Muscle Tissue of Animals: 

Origin of Muscle Tissue:

Muscular tissue in general develops from the mesoderm of the embryo but the muscles of the iris of the eye and myoepithelial cells of the salivary, mammary and sweat glands arise from the ectoderm of the embryo.

General Structure of Muscle Tissue:

Myo, sarco and motor are concerned with muscles. This tissue constitutes the muscles, made up of cells, which are in the form of contractile fibres varying in lengths. The fibres are bound together by connective tissues but they have no intercellular substance.Myoblasts give rise to muscle fibres. Myocytes (= sarcocytes) are muscle cells. Each fibre consists of fine fibrils called myofibrils, present in the cytoplasm known as sarcoplasm. Sometimes the muscle fibre is externally covered by a membrane, termed as sarcolemma.

Special Properties of Muscle Tissue:

The special property of muscular tissue is contractility i.e., the cells of muscular tissue can shorten considerably and return to the original relaxed state. The muscle cells contract in a definite direction. Another property of muscle is the electrical excitability. It is due to the energy stored in the electrical potential difference across the plasma membrane.

Functions of Muscle Tissue:

1. It brings about movements of the body parts and locomotion of the individual.

2. Muscles are responsible for peristalsis in tubular viscera, heartbeat, production of sound, etc.

3. Facial expression also depends on muscles.

4. It supports the bones and other structures.

5. Muscles are required for delivering a baby.

A whole muscle (Fig. 7.30) is covered by a connective tissue sheath, the epimysium. Beneath the epimysium each skeletal muscle consists of many muscle fibres arranged in bundles called fasciculi (sing, fasciculus or fascicle). Each bundle or fasciculus is surrounded by a connective tissue sheath, the perimysium and each muscle fibre or cell, is surrounded by a thin connective tissue sheath, the endomysium.

Types of Muscle Tissue:

There are present three types of muscle tissue:

1. Striped or striated or skeletal or voluntary muscles.

2. Un-striped or non-striated or visceral or smooth or involuntary muscles.

3. Cardiac muscles.

1. Skeletal Muscle Tissue (Striated or Striped Muscles):

Location:

These muscles are found in the limbs, body walls, tongue, pharynx and beginning of oesophagus and are under the control of animal’s will.

Structure:

These muscle fibres occur in bundles and are normally attached to the skeleton. Each muscle fibre is an elongated cell surrounded externally by a delicate membrane, the sarcolemma.

Just beneath the sarcolemma in each fibre many nuclei occur at irregular intervals. Thus, these fibres are multi-nucleated or syncytial in nature. The cytoplasm of each fibre (sarcoplasm) has a large number of myofibrils which are tightly packed.

Detailed Structure of Striated Muscle Fibre:

A myofibril has dark and light bands. The dark bands are also called A-bands (Aniso­tropic bands). The light bands are also called I-bands (Isotropic bands). At the centre of А-band, a comparatively less dark zone called H—Zone (= Hensen zone, named after Hensen who first described) is present.

In the centre of the H-zone is the M-line: The letter ‘M’ is from the German word Mittleschiebe (mittle = middle). Each I-band has at its centre a dark membrane called Z-line. The letter Z’ is from the German word Zwischenschiebe (zwischen = between, schiebe = disc). The Z-line is also called Z-disc, or Krause s mem­brane or Dobie’s line.

The part of the myofibril between two successive Z-lines is called sarcomere. Therefore, the sarcomere comprises А-band and half of each adjacent I-band. The sarcomere is the functional unit of myofibril. In fact each sarcomere is a bundle of thick and thin myofilaments. The thick myofilaments have diameters of about 150A, whereas the thin myofilaments have diameters of about 70A.

(i) Thick Myofilaments:

They consist mainly of myosin protein. They form cross bridges.

(ii) Thin Myofilaments:

They are composed of three different proteins- actin, tro­pomyosin and troponin.

Functions:

Skeletal muscles are un­der the control of animal’s will, Calcium is an essential element for the contraction of muscles. In the presence of calcium ions and energy from ATP, actin and myo­sin interact forming actomyosin which causes contraction of muscles.

2. Smooth Muscle Tissue (Non-striated Muscles):

Location:

Non-striated muscles are found in the posterior part of oesopha­gus, stomach, intestine, lungs, urinogenital tract, urinary bladder, blood vessels, iris of eye, dermis of skin and arrestor pili muscle of hair. Smooth muscles never connect with skeleton.

Structure:

These muscle fibres or cells are elongated and spindle shaped. Each fibre contains a single oval nucleus surrounded by the cytoplasm (sarcoplasm). In the cytoplasm the myofibrils are arranged longitudinally. There is no sarcolemma, however, the fibre is enclosed by plasma membrane.

Function:

These muscles help in peristalsis which happens in tubular viscera. Action of these muscles is controlled by autonomic nervous system and hence they are not under the control of the animal’s will.

Types:

Functionally smooth muscles are of two types — single-unit smooth muscles and multi-unit smooth muscles.

(i) Single-unit smooth muscles are composed of muscle fibres closely joined together. All the fibres of the single smooth muscle contract simultaneously as a single unit. These muscles are found in the walls of hollow visceral organs like gastrointestinal tract and urinary bladder.

(ii) Multi-unit smooth muscles are composed of independent muscle fibres and are not closely joined together. Their fibres contract more or less independently as separate units. Arrector pili muscles of skin dermis, ciliary and iris muscles in the eyes, and muscles of the walls of the large blood vessels are some examples of multi-unit smooth muscles.

3. Cardiac Muscle Tissue:

Location:

The cardiac muscles are found in the wall of the heart and in the wall of large veins (e.g., pulmonary veins and superior vena cava) where these veins enter the heart.

Structure:

These fibres show the char­acters of both un-striped and striped muscle fibres. Each fibre is a long and cylindrical structure which has a definite sarcolemma. The fibres are uninucleate and the nuclei lie near the centre. The fibres have some lateral branches, known as oblique bridges to form a contractile network. The myo­fibrils have transverse faint dark and light bands, which alternate with each other.

In this way cardiac muscle fibres are also striped, but having dark intercalated discs at intervals. The intercalated discs are specialized regions of cell membrane of two adjacent fibres. The intercalated discs function as boosters of contraction wave and permit the wave of muscle contraction to be transmitted from one cardiac fibre to another.

Unique Features:

Cardiac muscle fibres are supplied with both central and autonomic nervous system and are not under the control of the will of the animal. However, these muscles never get fatigued. Thus they are immune to fatigue. Blood capillaries penetrate the cardiac muscle fibres. They have very rich blood supply. They have the property of con­traction, even when they are isolated from the body temporarily.

Similarities between Cardiac and Skeletal Muscles:

Both cardiac and skeletal muscles are made up of elongated fibres which have numerous myofibrils. The myofibrils of cardiac muscle have the same structure as those skeletal muscle and are made up ofactin and myosin filaments. The cardiac and skeletal muscle fibres have dark and light bands. The connective tissue framework and the capillary network around cardiac muscle fibres are similar to those in skeletal muscle.

Similarities between Cardiac and Smooth Muscles:

Both cardiac and smooth muscles are uninucleate containing nucleus at the centre and are involuntary in function.

Comparison between Striated, Non-striated and Cardiac Muscle Fibres: