chapter 13 p3

Question 1

Synaptic cleft

Answer 1

• the gap which separates the axon of one neurone from the dendrite of the next neurone.
  It is approximately 20-30 nm across.

Question 2

Presynaptic neurone

Answer 2

neurone along which the impulse has arrived.

Question 3

Postsynaptic neurone

Answer 3

neurone that receives the neurotransmitter

Question 4

Synaptic knob

Answer 4

the swollen end of the presynaptic neurone.
It contains many mitochondria and large amounts of endoplasmic reticulum to enable it to manufacture neurotransmitters (in most cases).

Question 5

synaptic vesicles

Answer 5

vesicles containing neurotransmitters.
The vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft.

Question 6

Neurotransmitter receptors

Answer 6

receptor molecules which the neurotransmitter binds to in the postsynaptic membrane.

Question 7

structure of a synapse diagram

Answer 7

Question 8

Two types of neurotransmitter:

Answer 8

Excitatory:
Inhibitory:

Question 9

Excitatory:

Answer 9

these neurotransmitters result in the depolarisation of the postsynaptic neurone.
If the threshold is reached in the postsynaptic membrane an action potential is triggered.
E.g Acetylcholine

Question 10

Inhibitory:

Answer 10

these neurotransmitters result in the hyperpolarisation of the postsynaptic membrane.
This prevents an action potential being triggered.
E.g Gamma-aminobutyric acid (GABA) - found in some synapses in the brain.

Question 11

Synaptic transmission occurs as a result of the following:
p1

Answer 11

The action potential reaches the end of the presynaptic neurone
Depolarisation of the presynaptic membrane causes calcium ion channels to open
Calcium ions diffuse into the presynaptic knob
This causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane
Neurotransmitter is released into the synaptic cleft by exocytosis

Question 12

Synaptic transmission occurs as a result of the following:
p2

Answer 12

Neurotransmitter diffuses across the synaptic cleft and binds with its specific receptor molecule on the postsynaptic membrane
This causes sodium ion channels to open
Sodium ions diffuse into the postsynaptic neurone
This triggers an action potential and the impulse is propagated along the postsynaptic neurone.

Question 13

Neurotransmitter Removal in Synaptic Transmission:

Answer 13

Once a neurotransmitter has triggered an action potential in the postsynaptic neurone, it is important that it is removed so the stimulus is not maintained, and so another stimulus can arrive at and affect the synapse.
Any neurotransmitter left in the synaptic cleft is removed.
Acetylcholine is broken down by enzymes, which also releases them from the receptors on the postsynaptic membrane.
The products are taken back into the presynaptic knob.
Removing the neurotransmitter from the synaptic cleft prevents the response from happening again and allows the neurotransmitter to be recycled.

Question 14

Cholinergic Synapses and Acetylcholine

Answer 14

Cholinergic synapses use the neurotransmitter acetylcholine.
They are common in the CNS of vertebrates and at neuromuscular junctions - where a motor neurone and a muscle cell (an effector) meet.
If the neurotransmitter reaches the receptors on a muscle cell, it will cause the muscle to contract.

Question 15

Signal Transmission Process with Transmission across cholinergic synapses

Answer 15

Acetylcholine is released from the vesicles in the presynaptic knob
It then diffuses across the synaptic cleft where it binds with specific receptors in the postsynaptic membrane.
This triggers an action potential in the postsynaptic neurone or muscle cell.

Question 16

Acetylcholine Hydrolysis and Recycling:

Answer 16

Once an action potential has been triggered, acetylcholine is hydrolysed by a specific enzyme - acetylcholinesterase.
This enzyme is also situated on the postsynaptic membrane.
Acetylcholine is hydrolysed to give choline and ethanoic acid.
One molecule of acetylcholinesterase can break down around 25000 molecules of acetylcholine per minute.
The breakdown products are taken back into the presynaptic knob to be reformed into acetylcholine, and the postsynaptic membrane is ready to receive another impulse.

Question 17

diagram of Transmission across cholinergic synapses

Answer 17

Question 18

3 roles of synapses in the nervous system:

Answer 18

They ensure impulses are unidirectional.
As the neurotransmitter receptors are only present on the postsynaptic membrane, impulses can only travel from the presynaptic neurone to the postsynaptic neurone.

They can allow an impulse from one neurone to be transmitted to a number of neurones at multiple synapses.
This results in a single stimulus creating a number of simultaneous responses.

Alternatively, a number of neurones may feed in to the same synapse with a single postsynaptic neurone.
This results in stimuli from different receptors interacting to produce a single result.

Question 19

Summation and control:

Answer 19

Each stimulus from a presynaptic neurone causes the release of the same amount of neurotransmitter into the synapse.
In some synapses, however, the amount of neurotransmitter from a single impulse is not enough to trigger an action potential in the postsynaptic neurone, as the threshold level is not reached.
However, if the amount of neurotransmitter builds up sufficiently to reach the threshold then this will trigger an action potential.
This is known as summation.

Question 20

There are two ways summation can occur:

Answer 20

Spatial summation
Temporal summation

Question 21

Spatial summation

Answer 21

this occurs when a number of presynaptic neurones connect to one postsynaptic neurone.
Each releases neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone

Question 22

Temporal summation

Answer 22

this occurs when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period. - This builds up in the synapse until the quantity is sufficient to trigger an action potential

Question 23

Stimulation of the Nervous System by Drugs:

Answer 23

Many recreational and medical drugs cause their effects by acting on synapses.
This will result in the nervous system being stimulated or inhibited.
Drugs that stimulate the nervous system create more action potentials in postsynaptic neurones, resulting an enhanced response,
For example, if the targeted synapse is with a neurone that transmits an impulse from a sound receptor, the body will perceive a louder sound.

Question 24

Stimulation of the Nervous System by Drugs - These drugs may work by:

Answer 24

Mimicking the shape of the neurotransmitter - nicotine is the same shape as acetylcholine.
It can therefore bind to acetylcholine receptors on the postsynaptic membrane and trigger action potentials in the postsynaptic neurone.

Stimulating the release of more neurotransmitter.
For example, amphetamines.

Inhibiting the enzyme responsible for breaking down the neurotransmitter in the synapse.
For example, nerve gases stop acetylcholine being broken down.
This can result in a loss of muscle control.

Question 25

Inhibition of the Nervous System by Drugs:

Answer 25

Drugs that inhibit the nervous system create fewer action potentials in postsynaptic neurones, resulting in a reduced response.
For example, if the targeted synapse is with a neurone that transmits an impulse from a sound receptor, the body will perceive a quieter sound.

Question 26

Inhibition of the Nervous System by Drugs - these drugs may work by:

Answer 26

Blocking receptors - this means the neurotransmitter can no longer bind and activate the receptor.
For example, curare blocks acetylcholine receptors at neuromuscular junctions.
The muscle cells cannot therefore be stimulated, and the person suffers from paralysis.

Binding to specific receptors on the post-synaptic membrane of some neurones and changing the shape of the receptor such that binding of the neurotransmitter increases.
This therefore increases activity. An example of this is alcohol binding to GABA receptors.