Neurotransmitters-
Sodium ions diffuse into neuron, potassium ions diffuse out creating the action potential that drives the electrical signal across the neuron
Calcium ions enter axon terminal
Vesicle containing neurotransmitters are stimulated by electrical impulse to move toward the plasma membrane and exocytose the neurotransmitters out of the axon terminal into the synaptic cleft
The neurotransmitters attach to complementary receptors on the post-synaptic neuron and diffuse into the neuron, repeating the process again
The neurotransmitter itself does not diffuse into the neuron, it binds to a ligand-gated channel protein, causing it to open, allowing sodium ions to enter.
I don't know about the rest of it but don't worry - its not in our study design in that much detail anymore, we only need to know the basics.
Hydrophilic signalling molecules-
bind to complementary plasma membrane receptor on target cell
This activates the intracellular part of the receptor to activate an enzyme in the plasma membrane that catalyses production of the second messenger molecule. Second messenger molecule activates the first enzyme of the enzyme cascade, that then activates the next enzyme and this repeats to amplify the signal until it reaches the nucleus where the specific cellular response occurs (like synthesis of a specific protein)
All we need to know is:
1. Signalling molecule binds to membrane-bound receptor.
2. This causes the activation of a second messenger molecule.
3. This causes a signal cascade (think dominoes)
4. This causes a cellular response.
The intracellular part of a receptor does not necessarily cause the activation of an enzyme - if it did the enzyme would be considered the second messenger.
They do not necessarily travel to the nucleus (hydrophobic molecules generally do). Eg. Glucagon causes the activation of an enzyme that breaks glycogen down into glucose - nothing to do with the nucleus.
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