Responding to changes in the environment(6)

• Stimulus

  Something that can be detected by an organism, either internal in multicellular organisms or external in any organism
• Receptor


  An organ or specialised cell that can detect the change that is causing the stimulus
• Response


  As a result of the stimulus detected by the receptor, a response is caused, which may be movement of the organism or a change in behaviour
• Taxis


  A response that involves movement in a specific direction, either positive (towards the stimulus) or negative (away from the stimulus)
• Kinesis

  A response that involves movement, but in random directions, with both the speed and frequency of direction change increasing to increase the chance the organism will enter different conditions more rapidly
• Tropism


  Growth responses in plants controlled by a directional stimulus
• Indoleacetic acid (IAA)


  An important auxin produced in the tips and shoots of flowering plants that controls tropisms through its uneven distribution
• Phototropism in shoots


  If the shoot is illuminated from one side, auxins move towards the shaded part, causing elongation of the shaded side only, resulting in the shoot bending towards the light
• Gravitropism in roots


  IAA builds up on the lower side of the root, inhibiting growth and causing the upper side to grow faster, making the root bend downwards
• Reflex arc


  A rapid automatic response that can protect an organism from harmful stimuli, bypassing the brain so no decision has to be made
• Reflex arc


  Stimulus -> Receptor -> Sensory Neurone -> Intermediate Neurone -> Motor Neurone -> Effector -> Response
• Sensory neurone


  Carries the nerve impulse from the receptor to the spinal cord
• Intermediate neurone


  Located entirely in the spinal cord, relays the nerve impulse from the sensory neurone to the motor neurone
• Motor neurone


  Carries the nerve impulse from the spinal cord to the effector (muscle or gland)
• Receptor


  Detects changes in the internal and external environment, each specific to a particular kind of stimuli
• Types of receptors


  • Photoreceptors (detect changes in light), Mechanoreceptors (detect pressure and vibrations)
• Pacinian Corpuscle


  A type of mechanoreceptor located deep in the skin, mostly on fingers, soles of feet, and external genitalia, as well as in joints, tendons and ligaments
• Pacinian Corpuscle response


  Contains stretch-mediated sodium channels that open under pressure, causing depolarisation and an action potential
• Photoreceptors in the eye


  Light receptors located in the retina, specifically the fovea, that convert light into nerve impulses carried to the brain via the optic nerve
• Types of photoreceptors


  • Cones (involved in colour vision), Rods (involved in monochromatic vision)
• Cones


  Present at highest density in the fovea, contain the pigment iodopsin, require bright light, provide good visual acuity
• Rods


  Mainly concentrated outside the fovea, contain the pigment rhodopsin, very sensitive to light, provide low visual acuity
• Sinoatrial (SA) node


  The pacemaker of the heart, located in the wall of the right atrium, initiates the wave of electrical stimulation that causes the atria to contract
• What happens when an Electrical conduction passes through the heart

  Wave of excitation passes from SA node to atrioventricular (AV) node, then down the bundle of His and Purkyne fibres to cause ventricular contraction
• Accelerator nerve


  Part of the sympathetic nervous system, delivers higher frequency impulses to the SA node to increase heart rate
• Vagus nerve


  Part of the parasympathetic nervous system, delivers slower frequency impulses to the SA node to decrease heart rate
• Effect of Increased CO2 concentration

  Detected by chemoreceptors, increases heart rate to speed up blood flow to the lungs for CO2 expulsion
• Increased blood pressure


  Detected by baroreceptors, decreases heart rate to lower blood pressure
• Neurone


  Nerve cell that plays an important role in coordinating communication within the nervous system, with a cell body, dendrites, and axons
• Resting potential


  The polarised state of the neurone membrane at -70mV, maintained by the sodium-potassium pump
• Action potential generation


  Stimulus causes sodium channels to open, depolarising the membrane to +40mV, then potassium channels open to repolarise and hyperpolarise the membrane
• Saltatory conduction


  The faster movement of an action potential along a myelinated axon, jumping between nodes of Ranvier
• Factors affecting nerve impulse speed

  • Presence/absence of myelin, axon diameter, temperature
• Refractory period


  The short period after an action potential when the neurone membrane cannot be excited, ensuring unidirectional signalling
• All-or-nothing principle


  Either an action potential is produced or it is not, with a threshold value required for its generation
• Synapse


  Junction between two neurones that can prevent action potentials going the wrong way, amplify low frequency signals, and be inhibitory
• Action potential


  Either it is produced or it is not. A threshold value must be reached in order for an action potential to be created, with all action potentials being of the same strength.
• Synapse

  Junctions between two neurones. They have a greater role than just transmitting signals from one neurone to another.
• Roles of synapses

  • Prevent action potentials from going in the wrong direction
  • Amplify the effects of low frequency action potentials using summation
  • Some synapses can be inhibitory and prevent the movement of action potentials. Most though are excitatory.
• Action potential movement across a synapse

  1. Presynaptic membrane depolarises, calcium ion channels open, calcium ions enter presynaptic neurone

  2. Synaptic vesicles fuse with presynaptic membrane, neurotransmitter released into synaptic cleft

  3. Neurotransmitter diffuses across synaptic cleft, binds to receptors on postsynaptic membrane, sodium ion channels open

  4. Enzyme acetylcholinesterase hydrolyses acetylcholine, recycling neurotransmitter