Knowledge-18 Responses to stimuli

• Organisms respond to changes in their environment to improve their chances of survival.
• A stimulus is a change in the internal or external environment of an organism detected by a specialized sensory cell called a receptor.
• A coordinator formulates a response to a stimuli through effectors.
• in flowering plants , specific growth factors move from growing regions to others tissues , where they regulate growth in response to directional stimuli.
• The growth response to directional stimulus is called a tropism, plant growth can be towards or away from a stimulus.
• Indoleacetic acid (IAA) is a type of auxin , a plant growth factor.
• The effect of IAA on directional growth is different in roots and from in shoots , and depends on the concentration of the chemical in the tissues.
• IAA:

  1. A normal young plant is laid on its side in the dark . IAA is equally spread in the tissues.
  2. In the root , more IAA gathers on the lower side.
  3. The root grows more on the side with least IAA , making it bend towards gravity. When it has grown down IAA is evenly spread again.
  4. the shoot grows on the side with the most IAA.
• A taxis is a directional response to a stimulus. A positive taxis is movement towards a stimulus and a negative taxis is movement away from a stimulus.
• A kinesis is a non-directional response to stimulus and relies on speed and random changes in direction. The more unfavorable the stimulus the faster the organism will move away.
• Nervous system:

  • electrical impulse down the length of a neurone
  • a chemical messenger is released directly onto a target cell in response to an electrical nerve impulse
  • rapid , short-lived and localized.
• Hormone control:

  • hormones
  • blood system; target cells have receptors with specific complementary tertiary structures to the hormone.
  • slow , long-lasting and widespread.
• Reflexes are simple , neural responses to danger. They typically involve three neurons : a sensory neurone , relay neurone and a motor neurone. These form the reflex arc , the brain is not involved.
• Receptors respond to a specific stimulus. Stimulation of a receptor leads to the establishment of a generator potential.
• A Pacinian corpuscle is an example of a receptor. These corpuscles are found deep in the skin and joints and respond to physical stimuli like changes in pressure.
• A Pacinian corpuscle has a single neurone , the ending of which is surrounded by layers of tissue separated by gel.
• The cell surface membrane of the neurone has special channels called stretch mediated sodium channels.
• Generator potential in a Pacinian corpuscle:

  1. when the cell surface membrane of the neurone is deformed by change in pressure , the stretch mediated sodium channels in the cell surface widen.
  2. This allows sodium ions to diffuse through into the neurone , changing the potential difference across the membrane.
  3. The membrane depolarizes producing generator potential
  4. If sufficient sodium ions enter the cell , the potential difference will reach threshold potential and initiate an action potential. This moves the impulse along the neuron.
• Habituation means the skin gets used to a stimulus and we cease to be aware of it.
• Resolution of touch receptors is how close together in distance two stimuli have to be before they are detected as a single stimulus.
• Cones:

  • three different types that respond to different wavelengths of light to give colour vision.
  • each cone cell has its own bipolar neurone.( a neurone with only two extensions : one axon and dendron and sensory neurone)
  • contain pigment iodopsin and create generator potentials in high light intensities.
• Rods:

  • produces images in black and white (monochrome)
  • many rod cells share one bipolar neurone and a sensory neurone in the optic nerve , so there is a much greater chance of generator potential being produced from low light intensity.
• The innermost layer of human eye is called the retina. It has millions of light receptors , called rods and cones , that allow humans to sense light and colour with visual acuity.
• chemoreceptors are one of two types that are key to controlling heart rate.
• chemoreceptors:

  • located in the wall of the carotid artery and aortic arch
  • able to detect changes in blood pH that result from carbon dioxide dissolving in blood to form acid ,which lowers pH .
  • Able to increase frequency of impulses to the medulla oblongata in the brain when lower blood pH is detected. This increase impulses sent to the sinoatrial node to increase heart rate.
• Factors that affect heart rate:

  • exercise
  • adrenaline
  • blood pressure
  • limb movement
• heart rate can be measured by counting pulse per minute.
• Cardiac output = heart rate x stroke volume
• stroke volume is the blood pumped from the left ventricle per beat
• heart muscles can initiate its own contractions : it is myogenic.
• Myogenic stimulation happens as follows:

  1. An action potential is initiated by the sinoatrial node (SAN), a region of tissue in the right atrial wall that acts as a pacemaker.
  2. The action potential sets up a wave of electrical activity through wall of atria.
  3. The wave of electricity is channeled through the atrioventricular node (AVN) and down to the apex of the heart via purkinje fibers in bundle of HIS.
  4. The wave of excitation can then cause the heart muscle to contract from the apex , forcing blood out of the ventricles via the pulmonary artery.
• The SAN is connected to the cardiovascular centre in the medulla oblongata in the brain by the autonomic nervous system.
• The autonomic system influences heart rate by slowing it down via the parasympathetic nerve or increasing it via the sympathetic nerve.