Blood Brain Barrier and its structure

 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

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