🫘Biology Unit 9 Control Systems

• Stimulus

  Changes in the external or internal environment, such as light waves, pressure or blood sugar
• Receptor cells

  Detect stimuli, often part of sense organs like the ear, eye or skin. Have special receptor proteins on their cell membranes that do the sensing
• Coordinator

  The network of interneurones connecting the sensory and motor systems, the central nervous system (brain and spinal cord)
• Effectors

  Cells that effect a response, either muscles or glands
• Response

  Aids survival, includes movement, secretions from glands and muscle contractions
• Nerve cells/neurones

  • Have a cell body with extensions - dendrites carry impulses towards the cell body, axon carries impulses away
  • Axons and dendrons can be very long
  • Surrounded by Schwann cells that form a myelin sheath
• Types of neurone

  • Sensory
  • Motor
  • Relay
• Membrane potential

  Stable imbalance of Na+ and K+ ions across the cell membrane, always negative inside the cell
• Action potential

  1. Depolarisation - sodium channels open, sodium ions diffuse in, inside becomes positive

  2. Repolarisation - potassium channels open, potassium ions diffuse out, inside becomes negative again
• Resting potential

  The normal membrane potential of nerve cells, which is -70mV (inside the axon)
• Depolarisation

  The sodium channels open fully for 0.5ms, causing sodium ions to quickly diffuse in down their gradient, making the inside of the cell more positive
• Repolarisation

  The potassium channels open fully for 0.5ms, causing potassium ions to quickly diffuse out down their concentration gradient, making the inside more negative again
• Receptor cells

  Contain special receptor proteins that sense the stimulus
• Initiation of nerve impulse

  1. Correct stimulus causes sodium channel to open

  2. Sodium ions diffuse into cell

  3. Depolarisation of membrane potential

  4. Affects voltage-gated sodium channels nearby

  5. Starts action potential
• Propagation of nerve impulse

  1. Local reversal of membrane potential detected by surrounding voltage-gated ion channels

  2. Channels open when potential changes enough
• Ion channels


  • Have a refractory period after opening, where they cannot open again for about 2ms
  • Are either open or closed, no half-way position
• The action potential always reaches +40mV as it moves along an axon, and it is never attenuated (reduced) by long axons
• Strength of stimulus


  Indicated by frequency of nerve impulses, not size

  
• Weak stimulus causes low frequency of nerve impulses (around 10Hz), strong stimulus causes high frequency (up to 100Hz)
• Speed of nerve impulses

  • Affected by temperature, axon diameter, and myelination

  
• Nerve impulses in unmyelinated neurones have a maximum speed of around 1 m/s, in myelinated neurones they travel at 100 m/s

  
• Synaptic transmission


  1. Action potential reaches synapse

  2. Calcium channels open, calcium ions diffuse in

  3. Synaptic vesicles fuse with cell membrane, releasing neurotransmitters

  4. Neurotransmitters diffuse across synaptic cleft

  5. Bind to neuroreceptors in post-synaptic membrane

  6. Ion channels open, causing local depolarisation (post-synaptic potential)

  7. May initiate action potential in post-synaptic cell

  8. Neurotransmitter removed from synaptic cleft by breakdown or reuptake
• Types of synapses


  • Excitatory ion-channel synapses
  • Inhibitory ion-channel synapses
  • Non-channel synapses
  • Neuromuscular junctions
  • Electrical synapses

  
• Excitatory ion-channel synapse


  Has sodium channel neuroreceptors, opening causes local depolarisation (EPSP)

  
• Inhibitory ion-channel synapse


  Has chloride channel neuroreceptors, opening causes local hyperpolarisation (IPSP)
• Non-channel synapse


  Has membrane-bound enzyme neuroreceptors, activation causes production of intracellular messenger chemicals
• Non-channel synapses are involved in slow and long-lasting responses like learning and memory
• Neuromuscular junction


  Synapse between effector neurone and muscle cell, always uses acetylcholine as neurotransmitter
• Electrical synapse


  Membranes of two cells touch, allowing action potential to pass directly without neurotransmitter
• Synaptic integration


  1. Many synapses on one post-synaptic neurone

  2. Excitatory and inhibitory PSPs sum to form grand PSP

  3. If grand PSP above threshold, action potential initiated

  
• Spatial summation


  Summing of PSPs from different synapses over cell body and dendrites
• Temporal summation


  Summing of a sequence of PSPs at one synapse over a brief period
• Summation is the basis of the processing power in the nervous system
• Ways drugs can affect synapses

  • Mimic a neurotransmitter (agonist)
  • Stimulate the release of a neurotransmitter (agonist)
  • Open a neuroreceptor channel (agonist)
  • Block a neuroreceptor channel (antagonist)
  • Inhibit the breakdown enzyme (agonist)
• Synapses in different parts of the nervous system use different neurotransmitters and neuroreceptors, so drugs can target specific synapses

  
• Sympathetic nervous system response


  Increase in heart rate and blood pressure
• Dopamine


  Stimulates the reward system in the brain, leading to feelings of pleasure
• Endorphins


  Cause feelings of well-being, but lead to withdrawal symptoms and so cause addiction
• Cobra venom - neurotoxin

  1. Binds tightly and irreversibly to the acetylcholine receptors in neuromuscular junctions

  2. Stops the receptors from opening

  3. Stops skeletal muscles from contracting

  4. Victim is paralysed and killed if toxin reaches breathing muscles
• Lidocaine


  • Blocks voltage-gated sodium channels, stopping all action potentials
  • In low doses has only a local effect, acting as a local anaesthetic by inhibiting sensory neurones and as a muscle relaxant by inhibiting motor neurones
  • In high doses has more widespread effects and can be fatal