Synaptic Transmission

E1.

a)     Explain how pre-synaptic neurons can affect post-synaptic transmission of impulses (7)

Typically a presynaptic neurone excites a post synaptic neurone – the impulse is transmitted across the synapse. This is called an excitatory synapse. When the action potential reaches the area of the terminal buttons of the pre-synaptic neuron, it causes calcium ions to diffuse into the terminal buttons. Vesicles containing neurotransmitters fuse with the plasma membrane and release them into the synaptic cleft. The neurotransmitters bind with a receptor protein on the postsynaptic neurone membrane, this binding results in an ion channel opening and sodium ions diffusing through this channel. This initiates the action potential to begin moving down the post-synaptic neurone because it has been depolarized (made more positive).

However, some synapses are inhibitory synapses; the release of neurotransmitters into the cleft inhibits an action potential being generated in the post-synaptic neurone. At an inhibitory synapse the release of neurotransmitters into the synaptic cleft triggers the opening of ion channels, which allows Cl- ions to enter the neurone and K+ to leave. This makes the interior of the post-synpatic neurone more negative (hyperpolarised) and therefore less likely to initiate an action potential.


Mark Scheme: 
a. Pre-synaptic neurons can be excitatory or inhibitory
b. Chlorinegic  neurons released acetylcholine (standard wide use neurotransmitters)
c. Found in neuro-muscular junctions/ in autonomic nervous system/ most junction in voluntary nervous system
d. Adrengic nuerons release noradrenaline 
e. Found in sympathetic pathways 
f. Both types of neuron can be excitatory 
g. Neurotransmitters bind to receptors on post-synaptic membrane
h. Triggers opening of sodium channels / sodium moves across membrane
i. Causes depolarization 
j. Neurotransmitters are degraded/destroyed : acetyly choline esterase breaks down acetyl choline 
k. Other inhibitory neurotransmitters, e.g. GABA, dopamine 
l. Inhibitory neurotransmitters are less permeable to sodium / cause chloride ions to diffuse in
m. Causes hyperpolarisation
n. Potassium ions diffuse out 

b)     Explain the process of synaptic transmission (7)

At the far end of axons are swollen membranous areas called terminal buttons. Within these terminal buttons are many vesicles filled with neurotransmitters.

When an action potential reaches the area of the terminal buttons, it causes calcium ions to diffuse into the terminal buttons. Vesicles containing neurotransmitters fuse with the plasma membrane and releases the neurotransmitters into the synaptic cleft. Neurotransmitters diffuse across the synaptic cleft from the presynaptic neurone to the postsynaptic neurone.

Neurotransmitters bind with a receptor protein on the postsynaptic neurone membrane. This binding results in an ion channel opening and sodium ions diffusing in through this channel. This initiates the action potential to begin moving down the postsynaptic neurone because it’s been depolarized.

The neurotransmitter is degraded and broken into two or more fragments by specific enzymes. They’re then released from the receptor protein. The ion channel closes the sodium ions. The neurotransmitter fragments diffuse back across the synaptic gap to be reassembled into the terminal buttons of the presynaptic neurone. 


Mark Scheme: 
a. Presynaptic neurons pass the stimulus to post-synaptic neurons 
b. Presynaptic releases neurotransmitters into the synaptic cleft 
c. Process involves exocytosis
d. Exocytosis triggered by calcium ions entering into the bulb of the neuron
e. Neurotransmitter binds with the protein receptors on the post-synaptic cleft
f. Neurotransmitter binding causes ion channels to open
g. Ions diffuse into the cell
h. Causes depolarisation/hyperpolarisation of the pre-synaptic neuron
i. Outcome of the transmission depends on the type of receptor / channel opened
j. Specific example: sodium ions going into the post-synaptic neurone causes depolarisation
k. Specific example: chlorine ions going into the post-synaptic neurone causes hyperpolarisation
l. Neurotransmitters are destroyed by enzymes