5.3.9 Transmission Across a Cholinergic Synapse

Question 1

Explain transmission across a cholinergic synapse.

Answer 1

There are over 40 different known neurotransmitters

One of the key neurotransmitters used throughout the nervous system is acetylcholine (ACh)

Synapses that use the neurotransmitter ACh are known as cholinergic synapses

The detailed process of synaptic transmission using ACh is as follows:

The arrival of an action potential at the presynaptic membrane causes depolarisation of the membrane

This stimulates voltage-gated calcium ion channel proteins to open

Calcium ions (Ca2+) diffuse down an electrochemical gradient from the tissue fluid surrounding the synapse (high concentration of calcium ions) into the synaptic knob (low concentration of calcium ions)

This stimulates ACh-containing vesicles to fuse with the presynaptic membrane, releasing ACh molecules into the synaptic cleft by exocytosis

The ACh molecules diffuse across the synaptic cleft and temporarily bind to cholinergic receptors in the postsynaptic membrane

This causes sodium ion channels to open

Sodium ions to diffuse down an electrochemical gradient into the cytoplasm of the postsynaptic neurone

The sodium ions cause depolarisation of the postsynaptic membrane, re-starting the electrical impulse once the threshold is reached

The ACh molecules are broken down and recycled

This prevents the sodium ion channels staying permanently open and stops permanent depolarisation of the postsynaptic membrane,

The enzyme acetylcholinesterase catalyses the hydrolysis of the ACh molecules into acetate and choline

The choline is absorbed back into the presynaptic membrane and reacts with acetyl coenzyme A to form ACh, which is then packaged into presynaptic vesicles ready to be used when another action potential arrives

Question 2

Why is unidirectionality a key feature of a synapse?

Answer 2

Synapses ensure the one-way transmission of impulses

Impulses can only pass in one direction at synapses because neurotransmitter is released on one side and its receptors are on the other – chemical transmission cannot occur in the opposite direction

This prevents impulses from travelling the wrong way, back to where they were initiated

Question 3

Why is summation a key feature of a synapse?

Answer 3

Sometimes, a single impulse that arrives at a synaptic knob is insufficient to generate an action potential in the post-synaptic neurone because:

Only a small amount of acetylcholine is released into the synaptic cleft

This means only a small number of the gated ion channels are opened in the axon membrane

Therefore an insufficient number of sodium ions pass through the membrane

So the threshold potential is not reached

The small amount of acetylcholine attached to receptors is broken down rapidly by acetylcholinesterase

To overcome this, the effect of multiple impulses can be added together in a process known as summation

There are several benefits of summation

It allows for the effect of a stimulus to be magnified

A combination of different stimuli can trigger a response

It avoids the nervous system being overwhelmed by impulses

Synapses act as a barrier and slow down the rate of transmission of a nerve impulse that has to travel along two or more neurones

They only allow the impulses to pass on if there has been input from other neurones and receptors

There are two types of summation:

Temporal

Spatial

Question 4

Why is temporal summation a key feature of a synapse?

Answer 4

If multiple impulses arrive within quick succession the effect of the impulses can be added together to generate an action potential

A large amount of acetylcholine is released into the synaptic cleft

Therefore a large number of the gated ion channels open

So a sufficient number of sodium ions pass through the membrane

Question 5

Why is spatial summation a key feature of a synapse?

Answer 5

Multiple impulses arriving simultaneously at different synaptic knobs stimulating the same cell body can also generate an action potential through spatial summation

The multiple impulses result in a large amount of acetylcholine being released into the synaptic cleft which results in the generation of an action potential

Question 6

What are inhibitory and excitatory synapses?

Answer 6

Excitatory neurotransmitters can stimulate the generation of an action potential in a postsynaptic neurone

This is done by opening sodium ion channels in the postsynaptic membrane which causes depolarisation if a threshold is reached

inhibitory neurotransmitters can prevent the generation of an action potential in a postsynaptic neurone

They do this by opening potassium ion channels in the postsynaptic membrane which causes hyperpolarisation of the membrane

If a neurone is subject to both excitatory and inhibitory synapses the following happens:

Sodium ions enter the cell body following stimulation by the excitatory synapse

The stimulation of the inhibitory synapse causes potassium ions to diffuse out of the cell body

This cancels out the effect of the sodium ions entering

The threshold potential is not reached so no action potential is generated