Nervous + synaptic transmission

• What is a neurone?

  - a specialised cell adapted to carrying nerve impulses quickly from one part of the body to another
• Label a typical motor neurone

  Cell body- contains large amounts of rough ER + mitochondria

  
  

  
  

  Dendrites- carry nerve impulses to cell body

  
  

  
  

  Axon- long fibre which carries impulses away from cell body

  
  

  
  

  Schwann cells- wrap around axon providing isolation- make up myelin sheath

  
  

  
  

  nodes of ranvier- gaps between Schwann cells

  
• What is a nerve impulse?


  - temporary reversal of electrical p.d across axon membrane

  
  

  
  

  -reversal between two states : resting potential + action potential
• What is the electrical p.d during resting potential


  > -65 mV to -70mV

  
  

  inside of axon more negative than tissue fluid around it
• How is resting potential maintained?


  - movement of Na+ and K+ in and out of the axon , through the membrane
• How will ions move across the membrane of an axon?


  - ions charged so can't cross phospholipid bilayer by simple diffusion

  
  

  
  

  -need to use transport proteins + move via facilitated diffusion or active transport
• What type of proteins are embedded within the phospholipid bilayer ?

  intrinsic proteins:

  
  

  'Leak' channels - for either Na or K ions

  
  

  'Gated' channels- for either Na or K ions

  
  

  Sodium-potassium pumps
• How is resting potential established?


  - Active transport of 3Na+ out of axon and 2K+ into axon by sodium/potassium pumps

  
  

  -resulting in more K+ inside axon than out

  
  

  -membrane more permeable to K+ ions(x1000 more K+ leak channels) than to Na+ ions

  
  

  - K+ diffuse back out faster than Na+ diffuse back in

  
  

  -leading to pd of -65mV inside compared to out

  
  

  -membrane POLARISED
• What is an action potential?


  - stimulus can cause temporary reversal of charges across axon membrane

  
  

  -axon known as being depolarised

  
  

  -if axon depolarised enough can result in generation of an action potential
• Stages which cause action potential


  - resting potential

  -depolarisation

  -repolarisation

  -hyperpolarisation

  -refractory period
• Describe depolarisation


  - stimulus causes gated Na+ channels to open, making membrane more permeable to Na+ ions

  
  

  -Na+ rapidly diffuse into the axon, causing depolarisation

  
  

  inside of the membrane becomes charged to +40mV

  
  

  > gated sodium ion channels close, meaning the axon is again almost impermeable to Na+ ions.
• Describe Repolarisation


  > gated potassium ion channels open (i.e. the axon's permeability to potassium ions increases)

  
  

  
  

  -K+ ions diffuse rapidly out of the axon. (repolarisation).
• Describe hyperpolarisation


  >So many K+ ions diffuse out of the axon that they cause a more negative potential difference than normal, dropping the axon interior down to about -90mV

  
  

  - known as hyperpolarisation and causes the refractory period
• what is the refractory period?


  - a short period of time where that area of the axon membrane is recovering from its own depolarisation

  
  

  - Gated Na+ channels can NOT be opened in the membrane
• how is resting potential returned

  >gated K+ ion channels close and sodium-potassium pump restores the concentration gradients for Na+ and K+

  
  

  This returns the resting potential of the axon to -65mV. The resting permeability of the axon is restored.

  
  

  The whole process lasts 2-3 milliseconds.
• What is the all or nothing principle?

  - If initial stimulus not large enough (below the threshold), then the voltage-gated sodium channels will not open and the axon membrane will not become fully depolarised, i.e. no action potential will be generated.

  
  

  · If stimulus large enough (above the threshold), then action potential will be generated at a constant size and speed- all or nothing principle

  
  

  · Increasing initial stimulus will not produce a larger or faster action potential. It will, however, increase the FREQUENCY of action potentials.
• Gated sodium channels cannot be opened in the membrane that has entered a refractory period. Therefore an action potential can not be generated for a short period of time.

  
  

  Why is this important?

  > It ensures that action potentials travel in only one direction

  
  

  
  

  >It produces discrete, separate action potentials

  
  

  
  

  >It limits the number of action potentials
• What is saltatory conduction (myelinated neurones) ?


  - depolarisation can only occur at nodes of Ranvier

  
  

  - impulse jumps from node to node
• What factors affect speed of conduction


  - myelination + saltatory conduction

  
  

  action potentials don't need to be generated along entire length of axon only nodes of ranvier so increases speed of conduction

  
  

  Axon diameter- wider increases speed - less resistance to flow of ions + less leakage of ions

  
  

  Temperature- higher increases speed:

  
  

  -ions diffuse faster

  -enzymes involved in respiration work faster so more ATP for active transport
• What is a synapse?

  gap between the axon of one neurone and a dendrite of another

  
• What is the neurotransmitter used in a cholinergic synapse called ?

  - Acetylcholine

  
  

  --> made up of ethanoic acid + choline
• What is the neuromuscular junction?


  - synapse between motor neurone + a muscle

  
  

  - post-synaptic membrane is the sarcolemma of the muscle cell
• Describe synaptic transmission


  1.action potential at the synaptic knob OPENS calcium ion channels in the membrane which allows calcium ions (Ca2+) to diffuse in

  
  

  2. Influx of calcium ions causes synaptic vesicles to fuse with the presynaptic membrane and release ACh by exocytosis into the synaptic cleft

  
  

  3. ACh diffuses across the synaptic cleft and binds to receptors on the sodium ion channels in the postsynaptic membrane

  
  

  4. This causes Na+ channels to open and Na+ diffuse rapidly into the post synaptic cell, causing depolarisation. If the threshold is reached an action potential will be initiated

  
  

  5. To prevent the initiation of more action potentials in the postsynaptic membrane, by the continued presence of ACh, an enzyme, acetylcholinesterase, HYDROLYSES ACh

  
  

  6. The products diffuse back across the cleft and can be reabsorbed into the presynaptic neurone

  
  

  - ACh is re-formed and repackaged into vesicles.
• Explain why the presynaptic knob contains many mitochondria.


  Energy from ATP is required to generate neurotransmitter

  
  

  (also acetyl CoA produced in mitochondrial matrix during link reaction can be used to make more ACh, by providing an acetyl group)
• What are features of synapses?

  Unidirectionality - synapses can only pass impulses in one direction, the transmitter is only released by the presynaptic knob and receptors are found only on the postsynaptic cell membrane

  
  

  Spatial summation- number of dif presynaptic neurones together release enough transmitter at the same time

  
  

  Temporal summation: a single presynaptic neurone releases small amounts of neurotransmitter many times in a short period of time

  
  

  Inhibition
• Describe inhibition


  > On postsynaptic membranes of some neurones there are chloride ion (Cl-) channels which can be opened when activated by a certain neurotransmitter

  
  

  
  

  > causes Cl- ions to flood into the postsynaptic knob and make it more negative than it normally is at rest (hyperpolarisation)

  
  

  
  

  
  

  > This makes it less likely that the membrane can depolarise and so a new action potential cannot be generated. They are known as inhibitory synapse
• What are the effects of drug stimulants?


  - create, action potentials in postsynaptic neurones

  
  

  A drug could do this by:

  · mimicking the neurotransmitter, e.g. having a similar shape

  
  

  
  

  · causing the release of excess neurotransmitter

  
  

  · reducing the activity of the enzyme which breaks it down

  
  

  This causes an increase in the number of impulses sent along that neurone. Examples include caffeine, nicotine, amphetamines and cocaine.
• Effect of drug inhibit


  causing fewer action potentials in the postsynaptic neurone

  A drug could do this by:

  
  

  · inhibiting the release of the neurotransmitter

  
  

  · blocking the receptors for neurotransmitter on the postsynaptic membrane

  
  

  causes a reduction in the number of impulses sent along that neurone

  
  

  Examples include alcohol, cannabis
• 1. Nicotine is a similar shape to acetylcholine.

  Explain the effect nicotine would have on the nervous system

  Nicotine will bind to ACh receptors in the postsynaptic membrane

  
  

  > opening sodium channels and allowing sodium ions to diffuse into the postsynaptic neurone, initiating action potentials

  
  

  >More impulses will be generated in the CNS - excitatory effect
• Atropine is found in the flower Atropa belladonna, otherwise known as Deadly Nightshade or Belladonna. It has a similar shape to ACh and binds to its receptors, but does not allow the passage of sodium through the channels.

  Explain what effect atropine would have on the muscle at a neuromuscular junction?

  Atropine will bind to Ach receptors in the postsynaptic membrane but will not cause the sodium channels to open.

  
  

  >It will effectively block the receptor and prevent ACh from binding

  
  

  Therefore depolarisation will not occur and action potentials will not be generated in the muscle cell membrane. This prevents muscles from contracting, causing paralysis