Synaptic transmission

Question 1

What is a synapse?

Answer 1

• The point where one neurone communicates with another or an effector

Question 2

What is the structure of a synapse?

Answer 2

• synapses transmit information but not impulses from one neurone to another via chemical messengers called neurotransmitters
• neurones are separated by a small gap called the synaptic cleft
• neurotransmitters are stored in synaptic vesicles
• once the neurotransmitter is released from the vesicles, it diffuses across the synaptic cleft to the postsynaptic neurone

Question 3

What are the three features of a synapse?

Answer 3

• unidirectionality
• summation
• inhibition

Question 4

What is unidirectionality?

Answer 4

• synapses can only pass in one direction from the presynaptic neurone to the postsynaptic neurone

Question 5

What is summation?

Answer 5

• low frequency action potentials often lead to the release of insufficient concentrations of neurotransmitters to trigger a new action potential in the postsynaptic neurone
• they are able to release sufficient concentrations of neurotransmitters through a process called summation
• This can cause a rapid build up of neurotransmitters in the synapse in two methods (spatial and temporal)

Question 6

What are the two types of summation?

Answer 6

• spatial summation = This is when a number of presynaptic neurones together release enough neurotransmitters collectively to a postsynaptic neurone to meet the threshold value and cause an action potential
• temporal summation = A single presynaptic neurone releases many neurotransmitters over a short time period. If the neurotransmitters exceed the threshold value of the post synaptic neurone , then an action potential is triggered

Question 7

What is inhibition?

Answer 7

• Some synapses make it less likely that a new action potential will be created on the postsynaptc neurone. These are known as inhibitory synapses

Question 8

What are the steps of inhibitory synapses?

Answer 8

1. The presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channels on the post - synaptic neurone
2. The neurotransmitter causes the chloride ion protein channels to open
3. The chloride ions move into the postsynaptic neurone by facillitated diffusion
4. The binding of the neurotransmitter causes the opening of nearby potassium K+ channels
5. Potassium ions move out of the postsynaptic neurone into the synapse
6. The combined effect of negatively charged chloride ions moving in to the postsynaptic neurone and positively charged potassium ions moving out of the postsynaptic neurone into the synapse makes the inside more negative than the outside
7. The membrane potential increases to -80 compared to usual -65, this is called hyperpolarisation and makes it less likely for another action potential to occur as a large influx of sodium ions would be required to produce a new one

Question 9

What is the function of synapses

Answer 9

• they allow a single impulse along a neurone to initiate a new impulse in a number of different neurones of a synapse
• A number of impulses can be combined at a synapse which allows nerve impulses from receptors reacting to different stimuli to contribute to a single response

Question 10

What is a chollnergic synapse

Answer 10

• A synapse in which the neurotransmitter is called acetylcholine
• acetylcholine is a neurotransmitter made up of two parts which are acetyl (ethanoic acid) and choline
• cholinergic synapses are common in vertebrates

Question 11

What are the steps of cholinergic transmission

Answer 11

1. the arrival of an action potential at the end of the presynaptic neurone causes calcium ion protein channels to open and calcium ions to enter the synaptic knob by facillitated diffusion
2. The influx of calcium ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft
3. acetylcholine molecules diffuse across a narrow synaptic cleft very quickly as the diffusion pathway is short. Acetylcholine then binds to the receptor sites on sodium ion protein channels in the membrane of the postsynaptic neurone. This causes the sodium ion protein channels to open, allowing sodium ions (Na+) to diffuse in rapidly along a concentration gradient
4. The influx of sodium ions generates an action potential in the postsynaptic neurone
5. Acetylcholineesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl) which diffuses back across the synaptic cleft into the presynaptic neurone (recycling). In addition to recylcling the choline and ethanoic acid, the rapid breakdown of acetylcholine also prevents it from continously generating a new action potential in the postsynaptic neurone and so leads to discrete transfer of information across synapses
6. ATP released by the mitochondira is used to recombine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use. Sodium ion protein channels close in the absence of acetylcholine in the receptor sites

Question 12

What are the two effects that drugs have on synapses?

Answer 12

1. They stimulate the nervous system by creating more action potentials in postsynptic neurones. A drug may do this by mimicking neurotransmitters, stimulating the release of more neurotransmitter or inhibiting the enzyme that breaks down the neurotransmitter. This enhances the bodys response to impulses passed along the postsynaptic neurone
2. They inhibit the nervous system by creating fewer action potentials in postsynaptic neurones. A drug may do this by inhibiting the release of neurotransmitters or blocking receptos on sodium/potassium ion channels on the postsynaptic neurone. The outcome is to reduce the impulses passed along the postsynaptic neurone