Topic 6: Stimuli and Responses

• In order to survive, organisms must respond to what is going on around them - in doing this they increase their chance of survival
• Organisms respond to changes in their internal environment to make sure that the conditions are always optimal for their metabolism
• metabolism is all the chemical reactions that occur inside an organism
• Any change to the internal or external environment e.g. a change in temperature, light intensity or pressure is called a stimulus
• Simple mobile organisms e.g. woodlice have simple responses to keep them in a favourable environment: tactic or kinetic
• Taxis (tactic response) - directional movement in response to a stimulus. The direction of the stimulus affects the response
• Kinesis (Kinetic response) - non-directional (random) movement in response to a stimulus. The intensity of the stimulus affects the response.
• Receptors detect stimuli - they can be cells or proteins on cell surface-membranes. There are lots of different types of receptors that detect different stimuli e.g. baroreceptors detect changes in blood pressure. Receptors are specific to one kind of stimulus
• Effectors are cells that bring about a response to a stimulus, to produce an effect. Effectors include muscle cells and cells found in glands e.g. the pancreas. Receptors communicate with effectors via the nervous system or the hormonal system or both
• Sensory neurones transmit electrical impulses from receptors to the central nervous system (CNS) - the brain and spinal cord
• Motor neurones transmit electrical impluses from the CNS to effectors
• Relay neurones (also called intermediate neurones, interneurones or association neurones) transmit electrical impulses between sensory neurones and motor neurones
• Pacinian corpuscles are mechanoreceptors - they detect mechanical stimuli, e.g. pressure and vibrations. They are found in the skin and contain the end of a sensory neurone - called a sensory nerve ending wrapped in lots of layers of connective tissue called lamellae
• When a Pacinian corpuscle is stimulated, the lamellae are deformed and press on the sensory nerve ending. This causes the sensory neurone's cell membrane to stretch, deforming the stretch-mediated sodium ion channels. The channels open and sodium ions diffuse into the cell, creating a generator potential. If the generator potential reaches the threshold, it triggers an action potential
• Rods are mainly found in the peripheral parts of the retina and cones are mainly found packed togther in the fovea.
• Rods and cones contain different optical pigments making the sensitive to different wavelengths of light. Rods only give information in black and white (monochromatic vision) but cones give information in colour (trichromatic vision)
• woodlice show a tactic response to light - the move away from a light source - this helps them survive as it keeps them concealed under stones during the day (where they are safe from predators) and keeps them in damp conditions (which reduces water loss)
• woodlice show a kinetic response to humidity. In high humidity they move slowly and turn less often, so that they stay where they are. As the area gets drier, they move faster and turn more often, so that they move into a new area. This response helps woodlice move from drier air to more humid air, and then stay put. This improves their chance of survival - it reduces their water loss and it helps to keep them concealed
• when an electrical impulse reaches the end of a neurone, chemical messengers called neurotransmitters are secreted directly onto cells (e.g. muscle cells) - so the nervous response is localised.

  neurotransmitters are quickly removed once they have done their job, so the response it short-lived

  electrical impulses are really fast, so the response is usually rapid - allows animals to react quickly to stimuli
• • a simple reflex is a rapid, involuntary response to a stimulus.
  • the pathway of communication goes through the spinal cord but not through conscious parts of the brain, so the response happens automatically
  • bc don't have to spend time deciding how to respond - info travels very fast from receptors to effectors
• simple reflexes are protective - they help organisms to avoid damage to the body bc the response happens so quickly
• the pathway of neurones linking receptors to effectors in a simple reflex is called a reflex arc - three neurones involved:

  • sensory
  • relay
  • motor
• • stimulus
  • receptors
  • sensory neurone
  • CNS (relay neurone in spinal cord)
  • motor neurone
  • effectors
  • response
• real life example of a simple reflex: - hand withdrawal response to heat:

  • you touch a hot surface
  • thermoreceptors (heat receptors) in your skin detect the heat stimulus
  • a sensory neurone carries the impulse to the CNS
  • CNS - a relay neurone in your spinal cord carries the impulse to a motor nuerone
  • effectors - the motor neurone carries the impulse to muscle cells in your biceps
  • response - your biceps muscle contracts to pull your hand away from the heat source and stop your hand being damaged