B14 response to stimuli

• the sequence of events which leads to a response can be summarised as:

  stimulus -> receptor -> coordinator -> effector -> response
• a stimulus is a detectable change in the internal or external environment which leads to a response in the organism
• a receptor is an organ or specialised cell which detects a specific stimulus
• a coordinator uses information from the receptors to choose a suitable response to the stimulus
• an effector is a muscle or gland which carries out the response to the stimulus
• a taxis is a form of response which involves movement in a specific direction
• a taxis is when a motile organism responds to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable stimulus
• positive taxis = movement towards the stimulus

  negative taxis = movement away from the stimulus
• a kinesis is a form of response which involves movement in a random direction
• a kinesis is when a motile organism responds to environmental changes by changing its speed, and the rate at which it changes direction
• in a kinesis, speed and rate of changing direction are increased if the organism encounters unfavourable conditions, and are decreased if the organism encounters favourable conditions
• there is no such thing as positive or negative kinesis because the movement is not specifically towards or away from the stimulus
• a tropism is the growth of part of a plant in response to a directional stimulus
• phototropism is when plant shoots grow towards light, and plant roots grow away from light
• gravitropism is when plant shoots grow away from gravity, and plant roots grow towards gravity
• positive tropism = growth towards a stimulus

  negative tropism = growth away from a stimulus
• plants have to respond to changes in their internal and external environment to survive, but they are non-motile and have no nervous system, they respond to light, gravity and water
• plant responses involve plant growth factors which affect the growth of the plant tissues where they are released, and are made in cells throughout the plant
• IAA is a plant growth factor which is an auxin, it controls plant cell elongation
• IAA causes elongation of shoot cells and inhibits elongation of root cells
• phototropism in shoots process:

  • cells in the tip of the shoot produce IAA which is transported down the shoot
  • IAA is initially transported evenly throughout
  • light causes the movement of IAA from the light side to the shaded side
  • a greater concentration of IAA builds up on the shaded side than the light side
  • as IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side, the cells on the shaded side elongate more
  • the shaded side of the shoot elongates faster than the light side causing the shoot tip to bend towards the light
• gravitropism in roots process:

  • cells in the tip of the root produce IAA which is transported along the root
  • IAA is initially transported evenly throughout
  • gravity causes the movement of IAA from the upper side to the lower side of the root
  • a greater concentration of IAA builds up on the lower side than the upper side
  • as IAA inhibits elongation of root cells and there is a greater concentration of IAA on the lower side, the cells on the upper side elongate more
  • the upper side of the root elongates faster than the lower side causing the root tip to bend towards gravity
• reflexes are rapid automatic responses which can protect an organism from harmful stimuli, they bypass the brain so that no decision has to be made
• the general path of a reflex arc is:

  stimulus -> receptor -> sensory neurone -> intermediate neurone -> motor neurone -> effector -> response
• the sensory neurone carries the impulse from the receptor to the intermediate neurone in the spinal cord
• the intermediate neurone is located in the spinal cord, carries the impulse from the sensory neurone to the motor neurone
• the motor neurone carries the impulse from the intermediate neurone to the effector
• reflexes are important because:

  • they do not require the brain so leave it free to carry out more complex responses
  • they protect the body from harm
  • they are effective from birth and do not have to be learnt
  • they are fast because the neurone pathway is short with few synapses
  • they result in fast actions as there is no decision-making process
• pacinian corpuscles are sensory receptors which respond specifically to changes in mechanical pressure, by transducing the mechanical energy of the stimulus into a generator potential
• pacinian corpuscles have a single sensory neurone, located in the centre of layers of connective tissue with viscous gel between, surrounded by a blood capillary then a capsule
• pacinian corpuscles occur deep in the skin in the feet, fingers and genitalia
• the sensory neurones at the end of pacinian corpuscles contain stretch-mediated sodium channels in their plasma membranes
• pacinian corpsucle process:

  • at resting potential, the stretch-mediated sodium channels are too narrow to allow sodium ions to pass along them
  • when pressure is applied, the pacinian corpuscle is deformed and the membrane around the neurone becomes stretched
  • the stretching widens the sodium channels in the membrane, sodium ions diffuse into the neurone
  • the influx of sodium ions changes the potential of the membrane, it becomes depolarised, producing a generator potential
  • the generator potential creates an action potential which passes along the neurone to the central nervous system
• photoreceptors are light receptors found in the eye, on the retina
• in the eye, light enters through the pupil, the amount of light entering is controlled by the iris and muscles, then the lens focuses light onto the retina, onto a point called the fovea, where the photoreceptors cause nerve impulses to be sent to the brain on the optic nerve
• there are two types of photoreceptor, rod cells and cone cells
• rod cells are involved in monochromatic vision

  cone cells are involved in colour vision
• rod cells are present at greatest density outside the fovea

  cone cells are present at greatest density inside the fovea
• rod cells contain the pigment rhodopsin

  cone cells contain the pigment iodopsin
• rod cells are very sensitive to light so work in low-light conditions

  cone cells are not sensitive to light so require bright light to work