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.

The brain is often divided into four lobes based on anatomy and physiology: the frontal lobe, parietal lobe, occipital lobe, and temporal lobe. Each lobe controls various aspects of cognition and motor skills. The frontal lobe is associated with reasoning, speech, movement, and emotions. The parietal lobe is associated with orientation and recognition. The occipital lobe is associated with visual processing. The temporal lobe is associated with auditory processing and memory.

Broca's area is a small region of the frontal lobe located in the left hemisphere. This region of the brain is responsible for generating speech and articulation. Damage to this region of the frontal lobe could cause speech impairment. In contrast, Wernicke's area is located in the temporal lobe and is associated with comprehension of speech.

Spastic muscle activity is not related to the brain, but results from injury to motor neurons spanning from the spinal cord to the limbs.

Synapses are special regions where a neuron releases a signal to its target cell. Most commonly this signal is chemical (neurotransmitters), but it can also be electrical. The synapse is a gap between the neurons, and does not allow for direct contact. Signals are released from the axon of one neuron and must traverse the synaptic cleft before interfacing with receptors on the target cell.

Neurons do not directly release hormones into the blood stream and synapses do not offer protection to neurons.

Phototaxis is movement (taxis) in response to light (photo). Movement towards a source is positive; movement away from a source is negative. "Positive phototaxis" would be used to describe movement toward a light source.

Glial cells are non-neuronal cells that support neuron activity. Their functions include physical support of neurons, supply oxygen and nutrients, and take up excess neurotransmitters.