Neuronal communication

• what do all the neurones have in common

  • cell body
  • dendrons
  • axon
• what types of neurones are there?

  • sensory neurones
  • relay neurone
  • motor neurone
• what is the cell body in neurones?

  contains the organelle found in a typical animal cell including nucleus

  protein + neurotransmitter chemicals are made here
• what are the dendrons in neurones?

  carry the action potentials to surrounding cells
• what are the dendrons in neurones?

  conductive, long fibre that carries the nervous impulse along the motor neurone
• what do myelinated neurones have?

  Schwann cells which wrap around the axon to form the myelin sheath

  • this is a lipid and doesn't allow charged ions to pass through it
• what are the gaps in the myelin sheath called?

  nodes of Ranvier
• how does the action potential travel?

  The action potential jumps from node to node (saltaory conduction)

  = it travels faster as it only has to generate action potential at the nodes of Ranvier
• describe the neurones?


  explain the way they look

  
• sensory neurone

  • carry electrical impulses from the sensory receptor cells to the relay neurone (can be motor neurone)
  • long dendrons - sensory receptor cell - cell body of neurone -axon - to next neurone
• relay neurone 


  carry impulse between sensory + motor neurones

  short axons + dendrons

  not myelinated
• motor neurone 


  carry impulses from relay or sensory neurones to effector (muscle or gland)

  one long axon + multiple short dendrons
• sensory receptors 


  detect a stimulus

  cell are transducer = converting different types of stimuli into electrical nerve impulses

  • photoreceptor = light is stimulus
  • thermoreceptor (skin) = heat
• Pacinian corpuscle

  • pressure receptors deep in skin e.g. feet + fingers
  • sensory neurone has special channel proteins
• what do the membranes of Pacinian corpuscle contain?

  stretch-mediated sodium channels

  • open and allow Na+ to enter sensory neurone when stretched + deformed
  • pressure applied = deformed + stretches + widens Na+ diffuses in = establishment of generator potential
• Pacinian corpuscle

  
• stretch-mediated sodium channels

  
• resting potential 


  when a neurone is not conducting an impulse there is a difference between electrical charge inside + outside

  • more positive ions Na+ and K+ outside = inside more negative
  • 70mV
• ATP and Na+/K+ pumps

  • resting potential maintained by sodium-potassium pump involving active transport + ATP
  • pump moves 2K+ in + 3Na+ out
  • electrochemical gradient = K+ to diffuse out Na+ diffuse in
  • membrane more permeable to K+= more moved out = 70mV
• action potential 


  neurone's voltage increases beyond a set point from resting potential = generates nervous impulse
• depolarisation 


  due to the neurone membrane becoming more permeable to Na+
• action potential generated =moves along the axon like Mexican wave
• what happens

  
• If the depolarisation does not exceed -55mV an action potential + impulse are not produced + impulse are not produced
• large stimulus = response
• Refractory Period 


  when the membrane can not be stimulated due to the sodium channels recovering
• why is the refractory period important?

  1. ensures discrete impulses are produced = each impulse is separate
  2. action potential travels in one direction (two directions would = no response
  3. limits the number of impulse transmissions = stops over reaction to a stimulus
• what is a synapse?

  gaps between the ned of the axon of one neuron and the dendrite of another

  • the action potential is transmitted via neurotransmitter + diffuses across synapse
• Synapse

  
• Function of a synapse

  1. Action potential arrives at synaptic knob= depolarisation + Ca2+ channels open and Ca2+ diffuses into knob
  2. vesicles containing neurotransmitter move + fuse with presynaptic membrane . Neurotransmitter released to synaptic cleft
  3. neurotransmitter diffuses down conc gradient across cleft to post synaptic membrane. neurotransmitter binds by complementary shape to receptors on surface of post synaptic membrane
• Function of a synapse

  4. Na+ ion channel on post synaptic membrane open = Na+ diffuse in (if enough neurotransmitter = enough Na+ in ) above threshold post synaptic neurone = depolarised

  5. neurotransmitter = degraded + released from receptor = Na+ channel closes + post synaptic neuron goes back to resting potential

  • neurotransmitter transported back into presynaptic neurone - recycled
• Cholinergic synapse

  
• summation 


  rapid build up of neurotransmitters in the synapse to help generate an action potential by two methods ( spatial + temporal)
• spatial summation 


  many different neurones collectively trigger a new action potential by combing the neurotransmitter they released to exceed the threshold
• temporal summation


  one neurone releases neurotransmitter repeatedly over a short period of time to add up to enough to exceed the threshold value
• Inhibitory synapse

  • causes chloride ion to move into postsynaptic neurone + potassium ions to move out
  • this makes the membrane potential decrease -80mV = hyperpolarisation = action potential highly unlikely