Chapter 9: Response to Stimuli

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  • Response to Stimuli
    • Sensory reception
      • The ability to respond to a stimulus increases the chance of survival. = Greater chance of passing alleles to offspring = selection pressure favouring organisms with appropriate responses.
      • Taxes: a taxis is a simple response whose direction is determined by the direction of the stimulus.
      • Kineses: a kinesis is a form of response in which the organism does not move towards or away from a stimulus. Instead, they move more rapidly in more a unpleasant stimulus. This results in an increase in random movements which is designed to bring the organism back to more favourable conditions. E.g. Woodlice and temperature/humidity
      • Tropisms: a tropism is a growth movement of part of a plant in response to a directional stimulus. E.g. light = photo, gravity = geo, water = hyrdo.
      • Positive and negative
    • Nervous control
      • Nervous System
        • Peripheral Nervous System (PNS) - made up of pairs of nerves that originate from either the brain or spinal cord.
          • Motor Nervous System - motor neurones carry nerve impulses away from the CNS to effectors.
            • Autonomic Nervous System - carries nerve impulses to glands, smooth muscle and cardiac muscle and it is involuntary.
            • Voluntary Nervous System - carries nerve impulses to body muscles and is under voluntary (conscious) control.
          • Sensory Nervous System - sensory neurones carry nerve impulses from receptors to the CNS.
        • Central Nervous System (CNS) - made up of the brain and spinal cord.
          • Brain
          • Spinal Cord
      • A Reflex Arc
        • 1) The stimulus, e.g. heat from an object
          • 2) A receptor - temperature receptors in the skin on the back of the hand, which create a nerve impulse in a sensory neurone.
            • 3) A sensory neurone - passes the nerve impulse to the spinal cord.
        • 4) An intermediate neurone - links the sensory neurone to the motor neurone in the spinal cord.
          • 5) A motor neurone - carries the nerve impulse from the spinal cord to a muscle in the upper arm.
            • 6) An effector - the muscle in the upper arm, which is stimulated to contract
              • 7) The response - pulling the hand away from the hot object.
        • Stimulus, receptor, sensory neurone, intermediate neurone, motor neurone, effector, reponse
        • Importance of reflex arcs: Involuntary, brain free to carry out more complex responses and not overloaded.  Protect body from harmful stimuli, do not have to be learned.   Fast because neurone pathway is short with few synapses.
    • Control of heart rate
      • (Autonomic) Sympathetic nervous system (SNS): Stimulates effectors, speeding up any activity. Fight or flight response.
        • Systems oppose one another and create a balance.
      • (Autonomic) Para-sympathetic nervous system (PNS): Inhibits effectors, slowing down any activity. Controls activities under normal resting conditions. Replenishes body's reserves and conserves energy.
        • Systems oppose one another and create a balance.
      • Heart rate must be flexible to meet varying demands for oxygen.
        • Medulla Oblongata
          • Sinoatrial node (SAN) can increase heart rate by SNS.
          • Sinoatrial node (SAN) can decrease heart rate by PNS.
          • Control by Chemorecep-tors: found in the  wall of the carotid arteries (serve the brain). Sensitive to changes in pH of the blood which is the result of changes in CO2 concentratio-n.
            • When blood has a higher than normal conc. of CO2, pH is lowered (more acidic)
              • Chemorecep-tors in walls of carotid arteries and aorta detect this change and increase the frequency of nerve impulses to the centre in the medulla that increases heart rate.
                • This centre increases the frequency of impulses via the SNS to the SAN which increases the heart rate.
                  • The increased blood flow that this causes leads to CO2 being removed by the lungs so CO2 levels in blood return to normal.
                    • pH returns to normal. Frequency of nerve impulses is reduced. Heart rate returns to normal.
          • Control by Pressure Receptors: occur within the walls of the carotid arteries and the aorta.
            • Operate when blood pressure is higher than normal: they transmit a nervous impulse to the centre in the medulla oblongata that decreases heart rate. This centre sends impulses via the PNS to the SAN, which decreases the rate at which the heart beats.
            • When blood pressure is lower than normal: (opposite) nervous impulse to increase heart rate, SNS, increases heart rate.
    • Role of Receptors
      • Pacinian Corpuscle
        • Is specific to a single type of stimulus. E.g. mechanical pressure.
        • Produces a generator potential by acting as a transducer. Transducer converts stimulus into info the body can understand e.g. nerve impulses. Stimulus is always a form of energy and so is the nerve impulse.
          • Receptors therefore convert/trans-duce one form of energy into another. All receptors convert the energy of the stimulus into a nervou impulse known as a generator potential. E.g. pacinian corpuscle.
      • Receptors working together in the eye
        • Rod Cells
          • Rod-shaped, greater numbers than cone cells, distribution more at the periphery of the retina, absent at the fovea, give poor visual acuity, sensitive to low-intensity light
          • Share a single sensory neurone so respond to lower light intensities as a lower threshold value has to be exceeded to create a generator potential. = retinal convergence
            • To create a generator potential, rhodopsin (pigment in the rod cells) must be broken down. Low-intensity light is sufficient.
              • Cannot distinguish between separate sources of light since all rod cells link to a single bipolar cell meaning only one impulse is generated regardless of the amount of neurones stimulated.
                • Distribution is different in the fovea and periphery because the fovea is close to the front of the eye so gets higher light intensities  for the cone cells to work. (Converse is true  for rod cells)
        • Cone Cells
          • Cone-shaped, fewer numbers than rod cells, fewer at the periphery of the retina, concentrated at the fovea, give good visual acuity, not sensitive to low-intensity  light
          • Cone cells only respond to high intensity light as they are each connected to their own bipolar cell. Iodopsin (pigment in cone cells) requires higher light intensities to be broken down and create a generator potential.
            • Cone cells have good visual acuity because they have separate connections to bipolar cells so when each cone cell is stimulated a different impulse is sent.
              • Distribution is different in the fovea and periphery because the fovea is close to the front of the eye so gets higher light intensities  for the cone cells to work. (Converse is true  for rod cells)
      • Structure and function of Pacinian corpuscle: most abundant on the fingers, soles of feet and external genitalia. Appearance of an onion.
        • In its normal (resting) state, the stretch-mediated sodium channels of the membrane around the neurone of a Pacinian corpuscle are too narrow to allow Na+ ions to pass along them. = Resting potential.
          • When pressure is applied to the PC it changes shape and the membrane around its neurone becomes stretched.
            • This stretching widens the Na+ channels of the membrane and sodium ions diffuse into the neurone.
              • The influx of Na+ ions changes the potential of the membrane (becomes depolarised) producing a generator potential.
                • The generator potential in turn creates an action potential (nerve impulse) that passes along the neurone and then, via other neurones, to the CNS.

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