Lec 5
Physiology
Dr.HananLuay
The muscle
Objectives
1- Recognize the basic structure and morphology of skeletal
muscle.
2-Define sarcotubular system and determine its function.
3- State the electrical characteristics of skeletal muscles.
The muscles are excitable cells; they are machines to convert the
chemical energy to mechanical energy.
The muscle can be excited electrically, mechanically, chemically →
action potential (A.p.).
It differs from the nervous system by the fact that it has a contractile
mechanism which is activated by action potential.
Types of muscle:
1-Skeletal muscles: These are voluntary muscles attach to bones except
the tongue, superior portion of the esophagus, anal sphincter, and
diaphragm are also composed of skeletal muscle, but they do not cause
movements
of
the
skeleton.
Skeletal musclesSmoothmusclesCardiac muscles
2-Smoothmuscles: Involuntary muscle. It is Muscle of the viscera (e.g., in
walls of blood vessels, intestine, & other 'hollow' structures and organs
in the body).
3-Cardiac muscles: Muscle of the heart. Involuntary.
40% of the body is skeletal muscles and 10% are smooth and cardiac
muscles.
Characteristics of muscle:
excitability - responds to stimuli (e.g., nervous impulses)
contractility - able to shorten in length
extensibility - stretches when pulled
elasticity - tends to return to original shape & length after
contraction or extension.
Functions of muscle:
motion
maintenance of posture
heat production
The skeletal muscle:
It is that type of the muscles that is attached to bones & moves
skeleton, also called striated muscle (because of its appearance under
the microscope), it lacks anatomical and functional connection between
individual muscle fibers, and it is voluntary muscle (under voluntary
control).
Morphology:
It is composed of numerous fibers (building units of the muscular
system), which is made up of smaller subunits.
Each muscle fiber extends along the length of the muscle, and each is
innervated by one nerve fiber near the middle of the fiber.
The muscle fibers are arranged in parallel between the two tendon
ends, so that the force of contraction is additive also this allows each
fiber to be controlled individually so we can contract fewer or more
fibers and the strength of contraction will be graded.
The muscle fiber is a cylindrical single cell containing:
-multiple nuclei.
-Cell membrane (sarcolemma) fuses with the tendons at the muscle
ends.
-Sarcoplasm (intracellular fluid fills the spaces between the myofibrils
{K+, Phosphate, protein enzymes and mitochondria, sarcoplasmic
reticulum which controls the contraction(rapid contracting muscles
means extensive reticula)}
-Other organ cells
-Small muscle fibrils, which consist of filaments that are made up of
contractile proteins (actin and myosin).
Actin and myosin are large polymerized protein molecules, responsible
for actual muscle contraction.
The striations:
The myosin and actin filaments interdigitate and cause the myofibrils to
have alternate light and dark bands.
The light bands are only Actin filaments called I bands, they are isotropic
to polarized lights.
The dark bands contain Myosin (twice molecular weight of Actin) called
A bands; they are anisotropic to polarized lights, overlapping with Actin
filaments. (It does not change in contraction), each thick filament is
surrounded by 6 thin filaments in a regular hexagonal pattern.
So the striations are due to difference in the refractive index of the parts
of the muscle fibers.
The I band is divided in the middle by darker Z line (the actin filaments
are attached to the Z disc from which the filaments extend in both
directions to interdigitate with the myosin).
The A band is divided by the lighter H band, in the middle of it a
transverse line called M line.
On the ends of the myosin filaments are small projections called the
cross bridges, which interact with the actin filaments to cause
contraction.
The portion of the myofibrils (or the whole muscle fiber) that is between
2 successive Z discs is called Sarcomere.
On contraction, the length of it is about 2 micrometer (the actin
completely overlap the myosin, the tips of actin are just beginning to
overlap one another.
The sarcomere
is the smallest functional unit of the muscle; it is the
area between 2 Z lines. It increases in relaxation and decreases in
contraction.
eu Mro ireroirMrlr al raeuceloM contractile filaments:
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It is composed of 6 polypeptide chains, a/ 2 heavy (wrap spirally
around each other to form double helix called tail, one end of these
chains is folded bilaterally into a globular polypeptide structure called
head (2 heads) with 2N terminals, they contain actin binding sites and a
catalytic site that hydrolyse ATP and, b/ 4 light chains (are parts of the
head (help control the function of the head during contraction).
The tails are bundled together to form the body of the filaments, while
many heads hang outwards from the body.
Part of the body with the head extends to form arms (called the cross
bridges).
There are no cross bridges in the middle of myosin filaments
because the hinged arms extends away from the center.
The cross bridges are flexible at 2 points one where the arm leaves the
body, the other where the head attaches to the arm, these called hinges.
The thin filament is made of actin ,torponin and tropomyosin.
Actin molecules
:
Actin filament is made up of 2 chains of globular unit that form a long
double helix and contain binding sites for myosin. It is composed of
double stranded F actin protein molecule made in a helix, each strand is
composed of polymerized G actin molecule ,attached to each one
molecule of ADP , these are the active sites with which the cross bridges
of myosin interact.
Tropomyosin molecule
They are located in the groove between the two chains forming long
filaments overlying the binding sits of myosin. So in the resting state
they lie on the top of the active sites of the actin strands so no attraction
between actin and myosin.
Troponin
It is a protein attached intermittently at regular intervals along the sides
of tropomyosin molecules. It is a complex of 3 loosely bound protein
subunits:
Troponin I has a strong affinity for actin inhibits the interaction between
myosin to actin.
Troponin T binds troponin to tropomyosin.
TropomyosinC contains binding sites for calcium ions that initiate
contraction.
The troponin- tropomyosin complex is called the relaxating protein,
because it prevents the binding of actin to the heads of myosin and
leads to muscle relaxation.
The sarcotubular system
:
Because the skeletal muscle fiber is so large, the action potential
cannot flow deep within the muscle fiber to cause maximum muscle
contraction, current must penetrate deeply into the muscle fiber, and
this is by:
The T system (transverse tubules): it is a system of transverse tubules
in the form of letter T which is continuous with the membrane of the
muscle fiber, it starts from one side of the cell membrane to the
opposite side, so it is continuous with the extracellular space, and they
contain extracellular fluid inside ; they are present along the whole
length of the muscle fiber and is responsible for spreading of action
potential from the cell membrane to the interior of the muscle fiber, the
electrical currents around them create the muscle contraction.
The sarcoplasmic reticulum : it forms an irregular system of tubules
surrounding the myofibrils it has an enlarging ends or chambers called
terminal cisterns in close contact with the T system at the junction
between A and I bands.
The arrangement of the T system with the ciatern of the endoplasmic
reticulum at either side called Traid.
The sarcoplasmic reticulum contains excess amounts of calcium ions (in
the cistern) in high concentration which are released when the action
potential occurs in the adjacent tubules. After the contraction has been
occurred ,active calcium pump located in the walls of sarcoplasmic
reticulum pumps calcium back to the sarcoplasmic tubules (inside the
reticulum there is a protein called calsequestrin which can concentrate
and binds up to 40 times more calcium ions). In addition to that the
terminal cisterns help in muscle metabolism.