Response To Stimuli

Question 1

How does responding to the environment help organisms survive?

Answer 1

Animals increase their chances of survival by responding to changes in their external environment by avoiding g harmful environments (places that are too hot or cold for them).

They can also respond to changes in internal environment to make sure that the conditions are always optimal for their metabolism (all chemical reactions that go on inside them).

Plants also increase chances of survival by responding to changes in light, temperature, CO2, etc.

Any changes to internal or external environment is called a stimulus.

Question 2

What Is A Stimulus?

Answer 2

A stimulus is a detectable change in the internal or external environment of an organism that leads to a response in the organism.

This is a characteristic of life and increases chance of survival.

Question 3

What Is A Receptor?

Answer 3

Stimuli are detected by receptors.

Receptors are specific to one type of stimulus.

Some receptors are cells like photoreceptors and some are proteins on cell surface membranes (e.g. glucose).

Question 4

What Is A Coordinator?

Answer 4

A coordinator formulates a suitable response to a stimulus.

Coordination may be at the molecular level or involve a large organ such as the brain.

Question 5

What Is An Effector?

Answer 5

A response is produced by an effector.

This response may be at the molecular level or involve the behaviour of a whole organism.

One means of communication is via chemicals called hormones which is a relatively slow process found in both plants and animals.

Question 6

Means Of Communication In Organisms?

Answer 6

Plants and Animals - hormonal communication.

Just Animals - the nervous system.

Question 7

The Nervous System?

Answer 7

Justin animals.

Nervous systems usually have many different receptors and control affecters.

Each receptor and affecter is linked to a central coordinator. The coordinator acts like a switchboard connecting information from each receptor with the appropriate affect. The sequence of events can therefore involve either chemical control or nerve cells and may be summarised as:

Stimulus -> Receptor -> Coordinator -> Effector -> Response

Question 8

What Is A Taxis?

Answer 8

A taxi is is a simple response who’s direction is determined by the direction of the stimulus.

Positive Taxis - the organism moves towards the stimulus.

Negative Taxis - the organism moves away from the stimulus.

Question 9

Examples Of Taxis?

Answer 9

Single-celled algae will move towards light (positive phototaxis). This increases their chance of survival since being photosynthetic, they require light to manufacture their food.

Earthworms will move away from light (negative phototaxis). This increases their chance of survival because it takes them into the soil where they are better able to conserve water, find food and avoid some predators.

Some species of bacteria will move towards a region where glucose is more highly concentrated (positive chemotaxis). This increases the chance of survival because they use glucose as a source of food.

Question 10

What Is A Kinesis?

Answer 10

A kinesis is a form of response in which the organism does not move towards or away from a stimulus.

Instead, it changes the speed at which it moves and the rate at which it changes direction.

Question 11

Examples Of Kinesis?

Answer 11

Woodlice lose water from their bodies in dry conditions. When they move from a damp area into a dry one, they move more rapidly and change direction more often. This increases the chance of moving back into the damp area. Once back in the damp area, they slow down and change direction less.

However, if after some time spent changing direction rapidly, they are still in the dry area, their behaviour changes again. Instead, they move rapidly in straight lines, which increases the chances of moving through the dry area and into a new damp one. This increases the chance of survival.

Question 12

What is a choice chamber?

Answer 12

These investigate animal responses.

A choice chamber is a container in which a different environmental condition is created within it. An example of this is the woodlice example.

How to make a simple one:
- use a Petri dish and divide it into two components using a little piece of cardboard. Put a piece of fine mesh over the top of the divider and place the woodlice onto this mesh. The divider shouldn’t physically split the woodlice up because the mesh should sit on top of the divider.

1. To investigate light intensity, cover one half of the lid with black paper. Put a damp filter in both half of the Petri dish base. After 10 mins, take the lid off and record the number of woodlice on each condition. Try to maximise the amount of time the lid is off so the conditions are not affected by O2 concentration.
2. Repeat the experiment after gently moving the woodlice back to the centre of the dish. You should find the woodlice prefer dark and damp areas.

Investigate humidity by doing this but using dark paper on both sides. Make one of the papers dark and damp but one of the papers dark and dry. Use desiccating drying agent in the dry side.

Handle the woodlice with care for ethical reasons.
Wash hands after touching woodlice.
Put them in natural habitat after.

Question 13

What Is Tropism?

Answer 13

A tropism is the growth of a part of a plant in response to a directional stimulus.

The plant part grows toward (positive response) or away (negative response) from the stimulus.

E.g. plant roots might grow toward light (positive phototropism).
Plant roots might grow away from gravity (negative gravitropism).

Tropisms are controlled by IAA.

A tropism is the response of a plant to a directional stimulus (stimulus coming for a particular direction).

Plants respond to stimuli by regulating their growth.

A positive tropism is growth towards the stimulus.

A negative tropism is growth away from the stimulus.

Question 14

Examples Of Tropism?

Answer 14

Plant shoots grow towards the light (positive phototropism) and away from gravity (negative gravitropism) so that their leaves are in the most favourable position to capture light for photosynthesis.

Plant roots grow away from light (negative phototropism) and towards gravity (positive gravitropism). In both cases the response increases the probability that it will grow into the soil, where they are better able to absorb water and mineral irons.

Shoots are negativity gravitropic because they grow upwards.

Roots are positively gravitropic because they grow downwards.

Question 15

Plants Response To Light, Gravity And Water?

Answer 15

Light - shoots grow towards the light, e.g. they are positively phototropic because the light is needed for photosynthesis.

Gravity - plants need to be firmly anchored in the soil. Roots are sensitive to gravity and grow in the direction of its pull, e.g. they are positively gravitropic.

Water - almost all plant roots grow towards water, e.g. or positively hydrotropic, in order to absorb it for use in photosynthesis and other metabolic processes, as well for support.

Question 16

What Are Plant Growth Factors?

Answer 16

Plant growth factors are produced in small quantities.

Growth factors are hormone-like chemicals that speed up or slow down plant growth.

They’re produced in the growing regions of the plant (shoot tip and leaves).

They move to where they’re needed in the other parts of the plant.

Growth factors called auxins stimulate the growth of shoots by cell elongation (where cell walls become loose and stretchy, so the cells get longer). High concentrations of auxins inhibit growth in roots though. E.g. IAA.

Question 17

Why Are Plant Growth Factors Not Hormones?

Answer 17

Plant responses to external stimuli involve hormone-like substances.

These are called ‘plant growth factors’. They are called growth factors because:

• they exert their influence by affecting growth and they may be made by cells located throughout the plant rather than in particular organs,
• unlike animal hormones, some plant growth factors affect the tissues that release them rather than acting on a distant target organ.

Question 18

How does IAA effect phototropism and gravitropism in plants?

Answer 18

High IAA concentrations increases growth by elongation in shoots and inhibits growth in roots.

Phototropism -
Shoots - The high concentration of IAA in the shaded part of the plant tells the cells to elongate and grow toward the light.

Roots - High IAA concentration in the roots shaded side bends the root away from the light by inhibiting the root so it bends away from light.

Gravitropism -
Shoots - IAA concentration increases on the lower (beneath) side so the cells elongate and grow upwards.

Roots - IAA concentration increases on the lower (beneath side) of the root so that growth is inhibited and the root grows downwards.

Question 19

Phototropism In Flowering Plants By IAA?

Answer 19

If the shoots of flowering plants grow towards unilateral light, the following events happen:

1. Cells in the tip of the shoot produce IAA, which is then transported down the shoot.
2. The IAA is initially transported evenly throughout all regions as it begins to move down the shoot.
3. Light causes the movement of IAA from the light side to the shaded side of the shoot.
4. A greater concentration of IAA builds up on the shaded side of the shoot then on the light side.
5. As IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot, the cells on this side elongate more.
6. The shaded side of the shoot elongates faster than the light side, causing the shoot tip to bend towards the light.

Question 20

Other Things IAA Does?

Answer 20

IAA also controls the bending of roots in response to light.

However, where is a high concentration of IAA increases cell elongation in shoot, it inhibits cell elongation in roots.

For example, an IAA concentration of 10 ppm increases shoot cell elongation by 200% but decreases route cell elongation by 100%. As a result, in boots the elongation of cells is greater on the light side then on the shaded side and so rude spend away from light

Question 21

Gravitropism In Flowering Plants?

Answer 21

Response of a horizontally growing route to gravity:

1. Cells in the tip of the route produce IAA, which is then transported along the route.
2. IAA is initially transported to all sides of the root.
3. Gravity influences the movement of IAA from the upper side to the lower side of the root.
4. A greater concentration of IAA builds up on the lower side of the road then on the other side.
5. As IAA inhibits the elongation of root cells and there is a greater concentration of IAA on the lower side, the cells on this side elongate less than those on the other side.
6. The relatively greater elongations of cells on the other side compare to the lower side cause the root to bend downwards towards the force of gravity.

In shoots, the greater concentration of IAA on the lower side increases cell elongation and causes this side to elongate more than the upper side. As a result, the shoot grows upwards away from the force of gravity.

Question 22

Why do plants respond to stimuli?

Answer 22

Plants need to respond to stimuli to increase their chances in survival.

• They can sense direction of light and grow towards it to maximise their survival and photosynthesis.
• They can sense gravity, so their roots and shoots grow in the right direction.
• Climbing plants have a sense of touch and so they can find things to climb up and reach the sunlight.

Question 23

What is IAA?

Answer 23

Indoleacetic acid (IAA) is an important auxin.

It’s produced in the tips and shots in flowering plants.

IAA is moved around the plant to control tropisms - it moves by diffusion and active transport over short distances, and via the phloem over long distances.

This results in different parts of the plant having different concentrations of IAA. The uneven distributions of IAA means there is uneven growth.

Question 24

Role Of IAA In Elongation Growth?

Answer 24

The transport of IAA is in one direction, namely away from the tip of the shoots and roots where it is produced.

IAA has a number of effects on plant cells, including increasing the plasticity ability to stretch of the cell walls. The response only occurs in young cell walls where cells are able to elongate. As the cells mature, they develop greater rigidity, therefore older part of the shoot route will not be able to respond.

Question 25

Explanation As To Why IAA Increases Plasticity In Cells?

Answer 25

The increase of plasticity in cells (only cells that are able to elongate) is called: acid growth hypothesis.

It involves the active transport of hydrogen ions from the cytoplasm moving into spaces in the cell wall causing the cell wall to become more plastic allowing the cell to elongate by expansion.

(This leads onto the idea of IAA and gravitropism).

Question 26

Nervous System Has Two Major Divisions?

Answer 26

The central nervous system (CNS) which is made up of the brain and spinal cord.

The peripheral nervous system (PNS) which is made up of pairs of nerves that originate from either the brain or the spinal-cord. The peripheral nervous system split again into 2 systems.

Question 27

Peripheral Nervous System Is Divided Into?

Answer 27

The somatic nervous system which controls conscious activities like running and jumping.

The autonomic nervous system which control unconscious activities like breathing. This autonomic nervous system further splits into 2 systems:

Question 28

Autonomic Nervous System Further Divides Into?

Answer 28

Sympathetic nervous system which gets the body ready for fight or flight.

The parasympathetic nervous system which gets the body ready for rest and digest.

Question 29

What Is The Spinal Cord?

Answer 29

The spinal-cord is a column of nervous tissue that runs along the back and lies inside the vertebral column for protection.

Emerging at intervals along the spinal-cord, are pairs of nerves.

Question 30

What Is A Reflex Arc?

Answer 30

The pathway of neurones involved in a reflex is known as a reflex arc.

Reflex arcs, such as withdrawing your hand from a hot object, involve just three neurons. One of the neurones is in the spinal-cord and so this type of reflex also called a spinal reflex. The main stages of a spinal reflex arcs are on next flash card.

Question 31

Reflex Arc Stages?

Answer 31

1. The stimulus - heat from hot object.
2. A receptor - temperature receptors in the skin on the back of the hand, which generates nerve impulses in the sensory neuron.
3. A sensory neurone - Passes nerve impulses to the spinal-cord.
4. A coordinator (intermediate neurone) - links the sensory neuron to the motor neuron in the spinal-cord.
5. A motor neurone - carries nerve impulses from the spinal-cord to a muscle in the upper arm.
6. An effector - the muscle in the upper arm, which is stimulated to contract.
7. The response - pulling the hand away from the hot object.

Question 32

Importance Of Reflex Arcs?

Answer 32

1 They are in voluntary and therefore do not require the decision-making powers of the brain, leaving it’s free to carry out more complex responses. In this way, the brain is not overloaded with situations in which the response is always the same. Some impulses are nevertheless sent to the brain, so that it is informed of what is happening and can sometimes override the reflex,if necessary.

• They protect the body from harm. They are affective from birth and do not have to be learnt.
• They are fast, because the neurone pathway is short with very few, typically one or two, synapses where neurones communicate with each other. This is important in with role with reflexes.
• The absence of any decision making process also means the action is rapid.

Question 33

Features Of Sensory Receptors (e.g. Pacinian Corpuscles)?

Answer 33

They respond to changes in mechanical stimuli like pressure and vibrations - they called ‘mechanoreceoptors’.

As with all sensory receptors, Pacinian Corpuscle:

• Is specific to a single type of stimulus. In this case, it responds only to mechanical pressure. It will not respond to other stimuli.
• Produces a generator potential by acting as a transducer. All stimuli involve a change in some sort of energy. It is the role of the transducer to convert the change in form of energy by the stimulus into a form, nerve impulses, that can be understood by the body. Receptors in the nervous system convert the energy of the stimulus into a nervous impulse known as a generator potential. The Pacinian corpuscle well transducers the mechanical energy of the stimulus into a generator potential.

Question 34

Where Do Pacinuan Corpuscle Occur?

Answer 34

They respond to mechanical stimuli such as pressure.

They occur deep in the skin and almost abundant on the fingers, the soles of the feet and the external genitalia.

They also occur in the joints, ligaments and tendons, where they enable the organism to know which joints are changing direction.

The single sensory neuron of a Pacinian corpuscle is that the centre of layers of tissue, each separated by a gel. This gives it the appearance of an onion when cut in half.

Question 35

Structure And Function Of A Pacinian Corpuscle?

Answer 35

We saw that plasma membranes contain channel proteins that spanned them. These proteins have channels along which items can be transported. Each channel is specific. Sodium channels, for example, carry only sodium ions.

The sensory neuron ending at the centre of the Pacinian corpuscle has a specific type of sodium channel and in it’s plasma membrane. This is called a stretch-mediated sodium channel. These channels are so-called because their permeability to sodium changes when they are deformed, for example, by stretching.

The pacinian corpuscle contains the end of a sensory neurone. The nerve ending is wrapped in loads of layers of connective tissues called lamellae.

Question 36

The Pacinian Corpuscle Functions As Follows:

Answer 36

• In its normal resting state, the stretch-mediated sodium channels of the membrane around the neuron of a Pacinian capsules are too narrow to allow sodium ions to pass along them. In this state, The neuron of a Pacinian Corpuscle has a resting potential.
• The nerve ending is wrapped in loads of layers of connective tissues called lamellae. When it’s stimulated, the lamellae are deformed and press on the sensory nerve ending. This caused the sensory neurones cell membrane to stretch, deforming the stretch-mediated sodium ion channels.
• This stretching widens the sodium channels in the membrane and sodium ions diffuse into the neurone. This creates a generated potential.
• The influx of sodium ions changes the potential of the membrane (e.g. it becomes depolarised) thereby producing a generator potential. If the generated potential reaches the threshold, an action potential occurs.
• An action potential (nerve impulse) passes along the neuron and then, via other neurons, to the central nervous system.

Question 37

What Are Transducers?

Answer 37

Convert energy into a nerve impulse.

A photoreceptor is a type of transducer. And rod and cone cells are types of photoreceptor so they’re also a type of transducer.

Question 38

Photoreceptors?

Answer 38

Photoreceptors are light receptors in your eye.

Light will enter the eye through the pupil. The amount of light that enters is controlled by the muscles of the iris.

Light rays are focused by the lens onto the retina, which lines the insides of the eye.

The retina contains the photoreceptor cells - these detect the light.

The fovea is an area of the retina where lots of photoreceptors are present.

Nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve, which is a bundle of neurones.

Where the optic nerve leaves the eye is called the blind spot - there aren’t any photoreceptor cells here and so it’s not sensitive to light.

Question 39

How do photoreceptors convert light into electrical impulse?

Answer 39

1. Light enters the eye and hits the photoreceptors and is absorbed by the light-sensitive optical pigments.
2. Light bleached the pigments and this causes a chemical change which alters the membrane permeability to sodium ions.
3. A generator potential is created and if it Rachel’s the threshold, a nerve impulse is sent along a bipolar neurone. Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.
4. The eye has two types of photo receptor - rods and cones.

Question 40

Rod Cells?

Answer 40

A light receptor cell of the mammalian eye that is found on its innermost layer, the retina.

The millions of light receptors found in the retina are of two main types: rod cells and cone cells. Both are photoreceptors.

• Rod-shaped.
• Greater numbers than cone cells - 120 million in each eye.
• Distribution - more at the periphery of the retina, absent at the fovea.
• Give poor visual acuity.
• Sensitive to low intensity light.
• One type only,
• More sensitive than cone cells. This is because 3 rod cell joins one neurone so it takes less light to reach the threshold and trigger an action potential,
• Rods have low visual acuity because many rods join
  the same neurone, which means light from two points close together can’t be told apart,
• Very sensitive to light because many rods join one nerve one and so many weak generator potentials combine to reach threshold and trigger an action potential.

Question 41

Cone Cells?

Answer 41

A light receptor cell of the mammalian eye that is found on its innermost layer, the retina.

The millions of light receptors found in the retina are of two main types: rod cells and cone cells.

• Cone-shaped,
• Fewer numbers than rod cells. Around 6 million each eye.
• Fewer at the periphery of the retina and more concentrated at the fovea.
• Give good visual acuity.
• Not sensitive to low intensity light.
• Three types each responding to different wavelengths of light,
• Less sensitive than rods because one rod joins one neurone which means more light is needed to reach threshold to trigger an action potential,
• Cones have high visual acuity because cones are close together and one cone joins one neurone. This means when light from two points hits two neurones (one from each cone), two action potentials reach the brain and two points can be distinguished that are close together.

Question 42

The Automatic Nervous System?

Answer 42

The automatic nervous system controls the involve wintry activities of internal muscles and glands. It has two divisions:

• The sympathetic nervous system. In general, this stimulates affecters and so speeds of any activity. It acts rather like an emergency controller. It controls effectors when we exercise strenuously or experience experience powerful emotions. In other words it helps us cope with stressful situations by heightening our awareness and preparing us for activity (the fight or flight response).
• The parasympathetic nervous system. In general, this inhibits effectors and so slows down any activity. It controls the activities under normal resting conditions. It is concerned with conserving energy and replenishing the bodies reserves.

These two actions are opposites and so they are antagonistic. If one system contracts a muscle, the other relaxes it.

Question 43

The Hormonal System?

Answer 43

The hormonal system produces chemicals (hormones) that are transported via blood plasma to target cells.

Target cells have receptors on cell-surface membranes and change in concentration of hormones stimulates them. This results in a slower, less-specific form of communication between parts of an organism.

The responses are often long lasting and widespread.

E.g. control of glucose concentration in blood produces at a slow rate but lasts longer and is more widespread.

Question 44

The Nervous System?

Answer 44

The nervous system uses nerve cells to pass electrical impulses along their length.

They stimulate their target cells by secreting chemicals, known as neurotransmitters, directly onto them.

This results in rapid communication between specific part of an organism. The responses produced often short lived and restricted to a localised region of the body. An example of nervous coordination is a reflex action, such as the withdrawal of the hand from an unpleasant stimulus, such as heat. For obvious reasons, this type of action, is rapid, short lived and restricted to one region of the body.

Question 45

Myogenic?

Answer 45

A contraction is initiated from within the muscle itself.

E.g. the heart.

Question 46

Neurogenic?

Answer 46

A contraction is initiated from the outside and so the nervous impulses are outside the muscle.

Question 47

What Is The Sinoatrual Node?

Answer 47

Also know as ‘SAN’.

They are within the wall of the right atrium of the heart. It is from here that the initial stimulus for contraction originates. The sinoatrial node has a basic rhythm of stimulation that determines the beat of the heart. It is for this reason, it is often referred to as the pacemaker.

Question 48

Sequence Of Events That Controls The Basic Heart Rate?

Answer 48

1. The process starts in the Dino trail node (SAN) which is in the wall of the right atrium.
2. The SAN is a pacemaker - sets rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls.
3. A wave of electrical excitation spreads out from the sinoatrial node across both atria, causing them to contract. The heart contracts from top to bottom.
4. A layer of non-conductive collagen tissue (atrioventricular septum) prevents the wave crossing to the ventricles.
5. The wave of excitation enters a second group of cells called the atrioventricular node (AVN), which lies between the atria.
6. The atrioventricular node, after a short delay to make sure the atria have emptied, conveys a wave of electrical excitation between the ventricles along a series of specialised muscle fibres called ‘Purkyne tissue’ which collectively make up a structure called ‘the bundle of His’.
7. The bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity between the ventricles to the apex (bottom) of heart. The bundle splits into finer muscle fibres in the right and left ventricle walls, called the Purkyne tissue.
8. The bundle of his conducts the wave through the atrioventricular septum to the base of both ventricles, where the bundle branches into smaller fibres of Purkyne tissue.
9. The wave of excitation is released from the Purkyne tissue, causing the ventricles to contract quickly at the same time, from the bottom of the heart upwards.

Question 49

Modifying The Resting Heart Rate?

Answer 49

The resting heart rate of a typical adult human is around 70 bpm.

However, it is essential that this rate can be altered to meet a very in demand for oxygen.

During exercise, for example, the resting heart rate may need to be more than double.

Changes to the heart rate or controlled by a region of the brain called the Medulla Oblongata.

Question 50

How is heart rate controlled by brain and autonomic nervous system?

Answer 50

The SAN generates electrical impulses that cause the cardiac muscles to contract.

The rate at which the SAN fires (heart rate) is unconsciously controlled by the medulla oblongata.

Animals need to alter their heart rate to respond to internal stimuli (prevent fainting s due to lack of oxygen supply).

Stimuli are detected by pressure receptors and chemical receptors:

• pressure receptors called baroreceptors in the aorta and the carotid arteries (major arteries in neck). They’re stimulated by high and low blood pressure.
• there are chemical receptors called chemoreceptors in the aorta, the carotid arteries and in the medulla. They monitor the oxygen level in the blood and also CO2 and pH (indicators of O2 level).

Electrical impulses from the receptors are sent to the medulla along sensory neurones. The medulla processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones (part of the autonomic nervous system).

Question 51

The Medulla Oblongata?

Answer 51

This has two centres concerned with heart rate:

• Acentre that increases heart rate, which is linked to the sinoatrial node by the sympathetic nervous system.
• Centre that decreases heart rate, which is linked to the sinoatrial node by the parasympathetic nervous system.

Which of these centres is stimulated depends upon the nerve impulses they receive from the two types of receptor, which respond to stimuli of either chemical or pressure changes in the blood.

Question 52

What Are Chemoreceptors?

Answer 52

Chemoreceptors are found in the wall of the carotid arteries (the arteries that serve the brain).

They are sensitive to changes in the pH of the blood that result from changes in carbon dioxide concentration.

In solution, carbon dioxide forms of acid and therefore lowers pH.

Question 53

Process Of Control From Chemoreceptors?

Answer 53

1. When the blood has a higher than normal concentration of carbon dioxide, the bloods pH is lowered.
2. The chemoreceptors in the wall of the carotid arteries and the aorta detect this and increase the frequency of nervous impulses to the centre in the medulla oblongata that increases heart rate.
3. This centre increases the frequency of impulses via the sympathetic nervous system to the sinoatrial node. This, in turn, increases the rate of production of electrical waves by the sinoatrail node and therefore increases the heart rate.
4. The increased blood flow that this causes leads to more carbon dioxide being removed by the lungs and so the carbon dioxide concentration of the blood returns to normal.
5. As a consequence, the pH of the blood rises to normal and the chemoreceptors in the wall of the carotid arteries and aorta reduce the frequency of nerve impulses to the medulla oblongata.
6. The medulla oblongata reduces the frequency of impulses to the sinoatrial node, which therefore leads to a reduction in the heart rate.

Question 54

Process Of Control By Pressure Receptors?

Answer 54

Pressure receptors occur within the walls of the carotid arteries and the aorta. They operate as follows:

• When blood pressure is higher than normal, pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that decreases the heart rate. This centre sends impulses via the parasympathetic nervous system to the sinoatrial node of the heart, which leads to a decrease in the rate at which the heart beats.
• When blood pressure is lower than normal, pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that increases heart rate. This centre sends impulses via the sympathetic nervous system to the sinoatrial node, which increases the rate at which the heart beats.

Question 55

Principles Of Coordination?

Answer 55

There are two main types of coordination in animals: nervous system and hormonal system.

Question 56

Nervous coordination is…?

Answer 56

Localised - when an electrical impulse reaches the end of a neurone, neurotransmitters are secreted directly onto target cells - so the nervous response is localised.

Short-lived - neurotransmitters are quickly removed once they’ve done their job.

Rapid - electrical impulses are really fast and so the response and effect is rapid, unlike hormonal responses.

Question 57

Types of neurones and how they work?

Answer 57

Made up of network of neurones. Three types of neurones:

• sensory neurones transmit electrical impulses from the receptors to the central nervous system (CNS - the brain and spinal cord is the CNS).
• motor neurones transmit electrical impulses from the CNS to the effectors.
• relay neurones transmit electrical impulses between sensory and motor neurones.

How:

1. Stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neurone.
2. Electrical impulse reaches end of neurone and chemicals called neurotransmitters take the information across to the next neurone.
3. Impulse is sent to CNS (the coordinator) processes the information and sends impulses along motor neurone to an effector.

Question 59

How does heart respond to high blood pressure?

Answer 59

Baroreceptors detect high blood pressure.

Impulses are sent to the medulla, which sends impulses along the parasympathetic neurones.

These secrete acetyl chlorine (neurotransmitter) which binds to the receptors on the SAN.

Effector - cardiac muscles.
Response - heart rate is slowed to reduce blood pressure back to normal.

Question 60

How does heart respond to low blood pressure?

Answer 60

Baroreceptors detect low blood pressure.

Impulses are sent to the medulla, which sends impulses along sympathetic neurones.

These secrete noradrenaline (neurotransmitter) which binds to receptors on SAN.

Effector - cardiac muscles.
Response - heart rate speeds up to increase blood pressure back to normal.

Question 61

How does heart respond to high blood O2, low CO2 or high pH levels?

Answer 61

Chemoreceptors detect the chemical changes in the blood.

Impulses are sent to the medulla, which sends impulses along the parasympathetic neurones.

These secrete acetylcholine which binds to SAN.

Effector - cardiac muscles.
Response - heart rate decreases to return O2, CO2 and pH levels to normal.

Question 62

How does the heart respond to low blood O2, high CO2 or low pH levels?

Answer 62

Chemoreceptors detect chemical changes in the blood.

Impulses are sent to the medulla, which sends impulses along the sympathetic neurones.

These secrete noradrenaline which binds to receptors on SAN.

Effector - cardiac muscles.
Response - heart rate increases to return O2, CO2 and pH levels to normal.

Question 65

Why do receptors work?

Answer 65

Receptors communicate via the nervous system.

How?
1. When a nervous system receptor cell is in its resting state (not being stimulated), there’s a difference in charge between the inside and outside of the cell - generated by ion pumps and ion channels.

2. The potential difference when a cell is at rest is called it’s resting potential. When a stimulus is detected, the cell membrane is excite and becomes more permeable, allowing more ions to move in and out the cell and altering the potential difference.

The change in potential difference is due to a stimulus called the generator potential.

4. A bigger stimulus will excite the membrane more and cause a bigger movement of ions and a bigger change in potential difference and so a bigger generator potential is created.
5. If generator potential reaches past the threshold, it’ll trigger an action potential - when an electrical impulse moves along a neurone.

Action potentials are all the same size (all or nothing principle) so the strength of the stimulus is measured by the frequency of action potentials (number of action potentials triggered over a certain time).

If a stimulus is too weak, it won’t reach the threshold and there will be no action potential.