Acetylcholine affects movement, learning, memory and REM sleep. It has an excitatory affect on skeletal muscle fiber and an inhibitory affect on heart muscle fibers. 

Serotonin affects mood, sleep, appetite, impulsivity and aggression. If a person's level of serotonin is too high or too low it could affect mood and aggression. Depression has been directly linked to serotonin levels, and the most regularly prescribed Anti-Depressant drugs (SSRIs) regulat the serotonin re-uptake process, thus elevating the serotonin levels in patients.

Acetylcholine is among one of the best understood neurotransmitters. Acetylcholine plays a part in learning, memory, and motor control. It's responsible for muscular contraction upon release and sensing by the cell receptors; therefore, it is usually found between motor neurons and skeletal muscles. 

GABA is known to be a major inhibitory neurotransmitter. GABA is observed to be most active in the brain during times of rest and sleep. In a sense, it inhibits being awake and it can reduce the occurance of tremors and seizures. As a result, malfunctions linked to blocking the trnsmission of this neurotransmitter can result in tremors, seizures, and bouts of insomnia.

Synaptogenesis is the process of developing synapes. Synaptogenesis occurs throughout the lifespan and in spurts; this process results in the generation of dendrites and axons.

When synapses are unnecessary or redundant the brain eliminates them. This process is known as pruning. Pruning is a crucial part of the development process and follows each synaptogenesis spurt. 

An action potential is a rapid electrical charge that will propagate down through the neuron. This charge causes a continuous chain reaction through the neuron from the dendrites to the axon terminals by creating a threshold stimulus that allows rapid depolarization and repolarization via the movement of ions across the membrane. This represents the basic construct of how neuronal communication is possible. Although stimulus does seem like a viable answer, a neuron may sense a stimulus but may not propagate a neural impulse. In this case, the stimulus may not be strong enough to activate an action potential. This solicits the "all or nothing" behavior of action potentials. If the stimulus is slightly below the necessary threshold to elicit an action potential, then no impulse can be expected. Conversely, if a stimulus is just at the minimal requirement of the neuron's threshold, then an action potential may be expected; therefore, stimulus would be incorrect. Due to the fact that action potentials function on ionic concentration gradients, ionic equilibrium would also be an incorrect answer. While the terms absolute refractory period and hyperpolarization are related to action potentials, these are merely parts of an action potential that may be argued to be the reasoning to prevent an impulse from traveling backwards. This prevents an action potential from travelling back the way it came. These choices would also be incorrect answers because while they are important, they're only components to an action potential. 

The synaptic gap is where two neurons meet. Here neurotransmitters will be released from vesicles in the sending neuron to the receiving neuron. The receiving neuron will receive communication via sensing the neurotransmitters at receptor sites specific for that neurotransmitter. The release of neurotransmitter is stimulated once the action potential has propagated to the axon terminal. 

The neurotransmitter is released with the purpose of signaling and beginning action potentials for the receiving neuron. This chemical messenger must be quickly removed from the synapse to prevent continuous stimulation of the receiving neuron. The reuptake procedure is done from the sending neuron, meaning the remaining neurotransmitter is reabsorbed by the neuron it came from. This process will not create an action potential for the sending neuron as it would for the receiving neuron and is necessary in order to prevent overstimulation. 

Dendrites are the little branched hair-like structures attached to the cell body. They play the role of receiving information that will be propagated through the cell body and eventually through the axon hillock as a neural impulse if the stimulus warrants a great enough action potential. The axon is what the neural impulse will travel through to reach the axon terminals and ultimately pass along the message to the next neuron or target tissue via neurotransmitters released into the synapse. The synapse is the space between two neurons. Because action potentials do not propagate backwards (due to refractory periods), a stimulus is not expected to be sensed by the axon terminals. As a result, impulses will travel from cell body to axon terminals with the dendrites responsible for sensing stimuli.