5.3.5(Synapses 1)

State what structure is at the end of the sensory neurone

• The presynaptic neurone or synaptic knob

Describe the structure of the cholinergic synapse

• Synaptic cleft: 20-30nm wide

• Acetylcholine used as the neurotransmitter

Describe and explain how the structure of the cholinergic presynaptic bulb is adapted to its function

• Many mitochondria - synthesis lots of ATP for moving vesicles and other active processes

• Many smooth endoplasmic reticulum, which packages neurotransmitter into vesicle

• Large numbers of vesicles containing acetylcholine

• Voltage gated calcium channels on the plasma membrane

• Describe the structure of the post synaptic membrane

• Contains specialised receptors which have a the complementary shape to the shape of acetylcholine

• the receptor is also a voltage gated sodium channel which reacts to the acetylcholine

• Opens when acetylcholine binds to it

Describe and explain the process of transmission across the synapse

1. . Action potential reaches pre-synaptic neurone

2. Voltage gated calcium ion channels open

3. Calcium ions diffuse into the bulb

4. Calcium ions cause the synaptic vesicle to move to and fuse with the pre-synaptic membrane causing acetylcholine to be released by exocytosis

5. Acetylcholine molecules diffuse across the synaptic cleft

6. Acetylcholine molecules bind to the receptor sites on the sodium channels in the post-synaptic membrane

7. The sodium ion channels open

8. Sodium ions diffuse across the post-synaptic membrane into the post synaptic neurone

9. A generator potential or excitatory post-synaptic potential(EPSP) is generated

10. An action potential is generated in the post synaptic neurone if the threshold potential is met due to generator potentials combining

Describe and explain the importance of acetylcholinesterase

• Acetylcholinesterase is an enzyme, found in the synaptic cleft

• It stops the transmission of signals to the post synaptic neurone, so it does not continue to produce APs

• It hydrolyses the acetylcholine in the synaptic cleft into ethanolic acid and choline