3.6.2.2 Synaptic transmission

The point where one neurone ends and communicates with another/an effector is known as a synapse. Synapses transmit information from one neurone to another by means of chemicals (known as neurotransmitters). In between each neurone is a little gap known as the synaptic cleft in which the first neurone (the presynaptic neurone) releases the neurotransmitter into. Here, the axon is slightly swollen and termed the synaptic knob (this is at the end of the presynaptic neurone). The presynaptic knob contains synaptic vesicles (which the neurotransmitter is stored in), many mitochondria and endoplasmic reticulum (as these are required to  manufacture the neurotransmitter in the axon). The neurotransmitter is released from the synaptic knob into the synaptic cleft where it diffuses across the cleft to the post synaptic neurone which receives it with the help of specific receptor proteins on its membrane - synapses that produce new action potentials in this way are known as excitatory synapses. Synapses can only pass information in one direction only (they are unidirectional) - from the presynaptic neurone to the postsynaptic neurone. 

Summation:
Low frequency action potentials may lead to insufficient concentrations of a neurotransmitter which will not trigger a new action potential in the postsynaptic neurone. This can be overcome by the rapid build up of neurotransmitter in the synapse by summation in one of two methods:

• Spatial summation involves a number of different presynaptic neurones releasing enough neurotransmitter to exceed the threshold value to trigger a new action potential in one postsynaptic neurone
• Temporal summation involves a single presynaptic neurone releasing a neurotransmitter many times over a short period. A new action potential is triggered if the overall concentration of neurotransmitter exceeds the threshold value of the postsynaptic neurone.

Inhibition: 

It is also possible for synapses to make it less likely that a new action potential will be created on the postsynaptic neurone (these are inhibitory synapses):

• The presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channels on the postsynaptic neurone
• The neurotransmitter causes the chloride ion protein channels to open and chloride ions move into the postsynaptic neurone (by facilitated diffusion)
• The binding of the neurotransmitter causes potassium protein channels to open and potassium moves out of the post synaptic neurone into the synapse
• the effect of the negatively charged chloride ions moving in and the potassium ions moving out make the inside of the postsynaptic membrane more negative (and the outside more positive)
• The membrane potential increases
• this causes hyperpolarisation making it less likely that a new action potential will be created as a larger influx of sodium ions is needed to produce an action potential

Cholinergic synapse vs neuromuscular junction 

So we need to be able to compare the transmission across a cholinergic synapse and a neuromuscular junction...

Cholinergic synapses

A cholinergic synapse is a synapse in which acetylcholine is the neurotransmitter:

• An action potential arrives at the end of the presynaptic neurone. This causes calcium ion protein channels to open and calcium ions to enter the synaptic knob (facilitated diffusion)
• This influx of calcium ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft
• Acetylcholine molecules diffuse across the narrow synaptic cleft and bind to receptor sites on sodium ion protein channels in the postsynaptic neurone membrane
• This causes sodium ion protein channels to open and sodium ions diffuse in rapidly (facilitated diffusion)
• This influx of sodium ions generates a new action potential in the postsynaptic neurone
• Acetylcholinesterase hydrolyses acetylcholine into choline and acetyl (ethanoic acid) which diffuse back across the synaptic cleft into the presynaptic neurone. This also prevents the continuous generation of an action potential in the post synaptic neurone
• ATP released from mitochondria is used to recombine choline and acetyl (ethanoic acid). The acetylcholine is stored in synaptic vesicles for future use
• Sodium ion protein channels close

Cholinergic synapse structure/function:

• has neurotransmitters that are transported by diffusion
• has receptors that cause an influx of sodium ions upon binding with the neurotransmitter
• uses a sodium-potassium pump to repolarize the axon
• uses enzymes to break down the neurotransmitter
• can be excitatory or inhibitory
• links neurone-neurone or neurone-effector organ
• Motor neurones, sensory neurones, and intermediate neurones may be involved
• a new action potential may be produced along the post synaptic neurone
• acetylcholine binds to receptors on the membrane of the postsynaptic neurone

Neuromuscular junctions

This is the point where a motor neurone meets a skeletal muscle fibre. There are many junctions along the muscle to allow the whole muscle to contract at the same time. All muscle fibres supplied by a single motor neurone act together as a unit known as a motor neurone. This means we can control how much force the muscle exerts - e.g if we only want a light force we only stimulate a few units:

• A never impulse is received at the neuromuscular junction and the synaptic vesicles fuse with the synaptic membrane
• Acetylcholine is released into the synaptic cleft and diffuses across to the post synaptic membrane (this is the membrane of the muscle)
• This alters the membranes permeability to sodium ions and sodium ions enter rapidly which depolarizes the membrane
• Acetylcholinesterase hydrolyses acetylcholine into choline and acetyl (ethanoic acid) which diffuse back across the synaptic cleft into the presynaptic neurone. This ensures that the muscle is not over stimulated.

Neuromuscular junction synapse structure/function:

• has neurotransmitters that are transported by diffusion
• has receptors that cause an influx of sodium ions upon binding with the neurotransmitter
• uses a sodium-potassium pump to repolarize the axon
• uses enzymes to break down the neurotransmitter
• only excitatory
• only links neurone-muscle
• only motor neurones are involved
• the action potential ends here
• acetylcholine binds to receptors on the membrane of the muscle fibre