Synapses

Question 1

Type of synapses (2)

Answer 1

Separate neurones and send impulses between them

1) Electrical synapse- 3nm gap = small so electrical impulse transmitted directly
2) Chemical synapse- 20nm gap = too big and transmitted by neurotransmitters

Question 2

Functions of synapses

Answer 2

• Transmit impulses between neurones
• Filter out low-level stimuli
• Protect from over stimulation: acclimatisation after repeated stimulations
• Spatial summation- several neurones additively contribute to generate action potential for same post-synaptic neurone
• Temporal summation- frequent action potentials in pre-synaptic membrane build up to reach threshold potential/ value

Question 3

Synaptic transmission 1 (until exocytosis)

Answer 3

• Action potential arrives at axon terminal
• Causes membrane to become more permeable to Ca2+ ions as voltage-gated Ca2+ channels open
• Ca2+ ions rush/ flood in down the concentration gradient
• Ca2+ ions cause synaptic vesicles containing acetylcholine (neurotransmitter) to migrate towards pre-synaptic membrane
• Vesicle membrane fuses with the pre-synaptic membrane and a.ch released/ discharged into synaptic cleft by exocytosis

Question 4

Synaptic transmission 2 (until action potential)

Answer 4

• A.ch. molecules diffuse across synaptic cleft
• A.ch. molecules bind to complementary receptors on post-synaptic membrane
• Chemical/ ligand-gated Na+ channels open- protein changes shape
• Na+ ions flood in down gradient and membrane depolarised
• Action potential generated in post-synaptic membrane with sufficient depolarisation

Question 5

Synaptic transmission 3 (from new action potential)

Answer 5

• Active transport of Ca2+ ions out of synaptic knob = no more exocytosis
• A.ch. is broken down/ hydrolysed to acetyl (ethanoic acid) and choline by acetylcholinesterase
• Acetyl and choline diffuse back through pre-synaptic membrane
• Na+ channels close = no more action potentials
• Acetyl co-enzyme A re-synthesises a.ch in synaptic knob
• Or it is produced by sER
• Mitochondria- provide ATP for re-synthesis and packaging into vesicles

Question 6

Alzheimer’s disease effect on acetylcholine

Answer 6

Fewer synaptic knobs, so less enzymes and less acetylcholine synthesised