3.6.2.2 Synaptic transmission (A-level only) Flashcards
Define synapse.
= Junction between a neurone and another neurone or an effector cell (muscle / gland cell).
NB => tiny gap between cells at a synapse is called a synaptic cleft.
Outline the structures at a synapse.
Presynaptic neurone has a swelling called a synaptic knob - contains synaptic vesicles containing neurotransmitters.
Postsynaptic neurone has receptors specific to the neurotransmitter - receptors are only on the postsynaptic membrane so impulses are unidirectional as they can only travel in one direction.
Outline the general way an action potential is transmitted across a synapse.
- Action potential reaches end of a neurone, causes neurotransmitters to be released into synaptic cleft, which diffuse across to the postsynaptic membrane and bind to specific receptors.
- Trigger an action potential in a neurone / cause muscle contraction in a muscle cell / cause a hormone to be secreted (in a gland cell).
- Neurotransmitters removed from the cleft so response doesn’t keep happening —> taken into presynaptic neurone or broken down by enzymes (and products taken into neurone).
What is a cholinergic synapse?
= A synapse which uses acetylcholine (ACh) as its neurotransmitter.
How does ACh transmit the nerve impulse across a cholinergic synapse?
- Action potential arrives at synaptic knob of the presynaptic neurone.
- Action potential stimulates voltage-gated Calcium ion channels in the presynaptic neurone to open.
- Calcium ions diffuse into the synaptic knob (later removed by active transport).
- Influx of Ca2+ into synaptic knob causes the synaptic vesicles to move to the presynaptic membrane, which they then fuse with.
- Vesicles release ACh by exocytosis into synaptic cleft.
- ACh diffuses across synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane.
- Causes Na+ channels in the postsynaptic neurone to open.
- Influx of Na+ into the postsynaptic membrane causes depolarization —> 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 - broken down by acetylcholinesterase (AChE) and products (ethanoic acid and choline) reabsorbed by presynaptic neurone and used to make more ACh.
Comment on the excitatory nature of some neurotransmitters?
- Some are excitatory - depolarise the postsynaptic membrane, making it fire an action potential if the threshold is reached. ACh is an excitatory neurotransmitter at cholinergic synapses in the CNS.
Comment on the inhibitory nature of some neurotransmitters.
- Some are inhibitory - hyperpolarize the postsynaptic membrane (make it even more negative) - preventing it from firing an action potential. ACh is an inhibitory neurotransmitter at cholinergic synapses in the heart - can cause K+ channels to open on the postsynaptic membrane, hyperpolarizing it.
What is the significance of summation?
= Finely tunes the nervous response.
- If a stimulus is weak, only a small amount of neurotransmitter will be released from a neurone into the synaptic cleft —> may not be enough to excite the postsynaptic membrane to the threshold level and stimulate an action potential.
- –> important as instead of a number of neurones with weak generator potentials - we can add them together to produce one gp above the threshold level.
Define summation.
= When one postsynaptic neuron is excited/inhibited by more than one presynaptic neuron. Thus several neurons converge and release their neurotransmitters towards one neuron.
How does spatial summation work?
- Sometimes many neurones connect to one postsynaptic neurone.
- 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.
- Some neurones may release an inhibitory neurotransmitter —> total effect of all neurotransmitters might be no action potential
How does temporal summation work?
- 2 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.
=> both types of summation ensure synapses accurately process information, finely tuning the response.
Define neuromuscular junction (NMJ).
= A synapse between a motor neurone and a muscle cell.
How is a nerve impulse transmitted across a NMJ?
- NMJs use ACh, which binds to cholinergic receptors called nicotinic cholinergic receptors.
- Differences between normal cholinergic synapses and NMJs:
- Postsynaptic membrane has many folds that form clefts —> store AChE.
- Postsynaptic membrane has more receptors than other synapses.
- ACh is always excitatory at an NMJ —> when a motor neurone fires an action potential, normally triggers a response in a muscle cell
- –> not always the case for a synapse between two neurones.
How do agonist drugs affect the transmission of a nerve impulse across a synapse?
Agonist drugs are the same shape as neurotransmitters so they mimic their action at receptors.
=> more receptors activated.
=> Nicotine mimics acetylcholine so binds to nicotinic cholinergic receptors in the brain
How do antagonist drugs affect the transmission of a nerve impulse across a synapse?
Antagonist drugs block receptors so they can’t be activated by neurotransmitters.
=> fewer receptors activated (if any).
=> muscle cells may not be stimulated (if at NMJs) —> muscle paralysed.
