What a coordinated response is
Definition
Stimulus
A stimulus is a change in the environment, inside or outside the body, that is detected by a receptor.
Definition
Coordination
Coordination is the process by which the nervous system links information from receptors to the correct effectors so that different parts of the body work together as one response.
- A coordinated response is one in which the right effectors act, in the right order and by the right amount, because the CNS has processed information from receptors first.
- Without coordination each organ would act on its own, so muscles could pull against each other and a response could work against what the body needs.
- Coordination needs three things: information arriving from receptors, a central point where that information is compared, and a route out to the effectors.
- The nervous system supplies all three, which is why it can turn a single stimulus into a whole-body response.
Key Idea
The nervous system coordinates a response because every receptor sends its information to one place, and that same place controls every effector.
The pathway from stimulus to response
- Every coordinated response follows the same sequence of components.
- stimulus→receptor→sensory neurone→CNS→motor neurone→effector→response\text{stimulus} \rightarrow \text{receptor} \rightarrow \text{sensory neurone} \rightarrow \text{CNS} \rightarrow \text{motor neurone} \rightarrow \text{effector} \rightarrow \text{response}stimulus→receptor→sensory neurone→CNS→motor neurone→effector→response
- The receptor detects the change and turns it into an electrical impulse.
- The sensory neurone carries that impulse into the brain or spinal cord.
- Inside the CNS the impulse is passed across synapses to relay neurones, where it is compared with information from other receptors and with what is stored in the brain.
- The CNS then selects which motor neurones to fire, and each one carries impulses out to a particular effector.
- The effectors contract or secrete, and together their actions make up the response.
Common Mistake
- Do not name the effector as the stimulus, because the stimulus is the change that was detected, not the part that acts.
- Do not miss out the CNS, because a pathway that runs straight from a sensory neurone to a motor neurone is not coordinated by anything.
Receptors in different parts of the body
Definition
Receptor
A receptor is a cell or group of cells that detects a stimulus and converts it into an electrical impulse.
Definition
Sense organ
A sense organ is an organ containing groups of receptor cells that detect one particular type of stimulus.
- The nervous system can respond to many different kinds of change because it has several different types of receptor, each tuned to one stimulus.
- The eyes contain light receptors in the retina.
- The ears contain sound receptors, and also receptors that detect the position of your head so you can balance.
- The nose contains receptors that detect chemicals carried in the air, and the tongue contains receptors that detect chemicals dissolved in saliva.
- The skin contains separate receptors for touch, pressure, temperature and pain, which is why you can feel a light brush and a hot surface as different things.
- Receptors inside the body monitor internal conditions such as blood temperature, blood glucose concentration and the stretch of a muscle.
- All of them send the same kind of electrical impulse, so the CNS identifies the stimulus by which neurone the impulse arrives on, not by what the impulse looks like.
Example
Pressing gently on a closed eyelid makes you see a faint patch of light, because the brain treats any impulse arriving down the optic nerve as light however it was produced.
Why the nervous system can coordinate
- It reaches every part of the body, because nerves branch out from the spinal cord to every organ, muscle and area of skin.
- It has an enormous number of links between neurones, since one neurone in the CNS forms synapses with thousands of others.
- Those links let the impulse from one receptor be routed to any effector or to several effectors at once.
- It receives information from many types of receptor at the same time, so the CNS can weigh up what it sees, hears and feels before choosing a response.
- It can also block a response, because a relay neurone that is not stimulated enough does not pass the impulse on, so unimportant signals are filtered out.
- One stimulus can therefore produce a whole set of matched actions, such as several muscles contracting in sequence while a gland releases a substance.
Example
Catching a ball thrown towards you
- Light receptors in both retinas detect the ball and send impulses along the optic nerves.
- The brain compares the two images to judge how far away the ball is and how fast it is moving.
- Stretch receptors in your leg and back muscles report where your body already is.
- The brain sends impulses along motor neurones to leg muscles, arm muscles and hand muscles together.
- Those effectors move you into position, raise your arms and close your fingers at the moment the ball arrives.
- The catch works only because one control centre timed all of those separate muscles against each other.
Voluntary responses
Definition
Voluntary response
A voluntary response is a response that is consciously decided by the cerebrum before impulses are sent to the effector.
- In a voluntary response the impulse travels all the way up to the cerebrum, where a conscious decision is made before any motor neurone fires.
- That extra journey means the impulse crosses many more synapses, and each one adds a delay while the neurotransmitter diffuses.
- A voluntary response is therefore slower than an automatic one, but it can be changed to suit the situation and it can be learnt and improved with practice.
Reaction time
Definition
Reaction time
Reaction time is the time taken between a stimulus being detected by a receptor and the response being carried out by an effector.
- Reaction time measures how long the whole pathway takes, from the receptor detecting the stimulus to the effector acting.
- A typical voluntary reaction time is about 0.20 s0.20\ \text{s}0.20 s to 0.25 s0.25\ \text{s}0.25 s, and most of that time is spent crossing synapses rather than travelling along axons.
- Reaction time gets longer with tiredness, with alcohol, with some drugs, with distraction such as a phone, and with increasing age.
- Reaction time gets shorter with practice at the same task, and caffeine also shortens it for a while.
- This matters in real situations such as driving, where a longer reaction time means a longer thinking distance before the brakes are even touched.
Practical
Measuring reaction time with a falling ruler
- Aim: to measure reaction time and to find out how one chosen factor, such as which hand is used or whether the person is distracted, changes it.
- Apparatus: a 30 cm30\ \text{cm}30 cm ruler marked in millimetres, a chair with a flat-topped table, a results table and a conversion table or calculator.
- Variables: the chosen factor is the independent variable, the distance the ruler falls is the dependent variable, and the person being tested, the hand used, the starting position of the ruler, the height of the table and the surroundings are all controlled.
- Method, setting up:
- The person being tested sits upright with one forearm resting flat along the table so that only the hand hangs over the edge.
- They hold the thumb and first finger about 2 cm2\ \text{cm}2 cm apart, ready to close them but not moving.
- A partner holds the ruler vertically so that the 0 cm0\ \text{cm}0 cm mark is level with the top of the thumb, and checks that it hangs straight down between the fingers without touching them.
- The partner gives no countdown and no warning word, because a warning lets the brain prepare and shortens the measured time.
- Method, taking readings:
- The partner releases the ruler at a moment the tested person cannot predict, without pushing it downwards.
- The tested person catches the ruler as soon as they see it fall, keeping their eyes on the ruler rather than on the partner's hand.
- Read the mark level with the top of the thumb, in millimetres, with your eye at the same height as the thumb to avoid a parallax error.
- Record the distance, then repeat the drop at least ten times and calculate a mean, discarding any drop the person clearly anticipated or fumbled.
- Change only the chosen factor and repeat the whole set of drops with everything else kept the same.
- Results: the ruler falls further when reaction time is longer, so a distracted person or a tired person catches the ruler lower down.
- Maths: convert the mean distance ddd in metres into a time using t=2dgt=\sqrt{\dfrac{2d}{g}}t=g2d with g=9.8 m s−2g=9.8\ \text{m s}^{-2}g=9.8 m s−2, then compare the mean times for the two conditions.
- Watch out: anticipating the drop, resting the hand on the ruler, changing the starting gap between the fingers and letting the person watch the partner's arm instead of the ruler all make the results unreliable.
- Improving it: a computer test that records the time between a shape appearing on screen and a key being pressed removes the human reading of the scale and measures the time directly.
- Safety: use a plastic ruler rather than a metal one, keep the falling path clear of faces and never test anyone who does not want to take part.
Working out a reaction time
- The ruler measures a distance, so the distance has to be converted before you can compare reaction times.
- Always calculate the mean distance first, because converting each drop separately and then averaging the times gives a slightly different answer.
- Convert centimetres to metres before substituting, since 18 cm=0.18 m18\ \text{cm}=0.18\ \text{m}18 cm=0.18 m.
Example
Converting a ruler drop into a time
- Five drops give catch distances of 19 cm19\ \text{cm}19 cm, 22 cm22\ \text{cm}22 cm, 20 cm20\ \text{cm}20 cm, 21 cm21\ \text{cm}21 cm and 18 cm18\ \text{cm}18 cm.
- The mean distance is 19+22+20+21+185=20 cm\dfrac{19+22+20+21+18}{5}=20\ \text{cm}519+22+20+21+18=20 cm.
- In metres this is d=0.20 md=0.20\ \text{m}d=0.20 m.
- Substituting into t=2dgt=\sqrt{\dfrac{2d}{g}}t=g2d gives t=2×0.209.8t=\sqrt{\dfrac{2\times0.20}{9.8}}t=9.82×0.20.
- This works out as t=0.0408=0.20 st=\sqrt{0.0408}=0.20\ \text{s}t=0.0408=0.20 s.
- The reaction time is 0.20 s0.20\ \text{s}0.20 s, which is close to the value expected for an alert person.
Exam technique
Explaining a coordinated response
- Start with the stimulus in the words of the question, then name the sense organ or receptor that detects it.
- Say impulse rather than message or signal, because the mark scheme wants the correct term.
- Name the specific muscle or gland as the effector, since a vague ending loses the last mark in an explain question.
- When a question asks why one factor changed reaction time, quote the two mean values from the data before you explain them.
- If asked to improve the ruler method, name the change and its effect together, such as using an electronic timer so the reading no longer depends on judging a scale.
Limits of nervous coordination
- Nervous coordination is fast but short-lived, because a muscle relaxes as soon as the impulses stop arriving.
- It is also targeted, because a motor neurone ends on one particular effector rather than reaching the whole body.
- Changes that need to be held steady for hours or years are controlled by chemical messengers carried in the blood instead.
Self review
- Write out the seven stages of a coordinated response in order.
- Name the stimulus detected by receptors in the ear, the nose and the tongue.
- Give two features of the nervous system that allow one stimulus to produce a whole-body response.
- Explain why a voluntary response takes longer than an automatic one.
- Describe how you would use a falling ruler to compare two people's reaction times fairly.
- State three factors that make reaction time longer.
