The central nervous system-1

As its name suggests, the Central Nervous System (CNS) is situated in the middle of the body. It serves as the nervous system's command and integration hub. The brain and spinal cord are its two main structural components. They are located along the body's midsection.

Brain

The majority of the CNS, which weighs roughly 1300g in humans, is made up by the brain. It serves as the multifaceted hub for a variety of activities. It serves as the body's central hub for receiving, processing, and acting on impulses from all regions of the body. The adult brain is divided into four main sections:

• Brain stem
  • Medulla oblongata
  • Pons
  • Midbrain
• Cerebellum
  • Diencephalon

• Hypothalamus
  • Thalamus

• Cerebrum

Developmental Classification of Brain

• Fore brain (prosencephalon):
  • Cerebrum/telencephalon (cerebral hemisphere)
  • Diencephalon (thalamus and hypothalamus)
• Mid brain (mesencephalon)
• Hind brain (rhombencephalon):
  • Metencephalon (pons and cerebellum)
  • Myelencephalon (medulla oblongata)

Coverings of the Brain and Spinal Cord (Meninges)

The meninges are the connective tissue coverings that protect the brain and spinal cord. There are three layers in it:

• The dura mater
• The arachnoid mater
• The pia mater

The entire CNS is encircled by all three meningeal layers. Then, bone encircles the meninges. The bony vertebral foramen of the vertebrae that surround the spinal cord are lined with the spinal meninges, which form a tube-like covering around the cord.

Dura Mater

The hard outer layer is called dura mater. From the foramen magnum to the bottom edge of the second sacral vertebra, the spinal dura reaches. The epidural space, which is composed of fat and connective tissue to support and cushion the brain and spinal cord, is located between the dura mater and the vertebrae. Subdural space refers to the area between the dura mater and arachnoid mater. The dural matter is a double-layered membrane that, with some exceptions, is fused together and encloses the dural sinuses.

Arachnoid Mater

The thin membrane that separates the dura and pia maters is called the arachnoid mater. The second sacral vertebra is where the arachnoid mater likewise comes to an end. The arachnoid mater and dura mater are divided by the subdural space. Subarachnoid space refers to the region that exists between the arachnoid and pia mater. Cerebrospinal fluid is present within the subarachnoid space. Arachnoid villi, which are minute folds in the arachnoid layer, serve as one-way channels that let CSF diffuse from the subarachnoid space into the blood in the dural sinuses.

Pia Mater

The innermost fragile membrane with a good supply of blood vessels is called the pia mater. It is the only layer that adheres firmly to the spinal cord and brain. It reaches the first lumbar vertebra's lower border from the foramen magnum.

Blood Circulation in the Brain

The brain requires a constant flow of blood to provide the cells with the glucose and oxygen they require as well as to remove waste products from them. 20% of the glucose and oxygen the body uses is used by the brain. If the brain is deprived of oxygen for longer than 4 minutes, severe brain damage results.

The brain is supplied by two pairs of vertebral arteries and two pairs of internal carotid arteries. Numerous veins and the dural sinuses drain blood from the brain into the internal jugular veins.

Blood-Brain Barrier

The majority of the body's tissues allow for free movement of substances between the capillaries and the cells. The capillary wall's design and the connective tissue that surrounds it, however, restrict movement in the brain's tissues. The blood-brain barrier is made up of these structures. Thus, the capillary network in the brain tissues that restricts the free flow of blood and other substances is the blood brain barrier.

These capillaries are devoid of the pores or openings that can be seen in the delicate walls of other capillaries. Instead, the endothelial cells that make up the capillary wall overlap, resulting in walls that are two cells thick in some places. Additionally, astrocytes' foot processes (cytoplasmic extensions), which create tight junctions in the capillary walls, surround the capillary. The tight junctions' permeability properties are maintained by chemicals that the astrocytes secrete.ermeability properties intact.

This capillary wall, the basement membrane, and an astrocyte are the three passageways through which substances enter the brain tissue. It stops a lot of hazardous chemicals from entering the bloodstream and damaging brain cells. Alcohol, carbon dioxide, glucose, oxygen, and other substances can easily pass through the blood-brain barrier, but proteins and the majority of antibiotics cannot.

Cerebrospinal Fluid (CSF)

The fluid that surrounds and surrounds the brain and spinal cord is called cerebrospinal fluid (CSF), and it is clear and colorless. CSF is one of the bodily fluids that circulates. The choroid plexus, a web of capillaries in the walls of each ventricle, is where it is secreted. The four ventricles in the brain—two lateral ventricles (left and right), one third, and one fourth ventricle—are chambers filled with CSF. Ependymal cells line the ventricles. The largest ventricles inside each cerebral hemisphere are called lateral ventricles. An interventricular foramen connects each lateral ventricle to the third ventricle. The third ventricle is made up of a tiny passageway that runs through the thalamus region and connects the hemispheres. Between the brain stem and the cerebellum, the fourth ventricle connects to the spinal cord's central canal.

In an adult, the total volume of CSF is around 150ml. The volume is constant because it forms at a rate of about 20 ml per hour, or 1500 ml per day, and is absorbed at a similar rate. CSF typically has a pressure of 60 to 150 mmHg and a pH of 7.35. It contains sodium chloride, potassium, calcium, magnesium, glucose, proteins, and urea.

In the same way that CSF is continuously forming, it is also continuously being reabsorbed into the circulation. CSF enters the third ventricle from each lateral ventricle via the interventricular foramen (foramen of Monro) and exits the third ventricle via the cerebral aqueduct to enter the fourth ventricle. From the fourth ventricle, it travels through the cord's narrow central canal, which resembles a tube, and then exits into the subarachnoid regions. Through two lateral apertures known as foramen of Luschka and one median aperture known as foramen of Megendie, it communicates with the fourth ventricle. Then it moves slowly up and around the brain (in the subarachnoid spaces of their meninges), down and around the cord, and back to the blood.

Functions of CSF:

• It shields the spinal cord and brain from physical harm.
• By delivering oxygen, glucose, and other chemicals from the blood to neurons, it nourishes the brain tissues.
• By maintaining the fluid's volume and pressure, it serves as a shock absorber.
• Additionally, it aids in controlling the pressure inside the brain cavities.

Composition of CSF:

• Cell count -0-4cells/cmm
• Glucose- 40-60mg/dl
• Protein-20-45mg/dl
• Chloride- 720-750mg/dl
• Urea-12mg/dl
• Creatinine-1.5mg/dl
• Uric acid-1.5mg/dl
• Cholesterol-0.2mg/dl