Normal Brain Tissue: Structure, Cells, and Function

Brain tissue is the substance of the brain, an organ that governs thought, emotion, and control over the body. The human brain is comprised of billions of nerve cells arranged in patterns that coordinate our faculties. This network of cells and fibers allows for rapid communication between the brain and the rest of the body.

The Brain’s Cellular Workforce: Neurons and Glia

The brain’s functions are carried out by two primary categories of cells: neurons and glial cells. Neurons, also known as nerve cells, are the fundamental units of the brain and nervous system, with an adult brain estimated to contain around 86 billion. The structure of a neuron is specialized for transmitting information; it consists of a cell body (soma), dendrites, and an axon. The soma contains the nucleus, where genetic material is stored, while dendrites are branch-like extensions that receive signals from other neurons. The axon is a long projection that carries electrical impulses away from the cell body.

Glial cells, or neuroglia, are the support cells of the nervous system and are roughly as numerous as neurons. They perform a variety of functions that help maintain the brain’s environment and support neuronal activity. There are several types of glial cells in the central nervous system, each with a specific role:

• Astrocytes provide structural support and regulate the chemical environment of neurons.
• Oligodendrocytes are responsible for producing the myelin sheath, a fatty substance that insulates axons and increases the speed of electrical signal transmission.
• Microglia act as the brain’s resident immune cells, surveying for injury or infection and clearing cellular debris.
• Ependymal cells line the brain’s ventricles and are involved in producing cerebrospinal fluid (CSF).

Mapping the Brain: Grey Matter and White Matter

Brain tissue is visibly organized into two distinct types: grey matter and white matter. Grey matter is composed mainly of neuronal cell bodies, dendrites, and glial cells, giving it a pinkish-gray color. This tissue is where the processing of information occurs and forms the outer layer of the cerebrum, known as the cerebral cortex.

White matter is primarily made up of myelinated axons bundled together into tracts. The white appearance of this tissue is due to the high lipid content of the myelin sheath. White matter is located beneath the grey matter and serves to transmit signals between different brain regions and the spinal cord.

While in the brain grey matter is on the outside, this arrangement is reversed in the spinal cord, which has a core of grey matter surrounded by white matter. This organization reflects their distinct functions. Grey matter is responsible for processing and integration, while white matter is dedicated to communication.

How Healthy Brain Tissue Functions

The primary function of healthy brain tissue is to facilitate communication between its cells, a process known as neurotransmission. This occurs at specialized junctions called synapses, where signals pass from one neuron to another. When a neuron is activated, it generates an electrical impulse called an action potential, which travels down its axon. Upon reaching the axon terminal, the action potential triggers the release of chemical messengers called neurotransmitters into the synapse. These neurotransmitters then bind to receptors on the dendrites of the neighboring neuron, either exciting or inhibiting it, and thus passing on the message.

This constant communication requires a significant amount of energy. The brain, while only accounting for about 2% of the body’s weight, consumes approximately 20% of the body’s total oxygen and glucose. This high metabolic rate is necessary to fuel the active transport of ions across neuronal membranes, which maintains the electrical potentials required for signal transmission.

A characteristic of healthy brain tissue is its ability to adapt, a property known as synaptic plasticity. The connections between neurons are not fixed; they can be strengthened or weakened based on experience and activity. This continuous remodeling of synaptic connections is the cellular basis for learning and memory.

Keeping the Brain Environment Stable

The brain requires a highly controlled environment, maintained by several protective mechanisms. One of the most important is the blood-brain barrier (BBB), a highly selective semipermeable membrane that separates circulating blood from the brain’s extracellular fluid. The BBB is formed by the endothelial cells that line the brain’s capillaries, which are joined by tight junctions that restrict the passage of substances. Astrocytes also contribute to the integrity of the BBB by wrapping their “end-feet” around the capillaries.

Another component of the brain’s protective system is the cerebrospinal fluid (CSF). This clear fluid is produced by specialized ependymal cells in the choroid plexuses of the brain’s ventricles and circulates through the ventricles and the subarachnoid space. It serves several functions, including providing a cushion against mechanical injury, removing waste products, and maintaining a stable chemical environment for the brain’s cells.

The brain’s immune system also plays a role in maintaining a stable environment. Microglia, the resident immune cells of the brain, are constantly active, surveying their surroundings for signs of damage or infection. In a healthy brain, microglia help to clear away cellular debris and maintain a non-inflamed state.

Visualizing Normal Brain Tissue

Scientists and clinicians use a variety of techniques to visualize and study normal brain tissue. Non-invasive imaging, such as Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) scans, allow for the detailed examination of the brain’s structure. These methods can reveal the distinct boundaries between grey and white matter, the size of the ventricles, and other anatomical features of a healthy brain.

For a more detailed look at the cellular level, scientists use histology, which involves the microscopic examination of tissue samples. These samples are often obtained post-mortem, preserved, sliced into thin sections, and stained with various dyes. This process highlights different cellular components, such as neurons and glial cells, allowing researchers to study the organization of cells within the tissue.

These visualization methods are important for understanding the fundamental biology of the brain. They also provide a baseline against which to compare the changes that occur in various neurological diseases and disorders.