The nervous system has two major divisions: the central nervous system and the peripheral nervous system. The peripheral nervous system is further divided into the somatic nervous system and the autonomic nervous system. The autonomic nervous system can then be divided into the sympathetic and parasympathetic branches.

General: Central and Peripheral

Peripheral: Somatic and Autonomic

Autonomic: Sympathetic and Parasympathetic

The vagus nerve is responsible for slowing down the heart rate. It also increases digestive activity. Knowing this, we can conclude that the vagus nerve has a "rest and digest" function in the body. This means that it is part of the parasympathetic nervous system.

The sympathetic nervous system is responsible for stress responses, or "fight or flight." The somatic nervous system is under voluntary control, while the autonomic nervous system is involuntary. Both the sympathetic and parasympathetic divisions fall under the autonomic umbrella. The central nervous system includes only the brain and spinal cord.

Reflex arcs are unique neural pathways due to the fact they are constructed to cause voluntary muscles to move without a stimulus being integrated in the brain. Instead, the integration of the stimulus occurs in the spinal cord, where an efferent signal is immediately created towards the voluntary muscle. 

The sympathetic nervous system is responsible for the "fight or flight" response. The parasympathetic nervous system does the opposite (rest and digest). The enteric nervous system helps with digestion. The remaining two answer choices are too broad and do not answer the question as well as the sympathetic nervous system. Note that the sympathetic and parasympathetic nervous systems are branches of the autonomic nervous system, which itself is a branch of the parasympathetic nervous system.

The sodium potassium-pump is used in order to establish the negative resting potential in neurons. Since both sodium and potassium ions are positively charged, there needs to be more ions leaving the cell compared to ions entering. The pump accomplishes this by pumping three sodium ions out of the cell, while pumping two potassium ions into the cell. This loss of positive charge inside the cell results in the negative resting potential of neurons.

Thank about the net transfer of ions. Three positive sodium ions out of the cell for every two positive potassium ions into the cell is the same as one positive ion leaving the cell. When positive ions leave, the inside of the cell becomes more negative, helping the cell reach its resting potential of around –70mV.

In the beginning of an action potential voltage-gated sodium channels begin to open, allowing sodium ions to rush into the cell. This influx of positive ions results in a change in the polarity of the cell, making the voltage become positive inside the cell. This process is called depolarization.

Hyperpolarization comes after depolarization, and is caused by potassium ions leaving the cell interior. The removal of these positive ions causes the cell to become more negative than the resting potential.

Repolarization is the final process to return the cell to its resting potential. The sodium-potassium pump brings potassium ions back into the cell and removes the sodium ions, returning the cell to its normal resting state.

Action potentials are electrical signals transmitted by neurons. When a neuron is stimulated, a signal is transmitted down the axon. This signal is the action potential.

An action potential in a neuron can help to stimulate a muscle to contract, but the muscle itself will not conduct an action potential.

The axon is wrapped in fatty bundles called myelin sheaths that promote fast transmission of an electrical signal. The other structures listed here are not myelinated. 

The refractory period occurs when the cell repolarizes/hyperpolzarizes beyond the resting potential; that is, the membrane potential drops to a value more negative than when it is at rest. This prevents the firing of another action potential immediately after one has been fired. The other values represent the resting potential (), the threshold (), and values that are more positive, and are therefore incorrect.

As an action potential begins, there's a rapid influx of sodium in to cell, causing the cell's membrane potential to rapidly increase, depolarizing the cell. Once the cell has reached its action potential peak, the sodium channels begin to close. This closing activates the potassium channels. These channels allow potassium to leave the cell. Since potassium is a positive ion, as it leaves, the cell's membrane potential becomes more negative, repolarizing. The slight dip in the action potential curve, labeled as hyperpolarization, is result of the potassium channels lagging to close, and potassium loss is "overshot". As a result, too much potassium lost from the cell will cause the cell's potential to become more negative relative to its normal potential.