Synapses 13.5

Question 1

Synapse

Answer 1

The junction between two neurones that is necessary in order to keep transmitting the impulse. The impulse is further transmitted by neurotransmitters.

Question 2

Key features of synapses

Answer 2

Synaptic cleft, presynaptic neurone, post synaptic neurone, synaptic knob, synaptic vesicles, neurotransmitter receptors

Question 3

Synaptic cleft

Answer 3

A gap between the two neurones that is 20nm across and separates the axon of one and the dendron of the other.

Question 4

Presynaptic neurone

Answer 4

The neurone that the impulse arrived at before the synaptic cleft.

Question 5

Postsynaptic neurone

Answer 5

The neurone that receives the neurotransmitters after the synaptic cleft

Question 6

Synaptic Knob

Answer 6

The swollen end of the presynaptic neurone that contains many endoplasmic reticulum to produce neurotransmitters and also many mitochondria for the transport.

Question 7

Synaptic Vesicles

Answer 7

These are vesicles that contain the neurotransmitters that have been made in the synaptic knob. These will exit by exocytosis by fusing with the presynaptic membrane and releasing its contents

Question 8

Synaptic Receptors

Answer 8

These are receptor molecules present on the sodium ion channels embedded in the postsynaptic membrane that the neurotransmitters will bind to.

Question 9

Types of neurotransmitter

Answer 9

Excitatory and inhibitory

Question 10

Excitatory neurotransmitters

Answer 10

They result in the depolarisation of the post synaptic membrane so that an action potential is produced when the threshold is reached. An example is acetylcholine.

Question 11

Inhibitory neurotransmitters

Answer 11

They result in the hyperpolarisation of the postsynaptic membrane so that an action potential is inhibited. An example is GABA.

Question 12

How are impulses transmitted across synapses?

Answer 12

1. The arrival of the action potential at the end of the presynaptic neurone will result in calcium ion channels opening. This causes calcium ions to diffuse into the presynaptic knob.
2. The synaptic vesicles containing the neurotransmitter will fuse with the presynaptic membrane and will release the neurotransmitters into the synaptic cleft.
3. The neurotransmitters will diffuse across the synaptic cleft and bind to their specific receptors on the postsynaptic cleft.
4. This will cause sodium ion channels to open so sodium ions will diffuse in through the postsynaptic membrane and depolarise it which will trigger an action potential.
5. This will be propagated and the impulse will continue travelling to the CNS

Question 13

Why must you remove all the neurotransmitters from the synapses?

Answer 13

Once a neurotransmitter has triggered an action potential, it must be removed so the stimulus isn’t maintained and so that another stimulus can enter the synapse. Any left in the synaptic cleft will be broken down by enzymes and taken back to the presynaptic cleft. This is so that the response isn’t triggered over and over and also so neurotransmitters can be recycled.

Question 14

Cholinergic Synapses

Answer 14

This is using the neurotransmitter acetylcholine. They are found where a motor neurone and a muscle cell meet. When an impulse has reached the end of the neurone, and the Ca2+ channels have opened, the acetylcholine neurotransmitter vesicles will leave the synaptic knob by exocytosis. Here they diffuse down the synaptic cleft and bind to their receptors on the postsynaptic membrane of the muscle cell. This allows Na+ channels to open and Na+ to diffuse in which triggers an action potential in the muscle cell. After the action potential is made, acetylcholinesterase hydrolyses the acetylcholine into choline and ethanoic acid which is transported back to the synaptic knob where it will combine with ATP to form acetylcholine again and is stored for future use. The sodium ion channels will also close.

Question 15

Roles of synapses

Answer 15

• They ensure the impulse are travelling in one direction only as there are only receptors on the postsynaptic membrane.
• They can allow one neurone to trigger action potentials in multiple neurones so can create many simultaneous responses.
• They can also allow a number of neurones to feed into the same synapse to make a number of stimuli produce the same result.

Question 16

Summation

Answer 16

Each stimulus will release the same amount of neurotransmitters, but sometimes it doesn’t reach threshold level so you need multiple releases to allow the action potential to be triggered. This is summation

Question 17

Spatial Summation

Answer 17

This is when multiple neurones will release neurotransmitters to one postsynaptic neurone and will release enough to get above the threshold level and make an action potential.

Question 18

Temporal Summation

Answer 18

This is when one presynaptic neurone will release the same neurotransmitter multiple times in order for the threshold value to be reached.

Question 19

Harmful drugs effects on synapses

Answer 19

Drugs can stimulate the synapses and make more action potentials which can result in many effects. They can do it by mimicking the shape of a neurotransmitter and binding to the receptor, like nicotine is the same shape as acetylcholine. They can stimulate the release of transmitters like amphetamines. They can also inhibit enzymes from breaking the neurotransmitter down to make the action potential keep repeating

Question 20

Medicinal drug effects on synapses

Answer 20

They can block receptors so it can no longer bind to the neurotransmitter to cause pain for example. They can also bind to receptors to change their shape so they can no longer bind or can bind easier.