Question

Blood brain barrier in adults have cells ____ junctions between (a) tight (b) adhering (c) gap (d) none of these

Short answer

The blood-brain barrier in adults have cells with (a) tight junctions between them.

Step-by-step solution

Understanding the Blood-Brain Barrier

The blood-brain barrier (BBB) is a highly selective permeability barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system. Its main function is to protect the brain from chemicals and pathogens in the blood that may be harmful, while allowing essential nutrients to pass through.

Identify the Junctions in the BBB

The BBB is formed by endothelial cells that line the brain's capillaries. These cells are tightly packed together and have special junctions that restrict the passage of substances. The type of junctions are crucial in maintaining the selectivity of the BBB.

Determine the Type of Junction

There are various types of cell junctions that contribute to the function of tissues and organs. In the context of the BBB, the cells have tight junctions that restrict the movement of substances between the blood and the brain.

Key concepts

Tight Junctions

The remarkable ability of the brain to maintain a stable and protected environment is largely attributed to the presence of tight junctions within the blood-brain barrier (BBB). These tight junctions are specialized connections between endothelial cells in the brain's capillaries. Unlike other types of cell junctions, tight junctions form a near-impermeable seal, preventing most substances from passing through the spaces between the cells.

Think of tight junctions as the vigilant security guards at the gates of a fortress – the fortress, in this case, being the central nervous system. They meticulously control what can and cannot enter, making sure that harmful substances are denied access while allowing vital elements like oxygen and glucose to pass. These tight junctions are crucial for maintaining the delicate balance of neuronal activity by regulating the ionic composition of the extracellular fluid, which contributes to nerve impulse transmission.

The effectiveness of tight junctions is measured in terms of their 'leakiness,' ranging from 'leaky' to 'tight'. In the brain's capillaries, these junctions are exceptionally tight, reflecting the need for a highly selective barrier.

Understanding how tight junctions work, and what they allow to pass, can greatly benefit anyone trying to comprehend the intricacies of pharmacology and neurobiology. For instance, when developing treatments for neurological diseases, the permeability of these junctions can pose significant challenges for drug delivery into the brain.

Endothelial Cells

Endothelial cells are the building blocks of capillaries throughout the body, including the blood-brain barrier. These cells are not just passive liners of blood vessels; they play an active and critical role in vascular biology. In the BBB, endothelial cells are packed tightly to form a meticulous barrier that carefully regulates what can enter the brain's extracellular space.

The endothelium of the brain is unique compared to endothelial cells in other parts of the body. In the brain, they possess an impressive array of properties: they express transporters for essential nutrients, metabolize neurotransmitters, and contribute to the immune defense of the brain. Moreover, their tight junctions block the paracellular route, which is the space between the cells, ensuring that substances must be selectively transported through the cell membranes.

Endothelial cells in the BBB also have fewer vesicles compared to those in other tissues, which reduces the amount of material that can be transported via transcytosis (a method of transporting molecules within vesicles). They also have a unique enzyme, known as gamma-glutamyl transpeptidase, which is involved in the metabolism of glutathione and can protect the brain from oxidative stress.

Understanding how these cells maintain the protective shield of the BBB can offer insights into various neurological conditions where the barrier is compromised, such as multiple sclerosis, Alzheimer's disease, and stroke.

Central Nervous System

The central nervous system (CNS) is the command center of our body, comprising the brain and spinal cord. It processes information and orchestrates behavior by transmitting and receiving an elaborate network of nerve signals. However, for the CNS to function correctly, its environment needs to be precisely regulated, which is where the blood-brain barrier plays an essential role.

The CNS is particularly sensitive to fluctuations in the composition of its surrounding fluid. Substances in the blood, such as hormonal fluctuations or minor changes in electrolyte concentrations, which are inconsequential to most other tissues, can disrupt neurological function. Therefore, the BBB acts as both a guardian and a regulator, protecting neural tissue from harmful pathogens and maintaining homeostasis for optimal neural function.

In addition to protection, the CNS requires a steady supply of nutrients for energy and building blocks for growth and repair, which the BBB selectively allows to pass. Also, waste products that result from normal CNS activity need to be efficiently removed, which again is facilitated by the BBB.

For students exploring the fields of neuroscience, psychology, or medicine, understanding the CNS and its symbiotic relationship with the blood-brain barrier offers a fundamental insight into how the brain remains protected while performing complex tasks that make us who we are.