Final answer:
CNS neurons originate from the ectoderm during embryonic development. Neurulation forms the neural tube, which gives rise to the brain and spinal cord, establishing the foundation for adult CNS structures.
Step-by-step explanation:
The embryonic origin of central nervous system (CNS) neurons is the ectoderm, one of the three primary germ layers in early embryonic development. During the process of organogenesis and in an event called neurulation, specialized neuroectodermal tissues thicken into what is known as the neural plate. This neural plate then undergoes a transformation as its edges rise to form neural folds, which eventually converge to create the neural tube—this will become the CNS. This tube, lying atop the notochord, will give rise to the brain and the spinal cord as it develops.
By the fourth week, changes within the anterior part of the neural tube lead to the formation of brain structures via the creation of vesicles. Different parts of the neural tube will differentiate into various CNS components; for instance, dorsally located tissues are associated with sensory functions while ventrally located ones are connected to motor functions. These processes illustrate how the simple embryonic nervous system evolves into the more complex arrangement found in adult organisms.
Embryonic development is critical to understanding adult brain structure because the basic layout of the nervous system established during this time persists into adulthood. For example, the hollow center of the neural tube in embryos persists as ventricles in the adult brain, spaces where cerebrospinal fluid circulates.