Download Cardiac muscle

Name: Isamotu Ayoola Temitope
Department: Medicine and Surgery
Matric no: 14/MHS01/073
Course: Histology of basic tissues
Course code: ANA 203
HISTOLOGY OF MUSCLE TISSUE
Muscle tissue has a unique histological appearance
which enables it to carry out its function. In this particle,
we will examine the histology and how it relates to
contractility. Muscle tissue is composed of elongated cells
specialized for contraction and movement. Muscle tissue
is composed of cells differentiated for optimal use of the
universal cell property termed contractility. Microfilaments
and associated proteins together generate the forces
necessary for cellular contraction, which drives movement
The body contains three types of
muscle tissue: (a) skeletal muscle, (b) smooth muscle, and (c)
cardiac muscle. (Same magnification)
within certain organs and the body as a whole. Nearly all
muscle cells are of mesodermal origin and they
differentiate mainly by a gradual process of cell
lengthening with simultaneous synthesis of myofibrillar
proteins. Three types of muscle tissue can be
distinguished on the basis of morphologic and functional
characteristics and the structure of each type is adapted to
its physiologic role
Types of Muscle
There are three types of muscle:
Skeletal – Unlike most tissues, skeletal muscle does not
consist of individual cells. Rather it is formed from huge,
multinucleate muscle fibers, which develop by fusion of
many individual embryonic cells called myoblasts. Muscle
fibers can grow by continued nuclear division within the
fibers. Although skeletal muscle fibers are thus not
proper, individual cells, the term "muscle cell" is commonly
used to refer to one multinucleate fiber. Each individual
skeletal muscle fiber extends over much of the length of
the muscle in which it resides (up to many centimeters),
with a uniform diameter that is typically around 50 µm. In
contrast, cardiac muscle (which like skeletal muscle is also
striated) and smooth muscle both consist of single,
discrete cellular units, each with its own nucleus.
Cardiac – A form of striated muscle that is found only in
the heart. Identifying features are single nuclei and the
presence of intercalated discs between the cells. it also
has cross-striations and is composed of elongated,
branched individual cells that lie parallel to each other. At
sites of end-to-end contact are the intercalated disks,
structures found only in cardiac muscle. Contraction of
cardiac muscle is involuntary, vigorous, and rhythmic.
Cardiac muscle has several distinct characteristics.

Cardiac muscle is striated, like skeletal muscle. (For
brief description of striations, see skeletal muscle.)

Cardiac muscle consists of distinct, individual cells,
unlike skeletal muscle (which consists of immensely
long, multinucleate fibers).

Cardiac muscle cells are attached end-to-end by
specialized junctions called intercalated discs.
Smooth – Smooth muscle consists of individual cells
(leiomyocytes), each cell with its own nucleus.
The function of smooth muscle also differs substantially
from that of striated muscle.
Neurotransmitter activitation of smooth muscle is fairly
diffuse. There are no discrete, well-defined
neuromuscular junctions. The motor endings of
autonomic axons, where neurotransmitter is released, are
not closely associated with individual smooth muscle
fibers.
Electrical activitation of smooth muscle is passed from cell
to cell by gap junctions.
Smooth muscle of the gut can generate intrinsic rhythmic
contraction, independent from direct neural control. Input
from the autonomic nervous system increases or
decreases the level of this spontaneous activity. (In
certain other locations, such as the iris of the eye, neural
control is more direct and precise.)
Consult your textbook for more information, including
molecular details of contraction and control mechanisms.
Each smooth muscle cell (or "muscle fiber") is just a few
microns in diameter but may be two hundred microns
long. The nucleus is also elongated, often cigar-shaped.
In histologic sections, the full length of smooth muscle
nuclei is only apparent when the plane of section is
aligned with the long axis of the cells.
Muscle Fibers
As observed with the light microscope, longitudinally
sectioned skeletal muscle fibers show cross-striations of
alternating light and dark bands (Figure 10–7). The darker
bands are called A bands (anisotropic or birefringent in
polarized light); the lighter bands are called I bands
(isotropic, do not alter polarized light). In the TEM each I
band is seen to be bisected by a dark transverse line, the
Z line (Ger. Zwischenscheibe, between the discs). The
repetitive functional subunit of the contractile apparatus,
the sarcomere, extends from Z line to Z line (Figure 10–8)
and is about 2.5 m long in resting muscle. Myosin is a
much larger complex (molecular mass ~500 kDa). Myosin
can be dissociated into two identical heavy chains and two
pairs of light chains. Myosin heavy chains are thin, rod-like
molecules (150 nm long and 2–3 nm thick) made up of two
heavy chains twisted together as myosin tails. Small
globular projections at one end of each heavy chain form
the heads, which have ATP binding sites as well as the
enzymatic capacity to hydrolyze ATP (ATPase activity)
and the ability to bind actin. The four light chains are
associated with the head. Several hundred myosin
molecules are arranged within each thick filament with
their rodlike portions overlapping and their globular heads
directed toward either end.