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. 

Most mind-altering drugs will affect the neurons of the brain by interacting with neurotransmitters at the synapse. They may do this by imitating the shape of particular neurotransmitters, and attaching to the receptor sites associated with those chemicals. This would create artificial excitatory or inhibitory signaling in those neurons. They may also block the action of neurotransmitters, preventing signals which would otherwise have been sent. Finally, they may prevent the reuptake of neurotransmitters, allowing them to remain in the synapse for longer, thus extending and amplifying their effects. All of these may contribute to the experiences of those under the effects of such drugs. Mind-altering drugs do not have any impact on the resting potentials of neurons, or on the thresholds for activation of nerve cells. Though some drugs may exhibit toxic effects, these are more likely to affect the liver than brain cells, and would not be a likely cause of the experiences of the drug user regardless.

Sodium and potassium are crucial in the stages of the action potential. This is due to the fact that by controlling the concentrations of these positively charged ions within the cell, an electrical gradient may be effected across membrane of a neuron. By actively pumping sodium ions out of a cell, a neuron maintains a negative resting potential of approximately -70mV. During the action potential, ion channels open to allow sodium to flood into the cell along the direction of this electrical gradient. This in turn allows the propagation of the nerve impulse down the length of the axon, as positive ions activate more ion channels, not unlike a chain of dominoes. By subsequently controlling the concentration of potassium ions, and pumping sodium out of the cell, membrane resting potential is restored, allowing for the process to repeat itself once sufficient excitation is reached. Calcium ions have a role in the action potential as well, but it is much more specific, and limited to the release of neurotransmitters at the end of an action potential. Nitrogen and chlorine do not have crucial roles in the nerve impulse.

Sodium and potassium are crucial in the stages of the action potential. This is due to the fact that by controlling the concentrations of these positively charged ions within the cell, an electrical gradient may be effected across membrane of a neuron. By actively pumping sodium ions out of a cell, a neuron maintains a negative resting potential of approximately -70mV. During the action potential, ion channels open to allow sodium to flood into the cell along the direction of this electrical gradient. This in turn allows the propagation of the nerve impulse down the length of the axon, as positive ions activate more ion channels, not unlike a chain of dominoes. By subsequently controlling the concentration of potassium ions, and pumping sodium out of the cell, membrane resting potential is restored, allowing for the process to repeat itself once sufficient excitation is reached. Calcium ions have a role in the action potential as well, but it is much more specific, and limited to the release of neurotransmitters at the end of an action potential. Nitrogen and chlorine do not have crucial roles in the nerve impulse.

Nociceptors are sensory neurons that are activated by agitating or painful stimuli. Nociceptors respond to mechanical, thermal, and chemical stimuli to help protect the body from harmful interception. 

"Wernicke's aphasia" is a receptive aphasia that results in problems with language comprehension. People with Wernicke's aphasia speak rapidly and do not make sense. They often do not have insight into their language difficulties. On the other hand, "Broca's aphasia" is an expressive aphasia that results in difficulty generating language. A person with Broca's aphasia produces slow speech with incorrect grammar, making it difficult to understand. “Global aphasia” includes symptoms of both Wernicke's and Broca's aphasias. A person with “conduction aphasia” has difficulties with verbal repetition. Last, “agnosia” involves problems processing sensory information. 

The corpus callosum is a wide set of nerve fibers that connects the two hemispheres of the human brain. Agenesis of the corpus callosum, a rare birth defect, results in impaired cognitive abilities (e.g. key processes like face processing and other socially-important skills).

Substantia nigra is named for it's darker appearance relative to its surroundings. It is dark because of high amounts of neuromelanins in its tissues—an apparent byproduct of dopamine production. The substantia nigra is subdivided into two functionally distinct sections: the pars compacta and pars reticulata.