6.2.2 Synaptic transmission

Question 1

Structure of a neuromuscular junction.

Answer 1

Presynaptic membrane = synaptic knob; axon terminal of a motor neurone
Postsynaptic membrane = sarcolemma
Synaptic knob contains many mitochondria, and vesicles of acetylcholine.
Space between pre and post synaptic membranes = synaptic gap (or cleft).
Sarcolemma has ligand-gated sodium ion channels - these channels have acetylcholine receptors on them.

Question 2

Structure of a synapse.

Answer 2

The synapse includes both the presynaptic and post synaptic membranes, and the space between them (the synaptic cleft). The presynaptic cell contains vesicles of neurotransmitter, the post-synaptic membrane is embedded with ligand-gated sodium ion channels, which have receptor on them that are complementary to the neurotransmitters. Both cells contain many mitochondria.

Question 3

The sequence of events involved in transmission across a cholinergic synapse.

Answer 3

Depolarisation of presynaptic membrane by an action potential causes voltage-gated calcium ion channels to open and calcium ions enter to enter the synaptic knob by facilitated diffusion.

The influx of calcium ions causes synaptic vesicles to move toward and then fuse with the presynaptic membrane, releasing acetylcholine neurotransmitter into the synaptic cleft.

Acetylcholine diffuses across the synaptic cleft) and bind to receptors on the postsynaptic membrane, causing the opening of the ligand-gated sodium ion channels. Sodium ions enter the postsynaptic neurone by facilitated diffusion, leading to depolarisation.

If depolarisation is above threshold potential an action potential is generated.

Acetylcholine is broken down by acetylcholinesterase, a hydrolytic enzyme in the synaptic cleft.

Question 4

Unidirectionality

Answer 4

Synapses ensure that nerve impulses only travel in one direction, because neurotransmitter is only released from the pre-synaptic neurone, and neuroreceptors are only found on the post-synaptic membrane.

Question 5

Summation

Answer 5

Summation is the adding together of generator potentials. It allows a number of smaller depolarisations to summate to reach threshold potential.

There are two types: temporal (time) and spatial (space).

Temporal:
Not all generator potentials reach threshold. If they don’t, the sodium-potassium pump will soon restore resting potential. However if a number if successive action potentials in the presynaptic membrane lead to a succession of releases of neurotransmitter (ie several vesicles fusing one after the other), the postsynaptic membrane potential can rise with each wave of neurotransmitter faster than the sodium-potassium pump can restore resting potential, eventually allowing threshold to be reached. Threshold was reached because several action potentials occurred in a short time - hence temporal summation.

Spatial:
Several neurones can synapse with the same postsynaptic membrane. Although each on it’s own might not release enough neurotransmitter to trigger an action potential in the postsynaptic cell, the combined depolarisation from all three might.

The All or Nothing principle still stands, but summation allows for example:
Filtering out unimportant stimuli
Detection of very weak stimuli
Integration from multiple neurones (important in learning)

Question 6

Inhibitory synapses

Answer 6

Inhibitory synapses result in the postsynaptic membrane potential becoming lower than the resting potential (hyperpolarisation), so that more even sodium ions are required to reach threshold. Often they have ligand gated chloride ion channels on the postsynaptic membrane, so that when their neurotransmitter binds to the receptor, negatively charged chloride ions enter (as opposed to positively charged sodium ions). Sometime they open potassium channels, causing potassium ions to leave, which has the same effect.

Question 7

Differences between transmission across a cholinergic synapse and
across a neuromuscular junction.

Answer 7

Neurone to neurone / neurone to muscle.
Action potential in neurone / no action potential in sarcolemma.
No summation in muscle.
Muscle response always excitatory (never inhibitory).
Some neuromuscular junctions have different neurotransmitters (cholinergic always acetylcholine).