The central nervous system (CNS) consists of the brain, spinal cord, peripheral nerves, and other structures that coordinate and influence the activity of all parts of our body. Our eyes are considered part of the CNS because many parts of the eye actually begin as brain tissue during early embryonic development. Additionally, it takes many nerves and our brain to be able to perceive the light our eyes are taking in as physical objects. The fact that the eyes are part of the CNS is important because all structures of the CNS are protected by something known as the blood-brain-barrier or BBB.
The BBB consists of a highly selective, semi-permeable membrane that separates the circulating blood from the fluids and structures of the CNS. The BBB is formed by especially tight connections between the cells that line blood vessels in and around the CNS. The BBB functions to protect these very delicate structures. To do so, it selectively allows only a few select molecules, nutrients, and chemicals to pass, either by passive diffusion or selective transport. Water is an example of a molecule that can passively cross the BBB and larger molecules like glucose, amino acids, fats, and other nutrients that are essential to brain, neuronal, and eye function are selectively and actively transported. Therefore, not all nutrients make it to the CNS. Thus, to get nutrients into the brain, neurons, and the eye, we must selectively target nutritional interventions to compounds known to cross the BBB.