5.3.2

• Functions of neurones

  once a stimulus has been detected + its energy has been converted to a depolarisation of receptor cell membrane, impulse is then transmitted along neurones as an action potential

  → action potential is carried as a rapid depolarisation of membrane caused by influx of sodium ions
• 3 types of neurones

  • sensory neurones carry the action potential from sensory receptor to CNS
  • relay neurones connect sensory + motor neurones
  • motor neurones carry action potential from CNS to an effector such as muscle or gland

  
• Structure of neurones

  • many are very long so that they can transmit the action potential over a long distance
  • cell surface membrane has many gated ion channels to control entry + exit of sodium , potassium and calcium ions
  • Na/K pumps use ATP to actively transport sodium ions out of cell + potassium ions into cells
  • neurones maintain a potential difference across their plasma membrane
• Structure of neurones 2

  • cell body contains nucleus, many mitochondria + ribsomes
  • numerous dendrites connect to other neurones, the dendrites carry impulses towards cell body
  • an axon carries impulse away from body
  • neurones have myelin sheath , fatty layer, surrounding + insulates cells from electrical activity with neighbouring neurones

  →myelin sheath is composed of Schwann cells + are closely associated with neurone
• Differences

  • motor neurones

  → cell body in CNS have long axon that carries action potential

  • sensory neurones

  →long dendron carrying action potential from sensory receptor to cell body, positioned just outside the CNS

  • relay neurones

  →short dendrites + a short axon (number of dendrites + number of axon divisions are variable) ; conduct impulses in coordinated pathways
• Myelinated + Non-myelinated

  • 1/3 of the peripheral neurones in vertebrates are myelinated
  • remainder of the peripheral neurones + neurones found in CNS are non-myelinated
• Myelinated Neurones

  most sensory + motor neurones are associated with many Schwann cells, which make up a fatty sheath called the myelin sheath

  • Schwann cells are wrapped tightly around the neurone so the sheath actually consists of several layers of membrane + thin cytoplasm from Schwann cell
  • at intervals of 1-mm along the neurone there are gaps on the myelin sheath called nodes of Ranvier
• Myelinated Neurones 2

  • because myelin sheath is tightly wrapped around the neurone it prevents movement of ions across neurone membranes + therefore movement of ions across the membrane can only occur at the nodes of Ranvier

  → this means that the impulse, or action potential, jumps from one node to the next making the conduction more rapid
• Non-myelinated neurones

  also associated with Schwann cells, but several neurones may be enshrouded in one loosely wrapped Schwann cell

  → this means that the action potential moves along the neurone in a wave rather than jumping from node to node as seen in myelinated neurones

  the greater pain after the contact of like stubbing your toe is conducted by non-myelinated neurones
• Advantages of Myelination

  myelinated neurones transmit quicker than non-myelinated neurones

  • myelinated neurones carry action potentials from sensory receptors to the CNS + from the CNS to effectors and carry action potential over long distances

  → the increased speed of transmission means that the action potential reaches the end of the neurone much more quickly, enabling a more rapid response to a stimulus
• Advantages to myelination 2

  • non-myelinated neurones tend to be shorter + carry action potential only over a short distance

  → they are often used in coordinating body function such as breathing + action of digestive system + therefore increased speed of transmission is not so important