Download Muscles 443

Muscles
BCH 443
First Semester 1427-28
Types of Muscles
- Smooth muscle
- Striated muscle (skeletal muscle)
- Cardiac muscle.
Smooth Muscle
• Smooth muscle is
involuntary (they cannot be
controlled voluntarily).
• It lines the gut, blood
vessels, and reproductive
tract, in the wall of the
trachea, uterus, and bladder.
• The cells are tapered on the
ends.
• Their cells have a variable
length but are in the order of
0.1 mm.
• The contraction of smooth
muscle is controlled from
the brain through the
autonomic nervous system.
Skeletal Muscles
Functions of Skeletal Muscle
• Movement
• Posture
• Stabilizes joints
• Heat
• Protects organs
Striated muscles, are also called
skeletal muscles
• skeletal muscles are formed from a large number
of muscle fibres, that range in length from 1 to 40
mm and in diameter from 0.01 to 0.1mm.
• Each fibre forms a (muscle) cell and is
distinguished by the presence of alternating dark
and light bands. This is the origin of the
description "striated,"
• Striated muscles are connected to the bones via
tendons.
• Such muscles are voluntary and form an essential
part of the organ of support and motion.
• The striated muscle fibre corresponds to an
(unmyelinated) nerve fibre but is
distinguished electrophysiologically from
nerve by the presence of a periodic
transverse tubular system (TTS), a complex
structure that, in effect, continues the
surface membrane into the interior of the
muscle.
• Propagation of the impulse over the surface
membrane continues radially into the fibre
via the TTS, and forms the trigger of
myofibrillar contraction.
• The presence of the TTS affects conduction
of the muscle fibre so that it differs
(although only slightly) from propagation on
an (unmyelinated) nerve fibre.
Cardiac Muscle
• Cardiac muscle is found in the heart.
The cells are short, branched, and striated.
Intercalated disks are regions where cells join
together.
The blue arrows
in the
photograph point
to branches.
• Cardiac muscle is striated, but differs
in other ways from skeletal muscle:
• It is involuntary muscle.
• When excited, it generates a much
longer electric impulse than does
skeletal muscle, lasting about 300ms.
• Correspondingly, the mechanical
contraction also lasts longer.
• Cardiac muscle has a special property:
The electric activity of one muscle cell
spreads to all other surrounding
muscle cells, owing to an elaborate
system of intercellular junctions.
Structure of Skeletal Muscles
• Skeletal muscles consist of 100,000s of muscle cells (also
known as "muscle fibers") that perform the functions of the
specific muscle of which they are a part.
• The components of
skeletal muscle cells that
are specific to muscle
tissue are myofibrils.
• Each muscle fibre
("muscle cell") is
covered by a plasma
membrane sheath which
is called the
sarcolemma.
• Tunnel-like extensions
from the sarcolemma
pass through the muscle
fibre from one side of it
to the other in transverse
sections through the
diameter of the fibre.
These tunnel-like
extensions are known as
transverse tubules ("T
Tubules)
Sarcoplasm
• The cytoplasm present is muscle fibres (muscle
cells) is called sarcoplasm
• The sarcoplasm present in muscle fibres contains
many mitochondria.
• These mitochondria produce large amounts of
"ATP’, required for contracting muscles, moving
chromosomes during cell division, moving
structures with cells, transporting substances
across cell membranes, biosynthesis etc.
• Myoglobin is present in the sarcoplasm of muscle
fibres/cells. This reddish pigment results in the
distinctive colour of skeletal muscle, and stores
oxygen - until it is required by the mitochondria
for the production of ATP.
Sarcoplasmic reticulum
• Sarcoplasmic reticulum is a network of
membrane-enclosed tubules similar to
smooth endoplasmic reticulum (SER).
• Sarcoplasmic reticulum is present in muscle
fibres/cells and extends throughout the
sarcoplasm of the cell.
• The function of the sarcoplasmic reticulum
is to store calcium ions, which are
necessary for muscle contraction.
• The nuclei of muscle fibres ("muscle cells") are
located at the edges of the diameter of the fibre,
adjacent to the sarcolemma. A single muscle fibre
may have many nuclei.
Sarcomere: Contractile unit
• Each myofibril consists of two types of protein filaments called
"thick filaments", and "thin filaments". These two types of
filament have different structures .
• The thick filaments and the thin filaments within myofibrils
overlap in a structured way, forming units called sarcomeres.
• The extent to which the thick filaments and the thin filaments
overlap determines the sizes of the H zone, the I band, and the
A band of the sacromere formed by these filaments.
Central less dense zone-H;
Sarcomere repeated
every 2.3 µm.
Middle dark line- M line
Light Band
Dark Band
• In the diagram below the Z discs are
represented by the zig-zag lines that
form the boundaries between adjacent
sarcomeres
• Thick Filaments
Thick filaments (150 AO) are formed from a protein called
myosin which has important properties of elasticity and
contractibility.
The shape of the myosin molecules has the appearance of two
"hockey sticks" or "golf clubs" twisted together. These are the
myosin tail, and the myosin heads, or "crossbridges" .
Hydrolyzes ATP, binds Actin.
An Enzyme with ATPase activity: ATP +H2O
of energy for muscles
ADP +pi+ H+ source
• Thin Filaments (70 AO- Actin, Tropomyosin and Troponin):
The main component of the thin filaments is actin. Actin
molecules join together forming chains twisted into a helix
configuration. These molecules are very important to the
contraction mechanism of muscles because each actin
molecule has a single "myosin-binding site" The other two
protein molecules that form the thin filaments are called
troponin and tropomyosin.
The molecules of tropomyosin cover the myosin-binding
sites on the actin molecules when the muscle fibres are
relaxed.
• Actin and myosin are the two principal muscle
proteins, present in myofibrils.
• arranged in a ring-like structure, usually with six
(thin) actin strands surrounding a (thicker) myosin
fibril.
• run parallel and lengthwise.
• The myosin fibril has numerous small protrusions
called cross bridges.
• The actin strand is actually intertwined with an
even thinner, ribbon-like protein called
tropomyosin, and a smaller molecule, called
troponin, associates with tropomyosin in this
structure.
• Myosin and actin form the main contractile
elements of muscles.
This is because it is the binding of the thick
filaments to the thin filaments - and in particular
the positions of these points of attachment - that
controls the state of contraction/relaxation of the
muscle of which they are apart.
• Tropomyosin: Two strands α-helical rod.
– Located in the grooe between the two helical
strands of actin.
• Troponin: Complex of three polypeptide
chains TpC, TpI, and TpT. The troponin is
located in the thin filaments at intervals.
HISTOLOGIC FEATURES OF CARDIAC
MUSCLE.
The cardiac muscle resembles skeletal muscle
in having striated fibers. Cardiac muscle has
intercalated discs. Note the cross striations, the
central position of the nuclei, the intercalated
discs, and the branched fibers
The blue arrows
in the
photograph point
to branches.