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Biology A2 UNIT 5 > Response to Stimuli > Flashcards

Flashcards in Response to Stimuli Deck (55):


A detectable change in the internal or external environment of an organism that produces a response in the organism.

Ability to respond to a stimuli increases an organisms chance of survival.



Cells or organs that detect stimuli.


What do receptors do?

Transform energy of a stimulus into some form of energy that can be processed by the organism and leads to a response.



These carry out the response to a stimuli, can be cells, tissues, organs and systems.


How is a stimulus passed from receptor to effector?

Hormones or the nervous system


What is the sequence of events from stimulus to response?




A simple response whose direction is determined by the direction of the stimulus. As as result, a motile organism responds directly to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one.


Positive taxis.

Towards stimulus


Negative taxis

Away from stimulus.



A form of response in which the organism does not move toward or away from a stimulus. Instead the more unpleasant the stimulus, the more rapidly it moves and the more rapidly it changes direction.
Designed to bring an organisms back into favourable decisions.


EXAMPLES of taxis.

-Single celled algae will move towards light (+ve phototaxis). Increases chances of survival since they need light to survive.
-Earthworms will move away from light (-ve phototaxis). Increases chance of survival because it takes them into the soil, where they are better able to conserve water, find food and avoid predators.
- Some species of bacteria will move towards a region where glucose is more highly concentrated (+ve chemotaxis)/ Glucose source of food.


EXAMPLE of kinesis

WOODLICE- lose water from their bodies in dry conditions. When they are in a dry area they move more rapidly and change direction more often. Increase chances of moving to a different area. If this area happens to be moist they slow down and change direction less. More likely to stay where they are.



A growth movement of part of a plant in response to a directional stimulus. In almost all cases the plant part grows towards (+ve response) or away (-ve) from stimulus.


EXAMPLES of tropism.

-Plant shoots grow towards light (+ve phototropism) so that their leaves are in the most favourable position to capture light for photosynthesis.
-Plant roots grow away from light (-ve phototropism) and towards gravity (+ve geotropism). In both cases the response increases the probability that roots will grow into soil, where they can better absorb water and mineral ions.
-Plant roots grow towards water (+ve hydrotropism) so that , within the soil, root systems will develop where there is most water.


What are the two major divisions of the nervous system?

The central nervous system

The peripheral nervous system



Made up of brain and spinal cord.



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
Motor neurones



Carry nerve impulses from receptors towards CNS



Carry nerve impulses away from CNS to effectors


What is the motor nervous system divided into?

The voluntary nervous system
The autonomic nervous system



Carries nerve impulses to body muscles and is under voluntary control



Carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control.



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.



The nerve pathway in the body taken by an action potential that leads to rapid, involuntary response to a stimulus.


Describe the stages of the spinal reflex arc.

1. The stimulus- heat from a hot object
2. A receptor- Temperature receptors in the skin on the back of the hand, which create a nerve impulse in a sensory neurone.
3. A sensory neurone- Passes the nerve impulse to the spinal cord.
4. Intermediate neurone- links the sensory neurone to the motor neurone in the spinal cord.
5. Motor neurone- carries the nerve impulse 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 hand away from the hot object.


Why are reflex arcs important?

-Involuntary and therefore do not require the decision making powers of the brain, thus leaving it free to carry out more complex responses. Brain is not overloaded with situations in whihc the response is always the same. Some impulses are sent to the brain so it is informed of what is happening and can sometimes override this reflex if necessary.

-They protect the body from harmful stimuli. Effective from birth and do not have to be learned.

-They are fast, because the neurone pathway is short with very few, typically one or two, synapses (slowest link in nervous system).


What are the two divisions of the autonomic nervous system?




This stimulates effectors and so speeds up any activity. It acts rather like an emergency controller. Eg. when we exercise or experience powerful emotions it stimulates effectors. Helps us cope with stressful situations by heightening our awareness and preparing is for activity. (fight or flight)



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.


What is the resting heart rate of a typical adult?



What are changes to the heart rate controlled by?

The medulla oblongata


What are the two centres of the medulla oblongata?

-a centre that increases heart rate, which is linked to the sinoatrial node by the sympathetic nervous system.
-a centre that decrease heart rate,which is linked to the sinoatrial node by the parasympathetic nervous system.


What do receptors react to? (HR)

-Chemical changes in blood
-Pressure changes in blood



Found in wall of carotid arteries (serve the brain). They are sensitive to changes in pH of the blood that results from changes in CO2 conc. CO2 forms acid in solution so lowers pH.


Describe the process of control of the chemoreceptors.

-When blood has a higher than normal concentration of CO2, its pH is lowered.
-The chemoreceptors in the wall of the carotid arteries and the aorta detect this and increase the frequency of nervous impulses to the centre of the medulla oblongata that increases HR.
-This centre increases the frequency of impulses via the sympathetic nervous system to the SAN which, in turn, increases HR.
-The increased blood flow that this causes leads to more CO2 being removed from the blood by the lungs and so the CO2 level of the blood returns to normal.
-As a consequence the pH of the blood rises to normal and the chemoreceptors in the wall of the carotid arteries and the aorta reduce the frequency of impulses to medulla oblongata.
-The medulla oblongata reduces the frequency of impulses to the SAN, which therefore decreases HR to normal.



Occur within the walls of the carotid arteries and the aorta


Describe the control of pressure receptors.

-When blood pressure is higher than normal:
They transmit a nervous impulse to the centre in the medulla oblongata that decreases heart rate. This centre sends impulses via the parasympathetic nervous system to the SAN, which decreases HR.

-When PB is lower than normal:
They transmit a nervous impulse to the centre in the medulla oblongata that increases HR. This centre sends impulses via the sympathetic nervous system to the SAN node, which increases the rate at which the heart beats.


What do Pacinian corpuscles (receptor) respond to?

Changes in mechanical pressure. They occur deep in the skin and are most abundant in the fingers, the soles of the feet and the external genitalia. They also occur in joints an ligaments and tendons, where they enable an organism to know which joints are changing direction.


What are the features of a Pacinian Corpuscle?

-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 are forms of information, but unfortunately not forms that the body can understand. It is the role of the transducer to convert the information from the stimulus into a form that can be understood by the body, namely nerve impulses. Stimulus is always a form of energy, nerve impulse is too. Receptors therefore convert, or transduce, one form of energy to another. All receptors convert the energy of a stimulus into a nervous impulse known as generator potential.


Describe the structure of a Pacinian corpuscle.

The single sensory neurone of a Pacinian corpuscle is at the centre of layers of tissue, each separated by a gel. This gives it the appearance of an onion when cut vertically.

The sensory neurone has a special type of sodium channel in its plasma membrane. This is called a sodium-mediated channel.



Permeability to sodium changes when they change shape, eg by stretching.


How does the Pacinian corpuscle function?

-In its normal (resting) state, the stretch-mediated sodium channels of the membrane around the neurone of the Pacinian corpuscle are too narrow to allow sodium ions to pass along them. In this state, the neurone of the Pacinian corpuscle has a resting potential.
-When pressure is applied to the PC, it changes shape and the membrane around its neurone becomes stretched.
-This stretching widens the Na channels in the membrane and sodium ions diffuse into the neurone.
-The influx of sodium ions changes the potential of the membrane (i.e. it becomes depolarised), thereby producing a generator potential.
-The generator potential in turn creates an action potential (nerve impulse) that passes along the neurone and then, via other neurones, to the CNS.


What must the body have to distinguish between different intensities of a stimulus?

Range of receptors, each responding to a different intensity of stimulus.


Describe simply the role of receptors in the eye.

The light receptors in the eye are found on its innermost layer-the retina. The millions of light receptors found in the retina are of two main types: cone cells and rod cells. Both act as transducers by converting light energy into electrical energy of a nerve impulse.



Rod cells cannot distinguish between different wavelengths of light and therefore produce images in black and white. More numerous than cone cells; there are around 120 million in each eye.


Describe rod cells (structure and function)

Many rod cells share a single sensory neurone. Rod cells can therefore respond to light of a very low intensity. This is because a certain threshold value has to be exceeded before a generator potential is created in the bipolar cells to which they are attached. As a number of rod cells are attached to a single bipolar cell (=retinal convergence), there is a much greater chance that the threshold value will be exceeded than if only a single rode cell were attached to each bipolar cell. As a result, rod cells allow us to see in low light intensity,although only in black and white.


How do rod cells create a generator potential?

The pigment in rod cells (RHODOPSIN) must be broken down. Low intensity light is sufficient to cause this breakdown.


What is a consequence of many rod cells linking to a single bipolar cell?

Light received by rod cells sharing the same neurone will only generate a single impulse regardless of how many of the neurones are stimulated. This means they cannot distinguish between the separate sources of light that stimulated them. Two dots close together will appear as a single blob. Low visual acuity.


What do the three different cone cells respond to?

Different wavelengths of light.


How many cone cells are there in each human eye?

6 million.


Describe the structure of cone cells.

Have their own separate bipolar cell connected to a sensory neurone. means that the stimulation of a number of cone cells cannot be combined to help exceed the threshold value and so create a generator potential. As a result cone cell only respond to a high intensity of light.


What is the pigment found in cone cells and what does it need?

IODOPSIN- requires a higher light intensity to break down. Only light of a high enough intensity will break it down and create a generator potential.


Why can the brain distinguish between two separate sources of light stimulating cone cells?

Each cone cell has its own connection to a single bipolar cell, which mean that, if two adjacent cone cells are stimulated, the brain receives two separate impulses. The brain can distinguish between two separate sources of light that stimulated the two cone cells. This means that two dots close together will appear as two dots. Give accurate vision- good visual acuity.


Describe how cone cells and rod cells are distributed in the eye.

The distribution of rod cells and cone cells is uneven on the retina. Light is focused by the lens on the part of the retina opposite the pupil. This point is known as the fovea. The fovea therefore receives the highest intensity of light. Therefore, cone cells, but not rod cells, are found at the fovea. 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.