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DETECTING and RESPONDING to signals DETECTING and RESPONDING to signals RECEPTORS Receptors: Specialised structures capable of responding to specific stimuli by ... – PowerPoint PPT presentation

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Title: DETECTING and RESPONDING to signals

  • 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
  • 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.

  • 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

  • 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
  • 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.

  • 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.

  • The Nervous System The nervous 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 Inter-neurons 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
  • 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

Nerve Impulse Action Potential
  • The action potentialWhen 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//
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

Presynaptic membrane
Postsynaptic membrane
Receptor sites
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.

  • 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
  • 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 (anti-diuretic hormone, growth hormone
Hormones in Action
Neural or hormonal stimulation
Hormone fuses to specific receptors on target
Carried by bloodstream around the body
Target Cell
Endocrine gland
Hormone secreted
Hormones cont.
  • 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
  • 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
  • Growth away from the stimulus is called a
    Negative Tropism.
  • Stimulus Tropism
  • Light Phototropism
  • Gravity Geotropism
  • Touch Thigmotropism
  • Water Hydrotropism

Plants Sensing and RespondingStimuli
  • Plants do not monitor their internal environment,
    as do animals.
  • They are however sensitive to a number of
    environmental factors, both physical and chemical
  • 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
  • Hormonal responses in plants are relatively
  • 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.

  • 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.

  • 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.
  • 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.