Four Basic Types of Tissue Found in Human and Animals

The following points highlight the four basic types of tissue found in human and animals. The four types are: 1. Epithelial Tissue 2. Connective Tissue 3. Muscle Tissue and 4. Nervous Tissue.

Tissue Type # 1. Epithelial Tissue:

Epithelium is a tissue composed of layers of cells that line the cavities and surfaces of structures through­out the body. It is also the type of tissue from which many glands are formed. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies.

The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells. The epithelial tissue is made of closely packed cells arranged in flat sheets. Epithelia form the surface of the skin, line the various cavities and tubes of the body, and cover the internal organs.

Subsets of Epithelial Tissue:

Mucous membranes lining the inside of the mouth, the oesophagus and part of the rectum are lined by non-keratinized stratified squamous epithelium. Other, open to outside body cavities are lined by simple squamous or columnar epithelial cells.

Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands. The outer surface of the cornea is covered with fast-growing, easily regenerated epithelial cells. Functions of epithelial cells include secretion, absorp­tion, protection, trans cellular transport, sensation detection, and selective permeability. Endothelium (the inner lining of blood vessels, the heart and lymphatic vessels) is a specialized form of epithelium. Another type the mesothelium forms the walls of the pericardium, pleurae, and peritoneum.

Function:

The function of epithelia always reflects the fact that they are boundaries between masses of cells and a cavity or space.

The epithelium of the skin protects the underlying tissues from:

i. Mechanical damage

ii. Ultraviolet light

iii. Dehydration

iv. Invasion by bacteria

The columnar epithelium of the intestine:

i. Secretes digestive enzymes into the intestine

ii. Absorbs the products of digestion from the intestine.

Classification:

In humans, epithelium is classified as a primary body tissue, the other ones being connective tissue, muscle tissue and nervous tissue. Epithelium is often defined by the expression of the adhesion molecule e-cadherin, as opposed to n-cadherin, which is used by cells of the connective tissue.

Epithelial cells are classified by the following three factors:

Shape (of most superficial cells):

1. Squamous:

All squamous cells are cells with irregular flattened shape. A one-cell layer of simple squamous epithelium forms the alveoli of the respiratory membrane and the endothelium of capil­laries and is a minimal barrier to diffusion. Squamous cells can be found included in the filtration tubules of the kidneys and the major cavities of the body. These cells are relatively inactive metabolically and are associated with the diffusion of water, electrolytes and other substances.

2. Cuboidal:

As the name suggests, these cells have a shape similar to a cube, meaning its width is of the same size as its height. The nuclei of these cells are usually located in the center. The cuboidal epithelium forms the smallest duct glands and many kidney tubules.

3. Columnar:

These cells are taller than they are wide. Simple columnar epithelium is made up of a single layer of such cells. The nucleus is also closer to the base of the cell. The small intestine is a tubular organ lined with this type of tissue. Unicellular glands called goblet cells are scattered through­out the simple columnar epithelial cells and secrete mucus. The free surface of the columnar cells have tiny hair-like projections called microvilli. They increase the surface area for absorption.

4. Transitional:

This is a specialized type of epithelium found lining the organs that can stretch, such as the urothelium that lines the bladder and ureter of mammals. Since the cells can slide over each other, the appearance of this epithelium depends on whether the organ is distended or contracted: if distended, it appears as if there are only a few layers; when contracted, it appears as if there are several layers.

Stratification:

1. Simple:

There is a single layer of cells.

2. Stratified:

More than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the basal lamina. Stratified cells can usually withstand large amounts of stress.

3. Pseudo stratified with cilia:

There is only a single layer of cells, but the position of the nuclei gives the impression that it is stratified.

Specializations:

1. Keratinized cells:

Contain keratin (a cytoskeletal protein). While keratinized epithelium occurs mainly in the skin, it is also found in the mouth and nose, providing a tough, impermeable barrier.

2. Ciliated cells:

Have apical plasma membrane extensions composed of microtubules capable of beating rhythmically to move mucus or other substances through a duct. Cilia are common in the respiratory system and the lining of the oviduct.

3. Mesothelia:

These are derived from mesoderm:

i. Pleura:

The outer covering of the lungs and the inner lining of the thoracic cavity.

ii. Peritoneum:

The outer covering of all the abdominal organs and the inner lining of the abdominal cavity.

iii. Pericardium:

The outer lining of the heart.

4. Endothelial:

The inner lining of the heart, all blood and lymphatic vessels are derived from mesoderm. The basolateral surface of all epithelia is exposed to the internal environment (ECF). The entire sheet of epithelial cells is attached to a layer of extracellular matrix that is called the basement membrane or, better (because it is not a membrane in the biological sense), the basal lamina.

An epithelium also lines our air passages and the alveoli of the lungs. It secretes mucus which keeps it from drying out and traps inhaled dust particles. Most of its cells have cilia on their apical surface that propel the mucus with its load of foreign matter back up to the throat.

Tissue Type # 2. Connective Tissue:

As the name suggests, connective tissue holds everything together. It is characterized by the separation of the cells by non-living material, which is called extracellular matrix. Connective tissue is derived from mesoderm and is involved in structure and support.

The cells of connective tissue are embedded in a great amount of extracellular material. This matrix is secreted by the cells. It consists of protein fibers embedded in an amorphous mixture of protein- polysaccharide (‘proteoglycan’) molecules. Collagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making about 25% of the total protein content.

Blood, cartilage, and bone are usually considered connective tissue, but because they differ so substan­tially from the other tissues in this class, the phrase ‘connective tissue proper’ is commonly used to exclude those three. There is also variation in the classification of embryonic connective tissues.

Connective Tissue Proper:

Supporting connective tissue:

Gives strength, support, and protection to the soft parts of the body.

This includes:

Cartilage:

Ex. The outer ear.

Bone:

The human body contains 206 bones of different shapes and sizes. It is a very hard extra cellular matrix and a dynamic tissue.

The functions of bone include:

i. Maintenance of external form

ii. Weight bearing support

iii. Site for attachment of muscles

iv. Protection of internal organs

v. Stores calcium and phosphorus

vi. Site for hematopoietic system (in the bone marrow)

vii. Facilitates movements

Composition of bone:

Bone is a mineralized connective tissue. Its matrix contains both organic (35%) and inorganic (65%) material. The organic matter is mainly proteins. The inorganic or mineral compo­nent is mainly crystalline hydroxyapatite, Ca₁₀ (PO₄)₆(OH)₂ along with sodium, magnesium, carbonate and fluoride ions. 99% body’s calcium is in the bone which gives the strength to the bone.

The principle proteins found in bone are collagen type I and V. The non-collagen proteins include CS-PG I, II and III (chondroitin sulfate—proteoglycan), osteonectin, osteocalcin, osteopontin, bone sialoprotein and bone morphogenetic proteins. Components of bone are marrow, periosteum, endosteum and bone cells in various stages of devel­opment. The cells in the bone tissue are osteoprogenitor cells, osteoblasts, osteocytes and osteoclasts.

Osteoblasts:

They are mono-nucleated cells that synthesize most of the proteins (Vitamin D is essen­tial). They are responsible for the deposition of new bone matrix (osteoid) and subsequent mineralization. They control mineralization by the passage of calcium and phosphate ions. The membranes contain the enzyme alkaline phosphatase, that hydrolyzes organic phosphates to release phosphate ions.

During bone formation, the osteoblasts secrete collagen and non-collagen proteins to form the organic matrix. The matrix proteins become insoluble and deposition of calcium and phosphate occurs (mineralization) and become hydroxyapatite. As mineralization proceeds the osteoblasts are surrounded by bone tissue and differentiate into osteocytes.

Osteocytes:

They are star shaped cells (most abundant in bone). These cells are networked to each other via long processes that occupy tiny canals called canaliculi. These canaliculi exchange nutrients and wastes and maintain bone tissue. Hydroxyapatite, calcium carbonate and calcium phosphate are deposited around these cells.

Osteoclasts:

They are multinucleated cells. The osteoclasts play a key role in bone resorption (disso­lution of bone). Lysosomal enzymes digest bone matrix proteins at pH 4, and result in dissolution of bone matrix. Hydrogen ions are produced by the action of carbonic anhydrase II.

The products of bone resorption, mainly calcium and phosphorus ions are transferred to blood capillaries. Bone formation (Osteoblasts) is stimulated by parathyroid hormone, (PTH), vitamin D, androgens, growth hormone, insulin and inhibited by Cortisol. Bone resorption (Osteoclasts) is stimulated also by parathyroid hor­mone, thyroid hormones (T3), Cortisol, vitamin D and inhibited by calcitonin and estrogens.

Bone Disorders:

Osteoporosis:

There is progressive reduction in bone tissue per unit volume (bone density) causing skeletal weakness and fractures of various bones. Some factors like estrogens and interleukins-1 and 6 are involved in stimulating osteoclasts. Genetic and environmental factors are involved in bone resorption, including poor diet (calcium and vitamin D), smoking, alcohol consumption and lack of exercise.

Osteopetrosis (marble bone disease):

Increased bone density due to inability to resorb bone. It is due to the mutation in the gene coding for carbonic anhydrase II (CA II). Thus if CA II is deficient in activity in osteoclasts, normal bone resorption does not occur.

Rickets:

Rickets is a childhood disorder characterized by bone deformities due to defective mineral­ization of bone. Most commonly due to deficiency of vitamin D which stimulates the intestinal absorp­tion of calcium and phosphate.

Osteomalacia:

Is seen in adults that results from, demineralization of bone especially in women who have little exposure to sunlight often after several pregnancies. Dietary supplementation with egg, cod liver oil, liver can help recover this problem.

Binding connective tissue:

It binds body parts together. This includes:

Tendons:

Connect muscle to bone. The matrix is principally collagen, and the fibers are all oriented parallel to each other. Tendons are strong but not elastic.

Ligaments:

Attach one bone to another. They contain both collagen and the protein elastin. Elastin permits ligaments to be stretched.

Fibrous connective tissue:

It is distributed throughout the body. It serves as a packing and binding material for most of our organs. Collagen, elastin and other proteins are found in the matrix. Fascia is fibrous connective tissue that binds muscle together and binds the skin to the underlying structures.

Adipose tissue is fibrous connective tissue in which the cells have become almost filled with oil. The oil is confined within membrane-bound droplets. The cells of adipose tissue, called adipocytes, secrete several hormones, including leptin and adiponectin. All forms of connective tissue are derived from cells called fibroblasts, which secrete the extracellular matrix.

Embryonic connective tissues:

There are two types of embryonic connective tissues viz. Mesenchy­mal connective tissue and mucous connective tissue.

Disorders of connective tissue:

The various disorders of the connective tissue which can be both inherited and environmental are:

i. Marfan syndrome: A genetic disease causing abnormal fibrillin.

ii. Loeys-Dietz syndrome: A genetic disease related to Marfan syndrome, with an emphasis on vascular deterioration.

iii. Pseudoxanthoma elasticum: An autosomal recessive hereditary disease, caused by calcification and fragmentation of elastic fibres, affecting the skin, the eyes and the cardiovascular system.

iv. Systemic lupus erythematosus: A chronic, multisystem, inflammatory disorder of probable au­toimmune etiology, occurring predominantly in young women.

v. Fibrodysplasia ossificans progressiva: Disease of the connective tissue, caused by a defective gene which turns connective tissue into bone.

vi. Spontaneous pneumothorax: Collapsed lung, believed to be related to subtle abnormalities in connective tissue.

vii. Sarcoma: A neoplastic process originating within connective tissue.

Extra Cellular matrix (ECM):

The cells of connective tissue are embedded in a great amount of extracellular material (ECM). This matrix is secreted by the cells.

The main components of ECM are:

1. Proteins:

This includes:

(a) Structural fibrous proteins (collagen and elastin)

(b) Specialized proteins for cell adhesion (fibrillin, fibronectin & laminin)

2. Proteoglycans:

This includes, polysaccharide chains, called glycosaminoglycan’s (GAGs), often found covalently linked to proteins.

Collagens:

These are the major component of ECM, constituting 25% of all proteins in mammals. They are distributed in various tissues such as skin, bone, tendon, blood vessels, cornea, cartilage, inter­vertebral disks and vitreous body as fibril-forming. As a network forming, it is present in basement membranes and beneath squamous equithelium.

Collagens-Structure:

Most collagens are triple helical (three a chains) that are coiled in one another to form a thread like structure (fibril). Each chain is made of 1000 amino acids, rich in glycine and proline. Glycine, the smallest amino acid is found in every third position of the polypeptide chain which enables to form the triple helix. The repeating sequence of the helix is —Gly-X-Y where X is often proline Y is often hydroxy proline or hydroxy lysine (about 100 of each).

Proline and hydroxy proline confer the rigidity (strength) on the collagen molecule. Hydroxy proline and hydroxy lysine are formed by posttranslational hydroxylation catalyzed by prolyl hydroxylase and lysyl hydroxylase respectively. Ascorbic acid and a ketoglutarate are cofactors for the enzymes. Some of the hydroxy lysyl residues are modified by the addition of galactose or galactosyl-glucose through O-glycosidic linkage. Collagens are synthesized as pre collagens, then to pro collagen and finally converted to collagens.

Collagen diseases:

Some of the common diseases related to collagen of ECM are:

Ehlers-Danlos syndrome (EDS):

It is a genetic disorder in collagen molecule wherein type-Ill collagen is deficient. This can result from the deficiency of the enzyme hydroxylase or mutations in the amino acid sequences in collagen synthesis. It results in progressive deterioration of collagens, with different EDS types affecting different sites in the body, such as joints, heart valves, organ walls, arterial walls, etc. Skin and vascular problems are the major symptoms that are expressed.

Osteogenesis imperfecta (brittle bone disease):

It is also a group of inherited disorders, caused by insufficient produc­tion of good quality collagen to produce healthy, strong bones. It is characterized by bones that easily bend or frac­ture. Type-I disease presents in the early infancy with frac­tures and Type-II, is more severe and patients die in utero. Most patients have mutations in the gene which substitutes bulky side chain amino acids for glycine.

Elastin:

It is a connective tissue protein with rubber-like prop­erties present in the lungs, the walls of large arteries, bladder and elastic ligaments. They can be stretched to several times their normal length, but recoil to their original shape when the stretching force is relaxed.

Elastin is an insoluble protein synthesized from the precursor, tropo elastin which is a single gene. The polypeptide forms a random coil conformation. Pro elastin contains 700 amino acids, primarily glycine, alanine and valine, which are small and nonpolar.

It is also rich in proline and lysine but contains little hydroxy proline but no hydroxy lysine. Some of the lysyl residues are oxidatively deaminated to form aldehyde containing allysine residues. Three allysyl residues and one unaltered lysyl form the network called desmosine cross-link and this produces the rubber like elasticity.

Emphysema and α₁ antitrypsin:

In the normal lung, the alveoli are exposed to low levels of neutro­phil elastase, released from activated and degenerating neutrophils. This proteolytic activity can destroy the elastin in the walls of the alveoli of lungs. The most important inhibitor of this elastase is antitrypsin whose deficiency leads to the destruction of the elastin, caused by the action of elastase and results in a disease called emphysema.

Fibrillin, Fibronectin and Laminin:

Fibrillin is a large glycoprotein which is a structural component of micro fibrils. Marfan syndrome is due to mutation in the gene for fibrillin, which affects lens, skeletal system and cardiovascular system. Fibronectin is an important glycoprotein involved in cell adhesion and cell migration. Laminin is major protein component of renal glomerular and other basal laminas.

Proteoglycans and Glycosaminoglycan’s (GAGs):

Glycosaminoglycan’s are usually linked to proteins to from proteoglycans. GAGs occupy large amounts of space and form hydrated gels and resists compres­sion.

Their main functions are:

i. Act as structural components of the ECM

ii. Have specific interaction with collagen, elastin, fibronectin, laminin and growth factors

iii. As polyanions bind to polycations

iv. Contribute to the characteristic turgor of various tissues

v. Act as sieves in the ECM Facilitate cell migration

vi. Have role in compressibility of cartilage in weight-bearing (Hyalyronic acid and Chondroitin sulfate)

vii. Play role in corneal transparency (Keratin sulfate I and Dermatan sulfate)

viii. Have structural role in sclera (Dermatan sulfate)

ix. Act as anticoagulant (Heparin)

x. Are components of plasma membrane and act as receptors and participate in cell adhesion and cell to cell interactions (Heparin sulfate)

xi. Determine charge-selectiveness of renal glomerulus (Heparin sulfate)

xii. Are components of synaptic and other vesicles

Mucopolysaccharidoses:

These are hereditary disorders characterized by the accumulation of GAGs in various tissues, causing skeletal and extra cellular matrix deformities and mental retardation. They are caused due to the deficiency of one of the enzymes in the lysosomal degradation of heparin sulphate and Dermatan sulphate. Examples are Hunter syndrome, Hurler syndrome and Sanfilippo syndrome.

Tissue Type # 3. Muscle Tissue:

Muscle (from Latin musculus, diminutive of mus ‘mouse’) is contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Its function is to produce force and cause motion, either locomotion or movement within internal organs. Cardiac and smooth muscle contraction occurs without conscious thought and is necessary for survival.

The examples are the contraction of the heart and peristalsis which pushes food through the digestive system. Voluntary contraction of the skeletal muscles is used to move the body and can be finely controlled. The examples are movements of the eye, or gross movements like the quadriceps muscle of the thigh. There are two broad types of voluntary muscle fibers: slow twitch and fast twitch. Slow twitch fibers contract for long periods of time but with little force while fast twitch fibers contract quickly and powerfully but fatigue very rapidly.

Types of Muscle:

There are three types of muscle:

1. Smooth muscle or involuntary muscle:

Is found within the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and even the skin (in which it controls erection of body hair). Unlike skeletal muscle, smooth muscle is not under conscious control.

2. Cardiac muscle or involuntary muscle:

It is more akin in structure to skeletal muscle, and is found only in the heart. Cardiac and skeletal muscles are ‘striated’ i.e. they contain sarcomeres and are packed into highly regular arrangements of bundles; smooth muscle has neither. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, forming irregular angles (called intercalated discs). Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.

3. Skeletal muscle or voluntary muscle:

It is anchored by tendons to bone and is used to affect skeletal movement such as locomotion and in maintaining posture. Though this postural control is generally maintained as a subconscious reflex, the muscles responsible react to conscious control like non-postural muscles. The body of an average adult male is made up of 40—50% of skeletal muscle and that of an average adult female is made up of 30-40% (as a percentage of body mass).

Skeletal muscle is further divided into several subtypes:

(1) Type-I:

Slow oxidative, slow twitch, or ‘red’ muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity.

(2) Type-II:

Fast twitch muscle, has three major kinds that are, in order of increasing contractile speed:

(a) Type-IIa:

Like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.

(b) Type-IIx: (also known as type-IId):

This is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build­up of lactic acid).

(c) Type-IIb:

This is anaerobically glycolytic, ‘white’ muscle that is even less dense in mito­chondria and myoglobin. In small animals like rodents this is the major fast muscle type, explaining the pale color of their flesh.

Structure of Muscle:

Muscle is mainly composed of muscle cells. Within the cells are myofibrils; myofibrils contain sarcomeres, which are composed of actin and myosin. Individual muscle fibres are surrounded by endomysium. Muscle fibers are bound together by perimysium into bundles called fasci­cles; the bundles are then grouped together to form muscle, which is enclosed in a sheath of epimysium.

Muscle spindles are distributed throughout the muscles and provide sensory feedback information to the central nervous system. Skeletal muscle is arranged in discrete muscles, an example of which is the biceps brachii. It is connected by tendons to processes of the skeleton.

Cardiac muscle is similar to skeletal muscle in both composition and action, being comprised of myofibrils of sarcomeres, but anatomically different in that the muscle fibers are typically branched like a tree and connect to other cardiac muscle fibers through intercalated discs, and form the appearance of a syncydum.

Muscle contraction:

The three (skeletal, cardiac and smooth) types of muscle have significant differ­ences. However, all three use the movement of actin against myosin to create contraction. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by the nerves, the motor nerves and Moto neurons in particular.

Cardiac and smooth muscle contractions are stimulated by internal pacemaker cells which regularly contract and propagate contractions to other muscle cells they are in contact with. All skeletal muscle and many smooth muscle contractions are facilitated by the neurotransmitter acetyl­choline.

Diseases of the Muscle:

Neuromuscular disease:

Symptoms of muscle diseases may include weakness, spasticity, myoclonus and myalgia. Diagnostic procedures that may reveal muscular disorders include testing creatine kinase levels in the blood and electromyography (measuring electrical activity in muscles).

In some cases, muscle biopsy may be done to identify a myopathy, as well as genetic testing to identify DNA abnormalities associated with specific myopathies and dystrophies. Neuromuscular diseases are those that affect the muscles and/or their nervous control.

In general, problems with nervous control can cause spasticity or paralysis, depending on the location and nature of the problem. A large proportion of neurological disorders lead to problems with movement, ranging from cerebrovascular accident (stroke) and Parkin­son’s disease to Creutzfeldt-Jakob disease.

A non-invasive elastography technique that measures muscle noise is undergoing experimentation to provide a way of monitoring neuromuscular disease. The sound produced by a muscle comes from the shortening of actomyosin filaments along the axis of the muscle. During contraction, the muscle shortens along its longitudinal axis and expands across the transverse axis, producing vibrations at the surface.

Atrophy:

There are many diseases and conditions which cause a decrease in muscle mass, known as muscle atrophy, ex. Cancer and AIDS, which induce a body wasting syndrome called cachexia. Other syndromes or conditions which can induce skeletal muscle atrophy are congestive heart disease and some diseases of the liver.

During ageing:

There is a gradual decrease in the ability to maintain skeletal muscle function and mass, known as sarcopenia. The exact cause of sarcopenia is unknown, but it may be due to a combination of the gradual failure in the ‘satellite cells’ which help to regenerate skeletal muscle fibers, and a decrease in sen­sitivity to or the availability of critical secreted growth factors which are necessary to maintain muscle mass and satellite cell survival. Sarcopenia is a normal aspect of ageing, and is not actually a disease state.

Physical inactivity and atrophy:

Inactivity and starvation in rodents and mammals lead to atrophy of skeletal muscle, accompanied by a smaller number and size of the muscle cells as well as lower protein content. In humans, prolonged periods of immobilization, as in the cases of bed rest or astronauts flying in space, are known to result in muscle weakening and atrophy. Such consequences are also noted in small hibernating mammals like the golden-mantled ground squirrels and brown bats.

Tissue Type # 4. Nervous Tissue:

The function of the nervous tissue is to communicate between parts of the body. It is composed of neurons, which transmit impulses, and the neuroglia cells, which assist propagation of the nerve impulse as well as provide nutrients to the neuron. All the nervous tissues constitute the nervous system, which includes the brain, spinal cord and nerves.

Nervous tissue is specialized to react to stimuli and to conduct impulses to various organs in the body which bring about response to the stimuli. Nerve tissues (as in the brain, spinal cord and peripheral nerves that branch throughout the body) are all made up of specialized nerve cells called ‘neurons’, bound together by connective tissue.

A sheath of dense connective tissue, the epineurium surrounds the nerve. This sheath penetrates the nerve to form the perineurium which surrounds bundles of nerve fibers. Blood vessels of various sizes can be seen in the epineurium. The endoneurium, which consists of a thin layer of loose connective tissue, surrounds the individual nerve fibers. Glial cells are also part of nerve.

Neurons:

They function for the conduction of nerve impulses.

A typical neuron consists of:

A cell body which contains the nucleus

A number of short fibers termed dendrites, extending from the cell body

A single long fiber, the axon

Classification of Neurons:

The neurons are classified— Based upon function as:

(1) Sensory (or afferent) neurons:

Those that conduct impulses from the sensory organs to the central nervous system (brain and spinal cord) are called sensory (or afferent) neurons.

(2) Motor (or efferent) neurons:

These conduct impulses from the central nervous system to the effector organs (such as muscles and glands).

(3) Interneurons:

Those that connect sensory neurons to motor neurons are called interneurons (also known as connector neurons or association neurons).

Based upon structure as:

(1) Unipolar neurons:

Sensory neurons have only a single process or fibre which divides close to the cell body into two main branches (axon and dendrite). Because of their structure they are often referred to as unipolar neurons.

(2) Multipolar neurons:

Motor neurons, which have numerous cell processes (an axon and many dendrites), are often referred to as multipolar neurons. Interneurons are also multipolar.

(3) Bipolar neurons:

Bipolar neurons are spindle-shaped, with a dendrite at one end and an axon at the other. An example can be found in the light-sensitive retina of the eye.

Structure of a motor neuron:

A motor neuron has many processes (cytoplasmic extensions) called ‘dendrites’, which enter a large, grey cell body at one end. A single process, the ‘axon’, leaves at the other end, extending towards the dendrites of the next neuron or to form a motor endplate in a muscle. Dendrites are usually short and divided while the axons are very long and do not branch freely.

The impulses are transmitted through the motor neuron in one direction, i.e. into the cell body by the dendrites and away from the cell body by the axon. The cell body is enclosed by a cell (plasma) membrane and has a central nucleus. Granules called Nissl bodies are found in the cytoplasm of the cell body.

Within the cell body, extremely fine neurofibrils extend from the dendrites into the axon. The axon is surrounded by the myelin sheath, which forms a whitish non-cellular fatty layer around the axon. Outside the myelin sheath is a cellular layer called the neurilemma or sheath of Schwann cells. The myelin sheath together with the neu­rilemma is also known as the medullary sheath. This medullary sheath is interrupted at intervals by the nodes of Ranvier.

Neuronal Communication:

Nerve cells are functionally connected to each other at a junction known as synapse, where the terminal branches of an axon and the dendrites of another neuron lie in close proximity to each other but normally without direct contact. Information is transmitted across the gap by chemical secretions called neurotransmitters. It causes activation in the post-synaptic cell. The nerve impulse is conducted along the axon. The tips of axons meet other neurons at junctions called synapses, muscles (called neuromuscular junctions) and glands.

Glia:

Glial cells surround neurons. Once thought to be simply support for neurons (glia = glue), they turn out to serve several important functions.

There are three types:

1. Schwann cells:

These produce the myelin sheaths that surround many axons in the peripheral nervous system.

2. Oligodendrocytes:

These produce the myelin sheaths that surround many axons in the central nervous system (brain and spinal cord).

3. Astrocytes:

These are often star-shaped and the cells are clustered around synapses and the nodes of Ranvier where they perform a variety of functions viz. stimulating the formation of new synapses, modulating the activity of neurons, repairing damage and supplying neurons with materials secured from the blood. [It is primarily the metabolic activity of astrocytes that is being measured in brain imaging by positron-emission tomography (PET) and functional magnetic resonance imaging (fMRI)].

In addition, the central nervous system contains many microglia i.e. mobile cells that respond to damage (e.g. from an infection) by engulfing cell debris.