This lecture on muscular tissue explains the different types of muscular tissue, the mechanism involved in the contraction process and related aspects.
Outline of Lecture
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
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
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
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
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
Elongated, branching cells with irregular contours at their junctions
Narrow cisternae, coursing longitudinally
Anastomosing forming a plexiform pattern
Course mainly in transverse direction
Seen at the level of Z disc
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.
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.
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
Cell to cell junction
Type of innervation
Type of contraction
Regulation of contraction
Growth & Regeneration
Response to demand