Synaptic Transmission Flashcards
(8 cards)
What is a synapse?
Synapses are junctions between two neurons or between a neuron and a muscle, in which case it’s called a neuromuscular junction. These are found at the ends of axons, and most synapses outside the CNS use acetylcholine.
What is the process of transmission across a cholinergic synapse?
1) An action potential arrives at the synpatic knob of the presynaptic neuron.
2) This stimulates the Calcium (Ca2+) channels to open meaning the Ca2+ diffuses into the synapse.
3) Ca2+ increases in the synapse, the synaptic vesicles move to the presynaptic membrane and fuse with it, releasing acetylcholine with the synaptic cleft, known as exocytosis.
4) The acetylcholine diffuses across the synaptic clef and binds to receptors on the postsynaptic membrane.
5) This causes the Na+ channels to open so the Na+ diffuses into the postsynaptic neuron, this is known as depolarisation.
If the threshold is reached, a new action potential is triggered.
What is meant by the unidirectionality of synapses?
The impulse can only travel in one direction because receptors are only found on the postsynaptic membrane. Neurotransmitters can only be made and released from the presynaptic neuron’s side.
What is spatial summation?
When a number of presynaptic neurons connect to a single postsynaptic neuron. Together they release enough neurotransmitters to exceed the threshold value in the postsynaptic neuron and trigger an action potential.
What is temporal summation?
When a single postsynaptic neuron releases neurotransmitters many times over a short period of time. If the total amount is larger than the threshold value of the postsynaptic neuron, a new action potential is triggered.
What is meant by inhibition?
Some synapses make it less likely that a new action potential will be triggered in the postsynaptic neuron. Neurotransmitters may bind to Cl channels, causing them to open, allowing Cl to diffuse into the postsynaptic neuron which may cause K+ channels to open so K+ diffuses out making the membrane potential more negative.
Describe differences between cholinergic synapses and neuromuscular junctions.
CHLONERGIC SYNAPSE
Connects a neuron to another neuron. Neurons may be motor, sensory, or relay. May be excitatory or inhibitory. A new AP may be triggered at the postsynaptic neuron. Acetylcholine binds to receptors on membrane of postsynaptic neuron. Postsynaptic membrane has less receptors. Build-up of postsynaptic potential can lead to an action potential.
NEUROMUSCULAR JUNCTION
Connects a motor neuron to a bundle of muscle fibres. Only involves motor neurons. Only excitatory. Action potential only ends here. Acetylcholine binds to receptors on membrane of muscle fibre. Postsynaptic membrane has more receptors. A wave of depolarisation that spreads along the muscle fibre.
What are the effects of drugs on synapses?
- When drugs are a similar shape to a type of neurotransmitter, it can bind to receptors and trigger Na+ channels to open, triggering more action potentials.
- Act as blockers which prevents neurotransmitters from binding so fewer Na+ channels open so fewer action potentials are triggered.
- They can also inhibit enzymes that break down neutrotransmitters so more neutrotransmitter remains in the synaptic cleft that can bind to receptors so more action potentials can continue to be triggered.
