Stimuli & responses

• What are Pacinian Corpuscles?


  Mechanoreceptors (detect mechanical stimuli, e.g, pressure & vibrations). Found in skin. Contain sensory nerve ending wrapped in layers of connective tissue called lamellae.
• How do Pacinian Corpuscles work?


  When stimulated, lamellae are deformed & press on sensory nerve ending. Causes sensory neurone's cell membrane to stretch, deforming stretch-mediated sodium ion channels. Channels open & sodium ions diffuse into cell, creating a generator potential. If generator potential reaches threshold, it triggers action potential.
• What are receptors?


  Detect a specific stimulus. Can be cells or proteins. Receptors in nervous system convert energy of stimulus into the electrical energy used by neurones.
• What happens when nervous system receptor is in resting state?


  (Not stimulated) there's a difference in charge between inside(- charged) & outside(+charged) of cell- there's a voltage (potential difference) across membrane. Resting potential= potential difference when cell at rest. Resting potential generated by ion pumps & ion channels.
• What happens when a stimulus is detected?

  Cell membrane excited & becomes more permeable, allowing more ions to move in & out cell, altering potential difference. Generator potential=change in potential difference due to stimulus. Bigger stimulus excited membrane more, causing bigger movement of ions & bigger change in potential difference, so bigger generator potential produced.
• What is an action potential?

  Electrical impulse along neurone- triggered if generator potential big enough. Only triggered if generator potential reaches threshold level. Action potentials all 1 size, so strength of stimulus measured by frequency of action potentials in certain time period. If stimulus too weak generator potential won't reach threshold so no action potential.
• What are photoreceptors?


  Receptors in eye detect light. Retina contains them. Fovea is area of retina where there's lots of them.
• How do photoreceptors work?

  Light enters eye, hits photoreceptors & is absorbed by light-sensitive optical pigments. Light bleaches pigments, causing chemical change & altering membrane permeability to sodium ions. Generator potential created & if it reaches threshold, nerve impulse sent along bipolar neurone. Bipolar neurones connect photoreceptors to optic nerve- takes impulses to brain.
• What are the 2 types of photoreceptors in the human eye?

  Rods and cones
• Where are rods mainly found?

  Peripheral parts of retina
• Where are cones mainly found?

  Packed together in fovea.
• What colour do rods give info in?


  Black & white (monochromatic vision).
• What colour do cones give info in?

  Colour (trichromatic vision). 3 types of cones containing a different optical pigment: red-sensitive, green-sensitive & blue-sensitive. When they're stimulated in different proportions you see different colours.
• Why do rods & cones give info in different colours?

  Contain different optical pigments making them sensitive to different wavelengths of light.
• Why are rod cells very sensitive to light?

  Many rods join 1 bipolar neurone, so many weak generator potentials combine to reach threshold & trigger an action potential. They work well in dim light.
• Why are cones less sensitive to light?

  1 cone joins 1 bipolar neurone, so it takes more light to reach threshold & trigger an action potential. They work best in bright light.
• What is visual acuity?

  Ability to tell apart points that are close together.
• Why do rods give low visual acuity?

  Many rods join same bipolar neurone, which means light from 2 points close together can't be told apart.
• Why do cones give high visual acuity?

  Cones are close together & 1 cone joins 1 bipolar neurone. When light from 2 points hits 2 cones, 2 action potentials (1 from each cone) go to brain- can distinguish 2 points close together as 2 separate points.
• What 2 systems are the nervous system split into?

  Central nervous system (CNS-made of brain & spinal cord) & peripheral nervous system (made of neurones that connect CNS to rest of body).
• What 2 systems does the peripheral nervous system contain?

  Somatic (conscious activities) and autonomic (unconscious) nervous systems.
• What is the autonomic nervous system split into?

  Sympathetic (fight or flight) and parasympathetic (rest & digest) nervous systems. Involved in control of heart rate.
• What does myogenic mean?

  Cardiac (heart) muscle is myogenic- can contract & relax without receiving signals from nerves. This pattern of contractions controls the regular heartbeat.
• What is the sinoatrial node (SAN) ?

  Small mass of tissue in the wall of right atrium- like a pacemaker. Sets rhythm of heartbeat by sending out regular waves of electrical activity to the atrial walls. Causes right & left atria to contract at same time. Band of non-conducting collagen tissue prevents waves of electrical activity from being passed directly from atria to ventricles. Waves of electrical activity transferred from SAN to atrioventricular node (AVN).
• What does the AVN do ?

  Passes waves of electrical activity to bundle of His. Slight delay before AVN reacts to make sure atria have emptied before ventricles contract.
• What is the bundle of His?


  Group of muscle fibres responsible for conducting waves of electrical activity between the ventricles to the apex (bottom of heart). Bundle splits into finer muscle fibres in right & left ventricle walls called Purkyne tissue- carries waves of electrical activity into muscular walls of right & left ventricles causing them to contract simultaneously from the bottom up.
• What is the medulla?

  Part of brain that unconsciously controls rate at which SAN fires (heart rate). SAN generates electrical impulses that cause cardiac muscles to contract. Electrical impulses from receptors sent to medulla along sensory neurones. Medulla processes info & sends impulses to SAN along sympathetic or parasympathetic neurones.
• Why do animals alter their heart rate?

  To respond to internal stimuli (e.g, to prevent fainting due to low blood pressure or to make sure heart rate high enough to supply body with enough oxygen).
• How is internal stimuli detected?

  By pressure receptors & chemical receptors. Pressure receptors (baroreceptors) in aorta & carotid arteries. Stimulated by high & low blood pressure. Chemical receptors (chemoreceptors) in aorta, carotid arteries & medulla. Monitor oxygen & carbon dioxide level in blood & pH.
• What happens when a high blood pressure has been detected?

  Baroreceptors detect high blood pressure & send impulses along sensory neurones to medulla, which sends impulses along parasympathetic neurones. These secrete acetylcholine which bind to receptors on SAN. Causes heart rate to slow down to reduce blood pressure to normal.
• What happens when a low blood pressure has been detected?

  Baroreceptors detect low blood pressure & send impulses along sensory neurones to medulla, which sends impulses along sympathetic neurones. These secrete noradrenaline, which binds to receptors on SAN. Causes heart rate to speed up & blood pressure to increase to normal.
• High blood O2, low CO2 or high blood pH levels detected:

  Detected by chemoreceptors-send impulses along sensory neurones to medulla - sends impulses along parasympathetic neurones. These secrete acetylcholine which binds to receptors on SAN. Causes heart rate to decrease.
• Low blood O2, high CO2 or low blood pH levels detected:

  Detected by chemoreceptors-send impulses along sensory neurones to medulla-send impulses along sympathetic neurones. These secrete noradrenaline which binds to receptors on SAN. Causes heart rate to increase.
• What is a tropism?

  Response of a plant to a directional stimulus. Respond to stimuli by regulating their growth. Positive tropism= growth towards stimuli. Negative tropism= growth away from stimulus.
• What is phototropism?

  Growth of plant in response to light. Shoots are positively phototropic. Roots are negatively phototropic.
• What is gravitropism?

  Growth of plant in response to gravity. Shoots are negatively gravitropic & roots are positively gravitropic.
• What are specific-growth factors?

  Used by plants to respond to directional stimuli. Hormone-like chemicals that speed or slow plant growth. Produced in growing regions of plant (e.g, shoot & root tips) & move to where they're needed.
• What are auxins?

  Growth factors produced in tips of shoots & diffuse backwards to stimulate cell just behind tips to elongate. Cell walls become loose & stretchy so cells get longer. If tip of shoot removed no auxin available & shoot stops growing. Stimulates shoot growth but high concentrations inhibit growth in roots.
• What is indoleacetic acid (IAA) ?

  Auxin produced in tips of shoots & roots in flowering plants. Moved around plant to control tropisms (via diffusion & active transport short distance & phloem over long distance). Different parts of plant have different concentrations of IAA- uneven distribution = uneven growth of plant.
• What are effectors?

  Cells that bring about a response to a stimulus to produce an effect. Include muscle cells and cells in glands e.g, pancreas. Receptors communicate with effectors via nervous system or hormonal system.