Download NAME: AKPANWA JR., OWOANAM MATRIC NO: 14/MHS01/024

NAME: AKPANWA JR., OWOANAM
MATRIC NO: 14/MHS01/024
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
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
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 rhythmic
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.
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.