What do nerve junctions involve?
- Nerve junctions often involve several neurones.
- This could be several neurones from different places converging on one neurone.
- Or, one neurone sending signals out to several neurones that diverge to different effectors.
What causes an EPSP and what does it stand for?
- When one action potential passes down an axon to the synapse, it will cause a few vesicles to move to, and fuse with, the pre-synaptic membrane.
- This relatively small number of acetylcholine molecules diffusing across the cleft produces a small depolarisation.
- This is an excitatory post-synaptic potential.
What is summation?
- The effects of several EPSPs combine to increase the membrane depolarisation until it reaches the threshold.
- This combined effect is known as summation.
What is temporal summation?
Summation resulting from several action potentials in the same presynaptic neurone.
What is spatial summation?
Summation resulting from action potentials arriving from several different pre-synaptic neurones.
What is an IPSP and what do they do?
- An inhibitory post-synaptic potential.
- They reduce the effect of summation and prevent action potentials in the post synaptic neurone.
How can an IPSP be achieved?
- By opening chloride ion channels that allow chloride ions into the post-synaptic neurone.
- Or, by opening potassium ion channels that allow potassium ions out of the cell.
- In both cases, temporary hyperpolarisation is produced.
Describe in more detail what spatial summation consists of.
- Several pre-synaptic neurones converge to one post-synaptic neurone.
- This allows action potentials from different parts of the nervous system to contribute to generating an action potential in one post-synaptic neurone- so creating a particular response.
Give an example of when spatial summation could be useful.
When several different stimuli are warning is of danger.
How can the combination of several EPSPs be prevented from creating an action potential?
One IPSP.
One pre-synaptic neurone might diverge to several post-synaptic neurones. When might this be useful?
- This allows one action potential to be transmitted to several parts of the nervous system.
- This is useful in a reflex arc, one post-synaptic neurone elicits the response, while another informs the brain.
How do synapses ensure that action potentials are only transmitted in the correct direction?
-Only the pre-synaptic bulb contains vesicles of acetylcholine.
How can synapses filter out unwanted low-level stimulus?
Several vesicles of acetylcholine must be released to create an action potential in the post-synaptic neurone.
How can low-level action potentials be amplified by summation?
- If a low-level stimulus is persistent it will generate several successful action potentials in the pre-synaptic neurone.
- The release of many vesicles pf acetylcholine over a short period of time will enable the post-synaptic EPSPs to combine together to produce an action potential.
When is a synapse said to be fatigued and what does this cause?
- After repeated stimulation, a synapse may run out of vesicles containing acetylcholine.
- The synapse is said to be fatigued.
- This means that the nervous system no longer responds to the stimulus.
- We have become habituated to it.
Give examples of when synapses are fatigued so we become habituated to a stimuli and explain why this could be useful.
- Getting used to a smell or background noise.
- It may help to avoid overstimulation of an effector which could cause damage.
What is the creation and strengthening of specific pathways within the nervous system thought to be the basis of?
Conscious thought and memory.
Synaptic membranes are adaptable, how can a particular post-synaptic membrane be made more likely to fire an action potential?
-The post-synaptic membrane can be made more sensitive to acetylcholine by the addition of more receptors.
