Synapses

Question 1

What is the structure of a synapse?

Answer 1

Presynaptic Neurone - neurone where the impulse is from

Synaptic Cleft - the gap separating the axon of a membrane and the dendrite of the next

Postsynaptic Neurone - neurone receiving the neurotransmitter

Synaptic Knob - swollen end of the presynaptic neurone - has many mitochondria and lots of smooth ER to manufacture neurotransmitters

Synaptic Vesicles - vesicles containing neurotransmitters - the vesicles fuse with the presynaptic membrane and release their contents into the cleft

Neurotransmitter receptors - Receptor sites on the postsynaptic neurone that receive neurotransmitters

Question 2

What are the 2 types of neurotransmitter molecules?

Answer 2

Excitatory - these neurotransmitters result in the depolarisation of the post-synaptic neurone. If the threshold is reached, an action potential is triggered. E.g. ACh (acetylcholine)

Inhibitory - these neurotransmitters result in the hyperpolarisation of the post-synaptic neurone - preventing an action potential. E.g. GABA (gamma-aminobutyric acid) - found in synapses in the brain

Question 3

Why do synaptic transmissions occur?

Answer 3

1. Action potential reaches end of presynaptic neurone
2. Depolarisation of presynaptic membrane causes V-Gated Ca²⁺ to open and calcium ions diffuse in, down the electrochemical gradient, causing the synaptic vessels to fuse with the presynaptic membrane - neurotransmitters leave by exocytosis and diffuse across the synaptic cleft
3. After neurotransmitters bind to the receptor sites, this triggers the V-Gated sodium ion channels to open, sodium ions diffuse into the postsynaptic neurone triggering an action potential.

Question 4

What occurs after the action potential is triggered to the neurotransmitters?

Answer 4

After the action potential is triggered, the neurotransmitters must be removed from the receptor site to stop a constant stimulus. Room must be left for new stimuli. Neurotransmitters in the synaptic cleft are removed.

E.g. ACh is broken down by acetylcholinesterase (enzyme), which also releases ACh from the receptor sites.

Products are taken to the presynaptic knob - this prevents the response from happening repeatedly and the neurotransmitter can be recycled.

Question 5

What is a cholinergic synapse?

Answer 5

They use the neurotransmitter acetylcholine.

Common in vertebrate CNS’ and at neuromuscular junctions.

It is hydrolysed by acetylcholinesterase (which is situated in the postsynaptic membrane) giving ethanoic acid and choline.

The ATP in the presynaptic knob are released to react choline and ethanoic acid to make the ACh. ACh is stored in the synaptic vessels for future use

Question 6

What are the roles of synapses?

Answer 6

• Ensure impulses are unidirectional - neurotransmitters receptor sites are only on the postsynaptic membrane so the impulse can only travel in one direction
• Synapses can allow an impulse from one neurone to transmit to several synapses. Single stimulus creating multiple simultaneous responses
• Converse is several neurones can feed into a single post-synaptic membrane

Question 7

What is summation and what are the 2 types?

Answer 7

Sometimes the amount of neurotransmitter is not enough to cause postsynaptic membrane depolarisation as the transmitter is removed from the site too quickly or the threshold is not reached

There are 2 ways to reach the threshold to trigger an action potential - known as summation

Spatial Summation

Ensures several presynaptic neurones provide action potentials with enough neurotransmitters to trigger depolarisation as an individual one is not enough

Temporal Summation

Action potentials arrive from one presynaptic neurone in rapid succession (high frequency) to build up in the synapse to reach the threshold and trigger an action potential.