topic 6

• Stimulus is a detectable change in the environment that can be detected by cells and organisms
• Receptors are the cells which detect stimuli
• Taxis


  A simple response where an organism moves its entire body towards a favorable stimulus or away from an unfavorable stimulus
• Taxis


  • Positive taxis - organism moves towards a favorable stimulus
  • Negative taxis - organism moves away from an unfavorable stimulus
• Taxis


  • Earthworms show negative taxis away from light
  • Bacteria show positive chemotaxis towards certain chemicals
• Kinesis


  A simple response where an organism changes the speed at which it moves and the rate at which it changes direction
• Kinesis


  • If organism moves from favorable to unfavorable stimuli, it increases rate of changing direction
  • If organism is completely surrounded by unfavorable stimuli, it decreases rate of turning to move in a more straight line
• Kinesis


  • Woodlice increase rate of turning when moving from damp to dry area, but decrease rate of turning when completely in dry area
• Taxes and kinesis are simple responses seen in small animals to help them stay in favorable conditions
• Receptor


  Cells which detect stimuli
• Stimulus


  Detectable change in the environment
• Stimulus detection and response

  1. Receptor detects stimulus

  2. Stimulus triggers response
• Nervous system


  • Central nervous system (brain and spinal cord)
  • Peripheral nervous system (receptor cells, sensory neurons, motor neurons)
• Receptor


  Can only detect and respond to specific stimuli
• Key receptors in AQA specification

  • Pacinian corpuscle
  • Rods
  • Cones
• Action potential


  Generated potential that initiates a response
• Stimulus size


  Determines if action potential is generated
• Pacinian corpuscle


  Sensory neuron wrapped in layers of connective tissue with viscous gel
• Pacinian corpuscle


  • Responds to pressure stimulus
  • Found deep in skin, especially fingers and feet
• Pacinian corpuscle response to pressure

  1. Pressure deforms layers and stretches sensory neuron membrane

  2. Stretch-mediated sodium channels open

  3. Sodium ions flow in

  4. Action potential generated
• Receptors


  Cells which detect a change in the environment and respond to a specific stimulus
• If the stimulus is big enough, it will lead to an action potential
• Rods and cones


  Photoreceptors found in the retina
• Rods


  • Rod-like in shape
  • Do not distinguish between different wavelengths of light (colours)
  • Can detect light at very low light intensities
• Rhodopsin


  Protein pigment in rod cells that breaks down when absorbing light
• Triggering an action potential in rod cells

  1. Light intensity absorbed

  2. Rhodopsin pigment breaks down

  3. Threshold met in bipolar cell

  4. Action potential generated
• Retinal convergence


  Multiple rod cells connecting to one bipolar cell
• Spatial summation


  Adding together the impact of multiple rod cells to reach action potential threshold
• Advantage of retinal convergence in rods


  • Able to see in black and white even at low light intensities
• Disadvantage of retinal convergence in rods

  • Low visual acuity - cannot distinguish separate light sources
• Cones


  • Cone-shaped
  • Three different types with different colour pigments (red, green, blue)
  • Require higher light intensity to trigger action potentials
• Iodopsin


  Colour pigment in cone cells
• Advantage of cones


  • High visual acuity - can distinguish separate light sources
• Fovea


  Part of the retina that receives the highest light intensity
• The fovea has the highest concentration of cone cells
• The blind spot in the retina has no photoreceptors
• Myelinated motor neuron


  Structure and functions of its parts
• Cell body


  • Contains organelles like nucleus, mitochondria, ribosomes to make proteins and neurotransmitters
• Dendrites


  • Carry action potentials to surrounding cells
• Axon


  • Long conductive fiber that carries nerve impulses and has multiple action potentials