Stimuli, both internal and external, are detected and lead to a response Flashcards
Stimulus
Detectable change in the internal/external environment of an organism that leads to a response
Receptor
Detects stimulus, specific to one type of stimulus
Coordinator
Formulates a suitable response to a stimulus e.g nervous system/hormonal system
Effector
Produces a response to a stimulus e.g. muscles/glands
Reflex arc order
Stimulus → receptor → sensory neurone → coordinator (CNS/relay neurone) → motor neurone → effector → response
Reflex arc importance
Rapid (short pathway) because only 3 neurones and few synapses (synaptic transmission is slow)
Autonomic as it doesn’t involve passage to the brain - does not have to be learnt
Protects from harmful stimuli e.g. escape from predator/prevents damage to body tissues
Taxes
Directional responses by simple mobile organisms who move towards a favourable stimulus (positive taxis) or away from an unfavourable one (negative taxis)
e.g. woodlice show a tactic response to light. Move away from light → keeps concealed under stones during day away from predators, and in damp conditions which reduces water loss → improves chances of survival
Kineses
Non-directional responses by simple mobile organisms who change the speed of movement or the rate of direction change, in response to a non-directional stimulus
e.g. woodlice show a kinetic response to humidity
Positive and negative tropism in flowering plants
Tropism is the growth of a plant in response to a directional stimulus
Positive tropism is growth towards a stimulus
Negative response is growth away from the stimulus
Growth factors in flowering plants
A plant’s responses to external stimuli involves growth factors/hormone-like growth substances
Growth factors move from growing regions e.g. shoot tips/leaves where they are produced, to other tissues, where they regulate growth in response to directional stimuli e.g. auxins (such as IAA)
Indoleacetic acid (IAA)
Auxin
In roots, IAA inhibits cell elongation
In shoots, IAA promotes cell elongation
How IAA results in phototropism in shoots
Cells in tip of shoot produce IAA which is transported down the shoot (evenly initially)
IAA concentration increases on the shaded side and promotes cell elongation
Shoot bends towards light
How IAA results in gravitropism in roots
Cells in tip of root produce IAA which is transported down the root (evenly initially)
IAA concentration increases on the lower side of the root and inhibits cell elongation
Root curves downwards towards gravity
How does the Pacinian corpuscle function
In its normal state, the neurone Pacinian corpuscle has a resting potential as the stretch-mediated sodium channels of the neurone membrane are too narrow to allow sodium ions to pass along them.
Mechanical stimulus e.g. pressure deforms lamellae and stretch-mediated sodium ion channel
The sodium ion channels open and sodium ions diffuse into the sensory neurone
Greater pressure causes more channels to open and more sodium ions to enter, causes depolarisation which leads to a generator potential
If generator potential reaches threshold it triggers an action potential (nerve impulse)
What does the Pacinian corpuscle illustrate
Receptors respond only to specific stimuli – only responds to mechanical pressure
Stimulation of a receptor leads to the establishment of a generator potential. When threshold is reached, action potential sent, all-or-nothing principle.
Rod cells
Rod-shaped
Greater number than cone cells
More at the periphery of the retina, absent in fovea
One type of rod containing one pigment
Rods connected in groups to one bipolar cell/ganglion cell/neurone
Very sensitive to light (see in dim light)
Low visual acuity
Black & white (monochromatic) vision
Cone cells
Cone-shaped
Fewer numbers than rod cells
Concentrated at the fovea, fewer at the periphery of the retina
3 types of cones containing different optical pigments
One cone joins one neurone
Less sensitive to light (require bright light)
High visual acuity
Colour (trichromatic) vision
Differences in sensitivity to light in rod and cone cells
Rods are more sensitive to light
Rods connected in groups to one bipolar cell/ganglion cell/neurone (retinal convergence)
Spatial summation
Stimulation of each individual-cell alone is sub-threshold/insufficient but cells connected in groups means threshold more likely met/ exceeded to generate action potential
Cones are less sensitive to light/need higher intensity light
One cone joins to one neurone
No spatial summation
Differences in visual acuity in rod and cone cells
Cones give higher visual acuity
One cone joins to one neurone
If 2 adjacent cone cells are stimulated, brain receives 2 separate impulses (information)
It can distinguish between 2 separate sources of light
Rods give lower visual acuity
Rods connected in groups to one bipolar cell/ganglion cell/neurone (retinal convergence)
Spatial summation
Many neurones only generate one impulse/ action potential, regardless of how many neurones stimulated
It can’t distinguish between separate sources of light
Differences in sensitivity to colour in rod and cone cells
Cones allow colour vision
3 types of cones with different optical pigments that absorb different wavelengths/red/green/blue
Stimulation of different combinations/ proportions of cones gives a range of colour perception
Rods allow monochromatic vision since there is only one type of cone/pigment
Myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity.
Cardiac muscle is myogenic i.e. it can contract/relax without receiving electrical impulses from nerves
Myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity.
The role of the SAN in the bundle of His
Sinoatrial node (SAN) acts as a pacemaker and sends out regular waves of electrical activity across both atria
Causing right/left atria to contract simultaneously
(A layer of non-conductive tissue prevents wave crossing directly to ventricles)
Myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity.
The role of the AVN and Purkyne tissue in the bundle of His
Waves of electrical activity reaches the atrioventricular node (AVN) which delays impulse, allowing atria to fully contract and empty
AVN passes wave of electrical activity to bundle of His which conducts waves between ventricles to the apex of the heart, where the bundle branches into smaller fibres of Purkyne tissue
Ventricles contract simultaneously, from the bottom up
Role and location of baroreceptors (pressure receptors) and the roles of the autonomic nervous system and effectors in controlling heart rate
Baroreceptors located in aorta and carotid arteries
Baroreceptors stimulated by high/low blood pressure
Low BP → more frequent impulses to medulla/cardiovascular control centre → more frequent impulses sent to SAN along sympathetic neurones → more frequent impulses sent from SAN → cardiac muscle contracts more frequently so heart rate increases
High BP → more frequent impulses to medulla/cardiovascular control centre → more frequent impulses sent to SAN along parasympathetic neurones → less frequent impulses sent from SAN → cardiac muscle contracts less frequently so heart rate decreases
Role and location of chemoreceptors and the roles of the autonomic nervous system and effectors in controlling heart rate
Chemoreceptors located in aorta, carotid arteries and medulla
Chemoreceptors stimulated by blood pH/CO2 concentration/oxygen concentration (related to exercise)
High blood CO2 concentration/low pH/low blood O2 → more frequent impulses to medulla/cardiovascular control centre → more frequent impulses sent to SAN along sympathetic neurones → more frequent impulses sent from SAN → cardiac muscle contracts more frequently so heart rate increases
Low blood CO2 concentration/high pH/high blood O2 → more frequent impulses to medulla/ cardiovascular control centre → more frequent impulses sent to SAN along parasympathetic neurones → less frequent impulses sent from SAN → cardiac muscle contracts less frequently so heart rate decreases
Sensory neurones
Transmit nerve impulses from a receptor to an intermediate or motor neurone
One dendron that carries the impulse towards the cell body and one axon that carries it away from the cell body
Motor neurones
Transmit nerve impulses from an intermediate or sensory neurone to an effector such as a gland or a tissue
Long axon
Short dendrites
Intermediate (relay) neurone
Transmit electrical impulses between sensory neurones and motor neurones
