Topic 6

• Stimulus


  A detectable change in the environment
• Receptors


  Cells that can detect changes in the environment
• Tropism


  • A plant response to its surroundings via growth
  • Can be positive (grow towards stimulus) or negative (grow away from stimulus)
  • Stimuli are light and gravity
• Auxin (IAA)


  A growth factor that controls cell elongation in shoots and inhibits growth in roots
• Phototropism


  1. Shoot tip cells produce IAA

  2. IAA diffuses to shaded side

  3. Cells on shaded side elongate

  4. Shoot bends towards light
• Gravitropism


  1. IAA diffuses to lower side of shoot

  2. Lower cells elongate

  3. Shoot bends upwards against gravity
• Gravitropism in roots


  1. IAA moves to lower side

  2. Upper cells elongate

  3. Root bends downwards towards gravity
• Reflex


  Rapid automatic response to protect from danger
• Reflex arc


  Made up of 3 neurons: sensory, relay, motor
• Taxis


  Organism moves its entire body towards or away from a stimulus
• Kinesis


  Organism changes speed of movement and rate of direction changes in response to stimuli
• Receptors


  • Respond to specific stimuli
  • Can lead to action potentials or generator potentials
• Pacinian corpuscle


  • Pressure receptor in deep skin layers
  • Has stretch-activated sodium channels
• Rods


  • Photoreceptors in retina
  • Detect light at low intensities
  • Provide black and white vision with low acuity
• Cones


  • Photoreceptors in retina
  • Detect colour at high light intensities
  • Provide high visual acuity
• Cardiac muscle


  • Myogenic - contracts and relaxes on its own
  • Nervous system controls contraction speed
• Cardiac cycle control


  1. SA node depolarization spreads to atria

  2. AV node delays depolarization

  3. Depolarization travels down Bundle of His and Purkinje fibres to ventricles
• Wave of depolarization


  A non-conductive layer of tissue that separates the atria and ventricles and prevents the wave of depolarization traveling straight down to the ventricles
• Wave of depolarization from the AV node


  1. Travels down the bundle of His

  2. Travels up through the Purkinje fibers
• Delay in time for the apex of the heart and ventricle walls to contract


  • Gives the atria enough time to contract and force all the blood into the ventricles before the ventricles contract
  • Ventricles contract from the apex first and then move upwards, forcing all the blood out of the heart
• Cardiac muscle contraction and relaxation cycle

  1. Cells depolarize

  2. Cells repolarize and relax
• Medulla oblongata


  Part of the brain that controls the heart rate through the autonomic nervous system
• Parts of the autonomic nervous system

  • Sympathetic nervous system
  • Parasympathetic nervous system
• Sympathetic nervous system impulses

  Cause the SA node to release waves of depolarization more frequently, increasing the heart rate
• Parasympathetic nervous system impulses

  Cause the SA node to release waves of depolarization less frequently, decreasing the heart rate
• Chemoreceptors


  Detect changes in blood pH
• Pressure receptors


  Detect changes in blood pressure
• High respiratory rate


  Decreases blood pH due to excess carbon dioxide or lactic acid, increasing heart rate to remove the excess
• High blood pressure


  Increases impulses through the parasympathetic nervous system to decrease heart rate and lower blood pressure
• Low blood pressure


  Increases impulses through the sympathetic nervous system to increase heart rate and raise blood pressure
• Resting potential


  The difference in electrical charge inside and outside the axon when the neuron is not conducting an impulse
• Sodium-potassium pump


  An active transport pump that maintains the resting potential by pumping 2 potassium ions into the axon and 3 sodium ions out
• Action potential


  When the neuron's voltage increases beyond a set point from the resting potential, generating a nervous impulse
• Generation of an action potential

  1. Stimulus opens sodium ion channels

  2. Sodium ions diffuse in, increasing voltage

  3. Voltage-gated sodium channels close, potassium channels open, repolarization occurs

  4. Neuron returns to resting potential
• All-or-nothing principle


  If the stimulus is not large enough to reach the threshold potential, no action potential will be generated, but if the threshold is reached, a full action potential will always be produced
• Refractory period


  A period after an action potential when another cannot be immediately generated, ensuring discrete impulses and one-way conduction
• Factors affecting conduction speed

  • Myelination and saltatory conduction
  • Axon diameter
  • Temperature
• Synapses

  The gaps between neurons where neurotransmitters diffuse to generate action potentials in the next neuron
• Neurotransmitter release at synapses

  1. Action potential reaches synaptic knob

  2. Calcium channels open, calcium ions enter

  3. Neurotransmitter-containing vesicles fuse with membrane and release neurotransmitters into synaptic cleft
• Synapses


  Gaps between neurons