Structure & function of synapses

Question 1

What is a synapse?

Answer 1

• a synapse is the point where one neurone communicates with another or with an effector
• they are important in linking different neurones together and therefore coordinating activities

Question 2

What is the structure of a synapse?

Answer 2

• synapses transmit information, but not impulses, from one neurone to another by means of chemicals known as neurotransmitters
• neurons are separated by a small gap, called the synaptic cleft, which is 20-30nm wide
• the neurone that releases the neurotransmitter is called the presynaptic neurone
• the axon or this neurone ends in a swollen portion known as the synaptic knob
• this possesses many mitochondria and large amounts of endoplasmic reticulum
• these are required in the manufacture of the neurotransmitter which takes place in the axon
• the neurotransmitter is stored in the synaptic vehicles
• once the neurotransmitter is released from the vehicles it diffuses across to the postsynaptic neurone, which possesses specific receptor proteins on its membrane to receive it

Question 3

What are the features of the synapses?

Answer 3

-the basic way in which synapses function means they have a number of different features

Question 4

What is unidirectionality?

Answer 4

• synapses can only pass information in one-direction from the presynaptic neurone to the postsynaptic neurone
• in this way, synapses act like valves

Question 5

What is summation?

Answer 5

• low-frequency action potentials often lead to the release of insufficient concentrations of neurotransmitter to trigger a new action potential in the postsynaptic neurone
• they can however do so in a process called summation
• this entails a rapid build-up of neurotransmitter in the synapse

Question 6

What are the types of summation?

Answer 6

• spatial summation

- temporal summation

Question 7

What is spatial summation?

Answer 7

• spatial summation, in which 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

Question 8

What is temporal summation?

Answer 8

• temporal summation, in which a single presynaptic neurone releases neurotransmitter many times over a very short period
• if the concentration of neurotransmitter exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered

Question 9

What is the process of spatial summation?

Answer 9

• neurone A releases neurotransmitter but concentration is below threshold to trigger action potential in postsynaptic neurone
• Neurone B releases neurotransmitter but concentration is below threshold to trigger action potential in postsynaptic neurone
• Neurone A and B release neurotransmitter, concentration is above threshold and so action potential is triggered in the postsynaptic neurone
• low-frequency action potentials lead to neurotransmitter being broken down rapidly, concentration is below the threshold to trigger an action potential in the postsynaptic neurone
• high-frequency action potentials lead to release of neurotransmitter in a short time, concentration is above the threshold to trigger an action potential in the postsynaptic neurone

Question 10

What is inhibition?

Answer 10

-some synapses make it less likely that a new action potential will be created on the postsynaptic neurone known as inhibitory synapses

Question 11

How does the inhibitory synapse operate?

Answer 11

• the presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channels on the postsynaptic neurone
• the neurotransmitter causes the chloride ion protein channels to open
• chloride ions (CL-) move into the. postsynaptic neurone by facilitated diffusion
• the binding of the neurotransmitter causes the opening of nearby potassium (k+) protein channels
• potassium ions move out of the postsynaptic 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 postsynaptic membrane more negatively and the outside more positive
• the membrane potential increases to as much as -80mV compared with the usual -654mV 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 12

What are functions of synapses?

Answer 12

• synapses transmit information from one neurone to another

- in doing so they act as junctions

Question 13

Why is the fact that synapses act as junctions important?

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 combines at a synapse, this allows nerve imposes from receptors reacting to different stimuli to contribute to a single response

Question 14

What do you need to understand the basic functioning of synapses?

Answer 14

• when released, the neurotransmitter diffuses across the synaptic cleft to bind to specific receptor proteins which are found only one the postsynaptic neurone
• the neurotransmitter binds with the receptor proteins and this leads to a new action potential in the postsynaptic neurone, synapses that produce new action potentials in this way are called excitatory synapses