6A - Stimuli and Responses Flashcards
What types of changes do animals respond to?
Changes in:
• External environment
• Internal environment
Why do animals respond to changes in external environment?
To increase their chances of survival (e.g. by avoiding hazards).
Why do animals respond to changes in their internal environment?
To make sure that conditions are optimal for metabolism.
Do plants respond to changes in their environment?
Yes
What is a stimulus?
A change in the internal or external environment.
What detects a stimulus?
A receptor
What are receptors?
Cells or proteins on cell membranes that detect a stimulus.
Can receptors detect more than one type of stimulus?
No, they are specific to one type of stimulus.
What are effectors?
Cells that bring about a response to a stimulus.
What are the two ways receptors can communicate with effectors?
Via the:
• Nervous system
• Hormonal system
What is the nervous system made of?
A complex network of cells called neurones.
What are the types of neurones?
- Sensory neurones
- Relay neurones
- Motor neurones
What are sensory neurones?
Neurones that transmit electrical impulses from receptors to the CNS.
What are relay neurones?
Neurones that transmit electrical impulses from the sensory neurones to the motor neurones.
What are motor neurones?
Neurones that transmit electrical impulses from the CNS to effectors.
What are neurones?
Cells that transmit electrical signals.
Describe the path of an electrical signal in a non-reflex reaction.
Stimulus -> Receptor -> Sensory neurone -> CNS -> Motor neurone -> Effector -> Response
What does the CNS do in a non-reflex reaction?
- It acts as a coordinator
* Processes the information and sends impulses along motor neurones
What is the nervous system split into?
- Central nervous system (CNS) -> Brain + Spinal cord
* Peripheral nervous system
What is the CNS made up of?
Brain and spinal cord
What is the peripheral nervous system made up of?
The neurones that connect the CNS to the rest of the body
What are the two systems of the peripheral nervous system? What does each do?
- Somatic nervous system -> Conscious activities
* Autonomic nervous system -> Unconscious activities
What are the two divisions of the autonomic nervous system? What does each do?
- Sympathetic nervous system -> Gets the body ready for action (“flight or fight”)
- Parasympathetic -> Calms the body down (“rest and digest”)
Describe the entire structure of the nervous system with subdivisions.
Nervous system is split into:
• CNS
• Peripheral nervous system
Peripheral nervous system is split into:
• Somatic nervous system
• Autonomic nervous system
Autonomic nervous system is split into:
• Sympathetic nervous system
• Parasympathetic nervous system
What is a reflex?
Where the body responds to a stimulus without making a conscious decision to respond.
Why is a reflex response very fast?
No time is spent on deciding how to respond
What are reflexes useful for?
Protecting the body from dangerous stimuli because they are rapid.
What is the pathway of neurones in a reflex reaction called?
Reflex arc
Describe the path of a simple reflex arc.
- Receptor
- Sensory neurone
- Relay neurone
- Motor neurone
- Effector
(There are synapses between the neurones)
Describe the reflex arc for a heat-withdrawal response to heat.
- Thermoreceptors in the skin detect heat stimulus
- Sensory neurone carries impulse to relay neurone
- Relay neurone connects to motor neurone
- Motor neurone sends impulses to the effective muscles
- Muscles contract to withdraw body part and stop it being damaged
When and how is it possible to override a reflex reaction?
If there is a relay neurone involved, the brain can override the signal (e.g. tell the body to not move the hand away from heat)
What 3 words define the nervous system (as oppose to the hormonal system)?
- Localised
- Short-lived
- Rapid
What is nervous system communication localised?
When an electrical impulse reaches the end of a neurone, neurotransmitters are secreted directly onto target cells.
Why is nervous system communication short-lived?
Neurotransmitters are quickly removed once they’ve done their job.
Why is nervous system communication rapid?
Electrical impulses are very fast.
What are some changes that plants respond to?
- Light
- Gravity
- Touch (only climbing plants)
What is a tropism?
The response of a plant to a directional stimulus.
What is a directional stimulus?
A stimulus coming from a particular direction.
How do plants respond to stimuli?
By regulating their growth.
What is a positive and negative tropism?
- Positive -> Towards stimulus
* Negative -> Away from stimulus
What is the name for a plant’s response to light?
Phototropism
What is the name for a plant’s response to gravity?
Gravitropism
What are the tropisms of plant shoots?
- Positive phototropism
* Negative gravitropism
What are the tropisms of plant roots?
- Negative phototropism
* Positive gravitropism
What brings about plant responses to stimuli?
Growth factors
What are growth factors?
Chemicals that speed up or slow down the growth of plants.
Where are growth factors produced?
- In the growing parts of the plant (e.g. shoot tips)
* They then move to where they’re needed
What are auxins?
Growth factors that cause growth of shoots by cell elongation, but cause inhibition of growth in roots.
How do auxins cause cell growth?
Cell elongation
How do auxins affect different parts of a plant?
- Shoots -> Stimulate growth
* Roots -> Inhibit growth
What is an important auxins you need to know about?
IAA (Indoleacetic Acid)
What does IAA stand for?
Indoleacetic Acid
What is IAA?
An auxin produced in the tips and shoots of flowering plants.
How does IAA move over short and long distances?
- Short -> Diffusion + Active transport
* Long -> Phloem
Describe the process by which a shoot responds to light.
- IAA moves to shaded side of shoot
- So concentration is increased and cells elongate
- The shoot bends towards the light
Describe the process by which a shoot responds to gravity.
- IAA moves to the underside of shoots
- So concentration is increased and cells elongate
- The shoot bends away from gravity
Describe the process by which a root responds to light.
- IAA moves to shaded side of root
- So concentration is increased and growth is inhibited
- The root bends away from the light
Describe the process by which a root responds to gravity.
- IAA moves to the underside of roots
- So concentration is increased and growth is inhibited
- The shoot bends towards gravity
What type of organisms have simple kinetic responses?
Simple mobile organisms
What are the two types of simple response in simple organisms?
- Tactic
* Kinetic
What is another name for tactic responses?
Taxes
What is another name for kinetic responses?
Kineses
What is a tactic response?
When the organism moves either towards or away from a directional stimulus.
What is a kinetic response?
When the organism’s movement is affected by a non-directional stimulus.
What usually determines whether a response will be tactic or kinetic?
Whether the stimulus is directional or not.
What type of response do woodlice have to light?
Tactic - they move away from the light source.
What type of response do woodlice have to humidity?
Kinetic - in high humidity, they move less slowly and turn less often, while in dry conditions they move more quickly and turn more often
What is a choice chamber?
A container with different compartments in which different environments can be created to investigate how organisms respond to them.
Describe how to make a choice chamber.
- Take a Petri dish with a divider down the middle
- Create different conditions in each section
- A fine mesh is placed on top of the divider
- Petri dish lid is placed on top
Describe how a choice chamber can be used to investigate woodlice response to light intensity.
1) Construct a choice chamber
2) Cover one half of the lid and sides with black paper. This makes one side of the chamber dark.
3) Put damp filter paper in both sides of the base.
4) Place 10 woodlice in the centre of the chamber and shut the lid.
5) After 10 minutes, take the lid off and count the number of woodlice on each side.
6) Repeat by moving the woodlice to the centre (using a small paintbrush).
Describe how a choice chamber can be used to investigate woodlice response to humidity.
1) Construct a choice chamber
2) Put some damp filter paper in one side of the base and a desiccating agent in the other.
3) Put the lid on and leave for 10 minutes to stabilise.
4) Place 10 woodlice in the centre of the chamber and shut the lid.
5) After 10 minutes, take the lid off and count the number of woodlice on each side.
6) Repeat by moving the woodlice to the centre (using a small paintbrush).
What is the difference between tactic and kinetic responses?
Tactic responses are due to directional stimuli, while kinetic responses are due to non-directional stimuli.
How do woodlice respond to light and why?
- Tactic response
- Move away from light
- This conceals them under stones, which protects them from predators and keeps them in damp conditions
How do woodlice respond to humidity?
- Kinetic response
- In lower humidity, they move more slowly and turn less often
- This keeps the woodlice in high humidity, which protects them from water loss and keeps them concealed
What is a tactic response to light called?
Phototaxis
What are the different types of receptor?
- Cells
* Proteins on cell surface membranes
Give an example of a receptor that is a cell.
Photoreceptors
Give an example of a receptor that is a protein on a cell surface membrane.
Glucose receptors on pancreatic cells
What is the the resting potential of a receptor cell?
The potential difference across a cell membrane at rest.
What is a generator potential of a receptor cell?
The change in potential difference across a cell membrane due to a stimulus.
What maintains a resting potential?
- Ion pumps
- Ion channels
(NOTE: This is covered more in chapter 6B)
Describe how a receptor cell works.
- Resting potential is maintained by ion pumps and ion channels creating a difference in charge across the membrane
- A stimulus causes the cell membrane to be excited and more permeable, allowing more ions to move in and out of the cell, which alters the potential difference
- The change in potential difference is called the generator potential
- If the generator potential reaches the threshold, it triggers an action potential
- If the threshold is not reached, there is no action potential
Can the generator and action potentials vary in size?
- Generator -> Can vary depending on stimulus strength
* Action -> Always the same if triggered
In terms of action potentials, how is the strength of the stimulus measured?
It is the frequency of action potentials (NOT their size).
What is the name for the pressure receptors in your skin?
Pacinian corpuscles
What is the name for the light receptors in your eye?
Photoreceptors
What are the two types of receptor you need to know about?
- Pacinian corpuscle
* Photoreceptors
What type of receptors are Pacinian corpuscles?
- Mechanoreceptors
* They detect mechanical stimuli (pressure and vibrations)
Where are Pacinian corpuscles found?
In the skin.
Describe the structure of a Pacinian corpuscle.
- Sensory nerve ending of the sensory neurone is at the centre
- Sensory neurone goes off to one side
- Several layers of connective tissue called lamellae surround the nerve ending
What is the series of layers in a Pacinian corpuscle made of?
Connective tissue
What is the name for the series of layers in a Pacinian corpuscle?
Lamellae
Describe how a Pacinian corpuscle works.
- Pressure causes the lamellae to deform and press on the sensory nerve ending in the middle
- This causes the cell membrane to stretch, deforming the stretch-mediated sodium ion channels
- So these channels open and Na⁺ ions diffuse into the cell
- This creates a generator potential, which becomes an action potential if the threshold voltage is reached
What type of ion channels are involved in a Pacinian corpuscle?
Stretch-mediated sodium channels
Remember to practise drawing out the structure of a Pacinian corpuscle.
Pg 142 of revision guide
Where are photoreceptors found?
Retina of the eye
Describe the structure and functioning of an eye.
- Light enters through the pupil
- Size of pupil is controlled by the muscles of the iris
- Light is focused onto the retina by the lens
- Fovea is an area of the retina with many photoreceptors
- Nerve impulses are carried to the brain by the optic nerve
What is the fovea?
An area of the retina with many photoreceptors.
What is the blind spot?
The area where the optic nerve leaves the eye, so there are no photoreceptors (so it’s not sensitive to light).
Remember to practise drawing out a diagram of the eye.
Pg 142 of revision guide
How do photoreceptors work?
- Light hitting the photoreceptors is absorbed by light-sensitive optical pigments
- Light bleaches the optical pigments, causing a chemical change and altering the membrane permeability to sodium ions
- A generator potential is created by the movement of sodium ions
- If this reaches the threshold, a nerve impulse is sent along a bipolar neurone
Describe the pathway of a signal that is generated in a photoreceptor.
Photoreceptor -> Bipolar neurone -> Optic nerve -> Brain
Remember to practise drawing out the pathway of a signal generated in a photoreceptor.
See diagram pg 143 of revision guide
What is the bipolar neurone?
The neurone that connects a photoreceptor to the optic nerve.
What are the two types of photoreceptor in the human eye?
- Rods
* Cones
Where on the retina are rods and cones found?
- Rods -> Peripheral parts of the retina
* Cones -> Packed together in the fovea
Do rods and cones have the same pigments? What is the effect of this?
- No, they have different optical pigments
* This makes them sensitive to different wavelengths of light.
Describe the types of light that rods and cones are sensitive to.
- Rods -> Black and white
* Cones -> Colour
What is a good way of remembering with type of photoreceptor is sensitive to colour?
COnes COlour
Are all cone photoreceptors the same?
No, there are red-sensitive, green-sensitive and blue-sensitive.
What 3 types of colour-sensitive cone cells are there?
- Red
- Green
- Blue
What two factors can be used to compare the rod and cone cell performance?
- Sensitivity
* Visual acuity
What is visual acuity?
The ability to tell apart two points that are close together.
Describe how rod cells join to neurones.
Many rod cells join to a neurone
Describe how cone cells join to neurones.
One cone cell joins to a neurone
Remember to practise drawing out his rod and cone cells join to neurones.
Pg 143 of revision guide
Compare and explain the sensitivity of rod and cone photoreceptors.
Rods:
• Very sensitive to light
• Because many rods join one neurone, so many weak generator potentials can combine to reach the threshold and trigger an action potential
Cones:
• Less sensitive to light
• Because one cone joins to one neurone, so it requires more light to trigger an action potential
Compare and explain the visual acuity of rod and cone photoreceptors.
Rods:
• Low visual acuity
• Many rods join the same neurone, which means light hitting two points can’t be distinguished
Cones:
• High visual acuity
• Cones are close together and one cone joins to one neurone, so light hitting two points can be distinguished
Compare rods and cone photoreceptors.
Rods: • Black and white vision • High sensitivity • Low visual acuity • Found in peripheral parts of retina Cones: • Colour vision • Low sensitivity • High visual acuity • Found in fovea (centre of retina)
What can heart muscle be described as?
Myogenic - it can contract and relax without receiving signals from nerves.
Describe the process of myogenic contraction of the heart.
1) Sinoatrial node (SAN) in the right atrium wall sends out regular electrical signals to the atrial walls.
2) This causes the right and left atria to contract simultaneously.
3) A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles.
4) Instead the signal reaches the atrioventricular node (AVN).
5) There is a slight delay to allow the atria to empty before the AVN passes the signal down the bundle of His.
6) The bundle of His carries the signal to the apex (bottom) of the heart down the middle.
7) Purkyne tissue carries the wave of electrical activity into the muscular walls of the right and left ventricles, causing contraction from bottom up.
What is the bundle of His?
A group of muscle fibres in the septum of the heart that carries waves of electrical activity to the apex (bottom) of the heart.
What is Purkyne tissue?
The conductive tissue in the left and right ventricle walls that carries the waves of electrical activity up from the bottom of the heart.
What controls the rate at which the SAN generates electrical signals?
Medulla oblongata (a part of the brain)
Why do animals need to alter their heart rate?
In response to internal stimuli, such as low blood pressure (so that fainting does not occur).
What stimuli does heart rate need to respond to?
- Blood pressure
- Oxygen level
- Carbon dioxide level
- pH
What are the two types of receptor involved in detecting the stimuli that the heart rate needs to respond to?
- Pressure (Baroreceptors)
* Chemical (Chemoreceptors)
Where are baroreceptors found?
- Aorta
* Carotid arteries (major arteries in the neck)
Where are chemoreceptors found?
- Aorta
- Carotid arteries
- Medulla
What do baroreceptors detect?
• High/Low pressure
What do chemoreceptors detect?
- Oxygen level
- CO₂ level
- pH
Why do chemoreceptors detect changes in CO₂ levels and pH?
These are indicators of O₂ level.
In general, describe how the body responds to changes that the heart rate needs to respond to.
- Baroreceptors and chemoreceptors send electrical impulses along the sensory neurones to the medulla
- This information is processed and impulses are sent back to the SAN along sympathetic or parasympathetic neurones (part of the autonomic nervous system).
Describe how a response is generated to high blood pressure.
- Baroreceptors detect high pressure
- Impulses are sent to the medulla, which sends impulses along parasympathetic neurones.
- These secrete acetylcholine, which binds to receptors in the SAN
- The cardiac muscle slows down
Describe how a response is generated to low blood pressure.
- Baroreceptors detect low pressure
- Impulses are sent to the medulla, which sends impulses along sympathetic neurones
- These secrete noradrenaline, which binds to receptors in the SAN
- The cardiac muscle speeds up
Describe how a response is generated to high blood O₂ levels, low CO₂ levels or high pH levels.
- Chemoreceptors detect chemical changes in blood
- Impulses are sent to the medulla, which sends impulses along parasympathetic neurones
- These secrete acetylcholine, which binds to receptors in the SAN
- The cardiac muscle slows down
Describe how a response is generated to low blood O₂ levels, high CO₂ levels or low pH levels.
- Chemoreceptors detect chemical changes in blood
- Impulses are sent to the medulla, which sends impulses along sympathetic neurones
- These secrete noradrenaline, which binds to receptors in the SAN
- The cardiac muscle speeds up
What is the effect of acetylcholine on the SAN?
It slows down its firing.
What is the effect of noradrenaline on the SAN?
It speeds up its firing.
Which nervous system is involved in the control of heart rate?
Autonomic nervous system (made up of the sympathetic and parasympathetic systems)
