neurons

• less than a millimetre to up to a meter long
• structure


  cell body- has a nucleus, containing genetic material of a cell

  dendrites- branches that protrude from cell body, carry impulses from neighbouring neurones towards cell body

  axon- carries impulses away from cell body, down length of neurone. covered in fatty layer of myelin sheath.

  myelin sheath- protects axon and speeds up electrical transmission of impulse

  nodes of ranvier- speed up transmission of impulse (x200) by forcing it to 'jump' across gaps along the axon

  terminal buttons- at end of axon, and communicate with the next neurone in the chain across a gap known as a synapse
• if myelin sheath was continuous.. would have the reverse effect and instead slow down the electrical impulse, which is why it is segmented by gaps called nodes of ranvier.
• three types

  • sensory
  • relay
  • motor

  
• sensory


  neurone

  
• relay


  neurone

  
• motor 


  neurone

  
• sensory


  • carry messages from PNS to CNS
  • have long dendrites and short axons
  • carry nerve impulses from sensory receptors (vision, taste, touch) to the brain and spinal cord
  • when impulses reach the brain, they are translated into sensation (visual input, heat and pain) so that the organism can react appropriately
  • information that only travels as far as the spinal cord, allows reflex actions to occur quickly without delay of sending impulses to the brain
• relay

  • often lack myelin
  • connect sensory neurone to motor neurone
  • short dendrites and short axons
• motor


  • connect the CNS to effectors (muscles and glands)
  • they have short dendrites and axons
  • form synapses with muscles, and control their contractions
  • when stimulated, motor neurone releases neurotransmitters that bind to receptors on the muscle, triggering a response which leads to muscle movement
  • connects with the muscle/gland
• reflex arc


  the knee-jerk reflex is an example of a reflex arc:

  • a stimulus, such as a hammer, kits the knee
  • this is detected by sense organs in the PNS (peripheral nervous system), which conveys a message along a sensory neurone
  • the message reaches the CNS (central nervous system)
  • this then transfers the message to a motor neurone
  • this then carries the message to an effector, such as a muscle, which causes the muscle to contract and causes the knee to move or jerk
• synaptic transmission (stages 1-4)

  • stage one - electrical impulse arrives
  • stage two - vesicles move towards presynaptic membrane because of calcium
  • stage three - vesicles fuse with presynaptic membrane, releasing their contents to the synaptic gap
  • stage four - neurotransmitters diffuse across the gap
• synaptic transmission (stages 5-7)

  • stage five - neurotransmitters may reach postsynaptic membrane, and bind into receptors
  • stage six - if sufficient neurotransmitters bind onto the postsynaptic membrane, can cause a new electrical impulse, that can travel down the neurone towards the cell body (can only happen if the threshold is reached)
  • stage seven - synapse needs to be reset, enzymes unbind from receptors
• synaptic transmission 


  steps

  
• synaptic transmission 


  image

  
• synaptic transmission - impulse causes an influx of calcium, which makes the vesicles move
• exam q - why can an impulse only move one way? (synaptic transmission)

  • due to the different structures
  • only the post synaptic membrane can receive impulses as it is the only one membrane with receptors