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. 

Reuptake is reabsorption of neurotransmitter into the neuron. Chemically breaking down neurotransmitters makes them nonfunctional.

Synaptic vesicles are at the end of the presynaptic neuron, and they release neurotransmitter into the synapse. Dendrites are outer branched extensions of a neuron. Synapses are the structures that permit neuron transfer.

The synapse is the place where two neurons meet to transmit information. In other words, in between two neurons is the synapse, sometimes called the "synaptic gap". Terminal buttons are at the end of each axon and they secrete neurotransmitters. They are very close to the synapse, but not the right answer in this case. The myelin sheath encases the axon in order to insulate the cell, which speeds up the transmission of signals.

An action potential is a very brief shift or spike in a neuron's electrical charge that sends a message down the axon. An action potential is the can be described as a neuron "firing.” Action potentials occur after the resting potential and before a refractory period.

The synaptic cleft is the microscopic gap between neurons. The refractory period is a very brief period of time after an action potential in which another action potential cannot begin. One can think of the refractory period like a very brief rest period.

Neurons—also called nerve cells—are the basic links that allow communication within the nervous system. Neurotransmitters are chemical messengers that activate neighboring neurons, but they are not cells. Dendrites are part of a neuron—they are the branchy parts of the neuron that are specialized to receive incoming information. Hormones are also a type of chemical messenger; however, they are transmitted by the circulatory system and not the nervous system. 

The axon terminal of a neuron contains synaptic vesicles containing neurotransmitters. After an action potential, neurotransmitters are released and diffuse across the synaptic cleft, where they bind to receptors on the dendrites of another neuron. This can cause an action potential in the second neuron. Neurons are not directly connected to each other. Synaptic vesicles do not exist outside of the cell body.