Stimulus and Response

• Simple response:

  • a stimulus is a detectable change in the internal or eternal environment of an organism that produces a response in the organism.
  • a receptor detects the stimulus
  • a coordinator formulates a suitable response to a stimulus
  • an effector produces a response
• The ability to respond to a stimuli increases the chance of survival.
• Selection pressure favours organisms that are more able to respond.
• Taxis and Kinesis - simple response that can maintain a mobile organism in a favourable environment.
• Taxis - a simple response where the direction of response is determined by the direction of stimulus.
• Kinesis - A random response where movement is not related to the direction of stimulus
• During kinesis, in favourable conditions organisms move around and change direction less. This allows them to remain in favourable conditions for longer.
• Tropism - a growth response in plants where the direction of response is determined by direction of stimulus. Part of plant moves (not the whole plant).
• Phototropism in Shoots:

  • shoot tips produce IAA
  • in the presence of light, the IAA diffuses away from the light
  • this causes elongation of the shoot's cells on the shady side whilst the cells on the other side remain unstimulated
  • the shoot bends towards the light to increase photosynthesis
• In very high light intensities the shoot tip will produce high concentration of IAA and therefore, frow away from the light for protection.
• Gravitropism in Roots:

  • roots grow in the direction of gravity in order to absorb mineral ions from the soil
  • IAA diffuses to the cells at the bottom of the root tip but this time they inhibit elongation
  • the upper cells elongate normally so the root tip bends downwards
• Control of the Heart Rate - although the heart is myogenic (its beating occurs automatically), the rate at which it contracts is under the control of the sympathetic and parasympathetic nervous system
• 1. the SAN releases waves of electrical excitation which spread across the atria - causing them to contract.
  2. the electrical activity enters the AVN and there is a pause before it is re-emitted. This allows the atria to finish contracting.
  3. the electrical excitation cannot spread directly to the ventricles as there is a layer or insulatory tissue, it therefore spreads down the bundle of HIS to the base of the ventricles.
  4. The electrical activity then spreads up the Purkinje fibres causing the ventricles to contract from the bottom upwards.