Neurotransmitters in the human body are under tight control. Many drugs, such as anti-depressants or drugs for ADHD, limit neurotransmitter responses. Antagonistic molecules will inhibit neurotransmitters and are used in many drugs. These molecules structurally interact with receptor proteins, either blocking the active site or binding allosterically to alter the binding site shape. Antagonists can be competitive or uncompetitive.

In contrast, agonists are molecules that structurally resemble the ligand for a certain receptor and can bind to the active site to trigger a response. Nicotine, for example, is an agonist to certain acetylcholine receptors and can trigger these receptors.

Neurotransmitters, such as acetylcholine, are chemical signals that transmit action potentials from one neuron to another. This process occurs at the synapse, where a neurotransmitter is released from the presynaptic neuron. This neurotransmitter travels across the synaptic cleft and binds to a receptor on the postsynaptic neuron. Statement I is thus false.

The rate of propagation of a nerve signal is limited by the synapse because neurotransmitters must diffuse across the gap; statement II is true.

Calcium ions are very important in the release of neurotransmitters. Voltage-gated calcium channels are located on the axon of the presynaptic neuron. When an action potential reaches the synapse, calcium ions are allowed to enter into the presynaptic neuron. This influx of calcium ions interacts with vesicles containing neurotransmitters and causes them to release their contents into the synaptic cleft. Statement III is false because calcium ion channels are located on the membrane of presynaptic neuron, not postsynaptic neuron.

The SNARE proteins are responsible for allowing vesicles filled with neurotransmitter to fuse with the cell membrane at the synaptic cleft, resulting in exocytosis. Without these proteins, the neurotransmitter cannot propagate the signal to any other cells.

Neurotransmitter synthesis occurs via translation or synthesis in the smooth endoplasmic reticulum, depending on the identity of teh molecule. Resting potential is determined by the sodium-potassium pump, and action potential propagation relies heavily on voltage-gated sodium channels and myelin.

All of the choices could potentially be neurotransmitters.

Peptide neurotransmitters are proteins. An example of a peptide neurotransmitter is somatostatin. Nitric oxide is the most well-known gaseous neurotransmitter. Monoamines are molecules that contain an amine group connected to an aromatic ring. These molecules are derived from aromatic amino acids. Dopamine, norepinephrine, and epinephrine are very well-known monoamine neurotransmitters.  

Receptors on postsynaptic neurons are connected to ion channels. When the neurotransmitter binds to the receptor, the channel opens, making that neuron more or less likely to have an action potential depending upon which type of ion the channel allows to enter or exit the neuron. The result is either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP).

The parasympathetic division of the autonomic nervous system promotes the "rest and digest mode." The somatic nervous system controls voluntary skeletal muscles, but the parasympathetic nervous system controls involuntary smooth & cardiac muscles. The neurons of the parasympathetic nervous system release acetylcholine, which is a neurotransmitter that leads to a decrease in heart rate and blood pressure. Results of increased parasympathetic activity include: decreasing blood flow to skeletal muscles, increasing blood flow to the gut, constricting pupils, and glycogenesis. 

The correct answer is ACh (acetylcholine) because it is involved with muscle contraction. It is released at the neuromuscular junction, the site where the neuron and muscle meet.