THE BRAIN FROM TOP TO BOTTOM Click to enlarge NEURONS The human nervous system is composed of two types of cells: glial cells and neurons. Neurons are the ones that make us so clever. Like all the cells in the human body, every neuron has a membrane that surrounds its cytoplasm and a nucleus that contains its genes. Neurons also have small organelles that let them produce energy and manufacture proteins. But because the neurons’ main job is to transmit information, they also have two types of highly specialized extensions that distinguish them from other cells. Dendrites, with their tree-like branching structure, gather information and relay it to each neuron’s cell body. Axons are generally very long, and each neuron has only one. This axon carries information away from the neuron’s cell body toward other neurons , with which it makes connections called synapses . Axons can also directly stimulate other types of cells, such as muscle and gland cells. GLIAL CELLS The human nervous system is composed of two types of cells: glial cells and neurons . Compared with neurons, glial cells play a less obvious role in intercellular communication, so you tend to hear about them less often.

And when you do hear about them, it is often for the wrong reason, such as the outdated idea that while the brain contains about 100 billion neurons, it also contains 5 to 50 times as many glial cells. These latter figures are still cited frequently in neuroscience textbooks, but the consensus figure was actually revised downward early in the first decade of the 21st century, when several studies suggested that the ratio of glial cells to neurons is actually much lower: about 1 to 1 (in other word, that the brain contains just about 1 billion glial cells and 1 billion neurons). But that does not mean they are not essential. In fact, they are so essential that without them, the neurons could not work properly. The glial cells provide the neurons with nourishment, physical support, and protection. Glial cells also dispose of the waste materials generated when neurons die, and accelerate neural conduction by acting as an insulating sheath around certain axons. Definition of Glial cell Definition of Glial cell Brain Foods Slideshow Glial cell: A supportive cell in the central nervous system . Unlike neurons, glial cells do not conduct electrical impulses.

The glial cells surround neurons and provide support for and insulation between them. Glial cells are the most abundant cell types in the central nervous system. Types of glial cells include oligodendrocytes, astrocytes, ependymal cells, Schwann cells, microglia, and satellite cells. Last Editorial Review: 5/13/2016 Glial Cells Glial Cells What is RSS? BRAIN TEST Wanna get to the head of your class? Have your choice of college? Try Lumosity Brain Training for Students! Free Trial! ADHD TEST Glial Cells - Your Brain's Support Network You probably know about neurons. They're the brain cells charged with electricity that help you do your thinking. But do you know about glial cells? Glial cells don't get nearly as much press, but they're even more important to your brain's functioning than neurons. What Are Glial Cells Glial cells are known by many names - neuroglia or glia to name just a few. In the past, they were sometimes mistaken for brain cells but they're completely different. They're nervous system cells that are found in your brain and spinal cord.

You may be surprised that there are actually more glial cells than neurons in your brain. The Important Functions Of Glial Cells While neurons do the thinking, glial cells maintain your brain and keep house for them to operate the most efficiently. They have a number of important supportive functions. First of all, they hold your neurons in place. They insulate and protect them from outside influence. They also form a transportation network that brings oxygen and essential nutrients to your neurons. Neurons need these in order to perform their own complex functions. Finally, the glial cells destroy and carry away dead neurons that your brain doesn't need anymore. The Many Types Of Glial Cells Not all glial cells are the same. There are a number of different types that each has its own function. Astrocytes are found in the brain's capillaries. They manage the blood supply and keep other substances from entering the blood. Microglia are, as the name suggests, very small and their main duty is to remove wastes from the neurons. Oligodendocytes concern themselves primarily with insulating the neurons in a protective myelin sheath. There are other types as well, and researchers still continue to find new ones.

Glial Cells Fight Disease And Injury When you have a disease or injury that affects the brain, the glial cells fight back. Parkinson's Disease attacks glial cells, and they declare war and fight to the death against the illness. Traumatic injuries to the head can damage important areas of glial cells, which prevents oxygen from getting to the brain. Scientists are now studying these cells more closely to see how they are impacted by illness and injury. Their hope is to use this research to more successfully fight ailments of the brain. Glial cells are just as important to your brain's functioning as your neurons, if not more so. They're like the little army that supports your brain and bravely fights off all who try to harm it. Research on glial cells is ongoing and hopefully will lead to even better treatments for brain and nervous system disorders. Want To Learn More? We recommend investing in this guide to human physiology and anatomy . It will help you learn anatomy, including the brain, very efficiently and quicly!. Plus it's a fantastic reference for the entire human body with an extensive section on the brain.

SEER Training:Neurons & Glial Cells Neurons & Glial Cells Neurons & Glial Cells Neurons Neurons are the conducting cells of the nervous system. A typical neuron consists of a cell body, containing the nucleus and the surrounding cytoplasm ; several short radiating processes (called dendrites); and one long process (called the axon), which terminates in twiglike branches and may have branches projecting along its course. Cell Body In many ways, the cell body is similar to other types of cells. It has a nucleus with at least one nucleolus and contains many of the typical cytoplasmic organelles. It lacks centrioles , however. Because centrioles function in cell division, the fact that neurons lack these organelles is consistent with the amitotic nature of the cell. Dendrites and Axons An axon is a long, hair-like extension of a nerve cell that carries a message to another nerve cell. Dendrites are thread-like extensions of the cytoplasm of a neuron that receive signals from other neurons. Typically, as in multipolar neurons, dendrites branch into treelike processes, but in unipolar and bipolar neurons, dendrites resemble axons.

Glial Cells Glial (Neuroglial) cells do not conduct nerve impulses, but, instead, support, nourish, and protect the neurons. Glial cells are far more numerous than neurons and, unlike neurons, are capable of mitosis . For more information about anatomy of brain and CNS, go to the Nervous System section of the Anatomy & Physiology module on this Web site. Nervous Tissue Glial Cells Structure and Function By Regina Bailey What Are Neuroglia? Neuroglia, also called glial cells, are cells of the nervous system . They compose a voluminous support system that is essential to the proper operation of nervous tissue and the nervous system. Unlike neurons , glial cells do not have axons, dendrites or conduct nerve impulses. Neuroglia are typically smaller than neurons and are about three times more numerous in the nervous system. Glia perform a plethora of functions in the nervous system . These functions include providing support for the brain , assisting in nervous system repair and maintenance, assisting in the development of the nervous system, insulating neurons, and providing metabolic functions for neurons.

Neuroglia: Nervous Tissue Glial Cells There are several types of glial cells present in the central nervous system (CNS) and peripheral nervous system of humans. Six main types of neuroglia include: Astrocytes Astrocytes are found in the brain and spinal cord . They are star-shaped neuroglia that reside in endothelial cells of the CNS that form the blood-brain barrier . This barrier restricts what substances can enter the brain. Protoplasmic astrocytes are found in the gray matter of the cerebral cortex , while fibrous astrocytes are found in white matter of the brain. Other functions of astrocytes include glycogen storage, nutrient provision, ion concentration regulation, and neuron repair. continue reading below our video 10 Best Universities in the United States Ependymal Cells Ependymal cells are specialized cells that line the cerebral ventricles and central canal of the spinal cord . They are found within the choroid plexus of the meninges . These ciliated cells surround the capillaries of the choroid plexus and form cerebrospinal fluid.

Microglia Microglia are extremely small cells of the central nervous system that remove cellular waste and protect against microorganisms ( bacteria , viruses , parasites , etc.). Microglia are thought to be macrophages , a type of white blood cell that protects against foreign matter. They also help to reduce inflammation through the release of anti-inflammatory cytokines. Satellite Cells These glial cells cover and protect neurons of the peripheral nervous system . They provide structure and metabolic support for sensory, sympathetic, and parasympathetic nerves. Oligodendrocytes Oligodendrocytes are central nervous system structures that wrap some neuronal axons to form an insulating coat known as the myelin sheath. The myelin sheath, composed of lipids and proteins , functions as an electrical insulator of axons and promotes a more efficient conduction of nerve impulses. Schwann Cells Schwann cells are neuroglia that wrap around some neuronal axons to form the myelin sheath in peripheral nervous system structures. Schwann cells help to improve nerve signal conduction, assist in nerve regeneration, and aid in antigen recognition by T cells .

Oligodendrocytes and Schwann cells indirectly assist in the conduction of impulses as myelinated nerves can conduct impulses quicker than unmyelinated ones. Interestingly enough, the white matter in the brain gets its color from the large number of myelinated nerve cells that it contains. Animal Tissue Types To learn more about animal tissues, visit: Nervous Tissue - primary tissue of the central nervous system that is composed of neurons. Epithelial Tissue - covers the outside of the body and lines organs . Connective Tissue - supports and attaches tissues to other underlying tissues. Muscle Tissue - tissue capable of contraction and movement. Sources: Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Neuroglial Cells. Available from: The Root of Thought: What Do Glial Cells Do? - Scientific American Scientific American Mind The Root of Thought: What Do Glial Cells Do? Nearly 90 percent of the brain is composed of glial cells, not neurons.

Andrew Koob argues that these overlooked cells just might be the source of the imagination Advertisement | Report Ad Andrew Koob received his Ph.D. in neuroscience from Purdue University in 2005, and has held research positions at Dartmouth College, the University of California, San Diego, and the University of Munich, Germany. He's also the author of The Root of Thought , which explores the purpose and function of glial cells, the most abundant cell type in the brain. Mind Matters editor Jonah Lehrer chats with Koob about why glia have been overlooked for centuries, and how new experiments with glial cells shed light on some of the most mysterious aspects of the mind. LEHRER: Your new book, The Root of Thought , is all about the power of glial cells , which actually make up nearly 90 percent of cells in the brain. What do glial cells do? And why do we have so many inside our head? KOOB: Originally, scientists didn't think they did anything. Until the last 20 years, brain scientists believed neurons communicated to each other, represented our thoughts, and that glia were kind of like stucco and mortar holding the house together.

They were considered simple insulators for neuron communication. There are a few types of glial cells, but recently scientists have begun to focus on a particular type of glial cell called the 'astrocyte,' as they are abundant in the cortex. Interestingly, as you go up the evolutionary ladder, astrocytes in the cortex increase in size and number, with humans having the most astrocytes and also the biggest. Scientists have also discovered that astrocytes communicate to themselves in the cortex and are also capable of sending information to neurons. Finally, astrocytes are also the adult stem cell in the brain and control blood flow to regions of brain activity. Because of all these important properties, and since the cortex is believed responsible for higher thought, scientists have started to realize that astrocytes must contribute to thought. LEHRER: Why have glia been neglected for so long? KOOB: To understand this, you have to take a tour of the history of brain science. Glia were mainly a sidebar for 200 years in the struggle over the idea of the neuron. A few highlights were: In the late 18th century, scientists discovered the electrical properties of the neuron in the spine of frogs.

Neurons have long tethers that are easy to study called 'axons' that extend from the cell body from the brain into the spine and the spine out to the limbs and body. Similarly, neurons in the senses were linked to the neurons in the brain. This is where the notion of neurons as the base of our thoughts took root. In the mid-19th century, glia were just being discovered, and researchers figured the glial cells simply held the neurons together (glia is greek for glue). What I find sort of hilarious is that scientists stumbled upon a very numerous cell in the brain, an organ responsible for our thoughts and personality, but they were so focused on neurons that they concluded the new cell was worthless. In the late 19th century a staining method was developed to look at cells more effectively in the brain. A brilliant researcher from Spain, Santiago Ramon y Cajal, took it upon himself to study the brain from the perspective of neurons. He meticulously mapped out a scheme for how they process information and are connected, which led to "The Neuron Doctrine." ("The Neuron Doctrine" is a belief that neurons are responsible for our thoughts.) However, Cajal seemed inconvenienced by glial cells.

They were very numerous and obviously hanging out all over the cortex. Meanwhile, his brother Pedro, who was also a scientist, developed the theory that glial cells were 'support cells' that insulated neuron electrical properties. Cajal decided to back his brother's theory. And since 1906 when he won the nobel prize, this has been the dogma. LEHRER: Could you describe some of the early experiments that first led scientists to reconsider the role of glial cells? KOOB: Glial experiments didn't get going until the 1960s. All scientists knew about glia was that if you put neurons in petri dish, you had to have glia, or neurons would die. Then, Stephen W. Kuffler at Harvard, for reasons unknown, decided to test Pedro's accepted theory of insulation. This was around same time that cell counts in the brain revealed glial cells to be nearly 90% of the brain (this is where the neuron based idea that we only use 10% of our brain comes from). Kuffler is notable because he ironically established the Harvard 'neuro' biology department while he was performing these groundbreaking glial experiments.

Anyway, Kuffler took astrocytes from the leech and mud puppy and added potassium, something that is known to flow out of neurons after they are stimulated. He thought this would confirm Pedro's theory that glial cells were insulators. What he found instead was that the electrical potential of glial cells responded to potassium. Kuffler and colleagues found that astrocytes exhibited an electrical potential, much like neurons. They also discovered in the frog and the leech that astrocytes were influenced by neuronal ion exchange, a process long held to be the chemical counterpart to thought. Since then many researchers have completed experiments on the communicatory ability of glial cells with neurons, including in the late 80s and early 90s when it was discovered glial cells respond to and release 'neuro' transmitters. LEHRER: Why are calcium waves important? KOOB: In short, calcium waves are how astrocytes communicate to themselves. Astrocytes have hundreds of 'endfeet' spreading out from their body. They look like mini octopi, and they link these endfeet with blood vessels, other astrocytes and neuronal synapses.

Calcium is released from internal stores in astrocytes as they are stimulated, then calcium travels through their endfeet to other astrocytes. The term 'calcium waves' describes the calcium release and exchange between astrocytes and between astrocytes and neurons. Scientists at Yale, most notably Ann H. Cornell-Bell and Steven Finkbeiner, have shown that calcium waves can spread from the point of stimulation of one astrocyte to all other astrocytes in an area hundreds of times the size of the original astrocyte. Furthermore, calcium waves can also cause neurons to fire. And calcium waves in the cortex are leading scientists to infer that this style of communication may be conducive to the processing of certain thoughts. If that isn't convincing, it was recently shown that a molecule that stimulates the same receptors as THC can ignite astrocyte calcium release. LEHRER: You suggest that glia and their calcium waves might play a role in creativity. Could you explain? KOOB: This idea stems from dreams, sensory deprivation and day dreaming. Without input from our senses through neurons, how is it that we have such vivid thoughts?

How is it that when we are deep in thought we seemingly shut off everything in the environment around us? In this theory, neurons are tied to our muscular action and external senses. We know astrocytes monitor neurons for this information. Similarly, they can induce neurons to fire. Therefore, astrocytes modulate neuron behavior. This could mean that calcium waves in astrocytes are our thinking mind. Neuronal activity without astrocyte processing is a simple reflex; anything more complicated might require astrocyte processing. The fact that humans have the most abundant and largest astrocytes of any animal and we are capable of creativity and imagination also lends credence to this speculation. Calcium is also released randomly and without stimulation from astrocytes' internal stores in small bursts called 'puffs.' These random puffs can lead to waves. It is possible that the seemingly random thoughts during dreams and sensory deprivation experience could be calcium puffs becoming waves in our astrocytes. Basically, it is obvious that astrocytes are involved in brain processing in the cortex, but the main questions are, do our thoughts and imagination stem from astrocytes working together with neurons, or are our thoughts and imagination solely the domain of astrocytes?

Maybe the role of neurons is to support astrocytes. Nervous Tissue - Structure and Functions of Human Tissue Types 23 March 2016 - Combining motivational interviewing with cognitive behavioural therapy to treat anxiety Structure and Functions of Nervous Tissue Note: This is part of the section about the structure and function of different tissue types, which is related to the section about histology and cells incl. structure of animal cells , cell division , mitosis , meiosis . Nervous tissue consists of two main types of cells: neurons and neuroglia. Nerve cells, or neurones (also written "neurons") transmit nerve impulses that move information around the body. Neuroglia are also known simply as "glia" and have various functions in support of nerve cells but do not transmit nerve impulses themselves. More information about both of these types of cells follows below: Neurons Neurons (nerve cells) are sensitive to various types of stimuli such as heat/cold, light/dark, pressure. They transmit electrical nerve impulses thereby moving information around the body. There are several types of neurons, including motor neurones, sensory neurones and relay neurones.

Each neurone consists of an enlarged part called the cell body (or "perikaryon") which contains of nucleus of the cell and many "processes" called dendrites that extend away from the cell body and are important because they receive the (electrical signals called) nerve impulses. For more detailed information parts of neurones and their structures and functions see the page about neurones . Neuroglia / Glia Neuroglia are sometimes known as simply "Glia". They are not sensitive to stimuli and so do not generate or conduct nerve impulses. "Glia" or "Neuroglia" are therefore sometimes referred to as the "non-nervous cells of the nervous system". Their functions within the nervous system depend on the specific type of neuroglia: Types of Neuroglial Cell: Astroglial Cell Astrocyte Astroglial cells, also known as "astrocytes" are found throughout the Central Nervous System (CNS), that is in the tissues of the Brain and Spinal Cord. Structure: An astrocyte is a star-shaped cell that has many processes extending from its cell body into the surrounding network of nerve fibres. Function(s): Supply of nutrients to neurons.

Removes excess neurotransmitters Maintains appropriate balance of Ca2+ and K+ ions (which are important in passing nerve impulses at synapses). Helps migration of neurons during brain development. Aids formation of the blood-brain barrier. (Possibly participating in information storage processes) Ependymal Cell Ependymal cells are found in the Central Nervous System (CNS), that is in the tissues of the Brain and Spinal Cord. Structure: Function(s): Protection: Forms lining of the ventricles of the brain and central canal of the spinal cord. Forms cerebrospinal fluid (CSF). Aids circulation of cerebrospinal fluid (CSF). Ependymal cells form the extremely thin membrane called the ependyma that lines the ventricles of the brain and choroid plexuses. Microglial Cell Microglial cells are sometimes known simply as "microglia" and are found in the Central Nervous System (CNS), that is in the tissues of the Brain and Spinal Cord. Structure: Microglia are small glial cells. Function(s): Protects CNS neurons from disease - e.g. by clearing away debris and dead cells.

Therefore some texts describe microglia has having a "mainly scavenging" function. Microglia may be compared with macrophages (which are large scavenger cells, not specific to the nervous system). Oligodendrocyte Oligodendrocyte cells are found in the Central Nervous System (CNS), that is in the tissues of the Brain and Spinal Cord. Structure: Oligodendrocytes differ from astrocytes (listed higher-up this page) by having fewer and thinner processes and no gap junctions. Function(s): Forms supporting network around CNS neurons. Some texts describe oligodendrocytes as the CNS "equivalent" of Schwann Cells in the PNS because they produce myelin sheath around several adjacent axons of CNS neurons. Schwann Cell Schwann cells are found in the Peripheral Nervous System (PNS), that is around the nerves of the extremeties of the body, e.g. located in the skin. Structure: Morphology of brain cells: Neurons, Astrocytes, Microglia, ... Including pons, substantia nigra, medulla oblongata, etc.

Cell types / Brain & CNS Neurons The numbers of neurons varies extremly between species: the common fruit fly has about 100.000 neurons, whereas it is estimated that the human brain has about 1014 (100 billion) neurons. However these are estimates, nobody ever counted all the neurons, or delivered a plausible concept how to estimate their number. But, just to remember you, how wrong sience can be: it was printed in almost every biology / medicine text book, an often used question in tests, and broadly regarded as a fact for decades - that the human genome has about 100.000 genes. Until of course, the genes where actually counted: turns out, their number is about +/- 25.000 in humans and +/- 30.000 in mice. The main cell types characterizing the CNS are essentially neurons and glial cells, whereby the first are regarded as the cellular substrate of the cognitive abilities of the CNS. The basic morphology of a neuron consists of three main parts: The bulbous part of a neuron is called the soma (or perikaryon), and contains the cell nucleus. The word soma is Greek, meaning "body".

From the soma, one axon ( a ï¿½cable-likeï¿½ projection), often myelinated, extends, which propagates signals to other cells (neurons, muscle, etc.). The axon forms synapses at its terminal part, when it its ï¿½contactingï¿½ other cells. The soma is also covered by dendrites, which in most case are the ï¿½receivingï¿½ entity, which are connected to axon terminals of other neurons. It was long postulated, that in the adult mammalian brain, no new neurons are "born". However, it has been shown in humans and in mice that this dogma has been wrong. "This study was undertaken to investigate whether neurogenesis occurs in the adult human brain, in regions previously identified as neurogenic in adult rodents and monkeys. Human brain tissue was obtained postmortem from patients who had been treated with the thymidine analog, bromodeoxyuridine (BrdU), that labels DNA during the S phase.

Using immunofluorescent labeling for BrdU and for one of the neuronal markers, NeuN, calbindin or neuron specific enolase (NSE), we demonstrate that new neurons, as defined by these markers, are generated from dividing progenitor cells in the dentate gyrus of adult humans. Our results further indicate that the human hippocampus retains its ability to generate neurons throughout life." [Eriksson PS, Perfilieva E, Bjork-Eriksson T, Alborn AM, Nordborg C, Peterson DA, Gage FH.; Nature Medicine 1998 Nov;4(11) Source ï¿½ PubMed ] Glia Cell Types Summary The function of the brain is based on the ability of excitable cells to trigger and transmit electrical impulses. Against this background, it is maybe surprising, that the electrically excitable cells - the neurons - are numerically outnumbered by the so called glial cells within the central nervous system (CNS). The first to describe them was the German pathologist Rudolf Virchow (1821 - 1902) in 1846 He discovered cells in the CNS, which showed no features of neurons.

He assumed that it was this to be the connective tissue of the brain and described it as "Nervenkitt". Virchow assigned to two major roles to these cell types: support function for the nerve cells involvement in tissue repair A long time it was not recognized that the so called "Nervenkitt" was populated by different cell types. Only the development of new staining techniques (metal impregnation) in light-microscopy allowed further differentiation. Three different glia cell types have been found: astroglia or astrocytes oligoglia or oligodendrocytes microglia In recent years it has become increasingly clear, that these glial cells also take over other functions in the brain. They may communicate in a sort of network which complements and or supports the neural network. Likewise, the clarification of the ontogeny of glial cells is still a topic of research in neurobiology. Astrocytes Astrocytes belong to the 3 types of glial cells in the CNS (the others are microglia and oligodendrocytes). Astroglia is used as a synonym . These cell exhibt usually a star-shaped mmorphology, with many processes extending from the soma.

Astrocytes can be identified histologically since they express the intermediate filament glial fibrillary acidic protein (GFAP). Astrocytes perform a plethora of functions (and the list is constantly growing: biochemical support of neurons and other cell types, support of endothelial cells which form the blood-brain barrier, a major role in the repair and scarring process of the brain and spinal cord following mechanical / inflammatory injuries, a "guidance" for growing neurons / axons during the development of the brain. Previously in medical science, astrocytes have not been given an active role in the information processing activity of the brain, but this view has recently been challenged. Oligodendrocytes Oligodendrocytes are a variety of neuroglia cells within the central nervous system. In comparison to astrocytes oligodendrocytes are more gracile and form less cell ramifications. Their image in a microscopic cross section is reminiscent of trees with only a few branches - hence the name. Their cell processes surround the axons of nerve cells and isolate them from the environment.

In analogy to the Schwann cells of the PNS The cell body (not the branches) of the oligodendrocytes exists predominantly in the white matter, less frequently in the gray matter. The degeneration of the myelin sheath, which is formed by oligodendrocytes, is a characteristic morphological change in the disease multiple sclerosis. Microglia The name microglia was first (1920) introduced by Pio del Rio-Hortega (1882 ï¿½ 1945). Since the function and ontology of microglia was unknown at that time, and furthermore, the cells of the brain where divided into two types: neurons and ï¿½othersï¿½, which were termed by the generic expression ï¿½gliaï¿½ (e.g. astroglia and oligodendroglia ), the term microglia was a reference 1. to their shape 2. to their obviously non-neuronal characteristics. Microglia are a cell type, that is only found in the CNS (Central Nervous System; per human definition: brain and spinal cord).

Usually microglial cells are in a so called ramified state, in which they are immunologically inactive. However they can be ï¿½activatedï¿½ via various triggers (all caused by some kind of brain injury): elevated levels of pro-inflammatory cytokines (a combination of lipopolysaccharide (LPS) and interferon-gamma is one of the most effective) neuro-inflammatory and neuro-degenerative diseases, including Parkinsonï¿½s disease, Multiple Sclerosis, Alzheimerï¿½s, and Huntingtonï¿½s disease physical injury Once activated, microglia assume an amoeboid (round, oval) morphology, and display a behavior similar to macrophages. They act as antigen-presenting, phagocytic, and cytotoxic cells, all three a hallmark of cells of the immune system. Organization of Cell Types (Section 1, Chapter 8) Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy - The University of Texas Medical School at Houston Jack C. Waymire, Ph.D.

, Department of Neurobiology and Anatomy , The UT Medical School at Houston 8.1 Introduction to Neurons and Glial Cells The human nervous system is estimated to consist of roughly 360 billion non-neural glial cells and 90 billion nerve cells. Furthermore, there are hundreds of different types of neurons based on morphology alone. Often, neurons that look similar have strikingly different properties. For example, they utilize and respond to different neurotransmitter(s). This section reviews the cellular components of nervous tissue. Students should be able to describe neurons and glia, their morphological components as seen with the light and electron microscope, and some of the fundamental functional roles these cell types play in the nervous system. 8.2 Model Neuron Figure 8.1 Click on the parts of the Model Neuron to view structures. After reviewing the Model Neuron above, learn more about the function of each structure by clicking from the list below. 8.3 Cell Soma Click the identified structures on the model neuron to move to the related section. The region of the neuron containing the nucleus is known as the cell body, soma, or perikaryon (Figure 8.2). The cell body is the metabolic center of the neuron.

The interior of the soma consists of cytoplasm, a gel within a microtrabecular lattice formed by the microtubules and associated proteins that make up the cytoskeleton. Energy producing metabolism and the synthesis of the macromolecules used by the cell to maintain its structure and execute its function are the principal activities of the neuronal soma. As described in Chapter 6 , it also acts as a receptive area for synaptic inputs from other cells. Embedded within the neuronal cytoplasm are the organelles common to other cells, the nucleus , nucleolus , endoplasmic reticulum , Golgi apparatus , mitochondria , ribosomes , lysosomes , endosomes , and peroxisomes . Many of these cell inclusions are responsible for the expression of genetic information controlling the synthesis of cellular proteins involved in energy production, growth, and replacement of materials lost by attrition. Figure 8.2 (See enlarged view ) Diagrammatic representation of the neuron cell body or perikaryon emphasizing the endoplasmic reticulum, Golgi apparatus and cytoskeleton. Place cursor over image to identify organelles. 8.4 Dendrites Click the identified structures on the model neuron to move to the related section.

The membrane of the neuron functions as a receptive surface over its entire extent; however, specific inputs (termed afferents) from other cells are received primarily on the surface of the cell body and on the surface of the specialized processes known as dendrites. The dendritic processes may branch extensively and are often covered with projections known as dendritic spines. Spines provide a tremendous increase in the surface area available for synaptic contacts. The dendritic processes and spines of neurons are essentially expansions of cytoplasm containing most of the organelles found in the cell body. Dendrites contain numerous orderly arrays of microtubules and fewer neurofilaments (see below). The microtubule associated proteins (MAPs) in the dendrite have a higher molecular weight than those found in the axon. An example is MAP2. In addition, microtubules in dendrites have their positive ends toward the cell soma. Mitochondria are often arranged longitudinally. Rough endoplasmic reticulum and ribosomes are present in large but not small dendrites. The shape and extent of the "dendritic tree" of an individual neuron is indicative of the quantity and variety of information received and processed by that neuron.

The dendritic spines often contain microfilaments which is the cytoskeletal element responsible for changes in spine shape observed in some examples of synaptic plasticity. View an EM slide of a dendrite and a spine Information is received by the dendrite through an array of receptors on dendrite surface that react to transmitters released from the axon terminals of other neurons. Dendrites may consist of a single twig-like extension from the soma or a multi-branched network capable of receiving inputs from thousands of other cells. For instance, an average spinal motor neuron with a moderate-sized dendritic tree, receives 10,000 contacts, with 2,000 of these on the soma and 8,000 on the dendrites. 8.5 Initial Segment and Axon Hillock Click the identified structures on the model neuron to move to the related section. The cone-shaped region of the cell body where the axon originates is termed the axon hillock. This area is free of ribosomes and most other cell organelles, with the exception of cytoskeletal elements and organelles that are being transported down the axon. The neurofilaments in the axon hillock become clustered together as fascicles.

The region between the axon hillock and the beginning of the myelin sheath is known as the initial segment. In many cases, this region is the anatomical location for the initiation of the action potential. The area under the axolemma in this region has material that stains darkly when viewed by EM. This region is shown in Figure 8.4. At the distal-most end of the axon and its collaterales are small branches whose tips are button-shaped cytoplasmic enlargements called terminal boutons or nerve endings. 8.6 Axon Click the identified structures on the model neuron to move to the related section. The other type of process in the idealized neuron is the axon. Each neuron has only one axon and it is usually straighter and smoother than the dendritic profiles. Axons also contain bundles of microtubules and neurofilaments and scattered mitochondria. The most MAPs in an axon have a lower molecular weight than those in the dendrite. A predominant MAP in axons is tau. Microfilaments within the axon are usually associated with an area adjacent to the plasmalemma and often are the most dense at the nodes of Ranvier.

Beyond the initial segments, the axoplasm lacks rough endoplasmic reticulum and free ribosomes. The branches of axons are known as axon collaterales. The axon itself is often surrounded by a membranous material, called the myelin sheath, formed by glia cells. The myelin sheath acts to insulate the plasmalemma of the axon in a way that necessitates the more rapid spread of the depolarization of the plasmalemma and increases the speed of conduction of the nerve impulse (see Chapter 3 ). Figure 8.5 (see enlarged view ) Diagrammatic representation of the axon emphasizing the areas of microtubules, neurofilaments coursing within the cytoplasm. 8.7 Nerve Ending Click the identified structures on the model neuron to move to the related section. The part of the plasma membrane of the nerve ending that is specialized to form functional contacts with other cells is the synapse . Figure 8.6 View an EM of a nerve ending with spherical vesicles. When neurons interact with muscle fibers, the region of functional contact is called the neuromuscular junction or motor endplate ( Chapter 4 ).

According to the classical definition of synapse, when a nerve ending synapses on a dendrite or soma of a second neuron it is termed either an axodendritic or an axosomatic synapse, respectively ( Chapter 7 ). However, almost all possible combinations of pre- and postsynaptic elements have been found in the central nervous system. These different types of synapse are designated by combining the name of the structure of the presynaptic element with that of the postsynaptic structure. For example, when the transfer of information occurs from an axon to axon or from one terminal to another, the synapse involved is called an axoaxonic synapse. 8.8 Cellular Elements at the Typical Nerve Terminal Regions of functional contacts between neurons (synapses) have distinct morphological characteristics. Although a great deal of variation exists in the size and shape of boutons of individual neurons, synapses can be identified by the presence of the following: Figure 8.7 View an EM of a nerve ending with flat vesicles. A presynaptic complement of membrane-bound synaptic vesicles exists. These are spherical vesicles in excitatory nerve endings, shown in Figure 8.6.

In inhibitory neurons the synaptic vesicle are often flattened as shown in Figure 8.7. The nerve ending often has aggregations of dense material in the cytoplasm immediately adjacent to the membrane on the pre- and postsynaptic side of the junction (these are known as presynaptic density or postsynaptic density , respectively.) This dense material on the presynaptic side is thought to be the site of vesicle attachment. The dense material on the postsynaptic side is a site where receptor proteins and channels are prevalent. Many mitochondria are present, especially in the nerve terminal; and There is a distinct synaptic cleft or intercellular space of approximately 20-40 nm. An endoplasmic reticulum is present that regulates Ca2+ level. Endosomal membrane that functions in the recycling of synaptic vesicles. 8.9 Structural Variations Numerous variations of the "model" neuron described above exist. An important modification, which occurs especially in receptor neurons, involves the designation of a neuronal process as a dendrite or as an axon. Classically, the axon has been identified as the myelinated or unmyelinated process that transmits signals away from the cell body.

The classical view of the dendrite is that of an unmyelinated tube of cytoplasm which carries information toward the cell body. However, this distinction does not hold for ALL neurons. Some cells have a myelinated process that transmits signals toward the cell body. Morphologically the "dendrite" and the "axon" may, therefore, be indistinguishable. Neither the position of the cell body nor the presence or absence of myelin is always a useful criterion for understanding the orientation of the neuron. The region of impulse initiation is more reliable guide to understanding the functional focal point of the cell. This region is analogous to the initial segment of the model neuron, discussed above. Routinely the fiber or process, which contains the initial segment or trigger zone, is referred to as an axon. Note, as shown in Figure 8.8, the trigger zone does not have to be immediately adjacent to the cell body. Figure 8.8 Comparison of variations in the structure of neurons 8.10 Naming Neurons A number of conventions have evolved to classify and name neurons.

One of the oldest, devised by Golgi in the late 1800’s, is based on the complexity of the dendritic tree of the neuron. Through this approach cells are classified as unipolar, bipolar and multipolar neurons as shown in Figure 8.8 . Unipolar cells have only one cell process, and are primarily found in invertebrates. However, vertebrate sensory neurons are another form of this type of cell. Because these cells start out developmentally as bipolar neurons and then become unipolar as they mature, they are called pseudo-unipolar cells. Bipolar cells are present in the retina and the olfactory bulb. Multipolar cells make up the remainder of neuronal types and are, consequently, the most numerous type. These have been further sub-categorized into Golgi type II cells that are small neurons, usually interneurons, and Golgi type I cells that are large multipolar neurons. Cells are also named for their shape (e.g., pyramidal cells shown in Figure 8.9) or for the person who first described them (e.g., Purkinje cells shown in Figure 8.10).

More recently, cells have been named for their function or the neurotransmitter they contain (e.g., CNS norepinephrine cell groups covered in Chapter 12 ). This description is possible because of the development of histochemical and immunocytochemical methods to specifically identify the neurotransmitter type used by neurons. Many of the terms used in this section are defined below. Axolemma is the plasmalemma of the axon. Endoplasmic reticulum is a labyrinthine, membrane bounded compartment in the cytoplasm where lipids are synthesized and membrane bound proteins are made. In some regions of the neuron ER is devoid of ribosomes and is termed smooth ER. The smooth ER is involved in Ca2+ buffering and in the biosynthesis and recycling of synaptic vesicles as will be discussed in Chapter 10 . Endosome is a membrane-bounded organelle that carries materials ingested by endocytosis and passes them to lysosomes and peroxisomes for degradation. It also functions in the nerve ending to recycle synaptic vesicles. Golgi apparatus is a collection of stacked, smooth-surfaced membrane bound organelles where proteins and lipids made in the endoplasmic reticulum are modified and sorted.

Lysosomes contain enzymes that digest compounds that originate inside or outside the cells. They are involved in converting proteins to amino acids and glycogen to glucose, the basic nutrient of neurons. Their enzymes act at an acidic pH. As will be described later, they also serve as vesicles for reverse transport from axon terminals to the soma. Many lysosomes become degraded to lipofuscin granules, which accumulate as the organism ages and are regarded as neuronal refuse. Lysosomes form from the budding off the Golgi apparatus. They have a variety of membrane-bound shapes and sizes, ranging in size between 250 and 700 nm in diameter. Microfilaments are 7 nm in diameter filaments arranged as a paired helix of two strands of globular actin. Microfilaments are especially prominent in synaptic terminals, in dendritic spines, and in association with the axolemma. Microtubules are 20 to 25 nm diameter tubular structures that run in loose bundles around the nucleus and funnel into the base of the axonal and dendritic processes where they form parallel arrays distributed longitudinally.

They are made up of dimers of α and β tubulin subunits and contain associated proteins known as microtubule associated proteins (MAPS). The MAPS regulate the polymerization of tubulin subunits to form the microtubules. The dimers of α and β tubulin subunits polymerize to form proto-filaments arranged in an a helix such that 13 dimer subunits make up each full turn of the a helix. In addition, microtubules are not continuous, and each microtubule is composed of numerous 100 nm units. Microtubules are involved in axoplasmic transport (see below). Mitochondria are distributed ubiquitously throughout the cytoplasm of the entire nerve cell and are especially plentiful at presynaptic specializations. Neurofilaments are a type of intermediate filament found in nerve cells. Neurofilaments are involved in the maintenance of the neuron's shape and mechanical strength. Although neuronal neurofilaments are classified as intermediate filaments, their composition in neurons is different than that found in other cells. They are composed of three subunits that are arranged to form a 10-nm diameter tubule.

It is the neurofilament that stains with heavy metal to permit the visualization of neuronal shape. Neurofilaments run in loose bundles around the cell nucleus and other organelles and funnel into the base of the axonal and dendritic processes where they form parallel arrays distributed longitudinally. Neurofilaments are more abundant than microtubules in axons, whereas microtubules are more abundant than neurofilaments in dendrites. It is the neurofilaments that undergo modification in the Alzheimer's disease to form neurofibrillary tangles. Nucleolus is in the center of the nuclei of all neurons. It is a prominent, deeply stained spherical inclusion about one-third the size of the nucleus. The nucleolus synthesizes ribosomal RNA, which has a major role in protein synthesis. Nucleus of the neuron is large and round and is usually centrally located. In some cells, masses of deeply staining chromatin are visible in the nucleus. The nuclear membrane of neurons is like that of other cells - a double membrane punctuated by pores (nuclear pores) which are involved in nuclear-cytoplasmic interactions.

The nucleus in neurons is spherical and ranges in diameter from 3 to 18 micrometers depending on the size of the neuron. Neurons with long axons have a larger cell body and nucleus. As in other cells, the principal component of the nucleus is deoxyribonucleic acid (DNA), the substance of the chromosomes and genes. Peroxisomes are small membrane bounded organelles that use molecular oxygen to oxidize organic molecules. They contain some enzymes that either produce or degrade hydrogen peroxide. Plasmalemma of the neuron appears in the electron microscope as a typical bi-layered cellular membrane, approximately 10 nm thick. Postsynaptic density is darkly staining material of postsynaptic cell adjacent to the synapse. Receptors, ion channels, and other signaling molecules are likely bound to this material. Presynaptic density is the region of darkly staining material of the presynaptic membrane where synaptic vesicles are hypothesized to dock prior to fusion with the presynaptic membrane. Ribosomes are particles composed of ribosomal RNA and ribosomal protein which associate with mRNA and catalyze the synthesis of proteins. When ribosomes are attached to the outer membranes of the ER, the organelle is termed rough ER.

The rough ER, in laminae with interspersed ribosomes, is visible with the light microscope as Nissl substance. In light microscopic preparations, the appearance of Nissl substance varies in different types of neurons. It may appear as densely stained ovoids or as finely dispersed particles or aggregations of granules. Synapse is the junction that allows signals to pass from a nerve cell to another cell or from one nerve cell to a muscle cell. The synaptic cleft is the gap between the membrane of the pre- and postsynaptic cell. In a chemical synapse the signal is carried by a diffusable neurotransmitter. The cleft between the presynaptic cell and the postsynaptic cells is 20 to 40 nm wide and may appear clear or striated. Recent studies have indicated that the cleft is not an empty space per se, but is filled with carbohydrate-containing material. Synaptic vesicles are small spherical organelles in the cytoplasm of neurons that contain neurotransmitter and various proteins necessary for neurotransmitter secretion. Vesicles containing inhibitory neurotransmitter are often flat or elliptical whereas vesicles that contain excitatory neurotransmitter are usually more spherical.

8.12 Glial Cells and Function Figure 8.11 Types of neuroglia. Click on the different glial cells to view the details of their structure and function. The most numerous cellular constituents of the central nervous system are the non-neuronal, neuroglial ("nerve glue") cells that occupy the space between neurons. It has been estimated that there are roughly 360 billion glial cells, which comprise 80-90% of the cells in the CNS. This section will cover the general classifications of the neuroglial cells and describe some of the general properties that distinguish neuroglia from neurons. Neuroglia differ from neurons in several general ways in that they do not form synapses, View a protoplasmic astrocyte. Protoplasmic astrocytes are found primarily in gray matter. With silver or glial specific stains, their cell bodies and processes are very irregular. The processes may be large or very fine, sometimes forming sheets that run between axons and dendrites, and may even surround synapses. These fine sheet-like processes give the protoplasmic astrocyte cell body a "fuzzy" or murky appearance under the light microscope.

Bundles of fine fibrils may be seen within the cytoplasm. The nucleus of a protoplasmic astrocyte is ellipsoid or bean-shaped with characteristic flecks of chromatin. Specific types of intercellular junctions have been noted between the processes of protoplasmic astrocytes. These probably mediate ion exchange between cells. 8.15 Fibrous Astrocytes View a fibrous astrocyte. Fibrous astrocytes are found primarily in white matter, have a smoother cell body contour than do protoplasmic astrocytes as seen with glial-specific stains, and have processes that tend to emerge from the cell body radially. These processes are narrower and branch to form end feet on blood vessels, ependyma, and pia. Consequently, the processes of fibrous astrocytes do not form sheets and do not tend to conform to the shape of the surrounding neurons or vascular elements. The major distinguishing feature of fibrous astrocytes, as the name suggests, is an abundance of glial fibrils arranged in parallel arrays in the cytoplasm and extending into the processes.

In Nissl stains, the fibrous astrocytes have a nucleus essentially the same as that of the protoplasmic type with a flecked appearance. Intercellular adherences have also been observed between fibrous astrocytes. Figure 8.14 Astrocyte with end feed projecting to the surface of neurons, blood vessels, ependyma and the meninges. No single astrocyte would project to all of these structures. Both types of astrocytes function to support the neurons in their immediate vicinity. They provide a physical barrier between cells, maintain the ionic and pH equilibrium of the extracellular space around neurons, and continually modify the chemical environment of the neighboring cells. As shown in Figure 8.14, astrocytes form a complete lining around the external surface of the CNS (glial limitans) and around blood vessels (perivascular feet). During development, they form scaffolding along which nerve cells migrate to achieve their mature structure. During injury, the astrocytes proliferate and phagocytize dead cells. This often leads to the formation of glial scar. In addition to these general functions, astrocytes also act in more specialized ways to facilitate neuron function.

They metabolize neurotransmitters by removing them from the synaptic cleft. For example, the amino acid glutamate is taken up by astrocytes and inactivated by conversion to glutamine. Glutamine is then transported to the neuron to be re-synthesized into glutamate (see Chapter 13 ). More recent evidence indicates that the astrocytes can dramatically change size as part of their physiological regulation of the neuronal environment. These functions will be discussed in later sections. 8.16 Oligodendroglia Click on a glial cell to move to the related section. Oligodendrocytes are also located in both gray and white matter. They are the predominant cell type in white matter where they are often located as rows of cells between groups of neuronal processes. These are termed interfascicular oligodendroglia and are involved in the formation and maintenance of the myelin surrounding the neuronal processes nearby. In gray matter, oligodendroglia are usually located near neurons and, therefore, are known as perineuronal satellite cells. Cell bodies of oligodendroglia are often located near capillaries, but they lack the definite perivascular end feet characteristic of astrocytes.

The processes of oligodendrocytes are fewer and more delicate than astrocytes, and the cell body shape is polygonal to spherical. The oligodendrocyte nucleus is smaller than that of the astrocyte, is eccentrically located in the cell body, contains clumps of chromatin and can be stained by alkaline dyes. The cytoplasm of oligodendrocytes tends to be darker than that of astrocytes with silver stains, and does not contain glial fibrils (although they do contain microtubules). Figure 8.15 View an oligodendrocyte EM. The role of oligodendroglia in the central nervous system, particularly of the interfascicular oligodendrocytes, is the formation and maintenance of myelin. Myelin is the sleeve of membranous material described by Dr. Byrne, that wraps the neuronal axon as shown in Figure 8.15 to facilitate the conduction of the action potential through saltatory conduction. Myelin is composed of concentric layers of membranes compacted against one another with an internal (i.e. against the nerve fiber) and an external collar of cytoplasm.

As shown in Figure 8.15, a single oligodendrocyte contributes to the myelination of several adjacent nerve processes. Moreover, more than one oligodendrocyte contributes to the myelination of a single internode of an axon. The lamellae of myelin membranes result from the spiral wrapping of the axon by cytoplasmic processes of interfascicular oligodendroglia. Also, the oligodendrocyte forming a particular myelin internode (i.e. the myelin between two nodes) is seldom seen directly adjacent to the myelin-wrapped process. This is because thin cytoplasmic bridges connect the region of the oligodendrocyte cell body to the external wrap of myelin. It is important to note that the region of the axon exposed at the node of Ranvier is not bare. It may be the site of branching of the axon, the site of synaptic contacts, or it may be covered with various glial processes. The axon in the nodal region usually contains concentrations of organelles, especially mitochondria. In the Peripheral Nervous System (PNS), Schwann cells are responsible for the myelin formation.

These cells myelinate axons differently than the interfascicular oligodendroglia. As shown in Figure 8.16, they migrate around the axon, laying a membrane covering around the axon by squeezing out the cytoplasm of the Schwann cell. Also, every internode of a PNS axon represents a single Schwann cell. In addition unmyelinated axons in the PNS are also enclosed by membranes formed by Schwann cells. Figure 8.16 8.17 Ependyma Click on a glial cell to move to the related section. Ependymal cells are derived from the early germinal epithelium lining the lumen of the neural tube and thus are also ectodermal derivatives (along with neurons, astrocytes, and oligodendrocytes). Ependymal cells line the ventricles of the brain and the central canal of the spinal cord. They are arranged in a single-layered columnar epithelium, and have many of the histological characteristics of simple epithelium, which vary from squamous to cuboidal depending upon their location.

The ependyma forming the ventricular lining do not connect to a basal lamina, but rest directly upon underlying nervous tissue. As shown in Figure 8.17, the surface facing the ventricle contains many microvilli and cilia. These cilia move cerebrospinal fluid (CSF) in the ventricles. The lateral borders of the ependymal cells are relatively straight and form junctions with adjacent cells. Ependymal cells are modified in various regions of the ventricles into layers of cuboidal epithelium, which do lie on a basement membrane (formed by an outgrowth of the pia) over a rich bed of vasculature and connective tissue. This is the choroid plexus studied in the Laboratory that is responsible for the secretion, uptake and transport of substances to and from the CSF. Figure 8.17 View a microglia cell. Click on a glial cell to move to the related section. Microglia, in contrast to the other types of glial cells, originate from embryonic mesoderm.

They are present throughout the central nervous system, but tend to be inconspicuous in mature normal tissue and are difficult to identify with the light or electron microscope. They are more abundant in gray matter, and may compromise up to 5-10% of the neuroglia in the cerebral cortex. The general appearance of microglia is similar to oligodendrocytes, although they are smaller and have undulating processes with spine-like projections. Microglial nuclei are elongated or triangular and stain deeply with alkaline dyes. Following damage to nervous tissue, microglia proliferate and migrate to the site of injury where they clear cellular debris by phagocytosis. The reacting microglia have a swollen form with shortened processes and are difficult to discriminate from phagocytes from the periphery or migrating perivascular cells. It is estimated that at least one third of the phagocytes appearing in the area of a lesion are of CNS origin.

Question: Where in the human body are glial cells?
Answer: Brain