2. the nervous system

• types of neurones:

  1. sensory - transmit nerve impulses from receptors to the CNS made up of the brain and spinal cord
  2. motor - nerve impulses from the CNS to the effectors
  3. relay - between sensory neurones and motor neurones
• structure of neurones

  • cell body has extensions connecting to other neurones
  • dendrites and dendrons carry nerve impulses towards the cell body
  • axons carry nerve impulses away from the cell body
• sensory neurones

  
• Motor neurone

  
• Relay neurones

  
• axons are the other side of the cell body to the dendrites
• Impulses are received at the dendrites, which are carried to the cell body, which is then carried to the next neuron along the axon
• 1. stimulus detected by receptor cells and a nerve impulse is sent along a sensory neuron.
  2. impulse reaches end of neuron and neurotransmitters take the information across a synapse (gap) to the next neuron
  3. CNS processes the info decides what to do, then sends impulses along motor neuron to an effector
• sensory receptors convert the energy of a stimulus into electrical energy

  • they act as transducers - something that converts one form of energy into another
• when a nervous system receptor is in a resting state theres a difference in charge between the inside and outside of the cell

  • inside is negatively charged
  • There's a voltage across the membrane
  • voltage also known as the potential difference
• potential difference

  voltage of the membrane
• resting potential

  the potential difference when a cell is at rest and is generated using ion pumps and channels
• inside the cell has a relative negative charge at resting state
• outside the cell has a relative positive charge at resting state
• then a stimulus is detected the cell membrane is excited and becomes more permeable, which allows more ions to move in and out of the cell, altering the potential difference
• generator potential

  change in potential difference due to a stimulus
• ion

  particle with a positive or negative electrical charge
• a bigger stimulus excites the membrane more causing a bigger movement of ions and a bigger change in potential difference so a bigger generator potential is produced
• if generator potential is big enough it will trigger an action potential ( nerve impulse ) along a neurone
• an action potential is only triggered if the generator potential reaches a threshold level.

  • if stimulus is too weak the generator potential wont reach the threshold so theres no action potential
• Pacinian corpuscles are mechanoreceptors - they detect mechanical stimuli e.g. pressure of vibrations, and are found in the skin and contain a nerve ending

  • the nerve ending is wrapped in many layers of connecting tissues ( lamelle )
• when a Pacinian corpuscle is stimulated, the lamellae are deformed and press on the sensory nerve ending.

  • This causes deformation of stretch-mediated sodium channels in the sensory neurons cell membrane
  • Sodium ion channels open and sodium ions diffuse into the cell creating a generator potential and if threshold is reached it triggers an action potential