Response to Stimuli Flashcards
Stimulus
a detectable change in the internal or external environment of an organism which leads to a response.
Response
an action taken as a result of a stimulus
What is the advantage of responding appropriately to stimuli?
There is a selection pressure favouring organisms with more appropriate responses, as they are more likely to survive and reproduce, and pass their alleles on to the next generation.
What is a taxis?
a simple response, the direction of which is determined by the direction of the stimulus
Which type of organisms can show taxis?
Motile organisms
Taxis towards stimuls=
Taxis away from stimulus =
Positive taxis
Negative taxis
types of taxis:
light
chemical
heat
Phototaxis
Chemotaxis
Thermotaxis
What type of response is taxis?
Directional - organisms move towards or away from the stimulus.
What is kinesis?
A type of response in which the speed and rate of direction change of the organism changes.
What type of stimuli does kinesis involve?
less directional stimuli, eg humidity and temperature
Explain what happens in kinesis and why
Speed and rate of direction change increase when an organism crosses a division between a favourable and unfavourable environment. This raises the chances that the organism will return to a favourable environment more quickly.
Which organisms show kinesis
woodlice
tropism
the growth of part of a flowering plant in response to a directional stimulus
What do plants respond to?
Light, water, gravity
What types of tropisms do plants show?
phototropis, gravitropism, hydrotropism
What are tropisms controlled by?
Plant growth factors such as indoleacetic acid.
Explain positive phototropism in a plant shoot
- Cells in the tip of the shoot produce IAA, which is transported down the shoot.
- IAA is initially transported evenly down the shoot, but light causes the movement of IAA from the light side to the shaded side of the of the shoot.
- IAA causes elongation of shoot cells and as its concentration on the shaded side is greater, the cells on this side elongate more.
- The shaded side of the shoot elongates faster than the light side, so the shoot bends towards the light.
Which part of a plant shows negative phototropism?
The root
Which part of a plant shows positive phototropism?
The shoot
Exlain positive grapitropism in a plant root
- Cells in the root tip produce IAA, which is initially transported evenly through the root.
- Gravity causes IAAA to move from the upper side to the lower side of the root, so a greater concentration of IAA builds up on the lower side.
- IAA inhibits elongation of root cells, so cells on the lower side elongate less than cells on the upper side.
- The greater elongation of the upper side causes the root to bend downwards, towards the force of gravity.
Does IAA in the root promote or inhibit cell elongation?
inhibit
Describe the stages in a reflex arc.
⇒ stimulus ⇒ receptor ⇒ sensory neurone ⇒intermediate neurone ⇒ motor neurone ⇒ effector ⇒ response
Why are reflex arcs important?
involuntary, rapid, protect the body from harm. concious thought in the brain is not required, so the brain is free to carry out more complex responses.
What does a receptor do?
detect and respond to a specific stimulus
Give 2 examples of receptors
pacinian corpuscle, photoreceptors in the retina
What do pacinian corpuscles detects?
changes in mechanical pressure
Structure of a pacinian corpuscle
A single sensory neurone at the centre of layers of connective tissue (lamellae).
How is an action potential produced when pressure is applied to a pacinian corpuscle?
The lamellae become deformed and press on the sensory nerve ending. This deforms stretch-mediated sodium channels, causing them to open so sodium ions diffuse into the neurone. This creates a generator potential (depolarisation begins). if the threshold is reached, an AP is triggered.
2 types of photoreceptor cells
rods and cones
Sensitivity to light - rod cells
Rods are more sensitive to light/ respond to lower intensity light. More than 1 rod cell is connected to a single bipolar cell and sensory neurone, so many weak generator potentials can be combined to reach the threshold and trigger an AP. This is spatial summation.
Also, only a low intensity is required to break the pigment, rhodopsin, down.
What is visual acuity?
The ability to resolve points that are close together.
Sensitivity to light - cone cells
Cone cells are less sensitive to light. 1 cone cell is connected to 1 neurone, so the generator potentials from multiple cone cells can’t be combined to reach the AP threshold. Higher intensity light is required for this/ also to break down iodopsin pigment.
Visual acuity - cone cells
Cone cells - give high visual acuity as 1 cone is connected to 1 neurone, so stimulation of 2 adjacent cones means 2 impulses are sent to the brain. The brain can distinguish between light sources that are close together.
Visual acuity - rod cells
Rod cells - give low visual acuity because many rods are connected to the same bipolar cell and neurone, so only a single impulse will be sent to the brain regardless of how many rods are stimulated. The brain cannot distinguish between separate light sources that are close together.
Sensitivity to colour - rod cells
Rods can’t distinguish between different wavelengths of light so only give info in black and white. There is only 1 type of optical pigment, rhodopsin.
Sensitivity to colour - cone cells
There are 3 types of cone cells, each of which contains a different type of pigment which responds to a different range of wavelengths of light (iodopsin: red, green, blue). Iodopsin also requires a higher intensity of light for its breakdown.
Which type of photoreceptor cell responds to coloured light?
Cone cells
Distribution of rod and cone cells in the retina - describe and why?
rods are distributed more towards the periphery while cones are more concentrated at the fovea/ centre. This is because the fovea receives the highest intensity light, which is needed to stimulate cone cells.
Role of the autonomic nervous system
controls involuntary actions such as internal muscles and glands
Sympathetic nervous system
stimulates effectors and so speeds up activity.
Parasympathetic nervous system
this inhibits effectors, so slows down activity.
What is the heart formed of?
cardiac muscle
What does myogenic mean?
The heart contracts and relaxes without receiving signals from nerves.
Where in the heart is the stimulus for contraction initiated?
the SAN - a group of cells in the right atrial wall.
What is the role of the SAN?
acts like a pacemaker, setting the rhythm of the heart by sending out regular waves of electrical activity to the atrial walls.
Describe what happens during myogenic stimulation of the heart and transmission of waves of electrical activity.
- The sinoatrial node (SAN) acts like a pacemaker, setting the heart’s rhythm by sending out regular waves of electrical activity to the atrial walls.
- This causes the left and right atrial walls to contract.
- A layer of non-conductive tissue (atrioventricular septum) prevents this wave from crossing to the ventricles.
- The wave of excitation pass to the atrioventricular node (AVN) , which is between the atria.
- After a short delay, the AVN conveys a wave of electrical excitation between the ventricles and to the bottom of the heart, along the bundle of His. The delay allows the atria to empty before the ventricles contract.
- Purkyne tissue (divisions of BOH) carry the waves up the left and right ventricle walls, so they contract simultaneously, from the bottom up.
Why may the resting heart rate need to be altered?
to meet varying demands for oxygen - for example heart rate needs to increase during exercise.
Where are the centres that control changes to heart rate located?
The medulla oblongata
What is the centre than increases HR connected to?
The sympathetic nervous system
What is the centre than decreases HR connected to?
The parasympathetic nervous system
Where in the body are chemoreceptors located?
The carotid artery and aorta
Describe the role of chemoreceptors in controlling HR
⇒ Increased metabolic activity
⇒ More CO2 produced due to increased aerobic respiration.
⇒ Blood pH decreases. (increased acidity)
⇒ Chemoreceptors in in the carotid artery and aorta send impulses to the medulla oblongata.
⇒ The centre in the medulla oblongata which increases HR sends impulses to the SAN via the sympathetic nervous system.
⇒ SAN increases HR.
⇒ Increased blood flow removes CO2 faster.
⇒ CO2 levels return to normal.
The opposite can occur if blood pH rises too high.
Where in the body are pressure receptors located?
The carotid artery and aorta walls
What happens when BP is higher than normal?
Pressure receptors transmit nervous impulses to the centre in the medulla oblongata which decreases HR. This centre sends impulses via the parasympathetic nervous system to the SAN, which leads to a decrease in the rate of heart beats.
What happens when BP is lower than normal?
Pressure receptors transmit impulses to the centre in the medulla oblongata which increases heart rate. This centre send impulses via the sympathetic nervous system to the SAN, which increases the rate at which the heart beats.
Effector
muscle/ organ/ gland which produces a response
Coordinator
a suitable response to a stimulus.
Typical control pathway
stimulus => receptor => coordinator => effector => response
Explain how rod cells enable us to see in conditions of low light intensity.
Several rod cells are connected to each sensory neurone - the convergence principle/ spatial summation - weak generator potentials from the stimulation of several rod cells can be combined to reach the neurone’s AP threshold.
Explain how cone cells enable us to distinguish between objects close together.
Each cone cell connects to a single neurone, so there is no convergence - the brain receives information from each cone cell individually.
