stimuli and response

• Stimulus


  Any change in the internal or external environment
• Main types of neurones

  • Sensory neurones
  • Motor neurones
  • Relay neurones
• Reflex


  Rapid, automatic response to a stimulus without conscious decision
• Reflexes


  • Help protect the body because they are rapid responses
  • The pathway of neurones linking receptors to effectors in a reflex is called a reflex arc
• Reflex arc


  1. Sensory neurone carries impulses from receptor

  2. Relay neurone connects sensory and motor neurones

  3. Motor neurone sends impulses to effector
• If there is a relay neurone in the reflex arc, the reflex can be overridden by the brain
• Nervous system communication


  • Localised - neurotransmitters secreted directly onto target cells
  • Short-lived - neurotransmitters quickly removed
  • Rapid - electrical impulses are fast
• Electrical impulses are called nerve impulses or action potentials
• The nervous system is split into the central nervous system (brain and spinal cord) and the peripheral nervous system (neurones connecting CNS to body)
• Parts of the peripheral nervous system

  • Somatic nervous system - controls conscious activities
  • Autonomic nervous system - controls unconscious activities, with sympathetic and parasympathetic divisions
• Tactic responses


  The organism moves towards or away from a stimulus
• Tactic responses


  • Woodlice move away from light (phototaxis) to stay concealed and in damp conditions
• Kinetic responses (kineses)


  The organism's movement is affected by a non-directional stimulus
• Kinetic responses


  • Woodlice move slowly and turn less often in high humidity, move faster and turn more often in low humidity to find more humid areas
• Using a choice chamber to investigate animal responses

  1. Construct choice chamber

  2. Investigate effect of light intensity

  3. Investigate effect of humidity

  4. Can use a maze instead of choice chamber
• Positive gravitropism is the growth of a plant upwards in response to gravity
• Taxes


  Directional responses of organisms to stimuli
• Kineses


  Non-directional responses of organisms to stimuli
• The height of the plant is not given in the question
• The results obtained when investigating plant responses to stimuli depend on the height of the plant provided
• Receptors


  They detect what's going on and pass on information
• Receptors


  • Specific to one kind of stimulus
  • Many different types, each detect a different type of stimulus
  • Some are cells, some are proteins on cell surface
• How receptor cells that communicate information via the nervous system work

  1. Resting state not being stimulated, difference in charge between inside and outside of cell

  2. Ion pumps and ion channels generate potential difference

  3. Resting potential

  4. Stimulus causes movement of ions, change in potential difference

  5. Generator potential

  6. If generator potential reaches threshold, triggers action potential
• Pacinian corpuscles


  Pressure receptors in the skin that detect mechanical stimuli like pressure and vibrations
• How Pacinian corpuscles work

  1. Contain sensory nerve ending wrapped in layers of connective tissue

  2. Stimulation causes layers to deform, deforming nerve ending

  3. Deformation opens stretch-mediated ion channels, generating generator potential

  4. If generator potential reaches threshold, triggers action potential
• Photoreceptors


  Light receptors in the eye
• How photoreceptors work


  1. Light enters eye, focused by lens onto retina

  2. Photoreceptor cells (rods and cones) detect light

  3. Nerve impulses from photoreceptors carried to brain by optic nerve
• Rods and cones


  • Rods are more sensitive to light but give less visual acuity, cones are less sensitive but give higher visual acuity
• Where the optic nerve leaves the eye is the blind spot, as there are no photoreceptor cells there
• 144 b) PAs y +11= 2xx+!
• Control of Heart Rate


  How the body controls the beating of the heart
• Control of Heart Rate

  1. Cardiac muscle contracts and relaxes without receiving signals

  2. Contraction pattern controls the regular heartbeat

  3. Starts in the sinoatrial node (SAN)

  4. SAN acts as a pacemaker, sending out regular electrical activity

  5. Electrical activity passed to the atria

  6. Electrical activity prevented from passing directly to the ventricles

  7. Electrical activity passed to the atrioventricular node (AVN)

  8. AVN passes electrical activity to the bundle of His

  9. Bundle of His conducts electrical activity to the ventricles
• Sinoatrial node (SAN)


  • Located in the wall of the right atria
  • Sets the rhythm of the heartbeat by sending out regular electrical activity
• Atrioventricular node (AVN)


  • Responsible for passing the waves of electrical activity on to the bundle of His
  • Slight delay before reacting to ensure the atria have contracted before the ventricles
• Bundle of His


  • Group of muscle fibres responsible for conducting the waves of electrical activity between the atria and ventricles
  • Splits into Purkinje fibres in the right and left ventricle walls
• Purkinje fibres


  • Carry the waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract simultaneously
• Control of Heart Rate Involves the Brain and Autonomic Nervous System


  1. SAN generates electrical impulses that cause the cardiac muscles to contract

  2. Rate of SAN impulses is unconsciously controlled by the medulla oblongata in the brain

  3. Receptors detect internal stimuli like blood pressure and oxygen levels

  4. Receptors send electrical impulses to the medulla

  5. Medulla processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones
• Stimuli Detected by Receptors

  • High blood pressure
  • Low blood pressure
  • High blood O2, low CO2, or high pH
  • Low blood O2, high CO2, or low pH
• Stimuli Detected by Receptors


  Cause heart rate to speed up or slow down
• Baroreceptors


  Pressure receptors in the aorta and carotid arteries that detect high and low blood pressure