Topic 6- Synaptic transmission Flashcards
what is a synapse?
-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 responses
what do synapses transmit?
information but not impulses
what do neurones transmit information from one to another by?
chemicals called neurotransmitters
what are neurones separated by?
-synaptic cleft
-20-30 nm wide
what is the presynaptic neurone?
the neurone that releases the neurotransmitter
what is the synaptic knob?
the axon of the presynaptic neurone ends in a swollen portion
|> known as the synaptic knob
what does the synaptic knob possess?
This possesses many mitochondria and large amounts of endoplasmic reticulum
-These are required to manufacture neurotransmitter-> which takes place in the axon
where is the neurotransmitter stored?
in synaptic vesicles
Once the neurotransmitter is released from the vesicles, what happens?
it diffuses across to the postsynaptic neurone
what does the postsynaptic neurone possess?
specific receptor proteins on its membrane to recieve the neurotransmitter
Explain how synapses result in unidirectional impulse
(Feature of unidirectionality)
-neurotransmitter only made/ released from pre-synaptic neurone
-receptors only on post- synaptic neurone
Explain the feature of summation
-addition of a number of impulses converging on a single post-synaptic neurone
-causing a rapid buildup of neurotransmitter
-so threshold more likely to be reached to generate an action potential
why is summation needed?
low frequency action potentials release insufficient neurotransmitter to exceed threshold
Describe spatial summation
-many pre-synaptic neurones share one synaptic cleft/ post- synaptic neurone
-collectively release sufficient neurotransmitter to reach threshold to trigger an action potential
2) Another description:
-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
Describe temporal summation
-one pre- synaptic neurone releases neurotransmitter many times over in a short time
-sufficient neurotransmitter to reach threshold to trigger an action potential
2)Another description:
-a single presynaptic neurone releases neurotransmitter many times over a very short period
-if concentration of neurotransmitter exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered.
Describe inhibition by inhibitory synapses
-some synapses-> make it less likely that a new action potential will be created in the postsynaptic neurone
-presynpatic neurone releases a neurotransmitter that binds to chloride ions channels on the postsynaptic neurone
-neurotransmitter causes chloride ion channels to open
-chloride ions move into the postsynaptic neurone by FD
-binding of neurotransmitter causes potassium ion channels to open-
-potassium ions move out by FD from postsynaptic neurone into the synapse
-inside of axon has a more engrave charge relative to the outside
-below resting potential
-membrane potential increases to -85 mv compared -65mv at resting potential
-called hyperpolarisation
-|> less likely that an action potential will be created because a larger influx of sodium ions is needed to produce one
for Na+ required to enter depolarisation
where is a chemical (neurotransmitter made)?
only in the presynaptic neurone and not in the postsynaptic neurone
where is the neurotransmitter stored?
synaptic vesicles
what happens to the vesicles containing the neurotransmitter when an action potential reaches the synaptic knob?
-when an AP reaches the synaptic knob, the membrane of these vesicles fuse with the pre-synaptic membrane to release the neurotransmitter
what happens when the neurotransmitter is released from the pre-synaptic membrane?
-when released, the neurotransmitter diffuses across the synaptic cleft to bind to specific receptor proteins which are found only on the postsynaptic neurone
what happens when the neurotransmitter binds with the receptor proteins on 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
what is a cholingeric synapse?
Synapses that use the neurotransmitter acetylcholine
what is acetylcholine made up of?
Two parts:
-acetyl (more precisely ethnic acid) and choline
where ar cholingeric synapses common?
-in vertebrates
-where they occur in the CNS and at neuromuscular junctions (between neurones and muscles)
Describe the transmission across a cholingeric synapse
1)Arrival of action potential at the end of the presynaptic neurone causes calcium ion channels to open
-Calcium ions enter the synaptic knob by FD
2)Influx of calcium ions into pre synaptic neurone-> causes synaptic vesicles to fuse with presynaptic membrane
|> releasing acetylcholine into the synaptic cleft
3)-ACh molecules diffuse across synaptic cleft
-quickly-> diffusion path is short
-ACh binds to receptor sites on sodium ion protein channels in membrane of postsynaptic neurone
-causes sodium ion protein channels to open, allowing sodium ions to diffuse in rapidly
4)Influx of sodium ions generates a new action potential (if threshold is met)-> depolarisation occurs in the postsynaptic neurone
what happens to acetylcholine after synaptic transmission?
-it is hydrolysed by acetylcholinesterase into choline and acetyl
-these diffuse back across synaptic cleft into presynaptic neurone (=recycling)
-the rapid breakdown of ACh prevents it from continuously generating a new action potential in the postsynaptic neurone
|> to stop depolarisation-> if not removed, would keep bidding to receptors, causing depolarisation
What is role of ATP in the synaptic knob?
-ATP released by mitochondria is used to recombine choline and ethanoic acid into ACh
-Sodium ion protein channels close in the absence of ACh in the receptor sites.
Use examples to explain the effect of drugs on a synapse
-some drugs stimulate the nervous system, leading to more action potentials e.g
-similar shape to neurotransmitter
-stimulate release of more neurotransmitter
-inhibit enzyme that breaks down neurotransmitter-> sodium continues to enter
-some drugs inhibit the nervous system, leading to fewer action potentials e.g
-inhibit the release of neurotransmitter e.g prevent opening of calcium ion channels
-block receptors by mimicking shape of neurotransmitter
