Structure of a Neuron
The brain is a very complicated organ. In fact, we have yet to learn everything there is to know about the brain. However, we do know that it is comprised of highly specialized cells called neurons.
Neurons are the building blocks of the nervous system. They send and receive information all over the body using both chemical and electrical signals.
The most common way information is transmitted is through a single neuron electrically and then transmitted to the target cell chemically. The structure of neurons is designed for the most efficient transmission of these signals.
Anatomy of a Neuron
Although neurons look complicated, their design is actually quite simple. The neuron is broken up into two major regions:
- A region for receiving and processing incoming information from other cells
- A region for conducting and transmitting information to other cells
The type of information that is received, processed and transmitted by a neuron depends on its location in the nervous system. For example, neurons located in the occipital lobe process visual information, whereas neurons in the motor pathways process and transmit information that controls the movement of muscles. However, regardless of the type of information, all neurons have the same basic anatomical structure.
The Cell Body
The main portion of the neuron is called the soma, or cell body. In the center of the soma is the nucleus of the cell, which is where the chromosomes that contain all of the genetic material are stored. This is also the part of the cell that creates mRNA for cell replication.
Emerging from the soma are the dendrites and axons. The dendrites are, essentially, appendages that receive signals. Some CNS (central nervous system) dendrites have what are called dendritic spines, little knob-like structures that extend from the dendrite.
Detailed Structure of a Neuron
Dendrites and Synapses
Dendrites create one of the most well-known structures in the brain: the synapse. This is the site of interaction between the neuron and the target cell. Synapses can be located in several places and are classified based on their location:
- Axospinous – found on the dendritic spine
- Axodendritic – found on the dendrite itself
- Axosomatic - found on the soma (cell body)
- Axoaxonic – found on the axon, or tail
Neocortical Pyramidal Neuron
The axon can best be described as the tail of the neuron. It conducts and transmits information and in some cases may receive information.
Some axons have an intermittent coating known as the myelin sheath. This sheath is made of the plasma membrane of glial cells that form a lipid structure and are designed to increase the speed at which information is transmitted.
The gaps between the myelinated axon are called the nodes of Ranvier. At the end of the axon is the axon terminal which contains small vesicles packed with neurotransmitter molecules. These vesicles bind to receptors on the target cells when activated.
Both dendrites and axons are capable of forming multiple synapses. Although neurons only have one axon, this one axon can branch out extensively allowing it to distribute information to multiple target cells. Because of this, neurons can send and receive information to and from numerous targets.
The Myelin Sheath
As stated earlier, the myelin sheath is a multilayered lipid and protein structure that is made up of the plasma membrane of glial cells. In the peripheral nervous system (PNS), the Schwann cell is responsible for myelination. This cell can only myelinate one portion of one nerve cell. It accomplishes this by wrapping itself multiple times around the axon creating a multilayered sheath.
In contrast, oligodendrocytes are responsible for myelination in the central nervous system (CNS). These cells are capable of myelinating portions of up to 40 axons. They do this by extending a thin membrane and wrapping around the axon several times. To maintain this structure, these cells synthesize four times their own weight in lipids per day.
Demyelination in MS
The myelin sheath is the location of a number of diseases that cause degeneration of the myelin sheath, also called demyelinating, such as:
- Multiple Sclerosis
- Optic Neuritis
- Guillain-Barré syndrome
- Transverse Myelitis
- Central Pontine Myelinolysis
- Vitamin B-12 Deficiency
- Chronic Inflammatory Demyelinating Polyneuropathy
The degeneration of the myelin sheath causes degradation of the neural impulses that are transmitted along an axon. The systems affected by this degradation depend on the location of the degenerating myelin. For example, multiple sclerosis (MS) affects the neurons of the spinal cord as well as the brain leading to degradation of both motor and cognitive functioning.
Other Cells Associated With Neurons
Astrocytes are star-shaped cells that supply nutritional and physical support for the neurons. They also guide the migrating neurons to their adult destination during the developmental stage of the central nervous system.
These cells also provide services such as phagocytosis (cellular “trash removal”) and regulating the extracellular fluid along with providing a carbon source from lactate (via glucose metabolism) for the neurons.
Microglial cells, as their name suggests, are small. In fact, they are the smallest glial cells in the nervous system and act like immune cells, destroying micro-organisms and phagocytose cellular debris or “trash.”
The central nervous system and spinal cord are lined with ciliated cells called ependymal cells. The ependymal cells in the brain specifically secrete cerebrospinal fluid (CSF) into the ventricular system. The beating of their cilia efficiently circulate the CSF throughout the central nervous system.
© 2013 Melissa Flagg