Features of the synapse

Synapses permit the transmission of a nerve impulse from one neurone to another. In doing so, they act as junctions for the nervous system. They can allow...

• a single impulse to stimulate many other neurones into action, allowing one stimulus to create a number of different responses
• multiple impulses from different neurones to combine, forming a single impulse.

I need to appreciate that...

• Neurotransmitter is only produced in the pre synaptic neurone, not the post synaptic neurone
• Neurotransmitter is stored in synaptic vesicles and is released into the synaptic cleft when an action potential travels down the pre synaptic neurone
• Neurotransmitter will diffuse across the synaptic cleft once released by the vesicles
• The neurotransmitter binds with the appropriate receptor molecule on the post synaptic neurone, creating a new action down the axon

Unidirectionality
Synapses will only allow an impulse to travel in one direction, from the pre synaptic neurone to the post synaptic neurone.

Summation
Sometimes, the frequency of action potentials down the pre synaptic neurone causes insufficient neurotransmitter to be released, meaning that the action potential won't be replicated in the post synaptic neurone. The synapse has two ways around this:

• Spatial summation. This means the addition of neurotransmission in space. multiple neurones from different receptors can link with the same post synaptic neurone. Each pre synaptic neurone may not produce enough neurotransmitter to over come the threshold for a new action potential. But these neurones can accumulate their individual neurotransmitters to form an action potential.
• Temporal summation. This means the addition of neurotransmission over time. Several action potentials can release neurotransmitter from a particular pre synaptic knob, which can all add together to produce an action potential down the post synaptic neurone

Inhibition
On the post synaptic membrane of some neurones, there are chloride ion channels. These can be stimulated to open, which causes the negative ions to diffuse into the post synaptic neurone, making it more negative, in a process called hyper-polarisation. This makes it less likely that the post synaptic neurone will be stimulated into action, meaning chloride channels have an inhibitory effect.

Inhibition
On the post synaptic membrane of some neurones, there are chloride ion channels. These can be stimulated to open, which causes the negative ions to diffuse into the post synaptic neurone, making it more negative, in a process called hyper-polarisation. This makes it less likely that the post synaptic neurone will be stimulated into action, meaning chloride channels have an inhibitory effect.

A synapse is a point in the nervous system when the axon of a neurone meets the dendrite of another neurone or a target effector cell. They link individual neurones together to make the nervous system, and are important for coordinating activities.

Synapses use neurotransmitters to transmit an impulse across the gap, called the synaptic cleft, which is about 20-30 nanometres wide. The part of the neurone which releases neurotransmitter is called the pre-synaptic neurone. The end of the axon of the pre synaptic neurone swells, creating what is called the pre-synaptic knob. This part of the neurone contains lots of mitochondria and endoplasmic reticulum, which it uses to make a large amount of neurotransmitter in order to pass the nerve impulse across. The neurotransmitter is stored insynaptic vesicles, inside the synaptic knob. Theses vesicles can fuse which the membrane of the synaptic knob, causing neurotransmitter to diffuse across the synaptic cleft towards thepost synaptic neurone. Membrane bound sodium ion channels on the post synaptic neurone are capable of responding to the presence of neurotransmitter, which is a vital property of nerve transmission.