topic 6

• Stimulus


  A detectable change in the environment
• Receptors


  Cells that can detect changes in the environment
• Tropisms


  • Plant responses to their surroundings via growth
  • Can be positive (grow towards stimulus) or negative (grow away from stimulus)
  • Stimuli are light and gravity
• Auxin (IAA)


  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
• Positive taxis


  Organism moves towards a favorable stimulus
• Negative taxis


  Organism moves away from an unfavorable stimulus
• Kinesis


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


  Pressure receptor in skin with stretch-activated sodium channels
• Rods


  Photoreceptors in retina that detect black and white images at low light intensities
• Cones


  Photoreceptors in retina that detect colour at high light intensities
• Fovea


  Region of retina with highest concentration of cone cells
• Myogenic


  Cardiac muscle can contract and relax on its own without nervous system input
• Cardiac control


  1. SA node sets pace

  2. Depolarization wave spreads to AV node

  3. Delayed conduction to ventricles via bundle of His and Purkinje fibres

  4. Allows atria to contract first before ventricles
• Wave of depolarization


  1. Non-conductive layer of tissue separates the atria and ventricles

  2. Wave of depolarization travels down the bundle of His and up through the Purkinje fibers

  3. Causes a slight delay in the time it takes for the apex of the heart and ventricle walls to contract

  4. Allows the atria to contract and force blood into the ventricles before the ventricles contract

  5. Ventricles contract from the apex first and then move upwards
• Ventricles contracting from the apex first and then moving upwards


  Squeezes all the blood out of the heart, like squeezing toothpaste from the bottom up
• Repolarization of cells


  1. Causes the cardiac muscle to relax

  2. Cycle repeats when the SA node releases another wave of depolarization
• Medulla oblongata


  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
• Stimuli that change the heart rate

  • pH
  • Blood pressure
• Chemoreceptors


  Detect changes in pH
• Pressure receptors


  Detect changes in blood pressure
• Increased respiratory rate (e.g. during exercise)

  Decreases blood pH due to excess carbon dioxide and lactic acid, which increases the heart rate to remove the excess acid
• High blood pressure


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


  Increases impulses through the sympathetic nervous system to increase the heart rate and raise the blood pressure
• Myelinated motor neuron


  • Cell body contains organelles and neurotransmitter chemicals
  • Dendrites carry action potentials to surrounding cells
  • Axon is the conductive long fiber that carries nerve impulses
  • Schwann cells form the myelin sheath that insulates the axon, with gaps called nodes of Ranvier
• Resting potential


  The difference in electrical charge inside and outside the axon when the neuron is not conducting an impulse, typically -70 millivolts
• Maintenance of resting potential

  1. Sodium-potassium active transport pump pumps 2 potassium ions into the axon and 3 sodium ions out, creating an electrochemical gradient

  2. Potassium ions diffuse out more than sodium ions diffuse in due to more potassium ion channels
• Generation of action potential

  1. Stimulus opens sodium ion voltage-gated channels

  2. Sodium ions diffuse in, causing depolarization beyond the threshold potential

  3. More sodium channels open, further increasing the voltage

  4. Voltage-gated sodium channels then close, and potassium channels open, causing repolarization and an overshoot back to the resting potential
• All-or-nothing principle


  • If the stimulus is not large enough to reach the threshold potential (-55 millivolts), no action potential will be generated
  • If the threshold is reached, a full action potential will always be generated, with the same peak voltage
• Refractory period


  • Period after an action potential when another cannot be immediately generated, due to the voltage being too negative
  • Ensures discrete impulses, one-way conduction, and limits the number of impulses
• Factors affecting conduction speed

  • Myelination and saltatory conduction
  • Axon diameter
  • Temperature
• Synaptic transmission


  1. Action potential reaches the synaptic knob

  2. Depolarization of the synaptic knob opens calcium channels

  3. Calcium ions enter, causing neurotransmitter-containing vesicles to fuse and release neurotransmitters into the synaptic cleft

  4. Neurotransmitters bind to receptors on the next neuron, generating an action potential