3.6.1 - STIMULI AND RESPONSE
13 July 2021 12:08
Physiological responses generally follow a sequence which allows an STIMULUS
organism to respond to changes to internal or external environments. DETECTION
COORDINATION
Detection of change to internal or external environment provokes a response in sensory organisms who adjust to RESPONSE
respond to this change through modifying behaviour or physical factors
STIMULUS A stimulus is an agent capable of eliciting a physiological response, instigates change in activity or behaviour following detection.
Classified as internal or external depending on their origin
INTERNAL ▪ blood pressure EXTERNAL
▪ Light
Deviations from optimum condition or ▪ Gas partial pressures occur external to the organism, capable of
▪ Temperature
state causes homeostatic imbalance ▪ Ion/nutrient imbalance producing systemic responses in the body
▪ pressure
1 which elicits a sensation (pain, thirst ▪ foreign material and a readjustment of internal conditions.
▪ gravity
etc) that motivates behaviour that will ▪ hormones Perception of environmental change cause
▪ Mechanical stimulus
restore the body to normal. ▪ Enzyme action automatic or voluntary processes in order
▪ Eleftrical stimulus
▪ pH to restore equilibrium.
RECEPTION
Receptors detect stimuli and convey sensory PERCEPTION + COORDINATION Perception is recognition and interpretation of a sensation.
information about the nature of the stimulus
to coordination centres.
▪ sensory receptors convey information, processed and cognised at
2 4 main receptor classes: coordinating centres.
▪ Mechanoreceptors ▪ Centres of neural activity such as brain or spinal cord receive information
▪ Chemoreceptors through neural pathways via conduction of action potentials.
▪ thermoreceptors
▪ Sensory receptors (visual, olfactory) Non-reflexive response involves transmissiom though the central nervous system.
Signals travel to appropriate region of the cortex after being redirected by the
3
thalamus. redirecting it to neural centre dedicated to processing that particular sense.
RESPONSE
Animal systems have 2 levels of coordination:
Specialised response via coordinators, ▪ Chemical - glandular release of hormones allows regulation of homeostasis and
which may act on any level of organisation, coordination of bodily responses, non-specific pathway with wider effect
4 from cellular to the whole organism. ▪ Nervous - centralised system of coordination which provides rapid coordination
• carried out by effectors, which include using electrical impulses between the neural centre and the effector organs
glands, organs hormones, cells or muscles
REFLEX ARCS
▪ No processing involved in brain, minimises delay of routing signals
through neural pathway
▪ Enables immediate response to danger, protects organism
▪ amount of sensory inputs to brain limited, better processing as less
information to process
▪ Fewer synaptic transmissions (slowest part of pathway)
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, TAXIS
Innate behavioural response triggered by an external stimulus, resulting in
directional movement of the cell or motile organism.
POSITIVE movement towards source of stimulation (attraction)
NEGATIVE movement away from source of stimulation (repulsion)
UNFAVOURABLE
KINESIS
Modification to rate of non-directional movement
depending on proximity of organism or cell to its optima.
Increased rate of turning + decrease in speed FAVOURABLE
corresponds to favourable conditions
• Maintain motile organism in favourabble conditions.
• Non-directional, exhibits itself in circular movement + indirect pathways
ORTHOKINESIS change in speed of movement, decreases in favourable conditions
KLINOKINESIS modulation to rate of turning or frequency of direction change
increases in favourable conditions
TROPISM
Growth response of an organism (usually a plant) in response to a directional environmental stimulus. Tropism is typically associated with plants in
response to environmental factors which would be growth limiting.
• positive, growth towards the stimulus
Young shoots move in direction:
• Negative, directed growth causing movement away from the stimulus.
1. Against gravity (negative geotropism)
PHYTOHORMONES 2. towards unilateral sunlight in order to maximise light
Plant growth factors (phytohormones) are chemical messengers which absorbance for photosynthesis (positive phototropism)
regulate physiological processes.
• alteration of cell wall plasticity causes expansion or limits growth
Roots grow aligned with gravity (positive geotropism)
• Promotes axial elongation (shoots), lateral expansion (root swelling),
or iso-diametric expansion (fruit growth)
AUXINS
Class of phytohormone that positively influence cell enlargement. Unevenly localized growth produces bending, turning + directionalization of major organs
Controlled distribution within particular regions of the plant allows different degrees of expansion, correspond to identifiable patterns of growth.
• Indole-3-acetic acid (IAA) is the most abundant native phytohormone,
⇢ Unlike animals (hormone production restricted to glands) each plant cell is capable of producing hormones.
⇢ produced predominantly in young developing parts of plant such as the shoot apex, emerging leaves and developing seeds
Auxin localisation, strictly controlled directionality of auxin flow in response to gradients results from asymmetric cellular localization of auxin efflux carriers.
1 more auxin = higher H+ influx 3 cell wall becomes acidified 5 Imbalance in distribution, uneven growth
2 increased H+ causes decreased pH 4 Increased wall plasticity = swelling and elongation
Auxin produced at the tip, instead of being transported basipetally, is
transported laterally toward the shaded side. LIGHT Gravitational effect on auxin transport is greater
accumulation of auxin along the underside of the root,
1. Auxin broken down in sunlight, decreased causing downwards bending and root growth
concentration in areas exposed to incident light • shoots display negative gravitropism (when placed
2. direction of incident light is detected at the tip of on side, plant shoot will grow up)
the shoot via photoreception • Roots display positive gravitropism: grow down
3. Transport proteins are activated in plasma
membrane at lateral face of cells of shoot
1. statoliths affected by gravity, accumulation of
4. Auxin pumped out of efflux transporters,
starch in direction of gravitational pull
accumulates in cells on shady side.
5. Auxins activate proton pumps, decreasing pH in 2. Amyloplasts settling causes upregulation of IAA
the cells on dark side of plant. transporters
6. acidification of hydrogen crosslinks in cellulose, 3. Transporters move IAA in direction of gravitational
causes increased plasticity and swelling pull, efflux
7. Uneven growth causes bending toward the light. 4. inhibits root cell elongation , causing uneven
growth between root surfaces promoting
downwards bending into soil
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