Nervous system II

• Neurons


  Electrically excitable cells with a specialised structure that allows them to carry out their functions
• Neurons


  • Three-dimensional shape makes the immense numbers of connections within the nervous system possible
  • Use membrane potentials to generate electrical signals by controlling movement of ions across their membrane to create electrical currents
  • Small but big enough to allow for signalling between neurons and muscles
  • Cell membrane is responsible for regulating what crosses membrane

  
• Membrane potential


  The distribution of charge across the cell membrane, measured in millivolts

  
• Intracellular fluid has an excess of negative charge
• Excess negative ions are attracted to the excess positive ions outside the cell and vice versa
• Sodium and potassium ions


  Play the most important roles in generating the resting membrane potential

  
• Sodium-potassium pump moves three sodium ions for every two potassium ions
• Leak channels


  The only way ions can move across the membrane

  
• More potassium channels than sodium channels, so potassium can move more than sodium
• This maintains the membrane potential at -70 millivolts

  
• Action potentials


  Two types of changes in membrane potential: depolarisation (less difference in charge) and repolarisation (more difference in charge)

  
• Sequence of events during an action potential

  1. Membrane is sufficiently depolarised

  2. Triggers change in confirmation or shape of channel with inactivation gate swinging away opening channel

  
• Synaptic transmission


  Chemical synapses where neurotransmitters are released from the ends of the axons of one neuron and transmit a signal to a nearby cell
• Neurotransmitter causes a change in the cell the neuron synapses with, e.g. cause neuron to convey electrical signal causing muscle to contract or gland to secrete
• Two types of changes in membrane potential

  1. graded potentials: signs, over short distances
  2. action potentials: signal over long—distances in nerve, muscle