Glial cells, which are also known as neuroglia cells or sometimes simply glia, serve the function of supporting, nourishing and protecting the neurons. They are not involved in the breakdown or removal of unused neural pathways, nor are they any type of parasite. As for the function of a sheath, that better describes myelin that glial cells help to form.

The corpus callosum performs the task of communicating between the brain’s two hemispheres. It is the large band of neural fibers, which connects the two brain hemispheres and carries messages between them. The sympathetic and parasympathetic nervous systems are divisions of the autonomic nervous system and involve both hemispheres of the brain. The sensory cortex, while it spans both hemispheres, does not directly aid in communication between the two sides.

Just as it sounds, phyletic gradualism refers to a slow and steady pace of development. Punctuated equilibrium, on the other hand, best captures the theory described in this question. In the instances of critical periods, if the environment does not permit exposure, some losses in social learning cannot be regained even if the environment provides those learning exposures later. Saltation refers to the process of actually regressing in small steps during normal development.

On both micro and macro levels from the development of neurons to our actual body length, we experience small regresses or steps backward as part of normal healthy development. This process is called saltation. Phyletic gradualism and punctuated equilibrium refer to theories about the rate of development. In the instances of critical periods, if the environment does not permit exposure, some losses in social learning cannot be regained even if the environment provides those learning exposures later.

This can seem like a "which is the best response" type of question where more than one of these choices could be correct. Do not be fooled. The exchange of information or "impulses" in which dendrites and axons are involved is straightforward. Axons send out neural "messages" and dendrites receive. Sometimes it can be helpful to use a mnemonic device to remember: Axon has four letters like send. Dendrite has eight letters like receives.

The human brain is incredibly complex. “One or multiple neurotransmitters can result in one or multiple behaviors” is the best answer to this question. As an example, serotonin and dopamine both play an intimate role in the mood state and contingent behaviors associated with depression. While neurotransmitters can be difficult to balance, especially in people with a psychiatric disorder, there are real and predictable relationships at work between neurotransmitters and our behavior.

From studies of feral children seriously neglected in childhood who cannot speak or engage with others socially to investigations of kittens who have been permanently blinded by having their eyes covered for an extended period of time after birth, researchers have demonstrated that critical learning periods exist. In these critical periods, if the environment does not permit exposure, then some losses in social learning cannot be regained even if the environment provides those learning exposures later. Saltation refers to the process of actually regressing in small steps during normal development. While phyletic gradualism and punctuated equilibrium refer to theories about the rate of development.

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.