whether they can see using each eye
  3. Test the sensitivity of retinal cells
  4. Test the activity of nerves in the visual cortex in response to stimuli

 

The results:

 

  • Monkeys in Group 2 (both eyes blindfolded) had impaired vision
  • Monkeys in Group 3 (monocular deprivation) were blind in the deprived eye
  • Retinal cells were responsive in all groups
  • Cortical activity was reduced in parts of the brain that process information from the deprived eye
  • Adults undergoing the same tests showed no difference between groups. All could

The Conclusion:

There is a critical window for visual neural development, which requires stimulus from the eye. If this window is missed the monkey is blind, because of events happening in the brain, not the eye.

You need to know about these experiments because they all use animals

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5.8.11 – Ways in which animals including humans can learn

Association (classical conditioning):

 

US ® UR (Food ® Salivation)

Over time, if a neutral stimulus (CR) is played with the US, it becomes associated  with the US and begins to elicit the same response. Eventually, the animal learns

CS ® CR (Bell ® Salivation)

Pavlovian conditioning occurs by synapses between nerves growing together. This means that the sensory nerve carrying the message of the CS will always lead to the firing of the motor nerve, which triggers the CR.

Operant Conditioning:

 

This is very similar to classical conditioning except the animal learns by doing something i.e. it learns that an action has a certain outcome

A ® O (pushing a level ® food)

Habituation:

 

If the neutral stimulus is continuously present (not just before the US), but all the time, the animal learns to ignore the CS. The animal learns the bell signals nothing and it ignores the CS totally. This is called habituation.

If a nerve is frequently stimulated, the amount of Ca2+ that enters the pre-synaptic nerve gradually diminishes, until it is no longer enough to trigger vesicles to fuse  with the pre-synaptic membrane. This means no neurotransmitter is released, which results in no post-synaptic depolarisation. The effect is, essentially, that the stimulus is ignored.

Insight Learning:

 

In the early 1900s, Wolfgang Kohler performed insight experiments on chimpanzees. Kohler showed that the chimpanzees sometimes used insight instead of trial-and- error responses to solve problems. When a banana was placed high out of reach, the animals discovered that they could stack boxes on top of each other to reach it. They also realized that they could use sticks to knock the banana down. In another experiment, a chimp balanced a stick on end under a bunch of bananas suspended from the ceiling, then quickly climbed the stick to obtain the entire bunch intact and unbruised (a better technique than the researchers themselves had in mind).  Kohler’s experiments showed that primates can both see and use the relationships involved to reach their goals.

 

This type of learning is very difficult to explain using the Pavlovian model of conditioning. It is also difficult to explain using neuronal models of learning (i.e. synapses growing together through use) developed through studies on Aplysia. How insight learning occurs is unknown at the moment.

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5.8.10 – How to investigate visual perception in humans

The Muller-Lyer illusion;

Lines A and B are the same length, yet look different – why? The answer is that you have learned to process this kind of stimuli in a certain way. We live in a “carpentered world” of straight lines and we interpret line B as a corner (therefore larger than it appears, because it must be far away) and line A as a corner (therefore, smaller than it appears, because it must be close).

These optical illusions do not work on Zulus, which proves the illusion is caused by learned visual processing, rather than an innate function of the eye / brain.

 

 

 

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5.8.9 – The evidence that there exists a critical ‘window’ within which humans must be exposed to particular stimuli if they are to develop their visual capacities to the full

How to process stimuli correctly must be learned. The cortex is split into column of cells. When we are born, the columns overlap and are tangled. As we learn to process stimuli, the cells organise themselves into discrete columns, which no longer overlap. There is a “critical window” for this to happen (usually before puberty, younger for visual processing). If we miss the window, our brains will become “fixed” with tangled columns and won’t be able to process stimuli properly.

Hubel & Wiesel’s experiments prove this.

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