Topic 6 - A: Stimuli and Responses

• Survival and Response


  Organisms need to respond to their environment in order to survive
• Responding to the environment

  • Organisms increase their chances of survival by responding to changes in their external environment
  • Animals and plants respond in different ways
• Plant responses


  • Leaves move away from harmful stimuli
  • Plants growing in dark places grow towards light to increase their chances of photosynthesis
• Animal responses


  • Woodlice move to more humid areas to avoid drying out
• Stimulus


  A change in the environment, such as change in temperature, light intensity or pressure
• Tactic response


  The direction of the stimulus affects the response
• Kinetic response


  The intensity of the stimulus affects the response
• Organisms respond to changes in their internal environment as well as their external environment
• Nervous Communication


  Organisms use a nervous system to pass information between different parts of their body and coordinate responses
• Receptors and effectors


  • Receptors detect stimuli
  • Effectors bring about a response to a stimulus
• Neurones


  Nerve cells that transmit electrical impulses
• Nervous communication


  1. Stimuli detected by receptors

  2. Electrical impulses along neurones

  3. Neurotransmitters released at effectors

  4. Response occurs
• Simple reflex


  A rapid, involuntary response to a stimulus that does not involve the conscious brain
• Reflex arc


  The pathway of neurones linking receptor to effector in a simple reflex
• Simple reflexes are protective and help organisms avoid damage
• Responses in plants


  • Plants respond to stimuli by regulating their growth
  • Plants use growth factors to coordinate responses
• Tropism


  The growth response of a plant to a directional stimulus
• Tropisms


  • Phototropism - growth towards light
  • Gravitropism - growth in response to gravity
  • Thigmotropism - growth in response to touch
• Auxin


  A plant growth factor that stimulates cell elongation in shoots and inhibits it in roots
• Auxin concentration


  Higher in shaded parts of shoots, lower in roots, leads to uneven growth and tropisms
• High auxin concentrations inhibit root growth
• Auxin moves by diffusion and active transport to different parts of the plant
• Uneven distribution of auxin leads to uneven growth and tropisms
• The nervous system is made up of the central nervous system (CNS) which includes the brain and spinal cord, and the peripheral nervous system (PNS).
• Neurons are specialized cells that transmit electrical signals called action potentials.
• Sensory neurons carry information from sensory receptors to the CNS.
• Receptors


  Detect stimuli and pass information about stimuli along the nervous pathway
• Receptors


  • They are specific - they only detect one particular stimulus
  • There are many different types of receptor that each detect a different type of stimulus
  • Some receptors are receptor cells that connect to the nervous system
  • Some receptors are proteins on cell surface membranes
• How receptors work


  1. Receptors convert the energy of the stimulus into the electrical energy used by neurones

  2. Receptors have a resting potential when not being stimulated

  3. When a stimulus is detected, the cell membrane becomes more permeable, allowing more ions to move in and out, altering the potential difference - this is called the generator potential

  4. If the generator potential reaches a certain threshold level, it will trigger an action potential - an electrical impulse along a neurone

  5. The strength of the stimulus is measured by the frequency of action potentials triggered
• Action potentials are all one size, so the strength of the stimulus is measured by the frequency of action potentials the number of action potentials triggered during a certain time period
• If the stimulus is too weak

  The generator potential won't reach the threshold, so there's no action potential
• Pacinian corpuscles


  Mechanoreceptors that detect mechanical stimuli, e.g. pressure and vibrations
• Pacinian corpuscles


  • Found in the skin
  • Contain the end of a sensory neurone, called a sensory nerve ending
  • Sensory nerve ending is wrapped in layers of connective tissue called lamellae
• Pacinian corpuscle stimulation

  1. Lamellae are deformed

  2. Lamellae press on the sensory nerve ending

  3. Sensory neurone's cell membrane is stretched, deforming the stretch-mediated sodium ion channels

  4. Sodium ion channels open

  5. Sodium ions diffuse into the cell, creating a generator potential

  6. If generator potential reaches threshold, it triggers an action potential
• Inside of the membrane is negatively charged compared to the outside
• All parts of the membrane have sodium channels closed at rest
• Sodium channels open where stimulus is applied, so sodium ions diffuse into the cell
• All other parts of the membrane have sodium channels still closed
• Photoreceptors


  Receptors in your eye that detect light
• How light enters the eye

  1. Light enters the eye through the pupil

  2. Amount of light is controlled by the iris

  3. Light is focused by the lens onto the retina