Brain Cells Definition
Brain cells, also known as neurons, are the cells in the brain that send and receive electrical and chemical signals. They are the building blocks of the brain and transmit information throughout the central nervous system.
In addition to neurons, there are also glial cells that provide structure in the brain. The rest of the brain tissue is structural or connective called stroma which includes blood vessels, connective tissues, and other supporting structures.
What are Brain Cells?
Brain cells are the fundamental units of the brain and nervous system, responsible for receiving sensory input from the external world, sending motor commands to our muscles, and transforming and relaying electrical signals at every step in between.
There are two main types of cells in the brain: neurons and glial cells. Neurons are information messengers that use electrical and chemical signals to send information between different areas of the brain, as well as between the brain, spinal cord, and entire body.
They have three basic parts: a cell body or soma where the nucleus lies; dendrites that receive input from other cells; and an axon that sends messages from the cell.
Glial cells were originally thought to do nothing more than hold the nervous system together like glue. However, we now know that they have important functions such as communicating with each other and with neurons.
Glial cells also aid in maintaining homeostasis in the brain by removing debris and buildups. Astrocytes are star-shaped cells that surround neurons. They are the most abundant of all glial cells and have many important functions such as aiding in homeostasis maintenance.
Types of Brain Cells
The two main types of brain cells are neurons and glial cells.
What Are Neurons?
Neurons, also called nerve cells, are the fundamental units of the brain and nervous system. They are specialized, impulse-conducting cells that receive and send messages from the body to the brain and back to the body.
Neurons are electrically excitable cells that fire electric signals called action potentials. They communicate with other neurons via synapses in neural circuits and larger networks.
A neuron consists of a cell body and its processes, including dendrites that receive signals from other neurons and axons that send signals to other neurons or tissues.
Structure of Neurons
A neuron has three main parts: dendrites, an axon, and a cell body or soma. The dendrites are the tree-like branches of a neuron that receive input from other cells. The cell body is the trunk of the tree and contains the nucleus and other organelles.
The axon is a long, thin fiber that extends from the cell body and carries electrical signals called action potentials to other neurons or tissues.
A typical neuron consists of a compact structure called soma, filaments called dendrites that receive signals from other neurons, and a single axon that sends signals to other neurons or tissues. Neurons vary in size, shape, and structure depending on their role and location in the nervous system.
Cell body
The cell body, also known as soma or perikaryon, is the core section of a neuron that contains genetic information, maintains the neuron’s structure, and provides energy to drive activities.
It is the metabolic center of the neuron and houses the nucleus and other organelles. The soma is a bulbous, non-process portion of a neuron or other brain cell. It supports the dendrites and axon by providing nutrients and energy for their functions.
The cell body is responsible for integrating incoming signals from dendrites and generating outgoing signals to axons.
Dendrites
Dendrites are membranous tree-like projections that arise from the body of a neuron and extend about 2 μm in length. They are short, branching fibers that extend from the cell body of the nerve cell.
Dendrites usually branch extensively, forming a dense canopy-like arborization called a dendritic tree around the neuron. They receive input from other cells and conduct impulses toward the cell body.
Dendrites branch as they move towards their tips, just like tree branches do, and this branching allows them to receive input from multiple sources simultaneously.
Axon
An axon, also known as a nerve fiber, is a long, slender projection of a neuron that conducts electrical impulses known as action potentials to other neurons or tissues.
It is responsible for transmitting electrical signals to help with sensory perception and movement. Each axon is surrounded by a myelin sheath, which is a fatty layer that insulates the axon and helps it transmit signals over long distances.
An axon typically develops side branches called axon collaterals so that one neuron can send information to several others. Axons conduct electrochemical impulses or action potentials and are most commonly associated with sending information away from the cell body of the neuron.
Types of Neurons
Based on their roles, the neurons found in the human nervous system can be divided into three classes: sensory neurons, motor neurons, and interneurons.
Sensory Neurons
Sensory neurons, also known as afferent neurons, are the nerve cells in the nervous system that convert a specific type of stimulus via their receptors. They detect and respond to external signals from the environment, such as touch, sound, light, and temperature.
Sensory neurons transmit impulses from a receptor to a more central location in the nervous system.
For example, when you touch a hot surface with your fingertips, sensory neurons will be activated and send signals to the rest of the nervous system about the information they have received.
Sensory neurons consist of a cell body, axon, and dendrites. Dendrites are responsible for receiving signals from other neurons or tissues.
Motor Neurons
Motor neurons are a specialized type of brain cell called neurons located within the spinal cord and the brain. They are responsible for controlling muscle movement throughout the body.
Motor neurons come in two main subtypes: upper motor neurons and lower motor neurons. The upper motor neurons originate in the brain and travel downward to connect with the lower motor neurons.
Lower motor neurons, on the other hand, carry signals from the spinal cord to muscles and glands. Motor neurons allow us to move, speak, swallow, and breathe by sending commands from the brain to the muscles.
They comprise various tightly controlled complex circuits throughout the body that allow for both voluntary and involuntary movements.
Interneurons
Interneurons, also known as association neurons, are a type of neuron that is found exclusively in the central nervous system. They connect spinal motor and sensory neurons and transfer signals between them.
Interneurons lie between sensory and motor neurons and are also known as connector neurons or local circuit neurons. There are about 20 billion interneurons in the body, most of which are in the CNS but others are located in the peripheral nervous system.
Interneurons play a crucial role in processing information within the central nervous system by integrating signals from multiple sources and generating appropriate responses. They help to regulate reflexes, coordinate muscle movements and modulate sensory input.
What are Glial Cells?
Glial cells, also known as neuroglia or just glia, are a type of cell that provides physical and chemical support to neurons and maintains their environment. They are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system.
Glial cells consist of microglia, astrocytes, oligodendrocyte lineage cells, Schwann cells, and radial glial cells. Microglia act as immune cells in the brain and protect it from infections.
Astrocytes provide structural support to neurons and help regulate the chemical environment around them. Oligodendrocyte lineage cells produce myelin sheaths that insulate axons in the central nervous system while Schwann cells do so in the peripheral nervous system.
Radial glial cells are progenitor cells that can generate neurons, astrocytes, and oligodendrocytes. Glial cells play an important role in maintaining homeostasis within the nervous system by regulating neurotransmitter levels, modulating synaptic activity, and providing metabolic support to neurons.
Types of Glial Cells
There are several types of glial cells in the nervous system. In the mature central nervous system, there are three types of glial cells: astrocytes, oligodendrocytes, and microglial cells.
Oligodendrocytes
Oligodendrocytes are specialized glial cells that wrap themselves around neurons present in the central nervous system (CNS) and provide support and insulation to axons.
They are the myelinating cells of the CNS, which produce myelin sheaths that insulate axons and allow rapid saltatory conduction of nerve impulses.
Oligodendrocytes are the end product of a cell lineage that undergoes a complex and precisely timed program of proliferation, migration, differentiation, and myelination to finally produce the insulating sheath of axons.
They play an important role in maintaining homeostasis within the nervous system by regulating neurotransmitter levels, modulating synaptic activity, and providing metabolic support to neurons. Dysfunction or loss of oligodendrocytes can lead to several neurological disorders such as multiple sclerosis.
Astrocytes
Astrocytes are a sub-type of glial cells in the central nervous system that outnumber neurons by over fivefold.
They are star-shaped cells that maintain a neuron’s working environment by controlling the levels of neurotransmitters around synapses, regulating blood flow to the brain, and providing metabolic support to neurons.
Astrocytes contiguously tile the entire central nervous system (CNS) and exert many essential complex functions in the healthy CNS. During development, radial glial cells are the primary neural stem cells developing all neurons such as astrocytes, microglia cells, and oligodendrocytes.
Astrocytes play an important role in maintaining homeostasis within the nervous system by modulating synaptic activity, regulating extracellular ion concentrations, and removing excess neurotransmitters from synaptic clefts.
Dysfunction or loss of astrocytes can lead to several neurological disorders such as Alzheimer’s disease and epilepsy.
Ependymal Cells
Ependymal cells are a type of glial cell that lines the ventricles in the brain and the central canal of the spinal cord. They form a continuous epithelial sheet called the ependyma.
Ependymal cells develop from radial glia along the surface of the ventricles of the brain and spinal canal. They play a critical role in cerebrospinal fluid (CSF) homeostasis, brain metabolism, and clearance of waste from the brain.
Ependymal cells are involved in creating CSF, which is essential for cushioning and protecting the brain and spinal cord from injury. Dysfunction or loss of ependymal cells can lead to several neurological disorders such as hydrocephalus, which is characterized by an accumulation of CSF in the brain.
Microglia
Microglia are the primary immune cells of the central nervous system (CNS) and are similar to peripheral macrophages. They are cells of mesodermal/mesenchymal origin that migrate into the CNS to become resident macrophages within the unique brain microenvironment.
Microglia constantly patrol the cerebral microenvironment to respond to pathogens and damage. They act as immune sentinels that respond to pathogens and injury by changing their morphology, releasing cytokines, and phagocytosing cellular debris.
Microglia play a crucial role in maintaining homeostasis within the nervous system by regulating synaptic activity, modulating neurogenesis, and providing metabolic support to neurons.
Dysfunction or overactivation of microglia can lead to several neurological disorders such as Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis.
Losing Brain Cells
Brain atrophy, also known as cerebral atrophy, is the loss of brain cells called neurons and their connections with each other. This loss can lead to problems with thinking, memory, and performing everyday tasks.
There are two types of brain atrophy: focal and generalized. Focal atrophy affects cells in certain areas of the brain and results in a loss of function in those specific areas. Generalized atrophy affects cells all over the brain.
Brain atrophy occurs naturally as people age, but healthcare providers use the term “brain atrophy” when a person has more brain changes than expected for their age. Brain damage is an injury that causes the destruction or deterioration of brain cells. Brain damage can be caused by trauma, stroke, tumor, or other illnesses.
The symptoms of cerebral atrophy depend on which part of the brain is affected. Some common symptoms include memory loss, loss of coordination, localized weakness or paralysis, blurred or double vision, and difficulty speaking or understanding speech.
Life expectancy among patients with brain atrophy can be influenced by the condition that caused brain shrinkage. People with Alzheimer’s disease live an average of four to eight years after their diagnosis. Those with multiple sclerosis can have close to a normal lifespan if their condition is well-managed.
What is Neurogenesis?
Neurogenesis is the process by which new neurons are formed in the brain. It is most active during embryonic development and is responsible for producing all the various types of neurons of the organism, but it continues throughout adult life in a variety of organisms.
Until recently, neuroscientists believed that the central nervous system, including the brain, was incapable of neurogenesis and unable to regenerate. However, stem cells were discovered in parts of the adult brain in the 1990s, and adult neurogenesis is now accepted to be a normal process that occurs.
Adult neurogenesis is a process of generating functional neurons from adult neural precursors that occurs throughout life in restricted brain regions in mammals.
These regions are called neurogenic niches and include two discrete regions: the subventricular zone (SVZ) lining lateral ventricles and the subgranular zone (SGZ) within the hippocampal dentate gyrus (DG).
Cellular elements that form these neurogenic niches include endothelial cells, ependymal cells, astrocytes, microglia, mature neurons, and progeny of adult neural precursors. Vascular cells play a prominent role in regulating the proliferation of adult neural precursors.
Neurogenesis has been shown to occur not only in animals but also in humans. It has been linked to learning and memory as well as mood regulation. The discovery of adult neurogenesis has opened up new avenues for research into treatments for neurological disorders such as Alzheimer’s disease and depression.
