6.1 Internal and external stimuli Flashcards
What is a stimulus?
any change in the environment that is able to be detected, the organism will produce a response as a result of the stimulus
What increases survival?
the ability to respond to anything will increase an organisms chance of survival
What are the events involved in a response to stimuli?
Stimulus
Receptor
Processing
Effector
Response
What is a taxis?
- a movement in response to the direction of a stimulus.
- A taxis can be positive or negative.
- Taxes are named after the stimulus involved.
What does positive phototaxis mean?
a movement towards light
What does negative chemotaxis mean?
a movement away from a chemical
What are examples of phototaxis?
Algae move towards the light (+). This increases survival rate as they require light to manufacture food by photosynthesis.
Earthworms move away from the light (-). This increases survival as it takes them to the soil, where they are better able to conserve water, find food and avoid some predators.
What is an example of chemotaxis?
Some species of bacteria will move towards a region where glucose is more concentrate (+), and increases survival rate as glucose is a source of their food.
What is a kinesis?
a random movement, in which the rate of movement is related to the intensity of the stimulus but not to its direction.
What does an orthokinesis involve?
a change in the speed of movement
What does a klinokinesis involve?
a change in the rate of turning
What are tropisms?
- plants do not have a nervous system
- they show responses to stimuli in the way they grow.
- Plants will grow towards or away from a stimulus.
- can be positive or negative.
- A growth response is a tropism
What are receptors?
they detect the change, they are specific to one type of stimulus
What is a processing centre?
where the information is processed
What is an effector?
muscles or glands
What is a response?
the behaviour that is shown
What are innate behaviors?
- Involve responses that are not learned. They are determined by inherited nervous pathways
- As a result of these ‘built in’ pathways a given stimulus will always produce the same response
- They are selected for over many generations as they increase the chance of survival
- Taxes and kineses are example of innate behaviour
What are plant factors and where are they produced?
- chemicals that regulate plant growth response to directional stimuli
- produced in plant growing regions
- diffuse from cell to cell/phloem mass transport
Why do shoots show positive phototropism?
- indoleacetic acid diffuses to shaded side of shoot tip
- As IAA diffuses down shaded side, it causes active transport of H+ ions into cell wall
- Disruption to H-bonds between cellulose molecules and action of expansins make cell more permeable to water
- cells on shaded side elongate faster due to higher turgor pressure
- shoot bends towards light
Why do roots show positive gravitropism?
- gravity causes IAA to accumulate on lower side of the root
- IAA inhibits elongation of root cells
- cells on the upper side of the root elongate faster, so the root tip bends downwards
How does tropism happen in shoots?
- high IAA concentrations promote cell elongation, meaning; the dark side of shoot elongates and shoots grow towards the light(positive phototropism)
How does tropism happen in roots?
high IAA concentrations inhibit cell elongation, meaning; the lower side of the root becomes shorter and the roots grow downwards into the earth
What are tropisms controlled by?
- both phototropism and geotropism are controlled by the distribution of an auxin called IAA (indoleacetic acid) within the plant cells
- in gravitropism, IAA will accumulate on the lower side of the plant in response to the force of gravity
- in phototropism, light receptors trigger the redistribution of IAA to the dark side of the plant, the IAA moves by diffusion
What is the advantage of taxis and kinesis?
maintain mobile organism in optimum environment e.g. to prevent dessication
Many organisms respond to temperature and humidity via kinesis rather than taxis. why?
less directional stimuli; often no clear gradient from one extreme to the other
How could you recognise kinesis in an organisms movement?
- organism crosses sharp division between favourable and unfavourable environment; turning increases (return to the original favourable environment)
- if organism moves considerable distance into unfavourable environment: turning slowly decreases; begins to move in long, straight lines; sharper turns (lead organism to new environment)
How do kineses change?
- If an organism crosses a sharp change between favourable and unfavourable environments, rate of turning increases, raising its chances of a quick return.
- If it moves far into an unfavourable environment the rate of turning may slowly decrease so it moves straight, then turns very sharply. Important for less directional stimulus - humidity and temperature.
What is an example of kinesis?
- When woodlice move from a damp area to dry, they move more rapidly and change direction more often. This increases the chance of moving back to the damp area.
- They then slow down and change direction less.
- It prevents them drying out and so increases their chances of survival.
How is a response to a stimuli good?
- It increases the chances of survival for an organism.
- Organisms that survive have a greater chance of raising offspring and of passing their alleles to the next generation.
- So there is a selection pressure favouring organisms with more appropriate responses.
What is a coordinator?
A coordinator formulates a suitable response to a stimulus.
It may be at the molecular level or involve a large organ - brain.
How does communication occur?
- In large multicellular organisms it occurs via chemicals called hormones, a relatively slow process found in plants and animals.
- It also occurs by the more rapid nervous system.
What is the acid growth hypothesis?
This explains the effect of IAA on plasticity.
Hydrogen ions are actively transported from the cytoplasm into spaces in the cell wall causing the cell wall to become more plastic allowing the cell to elongate by expansion.
What are the divisions of the nervous system?
- the central nervous system
- the peripheral nervous system
What is the central nervous system made up of?
the brain and spinal cord
What is the peripheral nervous system made up of?
pairs of nerves that originate from either the brain or the spinal cord
What is the peripheral nervous system divided into?
- sensory neurones - which carry nerve impulses from receptors towards the central nervous system
- motor neurones - which carry nerve impulses away from the central nervous system to effectors
What are the divisions of motor neurones?
- the voluntary nervous system
- the autonomic nervous system
What does the the voluntary nervous system do?
carries nerve impulses to body muscles and is under voluntary control
What does the the autonomic nervous system do?
carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control, that is, it is involuntary
What is the spinal cord?
a column of nervous tissue that runs along the back and lies inside the vertebral column for protection. Emerging at intervals along the spinal cords are pairs of nerves.
What is the reflex arc?
The pathway of neurones involved in a reflex
What is the reflex arc of removing the hand from a hot object?
- the stimulus - heat from the hot object
- a receptor - temperature receptors in the skin on the back of the hand, which generates nerve impulses in the sensory neurone
- a sensory neurone - passes nerve impulses to the spinal cord
- a coordinator - links the sensory neurone to the motor neurone in the spinal cord
- a motor neurone - carries nerve impulses from the spinal cord to a muscle in the upper arm
- an effector - the muscle in the upper arm, which is stimulated contracts
- the response - pulling the hand away from the hot object
Why is it important that reflex arcs are involuntary?
they don’t require the decision making powers of the brain, leaving it free to carry out more complex responses. Therefore the brain isn’t 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
Why are reflex arcs important?
- they protect the body from harm. They are effective from birth and do not have to be learnt
- they are fast because the neurone pathway is short with very few synapses where neurones communicate with each other
- the absence of any decision-making process also means the action is rapid
What is a synapse?
the slowest link in a neurone pathway
Outline what happens in a simple reflex arc
Receptor detects stimulus
Sensory Neuron
Relay neuron in CNS coordinates response
Motor Neurone
Response by effector
What are the advantages of a simple reflex?
- rapid response to potentially dangerous stimuli since only 3 neurones involved
- instictive
What features are common to all sensory receptors?
- act as energy transducers which establish a generator potential
- respond to specific stimuli
What is the structure of a Pacinian corpuscle?
- single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule
- stretch mediate Na+ channels on plasma membrane
- capillary runs along base layer of tissue
Where are Pacinian corpuscle?
- they occur deep in the skin and are most abundant on the fingers, the soles of the feet and the external genitalia
- they also occur in joints, ligaments and tendons
What stimulus does a Pacinian corpuscle respond to?
- pressure deforms membrane causing stretch mediated Na+ ion channels to open
- if influx of Na+ raises membrane to threshold potential, a generator potential is produced
- Action potential moves along sensory neuron
What are the 2 types of photoreceptor cell located in the retina?
- cone cells
- rod cells
Where are rod and cone cells located in the retina?
Rod: evenly distributed around periphery but NOT in central fovea
Cone: mainly central fovea no photoreceptors at blind spot
Compare and contrast rod and cone cells.
Rod Cells:
- sensitive to low light intensity
- contain a pigment called rhodopsin
- we have about 125 million of them in our retina
- evenly distributed throughout the retina
- allow animals to see things in the dark that we might not be able to see
- all wavelengths of light detected
Cone cells:
- sensitive to different wavelengths of light
- Contain a pigment called iodopsin
- we have about 7 million of them in our retina
- concentrated in the centre of the retina in an area called the fovea
- Give animals a high visual activity
- red, blue, green wavelengths absorbed by different types of iodopsin
How are impulses initiated in rod and cone cells?
- they contain photosensitive pigments
- when the pigments absorb light their chemical structure is changed
- the cell membranes of the photoreceptors also become more permeable to sodium ions
Why do rod cells respond to low intensity light?
- rods are sensitive to low light intensity and can distinguish light from dark in a very dim room
- each rod is capable of detecting 1 photon of light
- rod cells converge on a bipolar neurone and their individual impulses are passed to the brain
- for an impulse to be transmitted to the brain via the optic nerve a certain potential difference must be reached
- one single rod cell may not be able to transmit an impulse but a number of rod cells ‘added together’ will
- this phenomenon is known as summation
What is the visual acuity of rod cells?
they give a low visual acuity
- light received by rod cells sharing the same neurone will only generate a single impulse to the brain regardless of how many of the neurones are stimulated.
- this means the brain cannot distinguish between the separate sources of light that stimulates them
- 2 dots close together cannot be resolved and so will appear as a single blob
What is the visual acuity of cone cells?
the number of impulses sent to the brain is dependent on the number of cone cells stimulated. Each cone cell sends a separate message, each rod cell does not
Outline the pathway of light from a receptor to the brain.
photoreceptor - bipolar neurone - ganglion cell of optic nerve - brain
What does myogenic mean?
contraction of heart is initiated within the muscle itself rather than by nerve impulses
Sate the name and location of the 2 nodes involved in heart contraction
- sinoatrial node - within the wall of the right atrium
- atrioventricular node - near lower end of right atrium in the wall that separates the 2 atria
What are the 2 divisions of the autonomic nervous system?
the sympathetic nervous system
the parasympathetic nervous system
What does the sympathetic nervous system do?
stimulates effectors and so speeds up any activity. It acts rather like an emergency controller. It controls effectors when we exercise strenuously or experience powerful emotions. It helps us to cope with stressful situations by heightening our awareness and preparing us for activity (fight of flight response)
What does the parasympathetic nervous system do?
inhibits effectors and so slows down any activity. It controls activities under normal resting conditions. It is concerned with conserving energy and replenishing the body’s reserves
How are heartbeats initiated and coordinated?
- SAN initiates wave of depolarisation (WOD)
- WOD spreads across both atria=atrial systole
- Layer of fibrous, non-conducting tissue delays impulse while ventricles fill and valves close
- AVN conveys WOD down septum via Bundle of His, which branches into Purkinje fibres along ventricles
- Causes ventricles to contract from apex upwards
What is the formula for cardiac output?
cardia output = stroke volume x heart rate
What is the autonomic nervous system?
system that controls involuntary actions of glands and muscles
Name the receptors involved in changing heart rate and state their location
Baroreceptors - detect changes in blood pressure: carotid body
Chemoreceptors - detect changes in pH e.g. due to increase in CO2 concentration : carotid body and aortic body
How does the body respond to an increase in blood pressure?
- Baroreceptors send more impulses to cardioinhibitory centre in the medulla oblongata
- More impulses to SAN down vagus nerve via parasympathetic nervous system
- Stimulates release of acetylcholine, which decreases heart rate
How does the body respond to a decrease in blood pressure?
- Baroreceptors send more impulses to cardioacceleratory centre in the medulla oblongata
- More impulses to SAN via sympathetic nervous system
- stimulates release of noradrenaline, which increases heart rate and strength of contraction
How does the body respond to an increase in CO2 concentration?
- Chemoreceptors detect pH decrease and send more impulses to cardioacceleratory centre of medulla oblongata
- More impulses to SAN via sympathetic nervous system
- Heart rate increases, so rate of blood flow to lungs increases, so rate of blood flow to lungs increases = rate of gas exchange and ventilation rate increase
