When some dyes, such as trypan blue are injected into the blood, they colour many tissues in the body but do not enter the CNS. Lipid soluble molecules tend to enter the brain whereas hydrophilic molecules do not.
The blood brain barrier however is more than a passive barrier. Brain capillaries contain tight junctions between the endothelial cells, and are surrounded by the end feet of astrocytes that touch capillaries and neuronal cell bodies.
Astrocytes are concerned with the nutrition of neurones, the metabolism of neurotransmitters, and causing vasodilatation in active regions of the brain.
Circumventricular Organs occur in areas where the barrier between CSF and neurones is deficient. These areas occur where molecules need to cross the barrier: in the neurohypophysis (involved in neurosecretion), and in chemosensitive areas where neurones taste the composition of the blood or CSF (e.g., for hormones or osmolality), or in the case of the area postrema, for substances that initate vomiting.
Some other specialised fluid compartments within the CNS contain fluids with unique compositions.
Image source: Christopher Reeve Foundation
Brain capillaries have a different structure to those elsewhere. Tight junctions in the vascular endothelium pose a barrier. The end feet of astrocytes also provide active cellular control over the passage of molecules. Lipid-soluble molecules pass more easily than hydrophilic ones.
Many dyes that can be injected into the blood stream do not colour the brain tissue; one example is trypan blue. This observation is the origin of the concept of the Blood -Brain Barrier: a means of only allowing certain molecules to cross into the brain from the blood, while excluding others.
Nowadays we recognise that the barrier between the blood and the brain is an active rather than a passive barrier, andthat its properties are determined by the activity of astrocytes.
Astrocytes not only form a cellular barrier between brain capillaries and nerve cells, but also participate in the metabolism of neurotransmitters and the regulation of local blood flow to active brain tissue.
Astrocytes have two types of cytoplasmic processes that
make contact with the cell bodies of neurones and their synapses, and
have processes, called end-feet, that make contact with brain blood vessels.
These extensions of the astrocyte allow the astrocyte to control the delivery of nutrients to the nerve cell as well as the removal of transmitters and waste products (such as ammonia which is combines with glutamte to form an innocuus compound, glutamine)
Astrocytes can also communicate with adjacent astrocytes using gliotransmitters that release calcium ons in adjacent astrocytes.
Image source: Armin Kübelbeck
The Astrocyte. Note the presence of 'end feet' on the blood vessel and the processes that extend towards nodes of Ranvier (and synapses).
Astrocytes have numerous projections that anchor neurons to their blood supply. They appear to regulate the external environment of neurons by a selective movement of ions and other chemicals.
Astrocytes are connected to each other by gap junctions, and communicate using calcium and other chemical messengers (gliotransmitters).
Astrocytes are an essential part of the blood-brain barrier, but also have other functions concerned the metabolism and removal of neurotransmitters, and with the redistribution of local blood flow towards active neurones.
Astrocytes can take up various chemicals released at synapses and recycle them. Two examples would be the neurotransmitters glutamate and GABA, released during synaptic transmission, and recycled by conversion to glutamine. Glutamate and GABA are re-synthesised from glutamine locally within the nerve terminal.
Glutamine is also released into the cerebral vessels and metabolised in the liver: hence one of changes seen in liver failure is a change in the EEG and brain function.
Astrocytes also release substances that regulate lthe diameter of local blood vessels, so that local blood flow is matched to the metabolic activity of neurones in that area.
Image source: Slideshare.com
The diagram shows the areas where the blood brain barrier is deficient. These are areas associated with secretory of chemosensitivity that require the movement of molecules in one or other direction across the barrier.
Circumventricular Organs: Areas where the blood-brain barrier is deficient
The circumventricular organs are sites where the blood brain barrier is defective. These sites have specialised functions, allowing certain molecules, such as hormones to move from the neurohypophysis into the blood stream, or for other molecules to move into chemoreceptive areas of the brain.
The sites where the blood-brain barrier is deficient are :
the hypothalamus and third ventricle (the organum vasculosum of the lamina terminalis),
the subfornical organ (involved in thirst),
the pineal gland (secretes melatonin)
the area postrema on the floor of the fourth ventricle.
There are also specialised ependymal cells- tanycytes - in some of these areas.
The area postrema is closely associated with the 'vomiting centre' - a region known to initiate vomiting
Image source: Open Clipart
The diagram shows the anterior chamber of the eye, containing aqueous humour. Aqueous humour is secreted by the ciliary body and passes through the pupil and is absorbed into the canal of Schlemm. The cornea is transparent and avascular, and aqueous humour provides the essential nutrients for this tissue. Changes in the dynamis of aqueous fluid can result in elevated intra-ocular pressure, a condition called glaucoma associated with loss of vision.
Other Fluid Compartments in the CNS.
Ependymal cells that line the cerebro-ventricular system present another barrier between the cerebro-spinal fluid (CSF) and the brain. CSF is produced by the choroid plexuses and has a significantly different composition to plasma.
The anterior chamber of the eye contains a fluid - aqueous humour - produced by the ciliary body. The aqueous humour provides nutrition of the avascular tissues of the cornea.
The inner ear also contains a compartment where the fluid composition is quite different from plasma. Endolymph contains high concentrations of potassium and is present in the cochlear duct and the vestibular apparatus.
These are examples of fluid compartments where composition of the fluid is determined by secretory epitheial cells .