TOPIC 6 - RESPONSE TO CHANGES

• Stimulus

  • Detectable change in the environment
  • detected by cells called receptors
• Nervous system structure

  • Central nervous system = brain and spinal cord
  • peripheral nervous system = receptors, sensory and motor neurones
• Simple reflex arc


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• Importance of simple reflexes


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• Tropism

  • Response of plants to stimuli via growth
  • can be positive (growing towards stimulus) or negative (growing away from stimulus)
  • controlled by specific growth factors (IAA)
• Specific tropisms


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• Indoleacetic acid


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• Phototropism in shoots


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• Phototropism in roots

  • Root tip produces IAA
  • IAA concentration increases on lower (darker) side
  • IAA inhibits cell elongation 
  • root cells grow on lighter side
  • root bends away from light
  • negative phototropism
• Gravitropism in shoots

  • Shoot tip produces IAA
  • IAA diffuses from upper side to lower side of shoot in response to gravity
  • IAA stimulates cell elongation so plant grows upwards
  • negative gravitropism
• Gravitropism in roots

  • Root tip produces IAA
  • IAA accumulates on lower side of root in response to gravity 
  • IAA inhibits cell elongation 
  • root bends down towards gravity and anchors plant
  • positive gravitropism
• Taxis

  • Directional response by simple mobile organisms
  • move towards favourable stimuli (positive taxis) or away from unfavourable stimuli (negative taxis)
• Kinesis


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• Receptors


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• Pacinian corpuscle

  • Receptor responds to pressure changes
  • occur deep in skin mainly in fingers and feet
  • sensory neurone wrapped with layers of tissue
• Pacinian corpuscle structure


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• How pacinian corpuscle detects pressure


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• Rod cells

  • Concentrated at periphery of retina
  • contains rhodopsin pigment
  • connected in groups to one bipolar cell (retinal convergence)
  • do not detect colour
• Cone cells

  • Concentrated on the fovea 
  • fewer at periphery of retina 
  • 3 types of cones containing different iodopsin pigments
  • one cone connects to one neurone
  • detect coloured light
• Rods and cones: describe differences in sensitivity to light

  • Rods are more sensitive to light
  • cones are less sensitive to light
• Rods and cones: describe

  differences in visual acuity

  • Cones give higher visual acuity
  • rods have a lower visual acuity
• Visual acuity

  • Ability to distinguish between separate sources of light
  • a higher visual acuity means more detailed, focused vision
• Rods and cones: describe differences in colour vision

  • Rods allow monochromatic vision (black and white)
  • cones allow colour vision
• Why rods have high sensitivity to light

  • Rods are connected in groups to one bipolar cell
  • retinal convergence
  • spatial summation
  • stimulation of each individual- cell alone is sub-threshold but because rods are connected in groups more likely threshold potential is reached
• Why cones have low sensitivity to light

  • One cone joins to one neurone 
  • no retinal convergence / spatial summation
  • higher light intensity required to reach threshold potential
• Why rods have low visual acuity

  • Rods connected in groups to one bipolar cell
  • retinal convergence
  • spatial summation
  • many neurones only generate 1 impulse / action potential -> cannot distinguish between separate sources of light
• Why cones have high visual acuity

  • One cone joins to one neurone
  • 2 adjacent cones are stimulated, brain receives 2 impulses
  • can distinguish between separate sources of light
• Why rods have monochromatic vision

  • One type of rod cell
  • one pigment (rhodopsin)
• Why cones give colour vision

  • 3 types of cone cells with different optical pigments which absorb different wavelengths of light
  • red-sensitive, green-sensitive and blue-sensitive cones 
  • stimulation of different proportions of cones gives greater range of colour perception
• Myogenic

  • When a muscle (cardiac muscle) can contract and relax without receiving signals from nerves
• Sinoatrial node

  • Located in right atrium and is known as the pacemaker 
  • releases wave of depolarisation across the atria, causing muscles to contract
• Atrioventricular node

  • Located near the border of the right / left ventricle within atria 
  • releases another wave of depolarisation after a short delay when it detects the first wave from the SAN
• Bundle of His

  • Runs through septum
  • can conduct and pass the wave of depolarisation down the septum and Purkyne fibres in walls of ventricles
• Purkyne fibres


  • In walls of ventricles
  • spread wave of depolarisation from AVN across bottom of the heart
  • the muscular walls of ventricles contract from the bottom up
• Role of non- conductive tissue

  • Located between atria and ventricles
  • prevents wave of depolarisation travelling down to ventricles
  • causes slight delay in ventricles contracting so that ventricles fill before contraction
• Importance of short delay between SAN and AVN waves of depolarisation

  • Ensures enough time for atria to pump all blood into ventricles
  • ventricle becomes full
• Role of the medulla oblongata

  • Controls heart rate via the autonomic nervous system 
  • uses sympathetic and parasympathetic nervous system to control SAN rhythm
• Chemoreceptors

  • Located in carotid artery and aorta
  • responds to pH / CO2 conc. changes
• Baroreceptors

  • Located in carotid artery and aorta
  • responds to pressure changes
• Response to high blood pressure

  • Baroreceptor detects high blood pressure
  • impulse sent to medulla
  • more impulses sent to SAN along parasympathetic neurones (releasing noradrenaline)
  • heart rate slowed