Neurons are nerve cells that transmit nerve signals to and from the brain at up to 200 mph. The neuron consists of a cell body (or soma)

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Brain Cells

The brain and spinal cord are made up of many cells, including neurons and glial cells. Neurons are cells that send and receive electro-chemical signals to and from the brain and nervous system. There are about 100 billion neurons in the brain. There are many more glial cells; they provide support functions for the neurons, and are far more numerous than neurons.

There are many type of neurons. They vary in size from 4 microns (.004 mm) to 100 microns (.1 mm) in diameter. Their length varies from a fraction of an inch to several feet.

Neurons are nerve cells that transmit nerve signals to and from the brain at up to 200 mph. The neuron consists of a cell body (or soma) with branching dendrites (signal receivers) and a projection called an axon, which conduct the nerve signal. At the other end of the axon, the axon terminals transmit the electro-chemical signal across a synapse (the gap between the axon terminal and the receiving cell). The word "neuron" was coined by the German scientist Heinrich Wilhelm Gottfried von Waldeyer-Hartz in 1891 (he also coined the term "chromosome").

The axon, a long extension of a nerve cell, and take information away from the cell body. Bundles of axons are known as nerves or, within the CNS (central nervous system), as nerve tracts or pathways. Dendrites bring information to the cell body.

Myelin coats and insulates the axon (except for periodic breaks called nodes of Ranvier), increasing transmission speed along the axon. Myelin is manufactured by Schwann's cells, and consists of 70-80% lipids (fat) and 20-30% protein.

The cell body (soma) contains the neuron's nucleus (with DNA and typical nuclear organelles). Dendrites branch from the cell body and receive messages.

A typical neuron has about 1,000 to 10,000 synapses (that is, it communicates with 1,000-10,000 other neurons, muscle cells, glands, etc.).

There are different types of neurons. They all carry electro-chemical nerve signals, but differ in structure (the number of processes, or axons, emanating from the cell body) and are found in different parts of the body.

  • Sensory neurons or Bipolar neurons carry messages from the body's sense receptors (eyes, ears, etc.) to the CNS. These neurons have two processes. Sensory neuron account for 0.9% of all neurons. (Examples are retinal cells, olfactory epithelium cells.)

  • Motoneurons or Multipolar neurons carry signals from the CNS to the muscles and glands. These neurons have many processes originating from the cell body. Motoneurons account for 9% of all neurons. (Examples are spinal motor neurons, pyramidal neurons, Purkinje cells.)

  • Interneurons or Pseudopolare (Spelling) cells form all the neural wiring within the CNS. These have two axons (instead of an axon and a dendrite). One axon communicates with the spinal cord; one with either the skin or muscle. These neurons have two processes. (Examples are dorsal root ganglia cells.)


Unlike most other cells, neurons cannot regrow after damage (except neurons from the hippocampus). Fortunately, there are about 100 billion neurons in the brain. 


Glial cells make up 90 percent of the brain's cells. Glial cells are nerve cells that don't carry nerve impulses. The various glial (meaning "glue") cells perform many important functions, including: digestion of parts of dead neurons, manufacturing myelin for neurons, providing physical and nutritional support for neurons, and more. Types of glial cells include Schwann's Cells, Satellite Cells, Microglia, Oligodendroglia, and Astroglia.

Neuroglia (meaning "nerve glue") are the another type of brain cell. These cells guide neurons during fetal development.

Plant Stem Cells, Leaf Cells, Root Cells

Plant cells differ structurally from the cells of most other organisms in a few key ways. Specifically, they are usually larger than animal cells and are surrounded by a rigid cell wall made from cellulose. They also often have a large central vacuole that takes up most of the cell, and if they carry out photosynthesis, the cells will have chloroplasts. This does not mean that all such cells are the same, and in fact, there are a number of different types of cells found in most plants.

Plants basically have three types of tissues, which are made up of different types of cells. Surface tissue forms the protective outer layer covering the plant. Fundamental, or simple tissues, are usually only composed of one type of cell and are normally grouped based on the level of thickness of the cell wall. Vascular tissues are complex tissues that consist of more than one type of cell. There are only two types of vascular tissue: xylem and phloem.

The surface tissue, or epidermis, of a plant is often only one cell thick, although it can be much thicker if the plant lives in a very dry environment and protection from water loss is crucial. It is made up of epidermal cells, which often have a very large vacuole. The cell wall that faces the outside of the plant is often thicker than cell wall that faces into the plant.

Epidermal cells in the leaves may be specialized as guard cells. These cells control the opening and closing of small holes in the leaves, called stomata. In this way, they regulate the movement of gases into and out of the plant. The function of epidermal cells that line the roots is water absorption from the soil. To increase the surface area, many epidermal cells grow long hair, or filaments, from their surface.

There are several types of fundamental tissues, including parenchyma, collenchyma and sclerenchyma. Parenchyma is made up of parenchyma cells and occurs in the roots, leaves and stems of plants. These plant cells are relatively unspecialized and contain large vacuoles and a thin cell wall. Within the leaves and stems, most of the chloroplasts are found in parenchyma cells. They give the cells their green color and allow photosynthesis to take place.

Collenchyma cells are longer than parenchyma cells, and their cell walls are much thicker. Their function is to provide support in young plants and in the stems and leaves of non-woody older plants. Sclerenchyma cells also provide support to plants, and they are far more specialized than collenchymas cells. They have a thick secondary wall that is hardened to strengthen the plant, and these cells are usually dead at maturity.

Xylem and phloem are the two types of vascular tissue found in a plant. Xylem is made up of parenchyma cells and two specialized cells called tracheids and vessel elements. Both tracheids and vessel elements are dead, and their function is to provide support and water transport from the roots up to the rest of the plant.

Phloem tissue is alive and is made up of parenchyma and sclerenchyma cells. In addition, it contains specialized plant cells called sieve tube cells and companion cells. The function of phloem is the transport of material throughout the plant in all directions, and they contain nonucleus and very little cytoplasm. Companion cells are closely associated with sieve tube cells and are thought to carry out cellular functions for both types of cell.

Parenchyma cells are a type of cell found within most plants. Like animals, plants have cells that are specialized for different functions. Parenchyma cells are simple cells that are not specialized, but they do occur within almost all plant tissues. Cells that are found within plants are often grouped into a specific type based on the size of the cell wall surrounding the cell and also if the cell is living or dead. Other types of cells that make up tissues within plants are collenchyma cells and sclerenchyma cells.

Each parenchyma cell is surrounded by a thin cell wall that contains cellulose. Within the cell well is the cell membrane, which controls what enters and leaves the cell. The center of the cell is filled by a very large vacuole and all the other organelles, including the nucleus and chloroplasts, are found pushed to the edge of the cell by the vacuole.

If the vacuole within the cell is full of water, it is said to be turgid. Packed together in the stems and leaves, turgid parenchyma cells provide support for herbaceous plants. These types of plants do not have a woody stem, so they die down to the soil level at the end of each growing season.

These cells are usually round, or spherical, in shape, but they can be pushed into other shapes by the cells that are surrounding them. Most processes of plant metabolism occur within parenchyma cells, and due to the large vacuole, they can be used to store food and water. When studying plant cells, these are often the types of cells that are observed due to their simplistic nature.

Almost every part of a non-woody plant has some parenchyma cells within it. Depending on where the cell is found, it carries out a different function. The fact that different functions occur with a parenchyma cell in different parts of the plant means that the structure of the cell can also vary.

The area where parenchyma cells are found within leaves is called the mesophyll. Due to the fact that they contain chloroplasts, the cells appear green. This means that photosynthesistakes place within these cells. During the process of photosynthesis, carbon dioxide and water is converted into glucose and oxygen. Energy for the process is obtained from sunlight.

Once glucose is produced, it can then be stored in the parenchyma cells found within other parts of the plant. In most cases, storage takes place within the roots of a plant. Food can also be stored within tubers, seeds and fruits that the plant produces.

Muscle Cells

A muscle cell is a special kind of cell that makes up an organism's muscle tissues. The muscles allow independent movement and regulate biological functions such as digestion and heartbeat. These cells are further specialized into distinct types based on their location and functions. All of them control movement by contracting; while much of this activity is involuntary, the muscles that direct the skeletal system can be consciously controlled. These muscles can be trained to carry out highly precise movements and strengthened through exercise.

Cellular Biology

All organisms are composed of structures called cells, many of which are microscopic. In complex creatures such as humans, these cells number in the trillions, and become highly specialized in early development. Nerve cells, for example, make up the brain and nervous system and can reach lengths of 3 feet (1 m), but are incapable of independent movement. Muscle cells, by contrast, have structures that allow for a wide range of motion, from the measured routines of a gymnast to the constant beating of the heart.

Types of Muscle Tissue

The medical term for a muscle cell is a myocete. During the embryonic, or pre-birth, stage of development, cellular bodies called myoblasts mature and develop into the various kinds of myocetes. In humans and higher animals, there are three kinds of muscle cells, corresponding to the major categories of muscle: skeletal, cardiac, and smooth.

Skeletal muscles, also called striated muscles, are governed by voluntary commands, and allow a broad range of body movements. Cardiac muscles keep the heart beating, and are capable of uninterrupted activity without fatigue. Smooth muscles, like cardiac muscles, are subject to involuntary commands, and are regulated by the brainstem located at the base of the skull. These smooth muscles ensure that the internal organs function normally, such as the muscular contractions that move food through the digestive tract.

The three kinds of muscle tissue can be identified easily by their organizing structures, which are particularly visible under a microscope. Skeletal muscle tissue, the most common kind of muscle tissue in humans and other large animals, has striations, or grooves, that mark out each individual muscle cell. These cells, sometimes called muscle fibers, extend the length of the muscle. This is necessary for the cells to perform their function efficiently.

Smooth muscle tissue, as the name implies, has a uniform appearance, similar to that of non-muscular tissue. The cells do not need to be as elongated as skeletal muscle fibers, because the motion created by these muscles is more gradual and requires less energy.

Cardiac muscle tissue has striations like that of skeletal muscles, but the cells are smaller, like those of smooth muscles. They also have a distinctive branched structure that is better suited to the task of constantly pumping blood through the heart. Otherwise, the two types of muscle are very similar.

Cell Structure

Muscle cells are made up of myofibrils, organic cable-like structures composed of essential proteins. Within the myofibrils are bundles of these proteins, arranged into thick and thin filaments within repeating sections known as sarcomeres. Responding to voluntary or involuntary nerve commands, these proteins slide past each other, causing the muscle cells to contract or relax and create movement. These mechanisms for motion are called actomyosin motors, referring to the proteins that compose them, actin and myosin.

All cells have a central organizing body called the nucleus. While most cells have just one, skeletal myocetes have several nuclei scattered along the length of the cell. This allows information and nutrients to be delivered more quickly throughout the cell. Cardiac and smooth muscle cells have the traditional single nucleus, although in smooth cells the nucleus is elongated, like the cells themselves.

Muscle cells can draw energy from proteins, fat, or glucose, a form of sugar created in the digestive process. Although most nutrients are distributed to muscles through the bloodstream, each muscle cell also stores a small amount of fat and glucose within itself as a ready source of energy, so the muscle can be used at any time.

Muscle Cells and Exercise

Certain types of exercise can cause muscle tissue to expand. The muscle cells themselves within the affected muscle will actually enlarge, as the increasing demand on the muscle caused by weight training, for example, triggers the release of biological growth hormones. The medical term for this type of muscle growth is called hypertrophy. This is different from hyperplasia, which is an increase in the actual number of muscle cells.

Hypertrophy can be stimulated by hormones like testosterone, which is why teenage boys may experience startling muscular changes, such as growth spurts, around puberty. This increase in muscle mass can also be stimulated artificially by injections of performance-enhancing drugs and hormones. Hormone injection can also have unforeseen health effects, including causing muscle hyperplasia. The abuse of these chemicals has become a legal and ethical issue in professional athletics.

Health experts recommend regular exercise to strengthen muscle cells for everyone, not just athletes. In addition to maintaining muscle strength, exercise has well-documented positive effects throughout the body, including improving a person's mood. Strenuous exercise sometimes causes muscle soreness, which is often caused by minute damage to muscle cells as a result of unaccustomed exertion. Regular exercise of the muscle typically reduces this soreness, as the tissue quickly adapts to new demands.

Bone Cells

Three distinct types of bone cells are present in bone tissue, each with their own crucial function. Working together, osteoblasts, osteoclasts, and osteocytes are responsible for the proper development and maintenance of the skeleton, as well as regulating levels of minerals present in the bloodstream and throughout the body. Two related types of cells, lining cells and osteogenic cells, are derived from osteoblasts but have their own key functions for proper bone health.


The cells responsible for the creation of new bone tissue are the osteoblasts. They are created in the marrow of the bone, which is the soft inner area containing the stem cells that also produce red and white blood cells. Working collectively, osteoblasts create a type of bone tissue called osteoid primarily from collagen, which is then mineralized. This means that calcium and other minerals adhere to the tissue, making the bone cells strong.

Although osteoblasts are essential in forming bones when a fetus is developing in the womb and as a child grows, these bone cells don't stop working even once a person has reached adulthood. Bones are constantly being broken down and built back up, with about 4% of all bone surfaces having active osteoblast activity at any time. This process is called remodeling. The regular development of new tissue allows bones to repair breaks or other injuries and change in response to the body's needs. In addition, the bones are subjected to stress through everyday use, and develop tiny microfractures that are constantly being fixed. Once bone tissue has been broken down and built back up again, most of the osteoblast bone cells are squeezed flat and no longer produce new tissue. They become lining cells and are used to help protect the underlying bone matrix. Lining cells are also key in regulating levels of minerals such as calcium and phosphate, allowing these substances to pass into and out of the bones as needed.


Osteoclasts are large bone cells formed in the marrow of the bone. Similar in structure to white blood cells, they are responsible for breaking down bone tissue, which is required for bone growth and healing. They start out as smaller cells called osteoclast precursors, but fuse together into osteoclasts with multiple nuclei when they find places on the bone that need to be broken down, a process called resorption.

Although the number of osteoclast cells is relatively small, they are vital not only for the formation of new bone but also for helping to regulate minerals in the bloodstream. As these cells break down bone, they release calcium and phosphate into the blood, where these minerals play an important role in many biochemical processes. Osteoclasts are also involved in the development of red blood cells in the bone marrow. Research also suggests that osteoclasts have immune receptors, and that there are close ties between the immune and skeletal systems. Exactly how the two interact is still being studied, although studies on autoimmune diseases like rheumatoid arthritis show how the immune system can affect bone resportion. Osteoclasts are linked to other diseases as well; when they break down bone faster than it can be rebuilt, for example, osteoporosis is the result.


After the new bone tissue has been built, those osteoblasts that don't become lining cells remain deep in the bone matrix and become osteocytes, cells with long branches through the bone tissue that form a network. Osteocytes function as a control center, directing mineral deposits and sending osteoclasts to start repairing damage to the bone tissue as needed. They also are responsible for signalling the release of minerals such as calcium into the bloodstream to maintain good health.

Osteocytes are the most common of the bone cells, and they can live for decades. Some are programmed to die naturally, but conditions like osteoarthritis and osteoporosis are linked with an increased level of cell death. In other words, when a higher number of osteocytes die, the bones become weaker.


Most bone cells are unable to divide and cannot reproduce. Osteogenic cells are bone cells capable of creating new osteoblasts and osteoclasts. They are located in the periosteum, which is the tissue surrounding the bone, and the bone marrow. An injury such as a fracture triggers cell production by the osteogenic cells, creating new osteoblasts and osteocytes to repair the damage as quickly as possible.


From a health perspective, pollen is both vital and annoying. It is an important part of plant reproduction and can result in things like many of the foods people enjoy eating. Yet certain forms of it also create allergic reaction, usually called hayfever, which can be difficult to experience and sometimes worsens with age.

These male cells of plants are analogous to things like animal sperm in their purpose because they frequently have to travel in order to create fertilization or pollination with other parts of plants. This traveling takes place in numerous ways. Wind can blow these cells, insects pick them up and deposit them elsewhere, they may ride in animal fur, and even humans carry them in hair and clothes.

Many people make assumptions about pollen that are not always accurate. Since some cells are larger than others they are highly visible, and seeing this, people with allergies may assume these are the worst allergens. Typically, that is not the case. Smaller, less easy to visualize cells are more likely to be inhaled easily and tend to be the greatest offenders in causing conditions like hayfever.

Another assumption is that these cells only come from one source, such as flowers, grasses, or trees. Actually, they come from many sources and people with hayfever might be allergic to a lot more than grass-based pollen, though ragweed cells are considered very likely to induceallergy. Yet many people are also significantly affected by these cells as produced by certain trees or flowers.

When people are allergic to pollen, what this really means is that contact, often through inhalation of pollen cells, causes the body to produce a histamine response. Exposure to these reproductive cells creates inflammation in the mucus membranes and can result in numerous symptoms, which include runny or itchy nose, post-nasal drip, itchy eyes, occasionally asthma, coughing, and others. There tends to be no fever in this immune response and people may not be allergic to hay.

Hay fever may have peak seasons, when the most pollen is present in the air. It may be hard to avoid, though people can take medications that help reduce histamine response. It is also helpful to minimize outdoor activities when high cell counts are reported, and to make sure to wash body and hair thoroughly after time spent outside. Since most forms of these irritating cells are microscopic, they’re not likely to be seen or felt on the body. An allergy sufferer may still know they’re present, anyway, by exhibiting allergic response.

Many regions publish useful counts of certain pollens to help people determine those times when allergic response is most likely. Yet many people don’t know specifically what plants create problems for them. Allergy testing can help to determine this, and can also rule in or out the possibility that allergies to other substances, like dust mites, might be resulting in hayfever symptoms too.


A bract is a part of a plant that may resemble a leaf or a petal. Structurally, a bract is most similar to a leaf, but it usually is slightly different from the plant's leaves. Some bracts are green while others are colored. Colored bracts can be quite brightly colored and are often mistaken for petals.

Bracts can be many shapes, sizes, and textures. They can be larger or smaller than the leaves and petals, and they are generally tougher. Their main function is to protect the flower from pests and harsh weather. When a flower first blooms, it is surrounded by the thick, green bracts. Some plants have two bracts while others have several. The flower blooms and grows out of the bracts, which remain on the plant and form the base of the flower.

A common bracted plant is the poinsettia. On plants like poinsettia and bougainvillea, the bracts are often referred to as “false flowers” because the plant's true flowers are so tiny and hard to see. Bracts that surround flowers in a cluster are called involucre. Poinsettia flowers are small and light green, and the grow at the center of the red involucre. Bougainvillea flowers are white, and about the size of a lentil.

Brightly colored bracts serve to both protect the flower and attract pollinating insects. Other common plants with colorful bracts and insignificant flowers include the dogwood, the vase plant, and the lollipop plant.

On flowers such as daisies and sunflowers, the bracts are the green part that holds the flower to the stem. Bracts of this type are called phyllaries. On grasses and grains, there are two types of bract. These types of plants grow with long clusters of flowers, called florets, at the top. Each floret eventually contains a seed. The flowers are surrounded by two thin, scaly bracts, with an inner bract called the palea and an outer bract called the lemma. The whole floret is surrounded by green leaf-like or spiky bracts called glumes.

Some bracts are adapted to very specific functions. The bracts of the passion flower are coated with a sticky, acidic substance that traps insects. The acid then breaks down and digests the insects to provide nutrients for the flower. On the Lobelia telekii, a tall, conical, furry-looking plant native to cold alpine regions of Africa, the blue-green fur is made up of bracts that act as insulators.

Skin Cells

Epithelial cells are a group of tightly compressed cells that layers itself on the internal and external surfaces of bodily organs and other surfaces found in the body. As a collective term, these cells are also referred to as a tissue called epithelium. These cells are also the primary composition of human skin.

The basic function of epithelial cells is to provide a protective layer for the organ they enclose. Cells of this type in the digestive system also can absorb nutrients that the body needs during the digestion process. They can also aid in the secretion of enzymes and hormones, as well as the excretion of unwanted byproducts, especially when located in areas such as the kidneys and sweat glands. Epithelial linings along the lungs help disseminate the oxygen in all parts of bodies. Special epithelial tissues around the sense organs such as the eyes, nose and tongue are made with nerve endings to heighten sensitivity.

These cells are categorized as either lining or glandular ephithelium cells. Lining epithelials further protect the organs by coating the cell’s basement membrane, another protecting sheet that prevents foreign bodies from invading the healthy organs. Glandular epithelial cells, on the other hand, coat the glands, such as the sweat and mammary glands.

Lining epithelial cells are further classified as simple or stratified epithelials. The simple type has only one layer of cells, and the stratified kind is composed of several cell layers, ranging from three to seven layers. Stratified cells are usually found on organs that can experience heavy attacks from chemical reactions or foreign bodies, such that the organs are not affected even if one layer of epithelial cells is destroyed. An epithelial cell can also take various shapes, depending on its location and function: flat, cube-like or column-shaped.

Epithelial cells are usually constructed to not have any blood vessels, so no physical pain is experienced when they are exfoliated and regenerated constantly, not just from the skin, but from all organs that have epithelia. Urine can be a vehicle for these cells to be excreted out of the body, which is why it is normal for these cells to be microscopically observed duringurinalysis. Elevated amounts of epithelial cells, however, can indicate problems such as bladder or urinary tract infection. Urine that is unusually cloudy and darker-colored might cause some concerns and a need for a thorough urinalysis.

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