Structure And Function Of Synapses

Question 1

What is a synapse

Answer 1

Where one neurone communicates with another or with an effector

Question 2

Be able to label a chemical synapse

Answer 2

Look on p360 in book

Question 3

How do synapses transmit information

Answer 3

By means of chemicals known as neurotransmitters

Question 4

What’s the small gap called that separates the neurones

Answer 4

The synaptic cleft

Question 5

What is the neurone that releases the neurotransmitter

Answer 5

Presynaptic neurone

Question 6

What’s the swollen end of the neurone called. What’s in it

Answer 6

Synaptic knob

Many mitochondria and endoplasmic reticulum - required for manufacture of the neurotransmitter which takes place in the axon

Question 7

Where is the neurotransmitter stored

Answer 7

Synaptic vesicles

Question 8

How does the postsynaptic neurone recurve the neurotransmitter once it’s released from the vesicles

Answer 8

It posses specific receptor proteins to receive it on its membrane

Question 9

Synapses are unidirectionality. What does this mean

Answer 9

They can only pass info in one direction - from the presynaptic neurone to the postsynaptic neurone

Question 10

What are the two methods of summation

Answer 10

Spatial summation - a number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone. Together they therefore trigger a new action potential

Temporal summation - a single presynaptic neurone releases neurotransmitter many times over a short period. It the concentration of neurotransmitter exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered

Question 11

Why does summation take place

Answer 11

When low frequency action potentials often lead to the release of insufficient concentrations of neurotransmitter to trigger a new action potential in the postsynaptic neurone

Question 12

Describe how inhibitory synapses operate

Answer 12

The presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channel on the postsynaptic neurone

The neurotransmitter causes the chloride ion protein channels to open

Chloride ions move into the postsynaptic neurone by facilitated diffusion

The binding of the neurotransmitter causes the opening of nearby potassium protein channels

Potassium ions move out of the post synaptic neurone into the synapse

The combined effect of negatively charged chloride ions moving in and positively charged potassium ions moving out is to make the inside of the post synaptic membrane more negative and the outside more positive

The membrane potential increases to as much as -80mV compared with the usual -65mV at resting potential

This is called hyperpolarisation and makes it less likely that a new action potential will be created because a larger influx of sodium ions is needed to produce one

Question 13

Synapses transmit info from one neurone to another. In doing so they act as junctions. What does this allow?

Answer 13

A single impulse along one neurone to initiate new impulses in a number of different neurones at a synapse. This allows a single stimulus to create a number of simultaneous responses

A number of impulses to be combined at a synapse. This allows nerve impulses from receptors reacting to different stimuli to contribute a single response

Question 14

What are excitatory synapses

Answer 14

Synapses that produce new action potentials

Question 15

Explain how a presynaptic neurone is adapted for the manufacture of neurotransmitter

Answer 15

It posses many mitochondria and large amounts of endoplasmic reticulum

Question 16

Explain how a postsynaptic neurone is adapted for the manufacture of neurotransmitter

Answer 16

It has a receptor molecules for neurotransmitters

Question 17

If a neurone is stimulated in the middle of its axon, an action potential will pass both ways along it to the synapses at each end of the neurone. However, the action potential will only pass across the synapse at one end. Explain why

Answer 17

Only one end can produce neurotransmitter and so this end alone can create a new action potential in the neurone on the opposite side of the synapse. At the other end there is no neurotransmitter that can be released to pass across the synapse and so no new action potential can be set up

Question 18

When walking along a street we barely notice the background noise of traffic. However we often respond to louder traffic noises, such as the sounds of a horn. From your knowledge of summation, explain this difference

Answer 18

The relatively quiet background noise of traffic produces a low-level frequency of action potentials in the sensory neurons from the Ear. The Amount of neurotransmitter released into the synapse is insufficient to exceed the threshold in the post synaptic neurone and to trigger an action potential and so noise is ignored.

Louder noises create a higher frequency and the amount of neurotransmitter releases is sufficient to trigger an action potential in the post synaptic neurone and so there is a response. (Temporal summation)

Question 19

When walking along a street we barely notice the background noise of traffic. However we often respond to louder traffic noises, such as the sounds of a horn. Suggest an advantage in responding to high level stimuli but not to low level ones

Answer 19

Reacting to low level stimuli that present little danger can overload the nervous system and so organisms may fail to respond to important stimuli. High level stimuli need a response because they are more likely to represent danger

Question 20

Explain why hyperpolarisation reduces the likelihood of a new action potential being created

Answer 20

As the inside of the membrane is more negative than at resting potential, more sodium ions must enter in order to reach the potential differnde of an action potential. It is more difficult for depolarisation to occur. Stimulation is less likely to reach the threshold level needed for a new action potential