14. Response to stimuli Flashcards
Define a stimulus
A detectable change in the external or internal environment of an organism that leads to a response in the organism
Define a taxis
A directional response to a directional stimulus
Define kinesis
A response to a change in stimulus by changing the amount of activity (speed/ rate of change of direction)
Define tropism
Growth of a part of a plant in response to a directional stimulus
Name 3 stimuli that plants respond to
- light
- gravity
- water
Name the plant growth factor that causes tropism
IAA (indoleacetic acid)
Describe phototropism in flowering plants
- Cells in shoot tips produce IAA which is then transported down the shoot, initially it is spread evenly
- Light causes the movement of IAA from the light to dark side of the plant
- A greater concentration builds on the dark side of the shoot relative to the light side
- IAA causes elongation of cells in shoots, so the cells on the dark side elongate more
- The shaded side grows quicker than on the light side which causes the shoot tip to bend towards the light
Describe gravitropism in flowering plants
- Cells in root tips produce IAA which is then transported along the shoot, initially it is spread evenly
- Gravity causes the movement of IAA from the upper side to lower side of the plant
- A greater concentration builds on the lower side of the root relative to the upper side
- IAA causes inhibition growth in of cells in roots so the cells on the lower side elongate less than the upper side
- The upper side grows quicker than on the lower side which causes the shoot tip to bend downward towards the light
Name the steps in a reflex arc
- Stimulus
- Receptor
- Sensory neurone
- Relay neurone
- Motor neurone
- Effector
- Response
Why are reflex arcs important
- They are involuntary, does not require decision making of brain so does not overload with stimuli in which their responses are always the same
- They protect you from harm
- Fast, as the neurone pathway is very short, typically 1 or two synapses and there is no decision making process (impulse being sent to brain)
What are the qualities of a reflex
- immediate
- innate
- protect you from harm
Name 2 features of receptors
- specific to one type of stimulus
- produces an action potential (transducer- converts energy of stimulus into electrical energy)
What is the receptor in the skin that responds to pressure
Pacinian corpuscle
Describe how the Pacinian corpuscle works
- At resting sate the stretch mediated sodium ion channels on the membrane of the corpuscle are too narrow to allow Na+ ions through. In this state the Pacinian corpuscle has a resting state
- When pressure is applied to the Pacinian corpuscle it is deformed
- This widens the stretch mediated sodium ion channels and sodium ions can diffuse into the neurone.
- This causes the membrane to depolarise, producing a generator potential
- This creates an action potential which passes along the neurone
Where are light receptors found
The retina
Name the two kinds of cells that are light receptors
- Rod cells
- Cone cells
Describe the structure of a Pacinian corpuscle
- sensory neurone at the centre
- surrounded by layers of connective tissue
- blood capillary then capsule layer
- sensory neurone has stretch-mediated sodium channel in the centre
What are the colour of the images produced by rod cells
Black and white
In what way are rod cells connected to sensory neurones
Many rod cells are connected to a single sensory neurone
What kind of light conditions are rod cells used to detect
Light of very low intensity
How rod rod cells see in lower light
- a number of rod cells are connected to a single bipolar cell (retinal convergence)
- due to summation there a much larger chance that the threshold value will be exceeded for rod cells
What must occur in order to create an action potential in rod cells
- pigment in the rod cells (rhodopsin) must be broken down
- there is enough energy in low intensity light to cause this breakdown
What is visual acuity
How clear eye sight is
Why do rod cells give low visual acuity
- many rod cells link to one sensory neurone
- rod cells sharing the same neurone will only generate a single impulse
- the brain cannot separate sources of light
- two close together dots cannot be resolved giving low visual acuity
How many different cone cells are there
3, each responding to a different range of wavelengths
Why do cone cells only respond to high intensity light
- each cone cell is connected to their own separate bipolar cell connected to a sensory neurone
- there is no summation to exceed the threshold value and create an action potential
What must occur in order to create an action potential in cone cells
- pigment in the cone cells (iodeosin) must be broken down
- only light of high intensity will provide enough energy to break it down
Why do cone cells give good visual acuity
- one cone cells link to one sensory neurone
- when two adjacent cone cells are stimulated the brain receives two separate impulses
- the brain can distinguish between sources of light
- two close together dots can be resolved giving good visual acuity
Describe the distribution of rod and cone cells on the retina
- cone cells are found on the fovea (which receives the highest intensity of light)
- the concentration of cone cells diminishes further away from the fovea
- at the peripheries of the retina where light intensity is at its lowest only rod cells are found
What is the benefit of having both types of light receptor
Mammals have good all-round vision in both day and night
Name the two divisions of the autonomic nervous system
- the sympathetic nervous system
- the parasympathetic nervous system
What is the sympathetic nervous system
- stimulates effectors so speeds up any activity
- helps us cope with stressful situations by heightening our awareness and preparing us for activity
What is the parasympathetic nervous system
- inhibits effectors and so slows down any activity
- controls activities under normal resting conditions
- concerned with conserving energy and replenishing the body’s reserves
What word can be used to describe how the sympathetic and parasympathetic systems work together
Antagonistic
What is an example of how the sympathetic and parasympathetic nervous systems work together
The control of heart rate
Why is the heart muscle known as myogenic
Its contractions are initiated from within rather than by nervous impulses from outside
What does the SAN stand for
Sinoatrial node
Where is the SAN located
Within the wall of the right atrium
Why is the SAN referred to as the pacemaker
It is where the initial stimulus for contraction originates. It has a basic rhythm of stimulation that determines the beat of the heart
Describe the sequence of events that controls the heart rate
- a wave of electrical excitations spreads out from the SAN across both atria causing them to contract
- the atrioventricular septum prevents the wave from crossing to the ventricles as it is non-conductive
- the wave of excitation enters a second group of cells called the AVN
- after a short delay the AVN produces a wave of electrical excitation between the ventricles along the bundle of His
- the bundle of his conducts the wave through the atrioventricular septum to the base of the ventricles where the bundle branches to smaller fibres of Purkyne tissue
- the wave of excitation is released from the Purkyne tissue causing the ventricles to contract quickly at the same time
What is the atrioventricular septum
A layer of non-conductive tissue between the ventricles and the atria
What does the AVN stand for
Atrioventricular node
Where is the AVN located
Lies between the atria and the ventricles
What is the bundle of His
A structure made of specialised fibres called Purkyne tissue
Through what structure does the electrical excitation produced by the AVN travel through
bundle of His, made from Purkyne tissue
In what region of the brain is the heart rate controlled by
Medulla oblongata
Describe two centres in the medulla oblongata that are concerned with heart rate
- one centre increases heart rate (linked to the sinoatrial node by the sympathetic nervous system)
- one centre decreases heart rate (linked to the sinoatrial node by the parasympathetic nervous system)
How do chemoreceptors detect changes of CO2 in the blood
- when the blood has higher concentration of CO2 its pH is lowered
- the chemoreceptors in the wall of the carotid arteries and the aorta detect this
How does impulses from the chemoreceptors increase heart rate
- the centre in the medulla oblongata increases the frequency of impulses via the sympathetic nervous system to the sinoatrial node
- this increases the rate of production if the electrical waves by the SAN and therefore increasing heart rate
How does the heart rate return to normal via chemoreceptors after heart rate has increased
- this increased blood flow leads to more CO2 being removed by the the lungs so CO2 levels return to normal
- pH of the blood rises to normal and the chemoreceptors in the wall of the carotid arteries and the aorta reduce the frequency of nerve impulses to the medulla oblongata
- the medulla oblongata reduces the frequency of impulses to the sinoatrial node which therefore leads to a reduction in the heart rate
How is heart rate controlled by pressure receptors when blood pressure is higher than normal
- pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that decreases heart rate
- this centre sends impulses via the parasympathetic nervous system to the sinoatrial node of the heart
- leads to a decrease in the rate at which the heart beats
How is heart rate controlled by pressure receptors 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 sends impulses via the sympathetic nervous system to the sinoatrial node
- increases the rate at which the heart beats
