Download NAME: AYODELE SAMUELSON AYOWOLE MATRIC NO: 14

NAME: AYODELE SAMUELSON AYOWOLE
MATRIC NO: 14/MHS01/034
DEPARTMENT: MEDICINE AND SURGERY
LEVEL: 200
HISTOLOGY OF MUSCLE TISSUE
Introduction
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 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
Functions of the Muscle.
1. contraction for locomotion and skeletal movement
2. contraction for propulsion
3. contraction for pressure regulation
Types of Muscle
There are three types of muscle:
Skeletal – is composed of bundles of very long, cylindrical,
multinucleated cells that show cross-striations. Their contraction is quick,
forceful, and usually under voluntary control. It is caused by the
interaction of thin actin filaments and thick myosin filaments whose
molecular configuration allows them to slide upon one another. The
forces necessary for sliding are generated by weak interactions in the
bridges between actin and myosin,A form of striated muscle that is under
voluntary control from the somatic nervous system. Identifying features
are
cylindrical
cells
and
multiple
CROSS SECTION OF SKELETAL MUSCLE
peripheral
nuclei.
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
rythmic
CROSS SECTION OF CARDIAC MUSCLE
Smooth – Skeletal muscle consists of muscle fibers, which are long,
cylindrical
multinucleated
cells
with
diameters
of 10–100
m.
Multinucleation results from the fusion of embryonic mesenchymal cells
called myoblasts (Figure 10–2). The long oval nuclei are usually found at
the periphery of the cell under the cell membrane. This characteristic
nuclear location is helpful in discriminating skeletal muscle from cardiac
and smooth muscle, both of which have centrally located nuclei. A form
of non-striated muscle that is controlled involuntarily by the autonomic
nervous system. The identifying feature is the presence of one
spindle-shaped central nucleus per cell. This article will deal mainly with
skeletal muscle.
CROSS SECTION OF SMOOTH MUSCLE
Composition of Skeletal Muscle
A muscle cell is very specialised for its purpose. One muscle
cell is known as a muscle fibre, and its cell surface membrane is known
as the sarcolemma.
Other organelles that are unique to muscle are T tubules, which are
invaginations of the sarcolemma that conduct charge when the cell is
depolarised.
Muscle cells also have a specialised endoplasmic reticulum – this is
known as the sarcoplasmic reticulum and contains a large store of
calcium ions.
Muscles also have an intricate support structure of connective tissue.
Each muscle fibre is surrounded by a thin layer of connective tissue
known as endomysium. These fibres are then grouped into bundles
known as fascicles, which are surrounded by a layer of connective tissue
known as perimysium. Many fascicles make up a muscle, which in turn is
surrounded by a thick layer of connective tissue knows as the epimysium.
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.
Ultra structural Appearance of Skeletal Muscle
The striated appearance of skeletal muscle fibres arises due
to the organisation of two contractile proteins or myofilaments, actin (thin
filament) and myosin (thick filament). The functional unit of contraction
is in a skeletal muscle fibre is the sarcomere, which runs from Z line to Z
line. A sarcomere is broken down into a number of sections:
Z line – Where the actin filaments are anchored.
M line – Where the myosin filaments are anchored.
I band – Contains only actin filaments.
A band – The length of a myosin filament, may contain overlapping actin
filaments.
H zone – Contains only myosin filaments.
The M line is inside the H zone which is inside the A band, whilst the Z
line is
inside the I band.
Sliding Filament Model
Myosin filaments have many heads, which can bind to sites
on the actin filament. Actin filaments are associated with two other
regulatory proteins, troponin and tropomyosin.
Tropomyosin is a long protein that runs along the actin filament, blocking
the myosin head binding sites.
Troponin is a small protein that binds the tropomyosin to the actin. It is
made up of three parts:
Troponin I - which binds to the actin filament.
Troponin T - which binds to tropomyosin.
Troponin C - which can bind calcium ions.
The unique structure of troponin is the basis
of
excitation-contraction coupling:
1. When depolarisation occurs at a neuromuscular junction, this is
conducted down the T tubules, causing a huge influx of calcium ions into
the sarcoplasm from the sarcoplasmic reticulum.
2. This calcium binds to troponin C, causing a change in conformation
that moves tropomyosin away from the myosin head binding sites of the
actin filaments.
3. This allows the myosin head to bind to the actin, forming a cross-link.
The power stroke then occurs as the myosin heads pivots in a ‘rowing
motion, moving the actin past the myosin towards the M line.
4. ATP then binds to the myosin head, causing it to uncouple from the
actin and allowing the process to repeat. Hence in contraction, the length
of the filaments does not change. However, the length of the sarcomere
decreases due to the actin filaments sliding over the myosin. The H zone
and I band shorten, whilst the A band stays the same length. This brings
the Z lines closer together and causes overall length to decrease.