Download DETECTING and RESPONDING to signals

DETECTING and
RESPONDING to
signals
RECEPTORS
Receptors: Specialised structures capable of responding to
specific stimuli by initiating signals in the nervous system or
triggering the release of a hormone.
Types of Receptors
 Chemoreceptors – These are stimulated by specific
chemicals in the external and internal environment.
 Mechanoreceptors – These are stimulated by anything
that changes the shape of the receptor.
 Photoreceptors – These detect light. In some animals
they also detect colour and form images.
 Thermoreceptors – These detect external heat and cold
through receptors near the surface and internal body temp.
deeper in the body by receptors in the major arteries and
hypothalamus.
Detecting Stimuli.
The intensity of a
stimulus must be
sufficient to reach the
threshold of the
receptor. This is the
weakest stimulus to
which the receptor can
respond. Receptors
then stimulate
effectors to produce a
response.
Responding
Responses in animals are based on sensory
information received from all parts of the body,
often requiring coordination from different parts
of the body. Internal communication involved in
homeostasis and regulation are carried out by the
nervous and hormonal systems.
 The nervous system carries messages rapidly along
nerve pathways.
 The hormonal, (ENDOCRINE), system is a slower
system that releases specific chemicals into the
bloodstream.
RESPONDING con’t.
 Misalignment detectors: These are detectors that
detect when a particular factor is out of line. They
monitor the precise factor of the internal
environment that is being controlled, eg. Oxygen
level in the blood or blood temperature in the brain.
 Disturbance Detectors: These warn of problems
before they arise. They detect the presence of
external or other internal changes that may result
in a change in the factor of the internal
environment being controlled.
Note: Disturbance and misalignment detectors allow for
a more precise control of internal factors than
misalignment detectors acting alone.
RESPONDING con’t.
 Effector organs include muscles and glandular
tissue. Muscle cells can be stimulated to contract
or can be inhibited restricting contraction. Glands
secrete biologically active substances such as
hormones and enzymes.
 Directionality is often an important aspect of
responsiveness. Some environmental stimuli,
particularly light and sound the direction from
which it comes is of the utmost importance. Often
the direction is determined by signals from a pair
of sensory organs such as eyes and ears.
NERVES - NEURONS
The Nervous System: The nervous
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system is present in animals but not
plants and is characterized by rapid
response.
It is composed of three complimentary
systems:
The Central Nervous System (CNS) –
the Brain and Spinal Cord where most
integration in the nervous system takes
place.
The Autonomic Nervous System –
includes nerves involved in
unconscious/involuntary responses.
Peripheral Nervous System – includes
sensory nerves and motor nerves.
Types of Nerve Cells
Nerve Cells: There are Three
main types of nerve cells.
They are:
 Sensory neurons – these conduct
messages from the receptors to the
CNS.
 Intermediate/Connector or Interneurons – these relay impulses from
the sensory to the motor neurons.
They are found in the CNS.
 Motor neurons – these relay
messages away from the CNS to the
effector organs, glands & muscles.
A Typical Nerve Cell
Neurons: All neurons are made up of three main parts:
 Cell body – contains the nucleus and the cytoplasm of the cell.
Messages received by the dendrites are sent to the cell body.
 Axon – an elongated section of the cell body that conducts
impulses away from the cell body and transmits messages to
other cells. Axons vary in length and branching.
 Dendrite – Fine branching extensions of the neuron that conduct
impulses toward the cell body and away from other cells.
Electrical Insulation
Myelin – rich in fats, forms an electrical insulating
layer around the axon, thus increasing the speed of
impulse conduction.
Schwann cells – cells outside the CNS that form a
tightly wrapped myelin sheath.
Node of Ranvier – gaps in the myelin sheath along
the axon. The sheath prevents ion flow across the
neuron membrane and forces the impulse to flow
from node to node. In this way impulses jump along
the axon.
Axon – speed of impulse travel is partly dependent
on the diameter of the axon. The larger axon
increases speed of conduction. (eg. squid have giant
axons with very rapid conduction speeds).
Nerve Impulse – Action
Potential
The action potential
When chemicals contact the surface of a neuron, they
change the balance of ions (electrically charged atoms)
between the inside and outside of the cell membrane. When
this change reaches a threshold level, this effect runs
across the cell's membrane to the axon. When it reaches
the axon, it initiates the action potential.
The surface of the axon contains hundreds of thousands of
miniscule mechanisms called ion channels. When the charge
enters the axon, the ion channels at the base of the axon
allow positively charged ions to enter the axon, changing the
electrical balance between inside and outside. This causes
the next group of ion channels to do the same, while other
channels return positive ions to the outside, and so on all
the way down the axon.
Action Potential
Great Website: http://outreach.mcb.harvard.edu/animations/actionpotential.swf
Action Potential – movement of ions
The Synapse
Synapse: Neurons never touch
each other. There is a gap or
junction between one neuron
and the next, known as a
synapse. The synapse consists
of the end of the axon of one
neuron and the start of a
dendrite of another neuron.
The Axon releases a chemical
called a neurotransmitter into
the synapse, which diffuses
across to the dendrites of the
other neuron. Receptors on the
dendrites combine with the
neurotransmitter and trigger a
nerve impulse in the next
neuron
Axon
Mitochondria
Presynaptic
membrane
Vesicles
Neurotransmitters
Receptor
sites
Postsynaptic
membrane
HORMONES – The Endocrine
System
 The Endocrine System consists of ductless
(endocrine) glands – specialised cells that secrete
hormones directly into the blood stream.
 Hormones are specialised chemicals produced in
minute amounts that are involved in the regulation
of many body processes. They circulate in the
bloodstream but can only be detected by specific
receptors on particular cells.
 Most hormones only affect the production of
enzymes, or structural proteins that affect
growth, development, reproductive cycles and
other processes in specific organs or tissues.
HORMONES IN ACTION
 Hormonal response may be slow acting
but its effects may be long lasting.
 The Hypothalamus gland in the brain is
the main control centre that regulates
hormones by sending nerve or hormonal
messages to the Pituitary gland.
 The Pituitary gland, in turn passes
“messages” via hormones to target
tissues around the body.
The Endocrine Glands
Comparison of Hormone Types
Type
Relative
Size
Movement
Examples
Fatty-acid
hormones
Small
Lipid-soluble, so they pass
directly through plasma
membranes.
Steroid
hormones:
Testosterone
, oestrogen,
progesterone
Amino-acid
hormones
Larger
Water-soluble, so they bind
to receptors on plasma
membranes. This activates
the second messenger
mechanism, cyclic AMP,
which causes the change
within the cell.
Insulin,
glucagon,
adrenaline,
thyroxine,
oxytocin,
ADH (antidiuretic
hormone,
growth
hormone
Hormones in Action
Hormone
Neural or hormonal
stimulation
Receptor
Hormone fuses to specific
receptors on target cells
Carried by bloodstream
around the body
Target Cell
Endocrine gland
Hormone
secreted
Response
Hormone
Hormones con’t.
Pheromones are chemical signals released outside the body. They target
organisms of the same species and are most commonly used to attract mates or
mark territory.
Comparing The Nervous And Endocrine Systems
Nervous System
Hormonal System
Type of
message
Electrochemical impulse
Chemical messenger
Speed of
message
Rapid
Slow
Transmission
Nerves / Neurons
Bloodstream
Duration of
response
Short
Long lasting
Target
Very specific in target
neurons, muscles or glands
More general, to target
tissues or organs in the
body
REGULATION IN PLANTS
 Plants are able to adjust their growth and
development in response to the environment.
 When a plant responds to an external stimulus the
plant exhibits a TROPISM.
 Growth towards a stimulus is called a Positive Tropism.
 Growth away from the stimulus is called a Negative
Tropism.
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Stimulus
Tropism
Light
Gravity
Touch
Water
Phototropism
Geotropism
Thigmotropism
Hydrotropism
Plants: Sensing and
Responding
Stimuli
 Plants do not monitor their internal environment, as do animals.
 They are however sensitive to a number of environmental factors, both
physical and chemical factors.
Physical Factors: include direction and wavelength of light, day/night
length (photoperiod), gravity, temperature and touch.
Chemical Factors: include water, carbon dioxide and specific chemicals,
for example ethylene gas (which ripens fruit).
 Directionality is often important in plant sensing. Eg. Shoots growing
towards light, Roots responding to gravity by growing down.
Responding
 Growth in plants is triggered by environmental factors. When the direction
of the growth is related to the direction from which the stimulus comes the
response is called a tropism. See previous lesson for various Tropisms.
Summary: Plant Hormonal
Responses Compared To Animals
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Hormonal responses in plants are relatively simple.
Plants have no endocrine system like animals.
Hormone secreting cells are not organised into
specialised glands.
Plant hormones are generally produced by cells
receiving appropriate stimuli.
Plant hormonal responses are much slower.
Plant hormones are distributed through:
1. From cell to cell
2. Transport pathways – usually the phloem
3. Even through the air.
FLOWERING
Plants flower, develop fruit and become dormant at
the most favourable times of the year. These are
controlled by the daily light length, or
PHOTOPERIOD. A response to the photoperiod is
called a photoperiodism. It is the night-length that
stimulates flowering.
 Short-day plants: long nights. Produce flowers when
the photoperiod is less than a critical value. So
flowering is prevented if the hours of daylight are too
long. These plants usually flower in late summer,
autumn or winter.
 Long-day plants: short nights. Will not flower until
the hours of daylight exceed a threshold value. They
tend to flower in late spring or early summer.
 In day neutral plants, the length of the photoperiod
is unimportant. A signal from the leaves, possibly a
hormone called florigen, causes development of buds.
DORMANCY & VERNALISATION
DORMANCY
 Some plants become dormant prior to winter. Lower
Temps and shortening days trigger changes that involve
loss of chlorophyll from leaves and withdrawal of
nutrients from leaves into stems and roots. Abscisic acid
is largely responsible for bud dormancy.
 Dormancy is broken by substantial rainfall, intense heat
(fire) to break seed coat, extended exposure to cold or
light or chemicals found in the digestive tracts of
animals.
VERNALISATION
 Some plants require exposure to cold before they can
complete their life cycle. Vernalisation is the period of
winter cold that stimulates flowering in many plants.