4. Nerves and Synapses Flashcards
Describe the release, action and inactivation of acetylcholine as a typical neurotransmitter.
Ach will be released across a synapse if Ca2+ floods into the cell from voltage gated channels being opened as a result of depolarisation of the presynaptic axon. The vesicles fuse with the membrane of the presynaptic neuron and Ach enters the synaptic cleft.
Ach will then cross the cleft and bind with Ach receptors on the post synaptic neuron where each receptor requires two molecules of Ach in order to cause any effect. After receiving the Ach, the sodium and potassium channels are opened to cause a post synaptic potential to occur.
After the brief presence of Ach has accomplished its role, it is rapidly cleared by the enzyme acetylcholinesterase which acts to hydrolyse the molecule.
What are the differences between pre- and post-synaptic potentials?
A pre-synaptic potential is a full action potential featuring the achievement of a threshold potential and full depolarisation followed by hyperpolarisation.
A post-synaptic potential however is the result of pre-synaptic depolarisation leading to NT release. This potential rises quickly to below threshold, but then declines much slower (proportionally to the rate of NT clearance). Thus, they have a much lower amplitude, but are longer in duration. Generally multiple PostSP will combine to cause an AP (summation)
Why is it important that there is a DIFFERENCE between the two (pre- and post-synaptic potentials)?
- Basically if there was no difference between the two, any and every stimulus moving as a presynaptic potential would cause the same response in a post synaptic potential and there would be no way to mediate the intensity of stimuli in the body.
- Additionally I would imagine this would make every aspect of the body hyperactive as every neuron responds to EVERY stimulus it is presented immediately.
What is summation; how does it work?
Summation is the combination of multiple postSP’s in order to reach threshold and cause a full action potential to occur in that neuron; can also be seen as the collective effect of NT’s from multiple presynaptic potentials.
Describe temporal summation
Occurs when two graded potentials from one presynaptic neuron occur close together in time (temporal = in time)
Describe spatial summation
Occurs as a result of multiple (or singular) nerve impulses from multiple presynaptic axons to the same postsynaptic axon.
What is the difference between excitatory and inhibitory synapses? Provide examples for both and explain what happens at the post-synaptic membrane
Firstly, let us refer to them by the names that the great Phil of Physiology has deemed them;
Excitatory synapses are called depolarising (excitatory) postsynaptic potentials (EPSP). An example would be Ach at the neuromuscular junction.
Inhibitory synapses are called inhibitory post synaptic potentials (IPSP). An example would be Ach at a junction with cardiac muscle.
Additionally, it is worth understanding that the difference exists between these two lies in two factors; firstly the neurotransmitter that is released across the cleft, and secondly the receptor picking up that NT on the postsynaptic membrane.
Essentially however, an IPSP will cause an increase in K+ efflux or Cl- influx to keep the interior of the cell more negative, while an EPSP will cause an increase Na+ influx or decreased K+ efflux to make the cell interior more positive.
The result of all this is that AP occurrence depends on the addition of different impulses from different synapses and their timings.
What is pre-synaptic inhibition?
This is when inhibitory fibres connect directly to a terminal button of excitatory fibres to cause hyperpolarisation of the excitatory fibre. This is a very selective mechanism; super specific as opposed to postsynaptic inhibition which is much more delocalised.
Why is pre-synaptic inhibition regarded as being selective and post-synaptic inhibition, non-selective?
Presynaptic inhibition is selective because it targets a specific presynaptic axon terminal button that is synapsing onto a post synaptic cell. Post synaptic inhibition is non-selective because it will just hyperpolarise the post-synaptic cell itself, therefore making any source of depolarisation less likely to result in an action potential.
Think of it this way @Alex Moore, presynaptic is closing off a road or locking a door - it’s selectively inhibiting that specific avenue of synapse; while post synaptic is making ALL doors and ALL roads harder to use. Presynaptic does not affect other avenues of approach, while postsynaptic indiscriminately inhibits all synapses onto the cell
