Topic 6

• Stimulus


  A detectable change in the environment
• Receptors


  Cells that can detect changes in the environment
• Tropisms


  • Plant responses to their surroundings via growth
  • Can be positive (grow towards stimulus) or negative (grow away from stimulus)
  • Stimuli are light and gravity
• Auxin (IAA)


  Growth factor that controls cell elongation in shoots and inhibits growth in roots
• Phototropism


  1. Shoot tip cells produce IAA

  2. IAA diffuses to shaded side

  3. Cells on shaded side elongate

  4. Shoot bends towards light
• Gravitropism


  1. IAA diffuses to lower side of shoot

  2. Lower cells elongate

  3. Shoot bends upwards against gravity
• Gravitropism in roots


  1. IAA moves to lower side

  2. Upper cells elongate

  3. Root bends downwards towards gravity
• Reflex


  Rapid automatic response to protect from danger
• Reflex arc


  Made up of 3 neurons: sensory, relay, motor
• Taxis


  Organism moves its entire body towards or away from a stimulus
• Positive taxis


  Organism moves towards a favorable stimulus
• Negative taxis


  Organism moves away from an unfavorable stimulus
• Kinesis


  Organism changes speed of movement and rate of direction changes in response to stimuli
• Pacinian corpuscle


  Pressure receptor with stretch-activated sodium channels
• Rods


  Photoreceptors that detect black and white images in low light
• Cones


  Photoreceptors that detect colour in high light intensity
• Fovea


  Region of retina with high concentration of cone cells
• Cardiac muscle


  Myogenic - contracts and relaxes on its own
• Heartbeat control


  1. SA node sets pace

  2. Depolarization wave spreads to AV node

  3. Delayed conduction to ventricles via bundle of His

  4. Ventricles contract from apex upwards
• Wave of depolarization


  1. Non-conductive layer of tissue separates atria and ventricles

  2. Wave of depolarization travels down bundle of His

  3. Wave of depolarization travels up through Purkinje fibers
• Wave of depolarization travels down bundle of His and up through Purkinje fibers


  Causes a slight delay in time for ventricles to contract
• Delay allows atria to contract and force blood into ventricles before ventricles contract
• Ventricles contract from apex to base, like squeezing toothpaste from bottom up
• Cells repolarize, causing cardiac muscle to relax, then cycle repeats
• Medulla oblongata


  Controls heart rate through autonomic nervous system
• Parts of autonomic nervous system

  • Sympathetic nervous system
  • Parasympathetic nervous system
• Sympathetic nervous system impulses

  Increase heart rate
• Parasympathetic nervous system impulses

  Decrease heart rate
• Chemoreceptors


  Detect changes in blood pH
• Pressure receptors


  Detect changes in blood pressure
• Increased respiratory rate


  Decreases blood pH due to excess CO2 and lactic acid
• Decreased blood pH


  Increases heart rate to remove excess CO2
• High blood pressure


  Increases impulses through parasympathetic nervous system to decrease heart rate
• Low blood pressure


  Increases impulses through sympathetic nervous system to increase heart rate
• Myelinated motor neuron


  • Cell body with organelles
  • Dendrites carrying action potentials
  • Axon conducting nerve impulses
  • Schwann cells forming myelin sheath
  • Nodes of Ranvier where axon is not insulated
• Resting potential


  Difference in electrical charge inside and outside axon when no stimulus, around -70mV
• Maintaining resting potential


  1. Sodium-potassium pump actively transports 2 K+ in and 3 Na+ out

  2. Creates electrochemical gradient causing K+ to diffuse out and Na+ to diffuse in

  3. More K+ diffusing out than Na+ diffusing in results in net negative charge inside
• Generation of action potential

  1. Stimulus opens Na+ voltage-gated channels

  2. Na+ influx depolarizes membrane beyond threshold

  3. More Na+ channels open, further depolarization

  4. Na+ channels close, K+ channels open, repolarization occurs

  5. Overshoot to -80mV, refractory period
• All-or-nothing principle


  • If stimulus not large enough to reach threshold, no action potential generated
  • If threshold reached, full action potential always generated
• Refractory period


  Period after action potential when another cannot be generated immediately