Synapses

Question 1

Summation:

Answer 1

• a process that combines these inputs to make the generation of an action potential in a postsynaptic neurone or effector cell more likely

Question 2

Why is summation needed?

Answer 2

• If synapse is weakly stimulated, it might not release enough neurotransmitter to reach the threshold necessary for an action potential to be generated in the postsynaptic neurone or effector cell.

Question 3

Two types of summation:

Answer 3

• Spatial summation
• Temporal summation

Question 4

Spatial summation:

Answer 4

• Multiple presynaptic neurones converge on a single postsynaptic neurone or effector cell
• The combined input of neurotransmitters can trigger postsynaptic firing
• Inhibitory inputs have the potential to prevent this firing

Question 5

Temporal summation:

Answer 5

• Repeated firing by a presynaptic neurone leads to continuous neurotransmitter release
• An increased amount of neurotransmitter makes it more likely to trigger postsynaptic firing

Question 6

Function of synapses:

Answer 6

• They transmit information through the release of neurotransmitter chemicals.
• A single impulse from a presynaptic neurone can initiate impulses in multiple postsynaptic neurones or effector cells.
• Impulses from several presynaptic neurones can be combined into a single postsynaptic response.

Question 7

What is a synapse?

Answer 7

• Junction where information is transferred from one neurone to another neurone / to an effector cell
• Designed to transmit in one direction

Question 8

Habituation:

Answer 8

• Run out of vesicles containing neurotransmitter
• Can’t pass any signals to neighbours
• Prevents overstimulation to effectors

Question 9

Structure of synapses:

Answer 9

• Presynaptic neurone
• Synaptic cleft
• Postsynaptic neurone
• Synaptic knob
• Synaptic vesicle
• Ca+2 channel
• Neurotransmitter
• Neurotransmitter receptor

Question 10

Presynaptic neurone:

Answer 10

This neurone releases neurotransmitters into the synapse.

Question 11

Synaptic knob:

Answer 11

The section at the end of the presynaptic neurone that contains the organelles needed for neurotransmitter production, like mitochondria to release energy.

Question 12

Synaptic vesicles:

Answer 12

These sacs within the synaptic knob store neurotransmitters until they are released.

Question 13

Synaptic cleft:

Answer 13

The gap between the presynaptic and postsynaptic neurones’ membranes.

Question 14

Postsynaptic neurone:

Answer 14

This neurone receives the neurotransmitters and can generate new action potentials.

Question 15

Neurotransmitter receptors:

Answer 15

These specific molecules on the postsynaptic membrane bind with the neurotransmitters.

Question 16

Excitatory and inhibitory synapses:

Answer 16

The same neurotransmitter can have different effects on postsynaptic neurones depending on whether the neurotransmitter is excitatory or inhibitory at a particular synapse.

Question 17

Excitatory neurotransmitter:

Answer 17

Effect on the postsynaptic membrane –> depolarisation
Action potential –> may trigger action potential if threshold potential reached
Examples –> acetylcholine is an excitatory neurotransmitter in the central nervous system and at neuromuscular junctions

Question 18

Inhibitory neurotransmitter:

Answer 18

• Hyperpolarisation
• Prevent action potential
• Acetylcholine is an inhibitory neurotransmitter at cardiac synapses

Question 19

Steps in synaptic transmission:

Answer 19

1. Action potential arrives at the presynaptic knob
2. This causes voltage-gated calcium ion (Ca2+) channels to open and Ca2+ flows into the presynaptic knob
3. This causes synaptic vesicles, which contain neurotransmitters, to move towards and fuse with the presynaptic membrane
4. The vesicles release neurotransmitters into the synaptic cleft through exocytosis, and the neurotransmitters rapidly diffuse across the synaptic cleft.
5. On reaching the other side, the neurotransmitters bind to receptor proteins on the postsynaptic membrane, causing the receptors to change shape.
6. This opens sodium ion channels in the postsynaptic membrane, leading to the depolarisation of the postsynaptic membrane as there is an influx of sodium ions.
7. If this depolarisation reaches a threshold level, an action potential is triggered in the postsynaptic neurone.

Question 20

Cholinergic synapses:

Answer 20

• Specific synapses that use acetylcholine (Ach) as their neurotransmitter

Question 21

After ACh binds to receptors and triggers a response:

Answer 21

1. ACh is broken down by the enzyme acetylcholinesterase into choline and ethanoic acid (acetate).
2. These breakdown products are then reabsorbed into the presynaptic knob via active transport.
3. They can then be recycled to synthesise more ACh.
4. Ach is transported into synaptic vesicles, ready for another action potential.

Question 22

Why is it important to remove neurotransmitters like ACh need to be removed from synaptic cleft?

Answer 22

• Prevent stimulus being maintained
• Allow another stimulus to affect the synapse
• Prevents continuous stimulation and allows for neurotransmitter recycling