JARINGAN DASAR HEWAN DAN MANUSIA

Tujuan : Mengenal tipe-tipe jaringan dasar yang ditemukan pada hewan.

PENDAHULUAN

Tubuh hewan terdiri atas jaringan-jaringan atau sekelompok sel yang mempunyai struktur dan fungsi yang sama. Jaringan dengan struktur yang khusus memungkinkan mereka mempunyai fungsi yang spesifik. Sebagai contoh, otot-otot jantung yang bercabang menghubungkan sel-jantung yang lainnya. Percabangan tersebut membantu kontraksi sel-sel dalam satu koordinasi (Campbell et al. 1999). Ilmu yang mempelajari jaringan disebut histologi. Jaringan didalam tubuh hewan mempunyai sifat yang khusus dalam melakukan fungsinya, seperti peka dan pengendali (jaringan saraf), gerakan (jaringan otot), penunjang dan pengisi tubuh (jaringan ikat), absorbsi dan sekresi (jaringan epitel), bersifat cair (darah) dan lainnya. Masing-masing jaringan dasar dibedakan lagi menjadi beberapa tipe khusus sesuai dengan fungsinya. Pada saat perkembangan embrio, lapisan luar kulit (germ layers) berdiferensiasi (dengan proses yang disebut histogenesis) menjadi empat macam jaringan utama, yaitu jaringan epitel, jaringan pengikat, jaringan otot, dan jaringan saraf.

1. Jaringan Epithelium

Jaringan epitel terdiri atas satu atau banyak lapis sel, yang menutupi permukaan dalam dan luar suatu organ. Secara embriologi, jaringan ini berasal dari lapisan ektoderm, mesoderm atau endoderm. Di bagian tubuh luar, epitel ini membentuk lapisan pelindung, sedangkan pada bagian dalam tubuh, jaringan epitel terdapat disepanjang sisi organ. Jaringan epitel dibedakan berdasarkan bentuk dan jumlah lapisan sel penyusunnya, yaitu (1) epithelium satu lapis (simple epithelium). Epithel ini terdiri atas sel-sel berbentuk pipih, kubus, dan silindris (batang). Epithelium pipih selapis ditemukan antara lain pada lapisan endotel pembuluh darah. Epithelium bentuk kubus ditemukan pada kelenjar tyroid dan pembuluh darah. Epithel berbentuk silindris (batang) ditemukan pada lambung dan usus. (2) Epithelium berlapis banyak (stratified epithelium) yang dibentuk oleh beberapa lapis sel yang berbentuk pipih, kuboid, atau silindris. Epithelium ini dapat ditemukan pada kulit, kelenjar keringat, dan uretra. Beberapa lapisan pada epithelium ini dapat berubah menjadi sel-sel yang memanjang dan disebut epithelium transisional. Epitel transisional ditemukan pada kandung

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kemih (vesica urinaria). Disamping itu, terdapat epithelium berlapis banyak semu (pseudostratified epithelium) yang ditemukan pada trakea.

Epitel pipih berlapis, seperti yang terdapat di permukaan kulit kita, mampu melakukan mitosis dengan cepat. Sel-sel baru hasil mitosis menggantikan sel-sel permukaan yang mati. Epitel ini juga sebagai pelindung organ terhadap abrasi oleh makanan yang kasar, seperti yang ditemukan pada esofagus. Sebaliknya, epitelium pipih selapis berukuran tipis dan lemah, yang cocok untuk pertukaran material dengan cara difusi. Epitel ini ditemukan pada dinding kapiler darah dan alveoli paru-paru (Campbell et al. 1999).

2. Jaringan Ikat

Jaringan ikat berfungsi untuk menunjang tubuh, dibentuk oleh sel-sel dalam jumlah sedikit. Jaringan ikat terdiri atas populasi sel yang tersebar di dalam matrik ekstraseluler. Secara embriologi, jaringan ikat berasal dari lapisan mesoderm. Sel-sel tersebut mensistesis matriks, dengan anyaman serat yang tertanam di dalamnya (Campbell et al. 1999). Jaringan ikat ini dapat dibedakan menjadi (1) jaringan ikat longgar dan (2) jaringan ikat padat, (3) jaringan lemak, (4) jaringan darah, (5) kartilago, dan (6) tulang.

Diantara enam tipe jaringan ikat, jaringan ikat longgar paling banyak ditemukan di dalam tubuh kita. Di dalam matriks jaringan ikat longgar ini hanya sedikit ditemukan serabut. Serabut penyusun jaringan ikat ini berupa kolagen. Fungsi utama jaringan ikat longgar adalah pengikat dan pengepak material, dan sebagai tumbuh bagi jaringan dan organ lainnya. Jaringan ikat longgar di kulit membatasi dengan otot (Campbell et al. 1999).

Jaringan ikat padat/fibrous mempunyai matriks yang banyak mengandung serabut kolagen. Jaringan ini membentuk tendon sebagai tempat perlekatan otot dengan tulang, dan ligamen sebagai tempat persendian tulang dengan tulang (Campbell et al. 1999).

Jaringan lemak mengandung sel-sel lemak. Jaringan ini digunakan sebagai bantalan, dan melindungi tubuh, serta sebagai penyimpan energi. Setiap sel lemak, mengandung tetes lemak yang besar. Didalam jaringan lemak, matriks relatif sedikit (Campbell et al. 1999).

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Darah adalah jaringan ikat yang tersusun sebagian besar cairan. Matriks darah disebut plasma, yang tersusun oleh air, garam mineral, dan protein terlarut. Sel darah merah dan putih tersuspensi di dalam plasma. Darah ini berfungsi utama dalam transpor substansi dari satu bagian tubuh ke bagian lain. Disamping itu, darah juga berperan dalam sistem kekebalan (Campbell et al. 1999).

Kartilago adalah jaringan ikat yang membentuk material rangka yang fleksibel dan kuat, terdiri atas serabut kolgen yang tertanam di dalam matriks. Kartilago banyak ditemukan pada bagian ujung tulang keras, hidung, telinga, dan vertebrae (ruas-ruas tulang belakang) (Campbell et al. 1999).

Tulang keras (bone) merupakan jaringan ikat yang kaku, keras, dengan serabut kolagen yang tertanam di dalam matriks (Campbell et al. 1999). Didalam matriks sel tulang terdapat kalsium yang dapat bergerak dan diserap oleh darah. Hal ini merupakan peran penting tulang dalam proses homeostasis kadar kalsium dalam darah. Sel tulang (osteosit) terdapat di dalam ruang yang disebut lakuna. Lakuna ini mengandung satu atau beberapa osteosit. Penjuluran yang keluar dari osteosit disebut kanalikuli. Kanalikuli dari satu sel berhubungan dengan sel lainnya, sebagai bentuk komunikasi sel. Satu osteon terdiri dari sejumlah lamela konsentris yang mengelilingi kanal sentral (kanalis Haversi). Pada individu yang masih hidup, kanal sentral ini berisi pembuluh darah.

3. Jaringan Otot

Secara embriologi, jaringan otot berasal dari lapisan mesoderm. Jaringan ini terdiri atas sel-sel yang memanjang atau berbentuk serabut yang dapat berkontraksi karena adanya molekul miofibril. Pada vertebrata, secara tipikal mempunyai tiga jenis otot , yaitu otot skelet (rangka), otot jantung (cardiac), dan otot polos (Campbell et al. 1999).

Otot skelet berstruktur bergaris melintang, berfungsi untuk menggerakkan rangka. Otot ini bersifat sadar (voluntary), karena mampu diatur oleh kemauan kita. Serabut ototnya mempunyai banyak nukleus yang terletak ditepi. Otot rangka mempunyai garis melintang yang gelap (pita anisotrop) dan garis terang (pita isotrop).

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Otot jantung merupakan otot bergaris melintang dan bercabang. Sifat otot ini tidak sadar (involuntary), karena kontraksinya tidak bisa diatur oleh kemauan kita. Nukleus terletak ditengah sel. Pada bagian ujung sel, terdapat sambungan rapat, yang membentuk struktur pembawa sinyal untuk kontraksi dari satu sel ke sel lainnya selama denyut jantung (Campbell et al. 1999).

Otot polos berbentuk seperti spindle. Kontraksi otot polos lebih lambat dinbbandingkan otot skelet, namun mereka mampu kontraksi dalam waktu lebih lama. Otot polos bersifat tidak sadar (involuntary), seperti otot jantung. Otot polos ditemukan pada banyak organ tubuh, diantaranya terdapat pada dinding pembuluh darah dan melapisi organ dalam seperti usus dan uterus. Membran plasmanya disebut sarkolema dan sitoplasmanya sering disebut sarkoplasma. Sitoplasma yang mengandung miofibril dengan ketebalan mencapai 1 mikron.

4. Jaringan Saraf

Jaringan saraf berperan dalam penerimaan rangsang dan penyampaian rangsang. Secara embriologi, jaringan ini berasal dari lapisan ektoderm. Jaringan ini terdapat pada sistem saraf pusat (otak dan sumsum tulang belakang) dan pada sistim saraf tepi. Ada dua macam sel, yaitu sel saraf (neuron) dan sel pendukung (sel glia). Neuron mengandung badan sel, nukleus, dan penjuluran atau serabut. Satu tipe penjuluran tersebut adalah dendrit, yang berperan dalam menerima sinyal dari sel lain dan meneruskannya ke badan sel. Tipe penjuluran sel saraf yang lain, disebut akson (neurit), yang berperan dalam meneruskan sinyal dari badan sel ke neuron lainnya. Beberapa akson berukuran sangat panjang, yaitu memanjang dari otak sampai ke bagian bawah abdomen (panjang 1/2 meter atau lebih). Transmisi sinyal dari neuron ke neuron lainnya umumnya dilakukan secara kimia. Selain neuron, ditemukan juga sel pendukung, seperti sel glia. Sel glia merupakan sel yang menunjang dan melindungi neuron. Sel-sel pendukung umumnya berperan dalam melindungi dan membungkus akson dan dendrit, sehingga membantu mempercepat transmisi sinyal (Campbell et al. 1999).

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ALAT DAN BAHAN

Alat dan Bahan :

1. Mikroskop cahaya

2. Preparat awetan epitelium pipih, kubus, dan kolumner selapis.

3. Preparat awetan jaringan ikat.

4. Preparat awetan otot polos, skelet, dan jantung.

5. Preparat awetan jaringan saraf.

CARA KERJA

Preparat Epitelium.

1. Mintalah preparat epitelium pipih, kubus, dan kolumner selapis pada asisten anda dan dengan menggunakan mikroskop, amati preparat dengan perbesaran lemah (10X10), kemudian dengan perbesaran kuat (10X40).

2. Gambar hasil pengamatan anda baik dengan perbesaran lemah dan perbesaran kuat. Dengan perbesaran kuat, amati setiap tipe epitelium : bentuk sel, jumlah inti, letak inti, dan ciri morfologi lainnya. Lengkapi gambar anda dengan keterangan.

Preparat Tulang Padat.

1. Mintalah preparat tulang padat pada asisten anda dan dengan menggunakan mikroskop, amati preparat dengan perbesaran lemah (10X10), kemudian dengan perbesaran kuat (10X40).

2. Gambar hasil pengamatan anda baik dengan perbesaran lemah dan perbesaran kuat. Dengan perbesaran kuat, amati satu buah sistem osteon, yang terdiri atas lakuna, kanal sentral, lamela tulang, kanalikuli, dan kanalis Haversi. Lengkapi gambar anda dengan keterangan.

Preparat Otot Polos.

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1. Mintalah preparat otot polos pada asisten anda dan dengan menggunakan mikroskop, amati preparat dengan perbesaran lemah (10X10), kemudian dengan perbesaran kuat (10X40).

2. Gambar hasil pengamatan anda dan beri keterangan selengkapnya.

Preparat Otot Skelet.

1. Mintalah preparat otot skelet pada asisten anda dan dengan menggunakan mikroskop, amati preparat dengan perbesaran lemah (10X10 ), kemudian dengan perbesaran kuat (10X40).

2. Amati preparat otot serat melintang irisan membujur dan irisan melintang, dengan menggunakan perbesaran kuat tentang bentuk sel yang berupa serabut dan adanya inti, garis gelap (anisotrop) dan garis terang (isotrop). Dimanakah letak intinya?

3. Gambar preparat anda dan beri keterangan selengkapnya.

Preparat Otot Jantung.

1. Mintalah preparat otot jantung pada asisten anda dan dengan menggunakan mikroskop, amati preparat dengan perbesaran lemah (10X10 ), kemudian dengan perbesaran kuat (10X40).

2. Amati preparat anda dengan menggunakan perbesaran lemah dan kuat dan bandingkan dengan preparat otot rangka.

3. Gambar preparat anda dan beri keterangan selengkapnya.

Preparat Jaringan Saraf.

1. Mintalah jaringan saraf pada asisten anda dan dengan menggunakan mikroskop, amati preparat dengan perbesaran lemah (10X10), kemudian dengan perbesaran kuat (10X40).

2. Gambar hasil pengamatan anda baik dengan perbesaran lemah dan perbesaran kuat. Dengan perbesaran kuat, amati satu neuron : badan sel, inti, akson, dan dendrit. Lengkapi gambar anda dengan keterangan.

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EPITHELIUM SATU LAPIS

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JARINGAN IKAT

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JENIS OTOT

SEL SARAF

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Epithelial Tissues Structure| Sqaumous Epithelium| Cubiodal Epithelium| Columnar Epithelium|

Stratified Epithelium| Functions of Epithelium|

Structure Epithelial tissue covers the whole surface of the body. It is made up of cells closely packed and ranged in one or more layers. This tissue is specialised to form the covering or lining of all internal and external body surfaces. Epithelial tissue that occurs on surfaces on the interior of the body is known as endothelium. Epithelial cells are packed tightly together, with almost no intercellular spaces and only a small amount of intercellular substance. Epithelial tissue, regardless of the type, is usually separated from the underlying tissue by a thin sheet of connective tissue; basement membrane. The basement membrane provides structural support for the epithelium and also binds it to neighbouring structures.

Types of Epithelial Tissue Epithelial tissue can be divided into two groups depending on the number of layers of which it is composes. Epithelial tissue which is only one cell thick is known as simple epithelium. If it is two or more cells thick such as the skin, it is known as stratified epithelium. Simple epithelium Simple epithelium can be subdivided according to the shape and function of its cells.

Squamous (pavement) epithelium.

Squamous cells have the appearance of thin, flat plates. The shape of the nucleus usually corresponds to the cell form and help to identify the type of epithelium. Squamous cells, for example, tend to have horizontall flattened, elliptical nuclei because of the thin flattened form of the cell. They form the lining of cavities such as the mouth, blood vessels, heart and lungs and make up the outer layers of the skin.

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Simple sqaumous epithelium Simple Cuboidal Epithelium.

As their name implies, cuboidal cells are roughly square or cuboidal in shape. Each cell has a spherical nucleus in the centre. Cuboidal epithelium is found in glands and in the lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium which produces the egg cells in the female ovary and the sperm cells in the male testes.

Simple cuboidal epithelium Simple Columnar Epithelium

Columnar epithelial cells occur in one or more layers. The cells are elongated and column-shaped. The nuclei are elongated and are usually located near the base of the cells. Columnar epithelium forms the lining of the stomach and intestines. Some columnar cells are specialised for sensory reception such as in the nose, ears and the taste buds of the tongue. Goblet cells (unicellular glands) are found between the columnar epithelial cells of the duodenum. They secrete mucus or slime, a lubricating substance which keeps the surface smooth.

Simple columnar epithelium Ciliated Columnar Epithelium

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These are simple columnar epithelial cells, but in addition, they posses fine hair-like outgrowths, cilia on their free surfaces. These cilia are capable of rapid, rhythmic, wavelike beatings in a certain direction. This movement of the cilia in a certain direction causes the mucus, which is secreted by the goblet cells, to move (flow or stream) in that direction. Ciliated epithelium is usually found in the air passages like the nose. It is also found in the uterus and Fallopian tubes of females. The movement of the cilia propel the ovum to the uterus.

Ciliated columnar epithelium Glandular Epithelium

Columnar epithelium with goblet cells is called glandular epithelium. Some parts of the glandular epithelium consist of such a large number of goblet cells that there are only a few normal epithelial cells left. Columnar and cuboidal epithelial cells often become specialised as gland cells which are capable of synthesising and secreting certain substances such as enzymes, hormones, milk, mucus, sweat, wax and saliva. Unicellular glands consist of single, isolated glandular cells such as the goblet cells. Sometimes a portion of the epithelial tissue becomes invaginated and a multicellular gland is formed. Multicellular glands are composed of clusters of cells. Most glands are multicellular including the the salivary glands.

Glandular epithelium Stratified Epithelium.

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Where body linings have to withstand wear and tear, the epithelia are composed of several layers of cells and are then called compound or stratified epithelium. The top cells are flat and scaly and it may or may not be keratinised (i.e. containing a tough, resistant protein called keratin). The mammalian skin is an example of dry, keratinised, stratified epithelium. The lining of the mouth cavity is an example of an unkeratinisied, stratified epithelium.

Stratified epithelium

Functions of Epithelial Tissue

Protection

Epithelial cells from the skin protect underlying tissue from mechanical injury, harmful chemicals, invading bacteria and from excessive loss of water.

Sensation

Sensory stimuli penetrate specialised epithelial cells. Specialised epithelial tissue containing sensory nerve endings is found in the skin, eyes, ears, nose and on the tongue.

Secretion

In glands, epithelial tissue is specialised to secrete specific chemical substances such as enzymes, hormones and lubricating fluids.

Absorption

Certain epithelial cells lining the small intestine absorb nutrients from the digestion of food.

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Excretion

Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine. Sweat is also excreted from the body by epithelial cells in the sweat glands.

Diffusion

Simple epithelium promotes the diffusion of gases, liquids and nutrients. Because they form such a thin lining, they are ideal for the diffusion of gases (eg. walls of capillaries and lungs).

Cleaning

Ciliated epithelium assists in removing dust particles and foreign bodies which have entered the air passages.

Reduces Friction

The smooth, tightly-interlocking, epithelial cells that line the entire circulatory system reduce friction between the blood and the walls of the blood vessels.

| epithelial tissue | connective tissue | muscle tissue | nervous tissue |

Connective tissue

Structure

Connective tissues function primarily to support the body and to bind or connect together all types of tissue. This tissue also provide a mechanical framework (the skeleton) which plays an important role in locomotion. Unlike epithelial tissue, connective tissue is characterised by the large amounts of intercellular substance (also called ground substance or the matrix) that it contains. Connective tissue are relatively few cells which are widely seperated from each other. These living cells are responsible for secreting the large amounts of intercellular ground substance (matrix). The matrix is a non-living material which may be liquid (eg. blood), semi-solid (eg. connective tissue) or solid (eg. bone). Embedded in the matrix are a variety of connecting and supporting fibres, eg. collagen fibres and elastic fibres. Classification of the basic connective tissue depends on the predominant fibre type present in each. Connective tissue can be divided into four main types.

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Muscle tissues  | Structure | Smooth Muscle Tissue | Skeletal Muscle Tissue | Cardiac (Heart)

Muscle Tissue |

Structure We know that living organisms can move on their own or can perform other types of movement. Muscle tissue has a ability to relax and contrast and so bring about movement and mechanical work in various parts of the body. There are other movements in the body too which are necessary for the survival of the organism such as the heart beat and the movements of the alimentary canal. Muscles can be divided into three main groups according to their structure, e.g.:

Smooth muscle tissue. Skeletal muscle tissue. Cardiac (heart) muscle tissue.

A whole muscle with manyfasciculi

Types of Muscle Tissue Smooth Muscle Tissue.

Smooth muscle tissue is made up of thin-elongated muscle cells, fibres. These fibres are pointed at their ends and each has a single, large, oval nucleus. Each cell is filled with a specialised cytoplasm, the sarcoplasm and is surrounded

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by a thin cell membrane, the sarcolemma. Each cell has many myofibrils which lie parallel to one another in the direction of the long axis of the cell. They are not arranged in a definite striped (striated) pattern, as in skeletal muscles - hence the name smooth muscle . Smooth muscle fibres interlace to form sheets or layers of muscle tissue rather than bundles. Smooth muscle is involuntary tissue, i.e. it is not controlled by the brain. Smooth muscle forms the muscle layers in the walls of hollow organs such as the digestive tract (lower part of the oesophagus, stomach and intestines), the walls of the bladder, the uterus, various ducts of glands and the walls of blood vessels .

Functions of Smooth Muscle Tissue

o Smooth muscle controls slow, involuntary movements such as the contraction of the smooth muscle tissue in the walls of the stomach and intestines.

o The muscle of the arteries contracts and relaxes to regulate the blood pressure and the flow of blood.

Smooth Muscle Tissue Skeletal Muscle Tissue.

Skeletal muscle is the most abundant tissue in the vertebrate body. These muscles are attached to and bring about the movement of the various bones of the skeleton, hence the name skeletal muscles. The whole muscle, such as the biceps, is enclosed in a sheath of connective tissue, the epimysium. This sheath folds inwards into the substance of the muscle to surround a large number of smaller bundles, the fasciculi. These fasciculi consist of

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still smaller bundles of elongated, cylindrical muscle cells, the fibres. Each fibre is a syncytium, i.e. a cell that have many nuclei. The nuclei are oval in shaped and are found at the periphery of the cell, just beneath the thin, elastic membrane (sarcolemma). The sarcoplasm also has many alternating light and dark bands, giving the fibre a striped or striated appearance (hence the name striated muscle). With the aid of an electron microscope it can be seen that each muscle fibre is made up of many smaller units, the myofibrils. Each myofibril consists of small protein filaments, known as actin and myosin filaments. The myosin filaments are slightly thicker and make up the dark band (or A-band). The actin filaments make up the light bands (I-bands) which are situated on either side of the dark band. The actin filaments are attached to the Z-line. This arrangement of actin and myosin filaments is known as a sacromere.

A myofibril with actin and myosin filaments

During the contraction of skeletal muscle tissue, the actin filaments slide inwards between the myosin filaments. Mitochondria provide the energy for this to take place. This action causes a shortening of the sacromeres (Z-lines move closer together), which in turn causes the whole muscle fibre to contract. This can bring about a shortening of the entire muscle such as the biceps, depending on the number of muscles fibres that were stimulated. The

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contraction of skeletal muscle tissue is very quick and forceful.

Functions of Skeletal Muscle Tissue

o Skeletal muscles function in pairs to bring about the co-ordinated movements of the limbs, trunk, jaws, eyeballs, etc.

o Skeletal muscles are directly involved in the breathing process.

Skeletal Muscle Tissue Cardiac (Heart) Muscle Tissue.

This is a unique tissue found only in the walls of the heart. Cardiac (Heart) Muscle Tissue shows some of the characteristics of smooth muscle and some of skeletal muscle tissue. Its fibres , like those of skeletal muscle, have cross-striations and contain numerous nuclei. However, like smooth muscle tissue, it is involuntary. Cardiac muscle differ from striated muscle in the following aspects: they are shorter, the striations are not so obvious, the sarcolemma is thinner and not clearly discernible, there is only one nucleus present in the centre of each cardiac fibre and adjacent fibres branch but are linked to each other by so-called muscle bridges. The spaces between different fibres are filled with areolar connective tissue which contains blood capillaries to supply the tissue with the oxygen and nutrients.

Functions of Cardiac (Heart) Muscle Tissue

o Cardiac muscle tissue plays the most important role in the contraction of the atria and ventricles of the heart.

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o It causes the rhythmical beating of the heart, circulating the blood and its contents throughout the body as a consequence.

Cardiac Muscle Tissue| epithelial tissue | connective tissue | muscle tissue | nervous tissue

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4. Nervous tissues | General | Structure of a Motor Neuron | Classification of Neurons | Funtions of

Nerve Tissue |

General All living cells have the ability to react to stimuli. Nervous tissue is specialised to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus. Nerve tissue (as in the brain, spinal cord and peripheral nerves that branch throughout the body) are all made up of specialised nerve cells called neurons. Neurons are easily stimulated and transmit impulses very rapidly. A nerve is

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made up of many nerve cell fibres (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 fibres. 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 fibres. Although the system forms a unit it can be divided into the following parts: the central nervous system (CNS) which consists of the brain and spinal cord, the nervous system consists of the nerves outside the CNS which connect the brain and spinal cord to the organs and muscles of the body and the automatic or involuntary nervous system consists of nerve centres and fibres inside as well as outside the central nervous system. There are three main types of neurons, which are classified according their function: Those that conduct impulses from the sensory organs to the central nervous system (brain and spinal cord) are called sensory (or afferent) neurons; those that conduct impulses from the central nervous system to the effector organs (such as muscles and glands) are called motor (or efferent) neurons. Interneurons (also known as connector neurons or association neurons) are those that connect sensory neurons to motor neurons.

Structure of a Motor Neuron

A motor neuron has many processes (cytoplasmic extensions), called dendtrites, 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 does not branched 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.

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A motor neuronNerve cells are functionally connected to each other at a junction known as a synapse, where the terminal branches of an axon and the dendrites of another neuron lie in close proximity to each other but never make direct contact.

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A Synapse

Classification of Neurons

On the basis of their structure, neurons can also be classified into three main types:

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.

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.

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.

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A diagram showing the different neurons

Functions of Nerve Tissue

Nervous tissue allows an organism to sense stimuli in both the internal and external environment.

The stimuli are analysed and integrated to provide appropriate, co-ordinated responses in various organs.

The afferent or sensory neurons conduct nerve impulses from the sense organs and receptors to the central nervous system.

Internuncial or connector neurons supply the connection between the afferent and efferent neurons as well as different parts of the central nervous system.

Efferent or somatic motor neurons transmit the impulse from the central nervous system to a muscle (the effector organ) which then react to the initial stimulus.

Autonomic motor or efferent neurons transmit impulses to the involuntary muscles and glands.

| epithelial tissue | connective tissue | muscle tissue | nervous tissue |

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