2.13–2.14Structure and function
The nervous system enables humans to react to their surroundings and to coordinate their behaviour.
Information from receptors passes along cells called neurones as electrical impulses to the central nervous system (CNS), which is the brain and spinal cord. The CNS coordinates the response of effectors, which may be muscles contracting or glands secreting hormones.
The sequence to learn, and to write out whenever a question describes a response:
stimulus → receptor → coordinator → effector → response
A reflex arc is the fast version of that sequence, and you should be able to explain how each structure in it relates to its function.
• The receptor detects the stimulus. • The sensory neurone carries the impulse to the CNS. • A synapse is the gap between two neurones. The impulse arrives, a chemical is released across the gap, and a new impulse starts in the next neurone — which is why synapses slow the journey slightly. • The relay neurone, in the spinal cord, passes it straight across. • The motor neurone carries the impulse out to the effector. • The effector — a muscle or gland — carries out the response.
Reflex actions are automatic and rapid, and they do not involve the conscious part of the brain. That is exactly why they matter: pulling your hand off something hot has to happen before you have decided anything, so bypassing conscious thought protects the body from damage.
Required practical 7
Plan and carry out an investigation into the effect of a factor on human reaction time
- The simple version: one person holds a ruler vertically between another's open finger and thumb, and drops it without warning.
- The distance the ruler falls before it is caught is converted to a time — a longer drop means a slower reaction.
- Choose one factor to change: caffeine, background noise, tiredness, which hand is used, or whether the person is distracted.
- Keep everything else the same, including the starting position of the ruler and the person catching.
- Repeat at least three times per condition, discard any obvious mistakes, and take a mean.
Watch out: Reaction times improve with practice, so run some trials first and use the later ones, or the second condition will always look faster whatever you changed. This practical is also about planning: the marks are in identifying the independent, dependent and control variables.
Worked example
A student's ruler drop distances are 18 cm, 22 cm and 20 cm. Calculate the mean, and explain why the mean is more useful than a single result.
- Mean = (18 + 22 + 20) ÷ 3.
- = 60 ÷ 3.
Answer: 20 cm. A single drop could be a lucky catch or a lapse in concentration; the mean of repeats reduces the effect of that random variation.
Reaction-time data comes with its own skills. You should be able to extract and interpret data from graphs, charts and tables about the functioning of the nervous system, and to translate information about reaction times between numerical and graphical forms.
In practice that means three things. Reading a table: compare like with like — the same person, the same hand — and quote the figures. Turning a table into a graph: the condition goes on the x-axis, the reaction time on the y-axis, and a bar chart suits conditions that are categories while a line graph suits a numerical factor such as number of hours of sleep. Reading a graph back: give the value with its unit, and say what it means about the response, not just that the bar is taller.
Common mistake
A reflex does not "go to the brain and back quickly". It goes through the spinal cord via a relay neurone and does not involve the conscious part of the brain at all. That is what makes it faster, and saying so is the mark.
- Central nervous system
- the brain and spinal cord, which coordinate the response of effectors to information received from receptors
- Reflex action
- an automatic and rapid response that does not involve the conscious part of the brain, which protects the body from damage
- Synapse
- the gap between two neurones; an impulse arriving causes a chemical to be released across the gap, starting a new impulse in the next neurone
- Effector
- a muscle that contracts or a gland that secretes a hormone, carrying out the response to a stimulus
2.10BThe brain
Biology only — not on the Combined Science specification
The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions. Three regions must be identifiable on a diagram, with their functions.
Higher tier only: neuroscientists have mapped regions of the brain to particular functions in three ways — by studying patients with brain damage and seeing which abilities were lost, by electrically stimulating different parts of the brain and observing the effect, and by using MRI scanning to see which regions are active during a task.
The complexity and delicacy of the brain make investigating and treating brain disorders very difficult. It is encased in the skull, so reaching it means surgery; the regions are not neatly separated; and damage caused during an operation is often permanent, because nerve tissue does not readily repair.
An evaluation question about brain surgery wants both sides: the benefit of removing a tumour or stopping seizures against the risk of permanent damage to speech, movement or personality — and the fact that a patient may have no better option.
2.11B–2.12BInvestigating and treating the brain
Biology only — not on the Combined Science specification
The brain is protected by the skull, which is exactly what makes it hard to study. Scanning is how that problem is got around, and two types of scan are named — they are not interchangeable, and questions test the difference.
A CT scan uses X-rays to build a detailed picture of the structure of the brain. It shows where the tissue is and what shape it is, so it reveals a tumour, a bleed or a damaged region. What it cannot show is whether that tissue is working.
A PET scan uses a radioactive tracer, injected into the blood, that collects where cells are most active. It shows function: which regions are busy while a person speaks, reads or moves, and which are unusually quiet. A PET scan can therefore detect a disorder that changes activity without changing structure, and the two scans are often used together.
Treating damage to the brain and spinal cord is difficult for reasons you should be able to state, not just assert.
The brain is encased in the skull, so reaching it means surgery that is itself dangerous. It is extremely complex, and regions have not been mapped completely, so a surgeon cannot always be sure what a piece of tissue does. Many drugs cannot cross the membranes between the blood and the brain, so a medicine that works elsewhere in the body may never reach the target.
Neurones of the central nervous system also do not readily repair themselves. That is why a spinal injury is usually permanent: once the neurones running through the spinal cord are severed, impulses can no longer pass between the brain and the body below the injury, and sensation and movement below that point are lost.
Common mistake
"A PET scan gives a clearer picture" is not the answer. The distinction is structure against function — a CT scan shows what the brain looks like, a PET scan shows what it is doing.
Before you practise
- The pathway is always stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response.
- A reflex is automatic and does not involve the conscious part of the brain, which is what makes it fast.
- At a synapse the signal crosses as a chemical, which diffuses across the gap and sets off an impulse in the next neurone.
- Cerebral hemispheres for thought and memory, cerebellum for coordination and balance, medulla oblongata for heart rate and breathing.
- A CT scan shows structure; a PET scan shows which regions are active.
- Neurones of the central nervous system do not readily repair, which is why spinal injuries are usually permanent.