The mechanism behind the refractory period—formally known as the after-hyperpolarization—for an action potential is not yet well understood, but is thought to be a built-in defense against sodium ion overload inside the voltage gate of a cell membrane. 

The correct answer is that dendrites are the sole receiver of stimulation because the soma of the neuron can also receive input from other cells. The other answers were wrong because they are true characteristic of the nervous system. Summation can be temporal or spatial depending on the amount of cells communicating with the neuron. One pre-synaptic cell can send input to the neuron multiple times to result in temporal summation, and multiple cells can send input to the same cell resulting in spatial summation. The total amount of input received by the dendrites and soma will sum in the initial segment of the axon, if the axon reaches the depolarization threshold, voltage gated sodium channels will open and set off a chain reaction that propagates the action potential to the axon terminal. This process is faster in cells with myelin because less of the axon needs to be stimulated to send the action potential to the terminal due to the insulation of segments by the myelin. 

The correct answer is voltage gated sodium channels. These cells are in the axon of the neuron and are only opened when the cell reach the depolarization threshold. Voltage-gated potassium channels are responsible for the re-polarization of the cell or the hyper-polarization. Voltage-gated calcium channels are apparent in the axon terminal of the neuron and play a role in the release of neurotransmitters. Sodium channels and potassium channels are in the dendrites and soma of the neuron. They interact to keep the cell at it's resting potential and inputs to the cell can affect the amount of sodium or potassium entering the cell. This can lead to the cell becoming depolarized or hyper-polarized, but do not themselves lead to the surge of activity that generates the action potential. 

The action potential begins at the axon hillock, a specialized region connecting the dendrite and the axon. Before the action potential may occur, an energy threshold must be surpassed. The axon hillock acts like a gateway, permitting the action potential to begin only once this minimum has been reached. From there, the action potential propagates down the axon of the neuron. This is the long, tail-like extension of the cell which connects to the dendrites of other neurons. At the end of the axon are the terminal buttons. It is here that neurotransmitters are released from the neuron, which travel across the synaptic cleft to bind to neighboring dendrites of other neurons. If these dendrites receive sufficient excitation, then they will release their own action potentials, and repeat the process; thus, the action potential sends information between nerve cells.

The "soma" is the name for the cell body of a neuron. This refers to the part of the neuron that houses the cell nucleus, and other organelles necessary for the life of the cell. This region is distinct from the dendrites, which are the branch-like structures that protrude from the soma. It is also distinct from the axon, the long tail-like structure which extends away from the cell body. "Myelin" is the name of a fatty substance which coats the axons of some nerve cells in order to insulate them. "Nerve" is a word that may be used interchangeably with 'neuron', particularly when referring to those in the peripheral nervous system. However it is not an alternate name for the cell body.

Myelin is a fatty coating that develops around the axons of some nerve cells in order to insulate them. This insulation serves to aid in the completion of action potentials. Glial cells exist in the brain, and aid in nourishing the neurons. Myelin does not serve this purpose. Myelin also does not protect against viral attack, nor that of other pathogens. Although myelin insulates the cells, it is not capable of speeding their rate of firing. Finally, myelin has no interaction with neurotransmitters, and does not increase the receptivity of a nerve cell to stimulation.

The nerve impulse within a neuron is primarily an electrical event. This is due to the fact that the cell becomes polarized and then proceeds through rapid depolarization and repolarization during and following the action potential. All of this is achieved through electrical gradients, which are maintained across the cell membrane in order to create potential energy. Communication between neurons, on the other hand, is achieved through the transmission of chemical information. Neurotransmitters released from the terminal button of one neuron cross the syaptic cleft to bind to receptor sites on neighboring dendrites of other nerve cells. The transmission of these chemicals delivers information, leading to excitation or inhibition of the receiving cells. "Axon and dendrite" does not correctly describe this relationship. "Neurotransmitter and action potential" seems more appropriate, but these two items are in the incorrect order to describe nerve impulse and interneuronal communication, respectively.

The electrical gradient of a nerve cell is maintained across its membrane through the the balancing of concentrations of ions on either side of the membrane. Ions are positively or negatively charged particles. Through the use of ion channels and pumps, an artificial electrical gradient is produced, with a greater concentration of positive ions outside the cell than in. During an action potential, these channels open and close at key moments to allow the propagation of an electrical signal down the axon, terminating with the release of neurotransmitters at the terminal buttons. Neurotransmitters do not play a role in maintaining the electrical gradient of the cell. They may transmit excitatory or inhibitory signals to other neurons, leading to changes in the resting potential of these cells, but they are not responsible for maintaining or creating the electrical gradients of the membrane potentials. Glial cells and myelin do not play roles in this. Glial cells support neurons by nourishing them and contributing towards other cell functions, while myelin is a fatty substance that sheathes some axons to insulate them. These do not contribute to the electrical gradient of the membrane.

The symptoms of alertness, stress, physical excitation and readiness, rapid breathing and so forth all describe the 'fight or flight' response to danger, which would be appropriate to the example of fleeing from a predator. This response is controlled by the sympathetic nervous system. The sympathetic nervous system is responsible for such involuntary behaviors as increasing heart rate, constricting blood vessels and raising blood pressure. These behaviors serve to benefit the organism in events demanding physical exertion and focus (e.g. hunting, fighting, or fleeing). The parasympathetic nervous system controls the opposing involuntary behaviors, which contribute to relaxing, digesting, and other states. The central nervous system refers to those neurons housed within the brain and spinal cord. This is not an applicable response to the question. The neurotransmitter dopamine is a part of the brain's reward system. It does not play a role in the fight or flight response.

Prior to the action potential, a higher concentration of sodium ions exists outside of the cell than within it, and a higher concentration of potassium ions exists within the cell than without it. Overall, the cell will have a negative charge relative to its surroundings, thus, an electrical gradient, or resting potential. The action potential begins with sodium ion channels open, allowing for a sudden rush of positively charged sodium ions into the cell, in order to compensate for the lower concentration of sodium and the lower charge within the cell. These gates close as the signal propagates through the cell, and in their wake potassium gates open, allowing these similarly positively charged ions to exit the cell, again due to the concentration gradients between the two environments. This, and the active pumping of sodium ions back out of the neuron helps to return the cell to its electrochemical resting state. Sodium is pumped out of the cell after the firing of the action potential, not at its beginning.Potassium being pumped into the cell is not part of the firing of an action potential. Typically the membrane potential of a neuron must exceed -55mV before an action potential can occur. The membrane may drop below -70mV after an action potential. This is called the refractory period, and during it, no further action potentials are possible. This is not the beginning of an action potential.