9.5-6 nervous transmission

• structure of a neurone

  • cell body
  • dendrons
  • Myelin sheath
  • node of ranvier
  • axon terminal
• cell body


  • contains nucleus and organelles
• dendrons


  • extensions from cell body
  • from dendrite to the cell body
• axon


  • extension of the cytoplasm
  • from cell body to axon terminal
• myelin sheath


  • consists of schwann cells around axon
  • insulates axon
  • prevents movement of ions
• node of ranvier


  gaps between schwann cells
• axon terminal


  • releases neurotransmitters
  • onto cells of target organs
• resting potential

  • Na+ channels closed but leaky - small amount of sodium ions diffuse into neurone
  • K+ channels closed but more leaky - more ions diffuse out of neurone
  • Sodium-Potassium ion pumps actively transport 3Na+ ions out and 2K+ ions in
  • more + ions are being pumped out
  • cytoplasm of axon is more negative than outside
  • stops action potential
  • membrane is polarised
  • -70mv
• stages of an action potential

  • depolarisation
  • repolarisation
  • hyperpolarisation
  • return to resting potential
• depolarisation

  • energy from stimulus received e.g pressure
  • Na+ gated channels open
  • Na+ ions diffuse down the electrochemical gradient INTO the cell via facilitated diffusion
  • potential difference across membrane now more +
  • -55mv = threshold potential reached. action potential initiated
  • Axon membrane is depolarised
  • other Na+ gated channels open
  • +40mv = max action potential reached. Na+ channels close. K+ channels open
• repolarisation and hyperpolarisation

  • Na+ channels close
  • K+ channels open - causing other K+ channels to also open
  • K+ ions diffuse OUT of neurone, down the electrochemical gradient
  • Charge of axon is lowered (potential difference across membrane is more -)
  • K+ channels are slow to close
  • overshoot of K+ ions diffusing out
  • potential difference across membrane more - than resting potential
  • charge in axon drops to -90mv
  • membrane is hyperpolarised
• refractory period 


  • time in which action potential cannot be generated and depolarisation cannot occur
  • occurs after hyperpolarisation (-80mv)
  • 2 types: Absolute and relative
• absolute refractory period:

  • membrane cannot be stimulated at all

  relative refractory period:

  • membrane needs a larger than normal stimulus
• importance of refractory period

  • limits frequency of impulse transmission
  • ensures unidirectional impulses (in one direction)
• propagation of unmyelinated neurone

  • Na+ ions diffuse into axon via gated channels
  • inside of axon is more positive
  • Na+ ions diffuse laterally (both directions) along axon
  • first section of membrane is depolarised
  • local current established
  • caused by Na+ ions causing more gated channels to open and diffuse in
  • so threshold is reached in next section of membrane
• impulses are conducted faster in myelinated neurones than unmyelinated neurones due to saltatory conduction
• saltatory conduction


  • Na+ gated channels are only present in the nodes of ranvier
  • myelin sheath prevents depolarisation of membrane in all other areas
  • because there are no Na+ channels there so no action potential
  • Na+ ions diffuse from one node to the other
  • so action potential jumps from node to node
• Factors affecting speed of transmission

  • axon diameter - larger the diameter, faster the rate of transmission, less resistance to ion flow
  • temperature - larger the temp, faster the rate of transmission, faster diffusion rate
• synaptic transmission


  • action potential reaches axon terminal - membrane is polarised
  • causes Ca+ gated channels to open
  • Ca+ ions diffuse into the presynaptic knob
  • which causes vesicles containing neurotransmitters (e.g. acetylcholine) to fuse with presynaptic membrane
  • neurotransmitters are released via exocytosis into the synaptic cleft and diffuse across
  • bind to neuroreceptors on the membrane of postsynaptic knob
  • causes Na+ channels to open and diffuse into knob
  • causing membrane to become depolarised
  • once threshold is reached, action potential is generated.
• Excitatory Post synaptic potential

  • Na+ channels open in post synaptic membrane
  • Na+ ions diffuse into membrane
  • post synaptic cell becomes more + charged
  • action potential occurs
• Inhibitory post synaptic potential

  • Cl- channels open in post synaptic membrane
  • chloride ions diffuse into membrane
  • post synaptic cell more - charged
  • action potential inhibited
• role of neurotransmitter - acetylcholine


  • causes muscle contraction
  • causes inhibition of parasympathetic neurones
  • e.g. decreased heart rate
• what happens to acetylcholine in synaptic cleft?


  • hydrolysed by acetylcholinesterase
  • into acetate and choline
  • diffuse back into presynaptic membrane
  • ATP used to reform them for storage in vesicle
• role of neurotransmitter - noradrenaline

  • increases rate of heart contraction
  • and breathing rate
  • increase force of skeletal muscle contraction
• nicotine


  mode of action:

  • mimics acetylcholine
  • binds to acetylcholine receptors

  effects:

  • increased heart rate and blood pressure
  • stimulates dopamine release
  • causing addiction
• lidocain


  mode of action:

  • blocks Na+ gated channels
  • cannot enter post synaptic neurone
  • action potential is not generated - inhibitory

  effects:

  • anaesthetic
  • pain signals prevented to brain
• cobra venom


  mode of action:

  • block acetylcholine receptors irreversibly
  • in post synaptic membrane
  • na+ channels permanently open

  effects:

  • paralysis (cant repolarise due to permanent opening)
  • death