Questions

1a) explain how pre-synaptic neurons can affect post-synaptic transmission of impulses [7]

Presynaptic neurons can either be exicitory or inhibitory. Specific types of neurons such as chlorogenic neurons release a type of neurotransmitter called acetylcholine.
The neurotransmitters are released when the action potential reaches the terminal buttons of the pre-synaptic neuron. This causes the calcium ion channels to open and thus difuse into the neuron. This causes the vesicles containing the neurotransmitters to to fuse with the plasma membrane in a process called exocytosis releasing the neurotransmitters into the pre-synaptic cleft.
The neurotransmitters then bind with receptors on the post synaptic cleft.
This binding action causes sodium ion channels or gates to open causing sodium ions to diffuse into the cells causing depolarization and this generates an action potential and thus the impulse is passed on.
However, in an inhibitory situation. The binding action of the neurotransmitters cause chlorine channels and potassium channels to open thus resulting in an influx of chlorine ions into the post synaptic neuron and potassium ions to diffuse out thus causing hyperpolarization.
The neurotransmitters are then deactivated/destroyed by enzymes and is recycled.
Examlpes of inhibitory neurotransmitters: GABA. 

1b) Explain the process of synaptic transmission [7]

Presynaptic neurons can either be excitory or inhibitory depending on the type of membrane receptor on the post-synaptic neurone.
Pre-synaptic neurons releases neurotransmitters into the synaptic cleft this is done by a process called exocytosis which requires energy. The fusing of the vesicles containing the neurotransmitter is triggered by the action potential triggering the opening of calcium ion channels. The influx of calcium ions is what triggers the vesicles to move and fuse with the membrane. Thus releasing the neurotransmitters into the synaptic cleft.
The neurotransmitters bind with the receptors on the post synaptic cleft. In an excitory situation the binding action causes sodium ion channels to open and causes sodium ions to diffuse into the cell causing depolarization causing an action potential to be created and the impulse to be passed on.

However, in an inhibitory situation the binding action causes chlorine and potassium ion channels to open causing chlorine ions to diffuse in and potassium ions to diffuse out thus causing hyperpolarization which prevents an action potential from being generated and thus inhibits the passing on of the impulse. The neurotransmitters are then destroyed/degraded/broken down by enzymes and recycled. An example would be the neurotransmitters acetyl chloline being broken down by the enzyme acetyl chline esterase.

Whether it is a excitatory or inhibitory situation will depend on the type of receptor on the pre-synaptic neuron.