Central nervous system

The nervous system has two main parts: the central nervous system and the peripheral nervous system. The central nervous system includes the brain and spinal cord. They are responsible for coordinating and integrating nervous information. Both the brain and spinal cord are protected by connective tissue membranes called the meninges and are suspended in cerebrospinal fluid. They are completely shielded by the skull and vertebral column.

Histologically, the central nervous system consists neurons, which are excitable cells supported by neuroglia. Neurons have cell bodies and long axons. The cell bodies, or soma, make up the gray matter, while the axons form the white matter. The gray matter appears grayish, while the white matter looks white due to the lipid material in the myelin sheaths around the axons.

The brain is located in the cranial cavity and is divided into three parts: forebrain, midbrain, and hindbrain:

• The forebrain includes two cerebral hemispheres (a.k.a. telencephalon) and the diencephalon nestled deep within it (containing structures like the thalamus and hypothalamus). The surface of the cerebral hemispheres is made of gray matter and is folded to create elevations (gyri) and depressions (sulci). Inside, there is a core of white matter that contains gray matter nuclei, the basal nuclei, deep within it.

• The midbrain, also called mesencephalon, connects the forebrain and hindbrain.

• The hindbrain (or rhombencephalon) is divided into the metencephalon (pons and cerebellum) and myelencephalon (medulla oblongata), with the medulla oblongata extending into the spinal cord.

The spinal cord resides within the vertebral canal and consists of 31 spinal segments, each giving rise to a pair of spinal nerves. It terminates inferiorly in the lumbar region, tapering off to form a cone-shaped region called the conus medullaris. Similar to the brain, the spinal cord also has gray matter and white matter. The gray matter, forming the inner core, is shaped like a butterfly with anterior and posterior horns. The anterior horns are motor, while posterior horns serve a sensory function. Additionally, in the thoracolumbar regions, there are lateral gray horns in the spinal gray matter, that provide sympathetic innervation. The gray matter is surrounded by white matter, which consists of neuronal axons and can be further divided into distinct regions such as the anterior, lateral, and posterior white columns.

The cranial and spinal nerves come from the brain and spinal cord, making up the peripheral nervous system. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves inside our body.