9 Response to stimuli

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  • 9 Response to simuli
    • Sensory Reception
      • Taxes
        • response who's direction is determined by the direction of the stimulus
          • positive/negative phototaxis
          • positive/negative chemotaxis
      • Kenisis
        • response causes more or less rapid movement
          • designed to bring the organism back to a more favourable condition
      • Tropisms
        • a growth movement in response to a directional stimulus
          • positive/negative phototropism
          • positive/negative geotropism
          • positive/negative hydrotropism
      • Stimulus
        • detectable change in internal or external environment that produces a response
          • detected by receptors
            • linked by a central co-ordinator
              • carried out by effectors
      • Response
        • what happens as a result of the stimuli
          • carried out by effectors
    • Nervous control
      • Nervous organisation
        • CNS - brain and spinal cord
        • PNS - pairs of nerves that originate from the brain and spinal cord
          • sensory neurone (to the CNS)
          • motor neurone (away from CNS)
            • voluntary (to body muscles)
            • autonomic  (to glands, smooth muscle and cardiac muscle)
      • The spinal cord
        • column of nervous tissue that runs along the back
      • A reflex arc
        • stimulus
          • receptor
            • sensory neurone
              • intermediate neurone
                • motor neurone
                  • effector
                    • responce
      • Importance of reflex arc's
        • they are involuntary
        • they protect the body from harmful stimuli - being effective since birth and do not need to be learned
        • they are fast, as the neurone pathway is short with very few synapses
    • Control of heart rate
      • The autonomic nervous system
        • the sympathetic nervous system
          • stimulates effectors and speeds up any activity
        • the parasympathetic nervous system
          • inhibits effectors and slows down any activity
      • Control of heart rate
        • control by chemoreceptors
          • found in the wall of the carotid arteries
          • sensitive to changes in pH due to CO2 conc
          • see page 148 AQA bio book
        • control by pressure receptors
          • found in the wall of the caratoid arteries and the aorta
          • when blood pressure is higher than normal, a nervous impulse is sent to the medulla oblongata that decreases heart rate - sends impulses via the parasympathetic nervous system to the sinoatrial node, decreasing heart rate
          • when blood pressure is lower than normal, they transmit a nervous impulse to the centre in the medulla oblongata that increases heart rate - sends impulses via the sympathetic nervous system to the sinoatrial node, increasing heart rate
        • controlled by a region of the brain called the medulla oblongata
          • two centres which link to the sinoatrial node by the sympathetic and parasympathetic nervous system
    • Role of receptors
      • Structure and function of the Pacinian corpuscle
        • stretch-mediated sodium channel
          • permeability to sodium changes when they change shape, e.g., by stretching
          • resting potential = sodium channels to narrow
            • pressure causes shape change and sodium channels to open, causing sodium ions to diffuse into the membrane
              • the influx of Na+ depolarises the membrane, producing  generator potential
                • the generator potential in turn creates an action potential that passes along the neurone, and then, via other neurones, to the CNS
      • Features of sensory reception as illustrated by the Pacinian corpuscle
        • responds to changes in mechanical pressure
        • is specific to a single type of stimulus
        • produces a generator potential by acting as a transducer
        • most abundant on the fingers, soles of feet and external genitalia, as well as tendons and ligaments
      • Receptors working together in the eye
        • light receptors are found in the retina
        • rod cells (b&w)
          • more at the periphery of the retina, absent at the fovea, gives poor visual acuity, sensitive to low light intensity
          • rhodopsin must break down to create a generator potential
        • cone cells (colour)
          • less at the periphery of the retina, concentrated at the fovea, gives good visual acuity, not-sensitive to low light conditions
          • iodopsin must break down to create a generator potential
          • 3:1
            • rod cells (b&w)
              • more at the periphery of the retina, absent at the fovea, gives poor visual acuity, sensitive to low light intensity
              • rhodopsin must break down to create a generator potential

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