B2. Synaptic Transmission Flashcards
Synapses and neurotransmitters
A synapse is the ________between a neurone and another neurone, or between a _________and an ___________cell, e.g. a muscle or gland cell. The tiny gap between the cells at a synapse is called the __________ _____. The ______________neurone (the one before the synapse) has a swelling called a synaptic ______. This contains __________ ________filled with chemicals called ____________________
A synapse is the junction between a neurone and another neurone, or between a neurone and an effector cell, e.g. a muscle or gland cell. The tiny gap between the cells at a synapse is called the synaptic cleft. The presynaptic neurone (the one before the synapse) has a swelling called a synaptic knob. This contains synaptic vesicles filled with chemicals called neurotransmitters
Figure 1: The structure of a typical synapse.
Effect of an action potential
When an action potential reaches the ____of a neurone it causes ___________________to be released into the ____________ ______. They _________across to the _______________ ________________(the one after the synapse) and ______to ____________ __________. When ________________ _____to _____________they might trigger an _______potential (in a neurone), cause muscle _______________(in a muscle cell), or cause a ___________to be ___________(from a gland cell).
Because the _____________are only on the ______________ _____________, synapses make sure impulses are ________________the impulse can only travel in one direction. __________________ are removed from the ______so the ___________ ________ ______ _____________, e.g. they’re taken back into the ______________ __________or they’re broken down by __________(and the products are taken into the __________).
When an action potential reaches the end of a neurone it causes neurotransmitters to be released into the synaptic cleft. They diffuse across to the postsynaptic membrane (the one after the synapse) and bind to specific receptors. When neurotransmitters bind to receptors they might trigger an action potential (in a neurone), cause muscle contraction (in a muscle cell), or cause a hormone to be secreted (from a gland cell).
Because the receptors are only on the postsynaptic membranes, synapses make sure impulses are unidirectional the impulse can only travel in one direction. Neurotransmitters are removed from the cleft so the response doesn’t keep happening, e.g. they’re taken back into the presynaptic neurone or they’re broken down by enzymes (and the products are taken into the neurone).
Acetylcholine
There are many different neurotransmitters. You need to know about one called acetylcholine (ACh), which binds to ________________receptors. Synapses that use acetylcholine are called ______________synapses.
There are many different neurotransmitters. You need to know about one called acetylcholine (ACh), which binds to cholinergic receptors. Synapses that use acetylcholine are called cholinergic synapses.
Cholinergic synapses - 1. Arrival of an action potential
An action potential arrives at the ____________ _____of the ____________ ___________. The action potential _____________ ________-______ __________ ____ ____________in the ______________ ___________to _____. _____________ _____(__) _________into the __________ ____. (They’re pumped out afterwards by _________ ___________.)
An action potential arrives at the synaptic knob of the presynaptic neurone. The action potential stimulates voltage-gated calcium ion channels in the presynaptic neurone to open. Calcium ions (Ca) diffuse into the synaptic knob. (They’re pumped out afterwards by active transport.)
Tip: Voltage-gated ion channels are channels that only open when the ___________difference across a membrane reaches a certain voltage.
Tip: Voltage-gated ion channels are channels that only open when the potential difference across a membrane reaches a certain voltage.
Cholinergic synapses - 2. Fusion of the vesicles
The ________of __________ions into the ___________ _____causes the ________ ________to ______with the ______________ __________. The ___________release the neurotransmitter _______________(____) into the ___________ ______by ____________.
The influx of calcium ions into the synaptic knob causes the synaptic vesicles to fuse with the presynaptic membrane. The vesicles release the neurotransmitter acetylcholine (ACH) into the synaptic cleft by exocytosis.
Exam Tip “The ______of calcium ions…” means that the calcium ions have flowed _____the __________ _____. You’ll lose out on marks in the exam if you talk about an influx of calcium ions out of the synaptic knob.
Exam Tip “The influx of calcium ions…” means that the calcium ions have flowed into the synaptic knob. You’ll lose out on marks in the exam if you talk about an influx of calcium ions out of the synaptic knob.
Cholinergic synapses - 3. Diffusion of ACh
ACh _________across the __________ ______and ______to ____________ _____________ _____________on the _____________ _________. This causes _________ ____ ___________ in the _______________neurone to open. The ______of __________ ____into the _______________ ____________causes ________________. An _________ ___________on the _____________ ______________is generated if the threshold is reached. _____is removed from the ___________ _______so the ____________doesn’t keep happening. It’s broken down by an enzyme called ______________________ (_____) and the products are __-_____________by the _____________ ___________and used to make more _____.
ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane. This causes sodium ion channels in the postsynaptic neurone to open. The influx of sodium ions into the postsynaptic membrane causes depolarisation. An action potential on the postsynaptic membrane is generated if the threshold is reached. ACh is removed from the synaptic cleft so the response doesn’t keep happening. It’s broken down by an enzyme called acetylcholinesterase (AChE) and the products are re-absorbed by the presynaptic neurone and used to make more ACh.
Tip: Exocytosis is the process by which a __________inside a cell moves to the ____-__________ ___________, _________with the ____________and ____________its contents __________the cell.
Tip: Exocytosis is the process by which a vesicle inside a cell moves to the cell-surface membrane, fuses with the membrane and releases its contents outside the cell.
Excitatory and inhibitory neurotransmitters
Neurotransmitters can be ___________, ___________or _____. Excitatory neurotransmitters ________________the _______________ membrane, making it fire an _______ ___________if the _____________is reached.
Inhibitory neurotransmitters _____________the _____________
____________(make the potential difference more negative), _____________ it from _______an ________ ____________-.
A synapse where inhibitory neurotransmitters are released from the presynaptic membrane following an action potential is called an _________ ____________.
Neurotransmitters can be excitatory, inhibitory or both.
Excitatory neurotransmitters depolarise the postsynaptic membrane, making it fire an action potential if the threshold is reached.
Inhibitory neurotransmitters hyperpolarise the postsynaptic membrane (make the potential difference more negative), preventing it from firing an action potential.
A synapse where inhibitory neurotransmitters are released from the presynaptic membrane following an action potential is called an inhibitory synapse
Excitatory neurotransmitters example
Acetylcholine is an ___________neurotransmitter (it ________to ____________ ____________to cause an _________ ___________in the ____________ ____________) at cholinergic synapses in the _____ and at __________________ ___________
Acetylcholine is an excitatory neurotransmitter (it binds to cholinergic receptors to cause an action potential in the postsynaptic membrane) at cholinergic synapses in the CNS and at neuromuscular junctions
Inhibitory neurotransmitters example
- GABA is an inhibitory neurotransmitter - when it ________to its _____________it causes _______________ ____ _________to ______on the _______________ _____________, ________________the neurone.
- Acetylcholine is an inhibitory neurotransmitter at _____________synapses in the _______. When it ______to _____________here, it can cause ______________ ____ ___________ to ______on the _________________ ____________, _________________it.
- GABA is an inhibitory neurotransmitter - when it binds to its receptors it causes potassium ion channels to open on the postsynaptic membrane, hyperpolarising the neurone.
- Acetylcholine is an inhibitory neurotransmitter at cholinergic synapses in the heart. When it binds to receptors here, it can cause potassium ion channels to open on the postsynaptic membrane, hyperpolarising it.
Summation at synapses
If a stimulus is weak, only a small amount of neurotransmitter will be released from a neurone into the ____________ _______. This might not be enough to excite the _______________ ______________to the _____________level and ______________an ________ ___________. Summation is where the effect of neurotransmitters released from _____neurones (or one neurone that’s stimulated a lot in a short period of time) is added together. It means synapses ______________process information, finely tuning the response. There are two types of summation: _____________ __________________ and _____________ _________________
If a stimulus is weak, only a small amount of neurotransmitter will be released from a neurone into the synaptic cleft. This might not be enough to excite the postsynaptic membrane to the threshold level and stimulate an action potential. Summation is where the effect of neurotransmitters released from many neurones (or one neurone that’s stimulated a lot in a short period of time) is added together. It means synapses accurately process information, finely tuning the response. There are two types of summation: spatial summation and temporal summation.
- Spatial summation
Spatial summation is where two or more presynaptic neurones release their neurotransmitters at the same time onto the _____ ____________ __________. The small amount of neurotransmitter released from each of these neurones can be enough ______________to reach the ___________in the ______________ ________and trigger an ________ ___________
Spatial summation is where two or more presynaptic neurones release their neurotransmitters at the same time onto the same postsynaptic neurone. The small amount of neurotransmitter released from each of these neurones can be enough altogether to reach the threshold in the postsynaptic neurone and trigger an action potential
Figure 3: One presynaptic neurone only releases a few neurotransmitters (left) but three presynaptic neurones release enough to trigger an action potential (right).
If some neurones release an inhibitory neurotransmitter then the total effect of all the neurotransmitters might be __ ________ ____________
Figure 4: If some presynaptic neurones release inhibitory neurotransmitters, it might prevent an action potential from being triggered (right).
If some neurones release an inhibitory neurotransmitter then the total effect of all the neurotransmitters might be no action potential
- Temporal summation
Temporal summation is where _____or more _______impulses arrive in _______ _______________from the _______ ______________ _________. This makes an _________ ____________ more ________because more neurotransmitter is released into the ____________ ______
Figure 5: The effects of temporal summation at a synapse.
Temporal summation is where two or more nerve impulses arrive in quick succession from the same presynaptic neurone. This makes an action potential more likely because more neurotransmitter is released into the synaptic cleft
Figure 7: The structure of a neuromuscular junction.
Drugs at synapses
Drugs can affect synaptic transmission. They can do this in various ways. For example, some drugs are the _____ _______as neurotransmitters so they _____their action at ________(these drugs are called _________). This means more ___________are _________.
Example
Nicotine mimics acetylcholine so binds to nicotinic cholinergic ___________in the brain.
Drugs can affect synaptic transmission. They can do this in various ways. For example, some drugs are the same shape as neurotransmitters so they mimic their action at receptors (these drugs are called agonists). This means more receptors are activated.
Example
Nicotine mimics acetylcholine so binds to nicotinic cholinergic receptors in the brain.
Drugs at synapses
Some drugs block receptors so they can’t be activated by neurotransmitters (these drugs are called _____________). This means ________receptors (if any) can be _____________.
Example
Curare blocks the effects of acetylcholine by blocking nicotinic cholinergic receptors at neuromuscular junctions, so muscle cells can’t be stimulated. This results in the muscle being _____________.
Some drugs block receptors so they can’t be activated by neurotransmitters (these drugs are called antagonists). This means fewer receptors (if any) can be activated.
Example
Curare blocks the effects of acetylcholine by blocking nicotinic cholinergic receptors at neuromuscular junctions, so muscle cells can’t be stimulated. This results in the muscle being paralysed.
Drugs at synapses
Some drugs inhibit the enzyme that _______ ______ neurotransmitters (they stop it from working). This means there are more neurotransmitters in the synaptic cleft to _____to ___________and they’re there for ________.
Example
Nerve gases stop acetylcholine from being broken down in the synaptic cleft. This can lead to loss of muscle ________.
Some drugs inhibit the enzyme that breaks down neurotransmitters (they stop it from working). This means there are more neurotransmitters in the synaptic cleft to bind to receptors and they’re there for longer.
Example
Nerve gases stop acetylcholine from being broken down in the synaptic cleft. This can lead to loss of muscle control.
Drugs at synapses
Some drugs stimulate the release of neurotransmitter from the ____________ __________so more ____________are ____________.
Example
Amphetamines ______a neurotransmitter called dopamine out of _________ ____________and into the __________ _______. This increases the effect of dopamine, e.g. It increases alertness.
Some drugs stimulate the release of neurotransmitter from the presynaptic neurone so more receptors are activated.
Example
Amphetamines force a neurotransmitter called dopamine out of synaptic vesicles and into the synaptic cleft. This increases the effect of dopamine, e.g. It increases alertness.
Drugs at synapses
Some drugs inhibit the release of neurotransmitters from the _____________ __________so fewer receptors are activated.
- Example
Opioids block __________ ____ _________in the ___________ ________. This means fewer _________ _____with the ____________ ________so ____neurotransmitter is released.
Some drugs inhibit the release of neurotransmitters from the presynaptic neurone so fewer receptors are activated.
- Example
Opioids block calcium ion channels in the presynaptic neurone. This means fewer vesicles fuse with the presynaptic membrane so less neurotransmitter is released.
