Human body systems, Year 11: homeostasis and the nervous system

Homeostasis

提示The word is built from Greek for 'same' and 'standing still'.

为什么The body is constantly disturbed: you eat, you exercise, the weather changes. Automatic control systems detect each change and correct it, so that conditions inside stay close to the level at which cells work best.

glucose; temperature; water

提示One is a sugar, one is measured in °C, and one you replace by drinking.

为什么These are the three conditions the GCSE course names. Each has its own automatic control system, and each matters because cells are damaged if the level drifts too far either way.

Enzymes and cells only work well within a narrow range

提示Think about what happens to the proteins that speed up the body's reactions when they get too hot.

为什么Every reaction in a cell is controlled by enzymes, and each enzyme works fastest at its optimum temperature and pH. Away from the optimum the reactions slow down, and at high temperatures enzymes are denatured, so the cell can no longer function.

coordination

提示They organise the response, like the control room that takes in reports and sends out orders.

为什么Every control system has three kinds of part. Receptors detect the change, a centre such as the brain or the pancreas receives that information and decides the response, and effectors carry the response out.

Muscles, which contract, and glands, which release (secrete) chemicals such as hormones

提示One kind produces movement; the other kind produces a substance.

为什么An effector is the part that carries out the response and so restores the best level. A muscle responds by contracting; a gland responds by secreting a chemical, which may be a hormone.

spinal cord

提示It runs down inside the backbone.

为什么The CNS is the coordinator of the nervous system. Information from receptors all over the body arrives there along neurones, and the CNS sends impulses out to the effectors that will respond.

electrical

提示The same kind of signal that travels along a wire.

为什么A neurone carries its message as a brief change that sweeps along the cell's long fibre. Because the signal travels within the cell rather than through the blood, it is very fast, which is why nervous responses take a fraction of a second.

Stimulus → receptor → coordinator → effector → response

提示The part that detects comes before the part that decides, and the part that acts comes after both.

为什么A receptor detects the stimulus and sends impulses to the coordinator, which is the brain or spinal cord. The coordinator sends impulses out to an effector, a muscle or a gland, and what the effector does is the response.

A stimulus

提示Light, sound, heat and touch are everyday examples.

为什么Receptor cells are each sensitive to one kind of change: light in the eye, sound in the ear, pressure and temperature in the skin. Detecting the change is the first step of every nervous response.

Receptors: light-sensitive cells in the eyes. Effectors: muscles in the arms and hands

提示Ask which part of you noticed the ball, and which part of you actually moved.

为什么The receptor cells in the eyes detect light from the ball and send impulses along neurones to the brain. The brain coordinates the response and sends impulses to the arm and hand muscles, which contract.

sensory; relay; motor

提示The first carries the signal in from the receptor, the last carries it out to the muscle, and one links them.

为什么The first neurone carries impulses from the receptor to the spinal cord. There a short linking neurone passes them straight across to the neurone that runs out to the effector, so the response happens without waiting for the conscious brain.

A synapse

提示The word begins with the same three letters as 'synchronise'.

为什么Neurones do not quite touch. When an impulse reaches the end of one neurone, a chemical is released that crosses the gap and starts a new impulse in the next neurone. A reflex arc has such a gap on each side of the relay neurone.

It is much faster, so less damage is done to the body

提示Think about what the heat would be doing to your skin during any delay.

为什么Reflex actions are automatic and rapid because the pathway does not involve the conscious part of the brain. That speed is the point: reflexes such as pulling away, blinking and coughing protect the body from harm.

The distance the ruler falls before it is caught; a shorter distance means a shorter reaction time

提示Read the scale at the point where the catcher's thumb ends up.

为什么The ruler starts with its zero mark level with the catcher's open finger and thumb. The slower the catcher reacts, the further the ruler drops before it is gripped, so the reading on the ruler can be converted into a time using a conversion table.

Independent: whether the person has had caffeine. Dependent: reaction time (the distance the ruler falls)

提示One is the thing the class changes on purpose; the other is the thing they then record.

为什么Everything else should be kept the same so that only the caffeine differs: the same person, the same hand, the same ruler and starting position, and the same surroundings. Otherwise a change in the result might have a different cause.

So the catcher cannot predict the drop; guessing early would give a falsely short reaction time

提示Imagine the dropper counted 'three, two, one' aloud before letting go.

为什么The test is meant to measure how long the catcher takes to respond to seeing the ruler move. If the catcher can anticipate the release, they start to close their fingers early and the result no longer measures that. Repeating the test and taking a mean reduces the effect of any single lucky or late catch.