Synaptic transmission (BIOL5)

Question 1

Define synapse

Answer 1

Junction between neurones or effector cells

Question 2

Features of synapses

Answer 2

• Synaptic cleft - gap b/w cells at synapse
• Pre-synaptic neurone has a synaptic knob - contains synaptic vesicles filled w/ NT
• When AP reaches end of neurone, causes NT release into synaptic cleft
  
  • Diffuse across pre-synaptic membr. and bind to spec. receptors
• When NT bind, trigger AP, muscle contraction or hormone secretion
• Impulses are unidirectional (one direction)
  
  • receptors only on post-synaptic membr.
• NT removed from cleft, so response only temporary:
  
  • Taken back to pre-synaptic neurone/broken down by enzymes

Question 3

Describe nerve impulses along the cholinergic synapse (acetylcholine)

Answer 3

1. AP arrives at synaptic knob at pre-syn neurone
2. Ca+ channels in pre-syn open
3. Ca+ diffuse into synaptic knob (then pumped out after by active transport)
4. Influex of Ca+ cause synaptic vesicles fuse w/ pre-syn memb
5. Vesicles release NT acetylcholine (ACh) into synaptic cleft - exocytosis
6. ACh diffuse across synaptic cleft, bind to cholinergic receptors on post-syn memb
7. Na+ channels open, causing AP on post-syn memb
8. ACh removed from synaptic cleft, so response stopped - b/d by enzyme AChE

Question 4

Features of neuromuscular junctions

Answer 4

• Synapse b/w motor neurone and muscle cell
• NT acetylcholine (ACh) bind to nicotinic cholinergic receptors
• Works in same way as cholinergic but..
  
  • post-syn has many folds, form clefts
    
    • store enzyme that b/d ACh - acetylcholinerase (AChE)
  • post-syn has more receptors!
    
    • when motor neurone fires AP, always triggers response in muscle cell

Question 5

Features of excitatory neurotransmitters

Answer 5

• Depolarise post-syn, making it fire AP if threshold reached
  
  • e.g ACh is excitatory NT - binds to cholinergic receptors, cause AP in post-syn

Question 6

Features of inhibitory neurotransmitters

Answer 6

• Hyperpolarise post-syn preventing firing AP
  
  • e.g. GABA bind w/ receptors causes K+ channels to open on post-syn

Question 7

Define summation

Answer 7

Where effect of NT released from many neurones is added together!

Question 8

Features summation

Answer 8

• If stimulus weak, small NT released into cleft not enough to excite post-syn/threshold to fire AP
• Summation allows this!

Question 9

Features of spatial summation

Answer 9

• Many neurones connect to one neurone
• Small amnt NT released fr each neurone added together
  
  • Reach threshold in post-syn to trigger AP
• If some neurones release inhibitory NT, total effect of NT had no AP

Question 10

Features of temporal summation

Answer 10

• 2 (or more) impulses arrive in quick succession from same pre-syn neurone
• Make AP likelier as more NT released into synaptic cleft

Question 11

Drugs may effect a neurone…

Answer 11

• Drugs same shape as NT
  
  • mimic their action, so more receptors activated
• Block receptors
  
  • can’t be activated by NT, fewers receptors active
• Inhibit enzyme that b/d NT
  
  • more NT in cleft bind to receptors, are there for longer
• Stimulate release of NT fr pre-syn neurone
  
  • more receptors active
• Inhibit release of NT fr pre-syn neurone
  
  • fewer receptors activated