Introduction to Human Physiology

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CELLS AND TISSUES

Although there are about 200 different cell types in the human body, they are generally classified into four types of tissue:

  1. epithelial
  2. connective
  3. muscle
  4. nerve.


Epithelial Tissue

Epithelial tissues consist of continuous sheets of cells that provide a protective covering over the whole body and contain various sensory nerve endings.

They also form the lining membranes of internal organs, cavities, and passageways and cover internal organs as shown in the illustration below from Campbell.

Figure 1 (From Campbell, pg. 780)

One surface of the epithelial sheet is always attached to an underlying layer, called the basement membrane, composed of a fibrous polysaccharide material produced by the epithehal cells themselves.

Epithelial cells are often specialized for synthesis and secretion of products for export. For example, glands are made up of modified epithelial cells that produce specific substances, such as perspiration, saliva, milk, hormones, or digestive enzymes. Gland cells may be isolated, such as the goblet cells that secrete the mucus that lubricates the linings of body cavities and passages, or clustered together in organs.

Epithelial tissues are classified according to the shape of the individual cells as:

Figure 2

The three types of epithelial cells that cover the inner and outer surfaces of the body.

Squamous cells, which usually perform a protective function, make up the outer layers of the skin and the lining of the mouth and other mucous membranes. there are usually several layers of these flat cells piled on top of one another. Cuboidal and columnar cells, in addition to lining varous passageways, are often involved in active transport and other energy-requiring activities.

Epithelial tissues may consist of only a single layer of cells (simple epithelium), as found in the inner lining of the circulatory system, or several layers (stratified epithelium), as found in the outer layer (epidermis) of the skin. Click here to find out about the skin and integumentary system.


Connective Tissue

Connective tissue binds together, supports, and protects the other three kinds of tissue.

Unlike epithelial tissue cells, the cells of connective tissue are widely separated from one another by large amounts of intercellular material, the matrix, which anchors and supports the tissue. The matrix comprises a ground substance, which is more or less fluid and amorphous (formless), and fibers synthesized by the tissue cells. There are several different types of fibers depending on the tissue:

  1. connecting and supporting fibers, such as collagen, which is a major component of skin, tendons, ligaments, and bones (see the diagram below from Campbell pg. 785)
  2. elastic fibers, which are found, for example, in the walls of large blood vessels; and
  3. reticular fibers, which form networks inside solid organs, such as the liver.

Figure 3 (page 782 Campbell )

The structure of collagen. A collagenous fiber (inset, TEM) is a bundle of many macrofibrils, each of which in turn is a bundle of many microfibrils. A microfibril consists of many tropocollagen helices, with each helix composed of three polypeptide chains twisted together.


 

While epithelial tissues are classified according to cell shape, connective tissues are grouped by the characteristics of their intercellular matrix such as the extracellular matrix of bone cells

Bone, like other connective tissues, consists of cells, fibers, and ground substance (Click here to see Figure 33-5 from Curtis).

It is distinctive in that the intercellular matrix is impregnated with hard crystals. Bone tissue, despite its strength, is amazingly light; our bones make up only about 18 percent of our weight.

Use the following linked words to find out how bone is dynamically remodeled or formed and the role of osteoclasts and osteoblasts in this hormonally controlled process.

 

Blood and lymph are connective tissues in which the matrix is plasma. Blood can become a solid or clot when necessary. This clotting process is important to understand


Muscle Tissue

Muscle cells are specialized for contraction.

Every function of muscle--from running, jumping, smiling, and breathing to propelling the blood through the arteries and ejecting the fetus from the uterus--is carried out by the contraction of muscle cells in concert. See muscle contraction in action.

Muscle tissue can be classified in various ways. Classifying by appearance yields two types, as shown in Figure 33-6: striated muscle, which has stripes under the microscope, and smooth muscle (no stripes).

Classifying by location, there are three categories: skeletal muscle, cardiac muscle, and, again, smooth muscle. The latter surrounds the walls of internal organs, such as the digestive organs, uterus, bladder, and blood vessels.

In a third type of classification, muscle tissue is categorized as voluntary or involuntary. Skeletal muscle is voluntary. Smooth muscle and cardiac muscle are not, except in rare individuals, under direct voluntary control.

Contraction of each of these types of muscle cell depends on the interaction of two proteins: actin and myosin. In skeletal and cardiac muscle, these proteins are arranged in regular, repeating assemblies, resulting in the characteristic striations.

Check out how muscle works.

Striated Muscle (skeletal muscle)

Striated muscle, as mentioned previously, includes both skeletal and cardiac muscle. Some 40 percent of a man's body, by weight, is skeletal muscle; women characteristically have less, around 20 percent.

A skeletal muscle is typically attached to two or more bones, either directly or by means of the tough strands of connective tissue known as tendons. Some of these tendons, such as those that connect the finger bones and their muscles in the forearm, are very long.

 

When the muscle contracts, the bones move around a joint, which is held together by ligaments and generally contains a lubricating fluid.

Figure 4

Most of the skeletal muscles of the body work in antagonistic groups, one flexing, or bending, the joint, and the other extending, or straightening, it (See figure to the right).

Also, two antagonistic groups may contract together to stabilize a joint. Such muscle action makes it possible for us (and others) to stand upright.

(From Figure 37-7 of Curtis)

A skeletal muscle, such as the biceps, consists of bundles of muscle fibers held together by connective tissue. Each fiber is a single cell, cylindrical or spindle-shaped (tapering at both ends), with many nuclei, formed by the fusion of a large number of small, mononucleate cells during embryonic development. These fibers are very large cells--50 to 100 micrometers in diameter and, often, several centimeters long.

 

Cardiac muscle

Cardiac muscle resembles skeletal muscle in its assemblies of actin and myosin and, thus, in its striated appearance. It differs from skeletal muscle in that its cells are usually mononucleate.

Smooth Muscle

Although smooth muscle cells also contain actin and myosin, the assemblies do not form a striated pattern.

Smooth muscle cells are spindle-shaped and mononucleate.

Functionally, smooth muscle contracts much less rapidly than skeletal striated muscle, its contractions are more prolonged, and it is under involuntary control. In most hollow organs, such as the intestines, smooth muscle fibers are organized into sheets arranged in two layers--an outer, longitudinal layer and an inner, circular layer--which can contract alternately, thus, for example, moving food along the intestinal tract (See Figure 5 below). In blood vessels, bundles of smooth muscle fibers encircle the arterial walls, constricting the vessels as the fibers contract.

Figure 5

Alternating contractions of circular and longitudinal muscles move food along the digestive tract, a process known as peristalsis.

Similar arrangements of muscles are involved in many other animal functions, including, for example, the locomotion of the earthworm.

Note how smooth muscle is arranged in the gut.

 


Nerve Tissue

The fourth major tissue type is nerve tissue.

The major functional units of nerve tissue are the neurons, which transmit nerve impulses.

Nerve tissue also comprises a second type of cell that supports and insulates neurons. Such cells are called neuroglia when they are in the central nervous system (brain and spinal cord) and Schwann cells in the peripheral nervous system. These supporting cells are believed to supply the neurons with nutrients and other molecules and play an important part in maintaining the ionic composition of nerve tissue, which is, as we shall see, central to its function.

Neurons are specialized to receive signals, from either the external or the internal environment, and to transmit them in the form of electrical impulses to other neurons, muscles, or glands.

Functionally, there are three types of neurons:

  • sensory neurons, which receive information and relay it to the central nervous system;
  • motor neurons, which relay signals from the central nervous system to effector organs (muscles or glands); and
  • interneurons, which transmit signals within the central nervous system.


Neurons.

(a) A motor neuron, characterized by an axon, that, in a large vertebra te, may be a meter or more in length.

(b) An interneuron, showing the complex system of dendrites and an axon with many branches. Such a nen ron forms connections-s ynapses-with many other nerve cells.

(c) A sensory neu ron, which transmits impulses from sensory receptors at the ends of the dendrite branches. In this type of neuron, the cell body is off to one side and the dendrite and axon form a single long fiber.

A neuron consists of the

  • cell body, which contains the nucleus and much of the metabolic machinery of the cell; the
  • dendrites, usually numerous, short, threadlike cytoplasmic extensions--processes--that receive stimuli from other cells; and the
  • axon, a single long process that carries the nerve impulse away from the cell body to other cells or organs.

A special junction called a synapse helps transmit a nerve impulse from one neuron to the next.

 

Both dendrites and axons are also called nerve fibers.

Neurons may reach astonishing lengths. For example, the axon of a single motor neuron may extend from the spinal cord down the whole length of the leg to the toe. Or a sensory neuron may send a dendrite down to the toe and an axon up the entire length of the spinal cord to terminate in the lower part of the brain. In an adult human, such a cell might be close to 2 meters long (5 meters in a giraffe).

Nerves are bundles of many nerve fibers from many neurons--usually hundreds and sometimes thousands. Each fiber is capable of transmitting a separate message, like the wires in a telephone cable.

Cross section of a nerve showing the separate fibers. These fibers, which may be axons or long dendrites, are insulated from one another by Schwann cells. In some cases, the Schwann cells have wrapped themselves around the nerve fibers, forming myelin sheaths, the dark borders. These fatty layers are opaque to electrons and so they appear black in electron micrographs. Under the light microscope (or to the unaided eye) they are a glistening white.

Reflex Arc

A reflex arc, showing the function of the three types of neurons.

The sensory neuron is stimulated by a sensory receptor.

It relays the signal to an interneuron, located entirely within the central nervous system.

From the inter-neuron, the signal is transmitted to a motor neuron that stimulates an effector, shown here as a muscle cell.

These basic components of the reflex arc are found in all vertebrates, from the simplest to the most complex.


Visit Neuroscience for Kids to further explore the Brain and Spinal Cord.


For a review of the sense organs visit the following sites:


Every organ in the human body contains these four tissue types, as shown, for example, in this illustration from the OnLine BioBook.

Human skin is an organ which has all the basic types of tissues.


Modified Ap. 19, 2003