Systems Biology
The blood-brain barrier refers to the highly selective boundary between the blood in capillaries and brain tissue. It is formed by tight junctions between capillary endothelial cells, which prevent most molecules from passing through. This barrier is crucial in protecting the brain from harmful substances while keeping neurotransmitters within the brain. However, this selectivity also makes it difficult for antibiotics and antibodies to reach the brain when needed. The blood-brain barrier is less restrictive around circumventricular organs, which require open communication with the bloodstream.
Another crucial component in brain function is cerebrospinal fluid (CSF), which circulates around the brain and spinal cord. CSF is produced by the choroid plexus and is essentially ultra-filtered plasma. The ependymal cells of the choroid plexus secrete CSF and have tight junctions between them, similar to the capillary endothelial cells of the blood-brain barrier. However, in the blood-CSF barrier, capillaries have open pores and allow more substances to pass through. This less restrictive barrier is clinically important, as it enables certain drugs injected into the CSF to reach the brain, providing a potential entry point for treatment when the blood-brain barrier is too restrictive.
Lesson Outline
<ul> <li>Blood-brain barrier <ul> <li>Highly selective boundary between blood capillaries and brain tissue</li> <li>Formed by tight junctions between capillary endothelial cells</li> <li>Prevents most molecules from passing through</li> <li>Protects brain from harmful substances and keeps neurotransmitters within the brain</li> <li>Makes it difficult for antibiotics and antibodies to reach the brain</li> <li>Less restrictive around circumventricular organs, allowing for open communication with the bloodstream</li> </ul> </li> <li>Cerebrospinal fluid (CSF) <ul> <li>Circulates around the brain and spinal cord</li> <li>Produced by the choroid plexus</li> <li>Essentially ultra-filtered plasma</li> </ul> </li> <li>Choroid plexus <ul> <li>Ependymal cells secrete CSF</li> <li>Have tight junctions between them, similar to capillary endothelial cells in the blood-brain barrier</li> </ul> </li> <li>Blood-CSF barrier <ul> <li>Capillaries have open pores, allowing more substances to pass through</li> <li>Less restrictive than the blood-brain barrier</li> <li>Enables certain drugs injected into the CSF to reach the brain</li> <li>Provides a potential entry point for treatment when the blood-brain barrier is too restrictive</li> </ul> </li> </ul>
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FAQs
The blood-brain barrier (BBB) is a highly selective barrier that separates the circulating blood from the brain's extracellular fluid, also known as the interstitial fluid, in the central nervous system. Its primary function is to protect the brain by preventing the entry of harmful substances, such as toxins and pathogens, from the bloodstream while allowing the transport of essential nutrients and molecules required for proper brain function.
The blood-brain barrier is mainly formed by capillary endothelial cells, which are connected by tight junctions. These tight junctions restrict the passage of substances between the cells. In addition to the endothelial cells, other components, such as the basal lamina and the end-feet of astrocytic glial cells, also play a role in maintaining the BBB's integrity and function.
The transport of substances across the blood-brain barrier is regulated through various mechanisms, including passive diffusion, carrier-mediated transport, and receptor-mediated transcytosis. Passive diffusion allows small lipophilic molecules and gases (like oxygen and carbon dioxide) to cross the barrier freely. Carrier-mediated transport utilizes special proteins to transport specific essential nutrients (e.g., glucose, amino acids) and other molecules into the brain. Receptor-mediated transcytosis involves the binding of a substance to specific receptors on the endothelial cells, allowing for its internalization and transport across the barrier.
Cerebrospinal fluid is a clear, colorless liquid that surrounds and cushions the brain and spinal cord, providing mechanical and immunological protection to these delicate structures. CSF serves several functions, including maintaining homeostasis in the central nervous system, providing nutrients to neural tissues, and removing waste products. The principal site of CSF production is the choroid plexus, which is a network of ependymal cells and blood capillaries located within the ventricles of the brain.
Circumventricular organs (CVOs) are specialized regions in the brain where the blood-brain barrier is either weak or absent. These regions are characterized by a high permeability to various substances due to the presence of fenestrated capillaries instead of the tight junctions typically found in the BBB. This unique feature allows the CVOs to serve as sites for monitoring blood-borne signals and facilitating communication between the bloodstream and the brain. Some key CVOs include the organum vasculosum of the lamina terminalis (OVLT), the subfornical organ (SFO), and the area postrema, which are involved in sensing osmotic changes, thirst regulation, and the induction of vomiting in response to toxins, respectively.
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