An action potential describes the event of an electrical impulse being activated by a given neuron once it is sufficiently polarized. We may think of an experience such as pain. If I prick my finger with a needle, I feel a small amount of pain. If, however, I unfortunately lose my fingertip due to a mechanical accident of some sort, I will feel much more pain. This difference in pain is not due to the strength of any one given action potential. An action potential either leads to an electrical impulse or it does not (in other words, it is all-or-none). There are no gradients in strength or degree; however, the number of action potentials occurring across neurons can have a cumulative effect (e.g., greater number of nerve cells involved in the more serious injury of losing a finger tip equates to a greater experience of pain).

While potassium—alongside sodium—plays a vital role in the functioning of neurons and in the exchange of neurotransmitters, it is not a neurotransmitter. Rather, it is involved in the shifting of polarity in the neuron that leads to an action potential. In other words, the amount of potassium present in a given neuron directly impacts meeting the threshold of an action potential. Serotonin, dopamine, and norepinephrine are neurotransmitters released by these action potentials.

In reading popular articles on brain science or, perhaps, by watching a documentary on brain injuries, we might have come across the terms sensory and motor neurons. Just as they sound, a sensory neuron passes along sensory information. Motor neurons, which are located close to the spine, assist with our motoric abilities (e.g., walking, grasping, pushing). Interneurons play the role of circuitry connectors between sensory and motor neurons. Because of this, they are also sometimes referred to as relay neurons. Finally, “outer neurons” are not one of the three major classifications of neurons. In fact, there are no such things as outer neurons.

The tiny space between a synapse of one neuron and the synapse of another is called the synaptic gap. It is also known as the synaptic cleft. It is in this space where neurotransmitters can be exchanged between neurons. It is important to note that not all neurotransmitter molecules emitted by a given synapse or necessarily received by the synapse across the synaptic gap. Multiple variables are at play. The gap is not needed for the neurons to have space to grow nor is the brain kept moist via these clefts.

This question is closely associated with the nature vs. nurture debate. Let us be reminded that both nature and nurture are at play in development including neuronal development. With that in mind, we can successfully conclude that the false statement in this series is, “by birth, neurons are fully developed including axons, dendrites and myelin.” While most of the neurons we will have throughout our life are present at birth, these neurons are in nascent form. That is, they are still building neural pathways and connections while other neural pathways and connections are being pruned away if left unused. This process is most prolific in the first three years of life, but continues across the lifespan.

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.