Stimuli, both internal and external, are detected and lead to a response

In a nutshell

Organisms survive better when they respond to changes in their surroundings. A stimulus is a change in the internal or external environment, and a receptor detects it so that an effector can bring about a response.

This subtopic covers three linked ideas: how plants and simple animals respond to a directional stimulus (tropisms, taxes and kineses, the simple reflex), how a receptor turns a stimulus into an electrical signal (the Pacinian corpuscle and the retina), and how the heart rate is controlled by the nervous system.

Assumed knowledge: Transport across cell membranes, Mass transport in animals.

Core content

Survival and response

A stimulus is a detectable change in the internal or external environment. Detecting and responding to stimuli increases an organism's chance of survival, for example by helping it find food, avoid predators or stay in favourable conditions.

The general pathway is always:

stimulus → receptor → coordinator → effector → response

Tropisms in flowering plants

A tropism is a growth response of a plant to a directional stimulus. The growth is controlled by specific growth factors that move from growing regions to other tissues, where they change the rate of cell elongation.

The growth factor you must know is indoleacetic acid (IAA), a type of auxin.

The single most important rule, and the one examiners see reversed most often:

• In shoots, IAA stimulates cell elongation.
• In roots, IAA inhibits cell elongation.

Phototropism (response to light):

1. IAA is produced in the shoot tip and is transported to the shaded side.
2. The higher IAA concentration on the shaded side stimulates cell elongation there.
3. The shaded side grows faster than the light side, so the shoot bends towards the light (positive phototropism). This gains the shoot more light for photosynthesis.

Gravitropism (response to gravity) in a shoot or root lying horizontally: IAA gathers on the lower side of both, but the effect is opposite because roots and shoots respond to IAA differently.

The effect of IAA concentration on roots and shoots

Roots and shoots do not just respond in opposite directions, they respond to different concentrations. Roots are far more sensitive to IAA: a concentration that stimulates a shoot will strongly inhibit a root.

The graph shows the general relationship: values above the line mean growth is promoted, values below it mean growth is inhibited. At the concentration that promotes shoots most (around 10^-5 mol dm^-3), root growth is strongly inhibited.

Still don't get it? · why high IAA makes a root bend downwards

Forget biology for a second. Imagine two runners in a three-legged race, tied together at the hips. If the runner on the left slows down, the pair does not stop, they curve to the left, because the right runner keeps going and swings the whole pair round towards the slow side.

A horizontal root is that pair. Gravity pulls IAA to the lower side, so the lower side has lots of IAA and the upper side has little.

Here is the step everyone skips: in a root, IAA is the thing that SLOWS growth down. So the lower side (lots of IAA) is the slow runner, and the upper side (little IAA) keeps growing fast. The fast upper side swings the root tip round and downwards, towards gravity.

In the exam, write it as a chain and never guess the direction: IAA moves to the lower side, high IAA inhibits cell elongation on the lower side, so the upper side elongates more, so the root bends downwards (positive gravitropism). The trap is thinking "more IAA means more growth". In roots it is the opposite.

Taxes and kineses

Simple mobile organisms keep themselves in a favourable environment using two kinds of simple response.

• A taxis is a directional response: the organism moves its whole body towards or away from a directional stimulus (for example, woodlice moving away from light is negative phototaxis).
• A kinesis is a non-directional response: the organism changes its speed of movement and rate of turning according to the intensity of the stimulus, but not in a set direction.

Kinesis works when the stimulus is not directional. For example, woodlice in dry air move faster and turn less, so they move in fairly straight lines and quickly leave the dry area. When they reach a humid area they slow down and turn more, so they tend to stay. The net effect is that they gather in the favourable, humid conditions.

Whichever it is, always link the behaviour back to survival: it keeps the animal in favourable conditions, near food, or away from danger.

The simple reflex

A simple reflex is a rapid, automatic response to a stimulus. It is protective: it reduces damage to body tissues (for example, pulling your hand away from a sharp object before you have consciously felt it).

A simple reflex uses only three neurones:

stimulus → receptor → sensory neurone → relay (intermediate) neurone → motor neurone → effector → response

The reflex is fast because it involves few neurones and does not rely on conscious processing by the brain (though the brain is usually informed afterwards). Because reflexes are automatic and do not have to be learnt, they protect the body before conscious thought could act.

Receptors: the Pacinian corpuscle

Receptors share two key properties, and the Pacinian corpuscle is the example used to show them:

• A receptor responds only to a specific stimulus (the Pacinian corpuscle responds only to mechanical pressure).
• Stimulation of a receptor sets up a generator potential.

A Pacinian corpuscle is found deep in the skin and in joints, ligaments and tendons. Its structure is a single sensory neurone ending wrapped in concentric layers of connective tissue called lamellae, separated by a gel. The neurone's membrane contains stretch-mediated sodium ion channels.

How pressure produces a generator potential:

1. At rest, the stretch-mediated sodium ion channels are closed.
2. Pressure deforms the lamellae, which stretches the sensory neurone's membrane.
3. The stretch-mediated sodium ion channels open, so sodium ions diffuse into the neurone.
4. This influx of positive ions depolarises the membrane, producing a generator potential.
5. A greater pressure deforms the membrane more, so more channels open and the generator potential is larger. If it reaches the threshold, an action potential is triggered and passes along the sensory neurone.

Receptors: rods and cones in the retina

The retina contains two types of light receptor, and their differences in sensitivity to light, sensitivity to colour and visual acuity come down to two things: their optical pigments and the way they connect to bipolar neurones on the way to the optic nerve.

Why rods give high sensitivity but low acuity: many rods connect to one bipolar neurone (retinal convergence). In dim light, each rod releases only a little neurotransmitter, but because several rods feed one bipolar neurone their effects add up (spatial summation) and can reach the threshold to fire an action potential. The cost is that the brain cannot tell which rod was stimulated, so two points close together produce one impulse and appear as one: low visual acuity.

Why cones give high acuity but low sensitivity: each cone has its own bipolar neurone, so two cones stimulated by two close points send two separate impulses and the brain sees two points: high visual acuity. But with no summation, a single cone needs high light intensity to fire, so cones do not work in dim light.

Colour vision: there are three types of cone, each with a pigment sensitive to red, green or blue light. Colours in between are seen when more than one type of cone is stimulated at the same time. Note the exact wording: it is a "green-sensitive cone", not a "green cone".

Still don't get it? · how retinal convergence gives high sensitivity but low acuity

Picture a call centre. Rods are like a room where twenty phones all ring through to ONE operator. Cones are like a room where each phone has its own operator.

Sensitivity first. Each rod in dim light is a very quiet ring, too quiet on its own to get the operator's attention. But because twenty quiet rings all reach the same operator, together they are loud enough to be answered. This "adding up of weak signals" is spatial summation, and it is why you can see in dim light: many rods pool their small responses to reach the threshold.

Now acuity, the price you pay. When the shared operator answers, they know a call came in, but not which of the twenty phones rang. So two rods stimulated by two dots that are close together give just one message to the brain, and the brain sees one dot. That is low visual acuity.

Cones do the opposite. Each cone has its own operator (its own bipolar neurone), so two nearby cones send two separate messages and the brain sees two separate dots: high acuity. But one cone alone, with no one to pool with, needs a loud ring (bright light) to be answered, so cones fail in the dark.

Exam wording: rods show retinal convergence and spatial summation, giving high sensitivity but low acuity; cones each connect to a separate (bipolar) neurone, giving high acuity but low sensitivity.

Control of heart rate

The heart is myogenic: it can contract and relax on its own, without a nerve telling it to. The heartbeat is started and coordinated by the heart's own conduction system.

The sequence of one heartbeat:

1. The sinoatrial node (SAN) in the wall of the right atrium sends out a wave of electrical activity, making both atria contract (atrial systole).
2. A layer of non-conducting tissue stops the wave passing straight to the ventricles.
3. The wave reaches the atrioventricular node (AVN), which delays it. This delay lets the atria finish emptying and the ventricles fill before the ventricles contract.
4. The AVN passes the wave down the bundle of His in the septum, and on to the Purkyne tissue (fibres).
5. The Purkyne fibres carry the wave to the apex, so the ventricles contract from the base (apex) upwards (ventricular systole), pushing blood up into the arteries.

Nervous control of heart rate. The rate of the SAN is adjusted by the medulla oblongata (the cardiac centre) using the autonomic nervous system. The receptors and pathways:

The point examiners most often want, and most often miss, is frequency: the medulla changes heart rate by sending a higher frequency of impulses (not "an impulse") along the sympathetic or parasympathetic nerve to the SAN.

Required practical 10: animal responses using a choice chamber or maze

This investigates the effect of an environmental variable (such as light, humidity or temperature) on the movement of an animal, for example woodlice.

• A choice chamber is divided into regions with different conditions (for example, damp versus dry, or light versus dark), with a mesh so the animals stay in the chamber but the conditions reach them.
• Place a set number of animals in the centre, then count how many are in each region at fixed time intervals.
• Control other variables so only the chosen variable differs. For a humidity test, keep light even and use a heat filter or dim light so heat from a lamp is not an extra stimulus.
• Repeat with more animals to make results more reliable, and use a chi-squared (χ²) test to check whether the distribution differs significantly from an even (random) one.

A maze can be used instead to test directional movement (taxis) by tracking which path the animals take.

Worked examples

Model answer: "Explain how heart rate increases during exercise." (up to 6 marks)

Each numbered point is a separate marking point, and they must be a linked chain in this order:

1. Exercising muscles respire faster, so blood carbon dioxide concentration rises and blood pH falls.
2. This is detected by chemoreceptors in the aorta and carotid arteries.
3. The chemoreceptors send impulses to the medulla (cardiac centre).
4. The medulla sends a higher frequency of impulses along the sympathetic nerve.
5. These impulses go to the SAN.
6. The SAN increases the rate at which it fires waves of electrical activity, so heart rate increases, removing CO₂ faster and delivering more oxygen to the muscles.

The marks that separate strong answers: naming the sympathetic nerve and, above all, saying more/higher frequency of impulses rather than just "impulses".

Calculation: cardiac output and stroke volume

Cardiac output is the volume of blood pumped by one ventricle per minute:

$\text{cardiac output} = \text{heart rate} \times \text{stroke volume}$

A person has a cardiac output of 5040 cm³ min^-1 and a heart rate of 72 beats min^-1. Find the stroke volume.

Rearrange for stroke volume:

$\text{stroke volume} = \frac{\text{cardiac output}}{\text{heart rate}} = \frac{5040}{72} = 70 \text{ cm}^3$

The common error is to multiply cardiac output by heart rate. Always check the answer is sensible: a stroke volume of tens of cm³ is reasonable, a value in the thousands is not.

Common exam mistakes

• Writing that IAA stimulates growth in roots. In roots a high IAA concentration inhibits cell elongation. The muddled logic "more IAA means more growth, so the root bends up" scores nothing.
• Giving the direction of a tropism without the causal chain. State: IAA moves to one side, then whether it stimulates or inhibits elongation there, then which side grows more, then the direction of bending.
• Calling a kinesis a taxis, or describing kinesis as directional. Kinesis is a non-directional change in speed and rate of turning that depends on stimulus intensity.
• Labelling the receptor as the effector in a reflex arc, or leaving the relay neurone out. The effector is the muscle or gland.
• Confusing a generator potential with an action potential. The generator potential must reach the threshold before an action potential is produced.
• Writing "sodium" instead of "sodium ions", forgetting they diffuse in, or dropping the word stretch-mediated from the sodium ion channels.
• Confusing sensitivity with acuity. Rods give high sensitivity and low acuity; cones the reverse.
• Writing "green cone" instead of "green-sensitive cone", or saying red-green-blue cones are all non-functional in colour blindness. Only the specific sensitive cone type is affected.
• Using "messages" or "signals" instead of impulses or action potentials, especially for rods, cones and heart rate. These are rejected.
• Saying the medulla sends "an impulse" to the SAN. The mark is for a higher frequency of impulses.
• Putting the AVN delay in the wrong context. The delay lets the atria empty and the ventricles fill before the ventricles contract.
• Not making comparisons comparative. Say "the upper side grows more than the lower side", not just "the upper side grows".

Key definitions

• Stimulus: a detectable change in the internal or external environment.
• Taxis: a directional response in which a mobile organism moves its whole body towards or away from a directional stimulus.
• Kinesis: a non-directional response in which an organism changes its speed of movement and its rate of turning according to the intensity of the stimulus.
• Tropism: a growth response of a plant in the direction of a directional stimulus.
• Simple reflex: a rapid, automatic (involuntary) response to a stimulus, involving three neurones (sensory, relay and motor).
• Generator potential: the depolarisation produced in the membrane of a receptor when it is stimulated, which triggers an action potential if it reaches the threshold.
• Myogenic: (of cardiac muscle) able to contract and relax on its own, without stimulation from a nerve.
• Retinal convergence: where several rod cells are connected to (synapse with) a single bipolar neurone.
• Spatial summation: where several rod cells connected to one bipolar neurone together release enough neurotransmitter to reach the threshold and generate an action potential.
• Visual acuity: the ability to tell apart two points that are close together (the sharpness of vision).
• Cardiac output: the volume of blood pumped by one ventricle of the heart in one minute (heart rate multiplied by stroke volume).

Specification

• I can explain how responding to environmental change increases an organism's chance of survival.
• I can explain the effect of different concentrations of IAA on cell elongation in roots and shoots, and use it to explain gravitropism and phototropism.
• I can distinguish taxis and kinesis as simple responses that keep a mobile organism in a favourable environment.
• I can describe the protective effect of a three-neurone simple reflex.
• I can carry out and interpret an investigation into the effect of an environmental variable on the movement of an animal using a choice chamber or maze (Required practical 10).
• I can explain that a receptor responds only to a specific stimulus and that stimulation sets up a generator potential.
• I can describe the structure of a Pacinian corpuscle and explain how deformation of stretch-mediated sodium ion channels sets up a generator potential.
• I can explain how differences in the pigments and connections of rods and cones account for differences in sensitivity to light, sensitivity to colour and visual acuity.
• I can describe myogenic stimulation of the heart and the roles of the SAN, AVN and Purkyne tissue in the bundle of His.
• I can explain the roles and locations of chemoreceptors and pressure receptors, and the roles of the autonomic nervous system and effectors, in controlling heart rate.

Related notes

• Nervous coordination
• Skeletal muscles
• Homeostasis
• Mass transport in animals