The myelin sheath is responsible for the fast proagation of signals down the neuronal tissue. It is a fatty layer that insulates the neuron, causing the action potential signal to jump or skip over portions of the axon, speeding up transmission. This myelin sheath gets laid down by oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system. Each other answer choice is a different part of the neuron itself; the neuron cannot synthesize its own myelin sheath.

The myelin sheath is the electrical insulator around neurons that increase the conduction velocity. In the peripheral nervous system, Schwann cells are responsible for the production of the myelin sheath. It is important to note that in the central nervous system, oligodendrocytes are responsible for producing the myelin sheath. The other cell types are neuroglia that have the following basic functions: astrocytes - support of neurons, damage repair, nutrient delivery/waste removal, ependymal cells - production of cerebrospinal fluid, microglia - immune-like neuroglia. 

Astrocytes are star-shaped cells found between neurons and blood vessels. They cover almost all of the capillaries in the brain and make contact with surfaces of neurons. They make up approximately 50% of the cells in the brain. Astrocytes are responsible for supporting neurons by maintaining the extracellular fluid, facilitating nutrient delivery and waste removal to and from neurons, maintaining the blood-brain barrier, and repairing damaged cells in the central nervous system. Schwann cells and oligodendrocytes are responsible for myelinating axons in the peripheral nervous system and central nervous system, respectively. Microglia are specialized macrophages that remove cellular debris, infectious agents and damaged neurons. Ependymal cells are epithelial-like glial cells in the central nervous system that line the ventricles and produce cerebrospinal fluid.

While both Oligodendrocytes and Schwann cells make myelin, oligodendrocytes produce myelin for the central nervous system (CNS), while Schwann cells make myelin in the peripheral nervous system (PNS). Oligodendrocytes are a type on neuroglia; their main function is to provide both support and insulation to axons of the CNS via the production of a myelin sheath. A single oligodendrocyte can extend processes to 50 axons, wrapping myelin around each axon. While Schwann cells provide a similar function, they can only wrap around one axon. 

Dendrites are the "branchlike" projections off of the soma of a neuron. Their purpose is to pick up information that can later be sent through the axon with action potentials. The more branching that is present allows greater surface area along with increased potential of picking up signals.

Oligodendrocytes help to create myelin sheaths, which help with conduction of signals.

The pineal gland secretes melatonin, it is also thought to have some role in regulating the pituitary gland. The thalamus is the center of pain perception and does not secrete hormones. The pituitary gland, also known as the "master gland" secretes many hormones except for melatonin, some example secretions are thyroid stimulating hormone, follicle-stimulating hormone, and growth hormone. Lastly, the hypothalamus secretes tropic hormones that regulate the activity of other glands.

The limbic system includes the amygdala, hippocampus, hypothalamus, and thalamus. Each of these structures secretes hormones that assist in processing emotions such as fear, and anger. The brainstem is responsible for basic life support. The cerebrum is the area dedicated to higher thinking. Lastly, the parietal lobes are where auditory signals are processed and they contain the primary somatosensory cortex. 

Melatonin is the hormone secreted by the pineal gland, it controls the circardian rhythm and is responsible for feeling sleepy. Corticotropin, released by the adrenal cortex is associated with cortisol release. Insulin is the most notable hormone associated with glucose metabolism and is released by the pancreas. Gastrin, cholecystokinin, and secretin assist digestion. 

Neurotransmitters will never enter the postsynaptic neuron. They will attach to receptors on the membrane of the postsynaptic neuron, release, and then be dealt with while in the synaptic cleft. In some instances, the neurotransmitter simply diffuses away from the synapse, but in most cases synaptic proteins help to recycle the molecules. Some enzymes will break the neurotransmitter into parts, which then reenter the presynaptic neuron and are reassembled. Acetylcholinesterase is a common example of this type of enzyme. Other neurons use membrane pumps to retrieve neurotransmitter from the synaptic cleft.