receptors / reflex arc

• What is a reflex?

  quick automatic response to a stimulus to protect from danger
• How is a reflex so quick ?

  only 3 neurones so only 2 synapses therefore v quick
• Reflex Arc Process in steps

  1. stimulus detected by specific receptors

  2) nerve impulse generates in sensory neurone

  3) nerve impulse travels to relay neurone which links sensory neurone to motor neurone

  4) motor neurone carries impulse to effector stimulated to respond e.g. pulling hand away
• Why are reflex arcs important?

  1) Since it is an involuntary response, it leaves the brain free to carry out more complex responses. (does not overload the brain).

  
  

  2) They protect the body from harmful stimuli. They also do not need to be learnt and are effective from birth.

  
  

  3) They are fast since the neurone pathways are short with only one or two synapses.
• What are taxes and kinesis?

  Simple responses that can maintain a mobile organism in a favourable environment
• What is taxes?

  When an organism moves it's entire body in response to a directional stimulus
• What is kinesis?

  a response where an organism changes the speed of movement and the rate it changes direction
• What happens when an organism moves from favourable to unfavourable conditions

  kinesis

  • rate of turning and movement increases to return to favourable conditions quickly
• What happens when an organism has been surrounded by negative stimuli for a while?

  kinesis

  • the rate of turning will decrease and rate if movement increases and therefore leave the area quicker
• positive taxis

  towards stimulus
• negative taxis

  away from stimulus
• are taxes directional?

  Yes
• are kinesis directional?

  No
• are tropisms directional?

  Yes
• Features of receptors

  Specific to one type of stimulus.Produces a generator potential by acting as a transducer (converts one form of energy into another)
• Structure of Pacinian Corpuscle

  End of neurone surrounded by layers of connective tissue separated by layers of gel,

  • sodium ion channels in membranes, stretch-mediated sodium channels
• Where do we find pacinian corpuscles

  fingers and feet
• function of pacinian corpuscles

  1. at resting state the stretch mediated sodium channels are too narrow for Na+ to pass through therefore at resting potential

  2) pressure is applied and the corpuscle is deformed and stretches membrane around neurone

  3) the stretching allows Na+ to diffuse into neurone

  4) influx of sodium ions depolarises the membrane and causes generator potential

  5) this in turn generates an action potential that passes along neurones
• Photoreceptors

  rods and cones (respond to light)