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Muscular Tissue

This lecture on muscular tissue explains the different types of muscular tissue, the mechanism involved in the contraction process and related aspects.

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Outline of Lecture
Muscular Tissue

Because a muscle fiber is not a single cell, its parts are often given special names such as

sarcolemma for plasma membrane

sarcoplasmic reticulum for endoplasmic reticulum

sarcosomes for mitochondria

sarcoplasm for cytoplasm


The tissues which we classify as muscles are those which in their differentiation have come to emphasize the property of contractility to a remarkable extent.

Muscle cells are known as fibers as there is elongation of cells in axis of contraction

All are derived embryonically from mesenchyme

Classification Of Muscles

On the basis of appearance of contractile cells

 Striated Muscles

Skeletal muscles

Cardiac muscles      

 Non-Striated Muscles

 Smooth muscles

Types of Muscle Tissue


Their cells are called fibers because they are elongated

Contraction depends on myofilaments



Plasma membrane is called sarcolemma

Sarcos = flesh

Lemma = sheath

Skeletal muscle

Connective Tissue Associated With Muscles Myofibrils

Structural & functional subunit of muscle fibers

Best seen in C/S of muscle fiber. In these situations they give the stippled appearance

Composed of thread like elements, called myofilaments

A myofibril consists of approximately 10,000 sarcomeres end to end


Thin filaments

Thick filaments

The functional unit of the myofibril is the

SARCOMERE, the segment of myofibrilbetween two adjacent Z lines


64 kilodalton protein

Consists of double helix of two polypeptides

Form filaments running in grooves  b/w F-actin molecules

In resting muscle, tropomyosin & troponin masks myosin binding sites on actin molecule


Complex of three globular subunits

Each tropomyosin molecule contains one troponin complex

Troponin C

Troponin T

Troponin I

Myosin 11

510 kilodalton protein

Composed of two polypeptide heavy chains& four light chains

Light chains are of two types; essential light chain, regulatory light chain. One molecule of each is present in association with myosin head

Globular head with two specific binding sites; one for ATP & one for actin


Aggregate in tail to tail to form bipolar thick filaments

Bare zone ——– H band

When a muscle contracts, each sarcomere shortens & become thicker, but the myofilaments remain the same length


The A Band The thick filaments are located at the center of a sarcomere, in the A band. The length of the A band is equal to the length of a typical thick filament. The A band, which also includes portions of thin filaments, contains the following three subdivisions

1.The M line. The central portion of each thick filament is connected to its neighbors by proteins of the M line. These dark-staining proteins help stabilize the positions of the thick filaments.

2.The H zone. In a resting sarcomere, the H zone, or H band, is a lighter region on either side of the M line. The H zone contains thick filaments but no thin filaments.


The I Band Each I band, which contains thin filaments but not thick filaments, extends from the A band of one sarcomere to the A band of the next sarcomere.

Z lines mark the boundary between adjacent sarcomeres. The Z lines consist of proteins called connectins, which interconnect thin filaments of adjacent sarcomeres.

From the Z lines at either end of the sarcomere, thin filaments extend toward the M line and into the zone of overlap. Strands of the protein titin extend from the tips of the thick filaments to attachment sites at the Z line

Titin helps keep the thick and thin filaments in proper alignment; it also helps the muscle fiber resist extreme stretching that would otherwise disrupt the contraction mechanism

Sliding Filament Theory

The Force of contraction is generated by the process that slides the actin filament over the myosin filament

The length of the thick and thin filaments do not change

The length of the sarcomere decreases as actin is pulled over myosin

The Contraction Cycle

Regulation of contraction

Sarcoplasmic reticulum

Transverse tubules

Cardiac Muscles

Elongated, branching cells with irregular contours at their junctions




Sarcoplasmic Reticulum

Narrow cisternae, coursing longitudinally

Anastomosing forming a plexiform pattern

T tubules

Wider lumen

Course mainly in transverse direction

Seen at the level of Z disc


Intercalated Discs

These represent the attachment site between cardiac muscle cells.

Under L/M, disc appears as a densely staining linear structure that is oriented transversely to the muscle fiber. It consists of short segments arranged in a step like fashion.

Under TEM, represent the complex set of intercellular junctions

It consists of a transverse component and a lateral component.

Both components of disc contain specialized cell to cell junctions between adjoining muscle cells.

Fascia adherens

Maculae adherentes

Gap junctions

An intercalated disc is an undulating double membrane separating adjacent cells in cardiac muscle fibers. Intercalated discs support synchronized contraction of cardiac tissue. They can easily be visualized by a longitudinal section of the tissue.

Three types of membrane junctions exist within an intercalated disc—fascia adherens, macula adherens (aka desmosomes), and gap junctions.

Fascia adherens are anchoring sites for actin, and connects to the closest sarcomere.

Macula adherens stop separation during contraction by binding intermediate filaments joining the cells together also called a desmosome.

Gap junctions allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle.

Smooth Muscles

Consists of fusiform or spindle shaped cells with abundant cytoplasmic whose central thickest portion, the nucleus lies

Composed of non striated cells, each of which is enclosed by a basal lamina and a network of reticular fibers.

Generally occur in the form of bundles or sheets of elongated fusiform cells with finally tapered ends.



Outlines of cell





In smooth muscle cells, bundles of myofilaments crisscross obliquely through the cell, forming a latticelike network.

These bundles consist of thin filaments  (5-7 nm) containing actin and tropomyosin & thick filaments (12-16 nm) consisting of myosin.

Plasma membrane in high resolution TEM is characterized by numerous membranous caveolae which protrude in cytoplasm

Thickened appearance of plasma membrane because of:

Densities along its internal aspect

Prominent external lamina over its outer aspect

Comparison of three muscle types

Structural Features

Muscle cell


C.T. components




T tubules

Cell to cell junction

Special features

Special Features

Type of innervation

Efferent innervation

Type of contraction

Regulation of contraction

Growth & Regeneration


Response to demand





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