BLOOD-BRAIN BARRIER
Blood-brain barrier (BBB) is a neuroprotective structure that prevents the entry of many substances and pathogens into the brain tissues from blood. It was observed more than 50 years ago, that when trypan blue, the acidic dye was injected into living animals, all the tissues of body were stained by it, except the brain and spinal cord. This observation suggested that there was a hypothetical barrier, which prevented the diffusion of trypan blue into the brain tissues from the capillaries. This barrier was named as blood-brain barrier (BBB). It exists in the capillary membrane of all parts of the brain, except in some areas of hypothalamus.
STRUCTURE OF
BLOOD-BRAIN BARRIER
Tight
junctions in the endothelial cells of brain capillaries are responsible for BBB
mechanism. In capillaries of other organs, adjacent endothelial cells leave the
cleft called fenestra, which allows
transcytosis
of several substances through endothelium. However, in
capillaries of brain, fenestra
are absent
because, the endothelial cells fuse with each other by tight junctions. Tight
junctions are formed between endothelial cells of the capillaries at childhood.
At the same time, cytoplasmic foot processes of astrocytes (neuroglial cells)
develop around capillaries and reinforce the barrier. Astocytes envelop the
vasculature almost completely. Pericytes also form the important cellular constituent
of BBB. These cells play an important role in formation and maintenance of
tight junction and structural stability of the barrier. In brain, pericytes function
as macrophages and play an important role in
the
defense.
FUNCTIONS OF
BLOOD-BRAIN BARRIER
BBB acts
as both a mechanical barrier and transport mechanisms. It prevents potentially
harmful chemical substances and permits metabolic and essential materials into
the brain tissues. By preventing injurious materials and organisms, BBB
provides healthy environment for the nerve cells of brain.
Substances which can
Pass through Blood-Brain Barrier
1. Oxygen
2. Carbon
dioxide
3. Water
4. Glucose
5. Amino
acids
6.
Electrolytes
7. Drugs
such as L-dopa, 5-hydroxytryptamine sulfona mides, tetracycline and many
lipid-soluble drugs
8.
Lipid-soluble anesthetic gases such as ether and nitrous oxide
9. Other
lipid-soluble substances.
Substances which
cannot Pass through Blood-Brain Barrier
1.
Injurious chemical agents
2.
Pathogens such as bacteria
3. Drugs
such as Penicillin and the catecholamines.
Dopamine
also cannot pass through BBB. So, parkinsonism is treated with L-dopa, instead
of
dopamine. Bile
pigments: However, since the barrier is not well developed in infants, the bile
pigments enter the brain tissues. During jaundice in infants, the bile pigments enter brain and causes damage of basal ganglia, leading to kernicterus.
BLOOD-CEREBROSPINAL FLUID BARRIER
Blood-CSF
barrier is the barrier between blood and cerebrospinal fluid that exists at the
choroid plexus.
The
function of this barrier is similar to that of BBB. It does not allow the movement
of many substances from blood to cerebrospinal fluid. It allows the movement of
only those substances which are allowed by BBB.
APPLIED PHYSIOLOGY –HYDROCEPHALUS
Abnormal
accumulation of CSF in the skull, associated with enlargement of head is called
hydrocephalus.
During
obstruction of any foramen, through which CSF escapes, the ventricular cavity
dilates and this condition is called internal hydrocephalus. It is also known as non-communicating hydrocephalus.
On the
other hand, if the arachnoid villi are blocked, external or communicating
hydrocephalus occurs.
Hydrocephalus
along with increased intracranial pressure causes headache and vomiting. In
severe
conditions,
it leads to atrophy of brain, mental weakness and convulsions
0 Comments