The blood–brain barrier (BBB) is a highly selective semipermeable border of endothelial cells that regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system, thus protecting the brain from harmful or unwanted substances in the blood. The blood–brain barrier is formed by endothelial cells of the capillary wall, astrocyte end-feet ensheathing the capillary, and pericytes embedded in the capillary basement membrane. This system allows the passage of some small molecules by passive diffusion, as well as the selective and active transport of various nutrients, ions, organic anions, and macromolecules such as glucose and amino acids that are crucial to neural function.
Solute permeability at the BBB vs. choroid plexus
Part of a network of capillaries supplying brain cells
The astrocytes type 1 surrounding capillaries in the brain
Sketch showing constitution of blood vessels inside the brain
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. In vertebrates, a small part of the brain called the hypothalamus is the neural control center for all endocrine systems. The brain is the largest cluster of neurons in the body and is typically located in the head, usually near organs for special senses such as vision, hearing and olfaction.
It is the most energy-consuming organ of the body, and the most specialized, responsible for endocrine regulation, sensory perception, motor control, and the development of intelligence.
The brain of a chimpanzee
Cross section of the olfactory bulb of a rat, stained in two different ways at the same time: one stain shows neuron cell bodies, the other shows receptors for the neurotransmitter GABA.
Neurons generate electrical signals that travel along their axons. When a pulse of electricity reaches a junction called a synapse, it causes a neurotransmitter chemical to be released, which binds to receptors on other cells and thereby alters their electrical activity.
Fruit flies (Drosophila) have been extensively studied to gain insight into the role of genes in brain development.