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.

Neurotransmitters are released from the terminal button of a neuron, and then travel across the gap to reach the dendrites of neighboring neurons. Neurons do not actually physically touch. Rather, minute gaps separate them. These gaps are known as the synaptic clefts. The synapse may also describe this juncture between cells. Neurotransmitters play a key role in this region. On the other hand, the soma is the cell body of a neuron. It does not have specific interactions with neurotransmitters, other than to receive excitatory or inhibitory signals from them, along its dendrites. The axon is the tail-like limb along which action potentials are sent. Though neurotransmitters are contained within the synaptic bulbs at the ends of these, they are not within the axons themselves. The axon hillock is a location between the axon and soma which serves as a gateway to initiating an action potential. It does not relate to neurotransmitters.

The sympathetic nervous system is that which prepares the body for action. It contributes to focus, increased heart rate, blood pressure, and breathing rate, and muscular activation. It also contributes to stress, and general 'readiness' of the body. This system is directly involved in the 'fight or flight' response, which readies an organism for the physical activity necessary or survival. The parasympathetic nervous system does the opposite of the sympathetic nervous system: it contributes to relaxation of the body, reduction of heart and breathing rates and blood pressure, the initiation of digestion, and other physiological responses associated with relaxation and a state of calm. The central nervous system refers to the neurons housed within the brain and spinal cord. It is not the appropriate term for either of the systems described in the question, although many of the neurons within the central nervous system will be implicated in the para and sympathetic nervous systems. Likewise, the peripheral nervous system is also not the appropriate term in this question, though the nerves comprising it will be implicated in both responses. The peripheral nervous system refers to the nerve cells in the body not within the brain or spinal cord.

Action potentials either occur or they do not. There is no such thing as a "half action potential", or a 'weak' versus 'strong' action potential. They occur when the threshold of the neuron has been exceeded, and once an action potential begins, it will go until completion. For this reason, 'all or nothing' is an apt description of their nature. Neurons do not fire continuously: there will be rests between separate action potentials, and at certain times some neurons may not need to fire at all. They do not all need to be active or inactive simultaneously. An electrical transmission does not cross the synapse. Rather, the electrical signal of the action potential ends at the terminal buttons of the axon, at which point chemical neurotransmitters are released across the synapse.