Topic 6a Muscular System- Dr. Basu’s Easy Anatomy & Physiology Lecture

Topic 6a Muscular System- Dr. Basu’s Easy Anatomy & Physiology Lecture


Hi everyone, welcome to the human anatomy
class. This is the first lecture of topic six, the
muscular system. The main function of muscles in the human
body is to provide movement. There are three types of muscles, skeletal
muscle, smooth muscle, and cardiac muscle. This table is comparing the three types of
muscles. The body location of the skeletal muscles
are attached to the bones. The cardiac muscles are present in the heart
wall. While smooth muscles are presented in the
wall of the hollow organs. Coming to sell shape in appearance, skeletal
muscle are long cylindrical multinucleate they have striations, that means stripes. Cardiac muscles are branched, short, they
are uninucleate. they also have stripes, and they have intercalated
discs. Smooth muscles are single shape, uninucleate,
and no striations. The third characteristic is, what are the
connective tissue components. Skeletal muscle has endomysium, perimysium,
and epimysium. Cardiac muscle only has endomysium. Smooth also has endomysium. Control of contraction. Skeletal muscles are voluntary. That means they’re under your conscious
control. Cardiac and small muscles are in voluntary. What about the speed of contractions. Skeletal muscles can contract slowly or very
fast. Cardiac muscle contraction is very slow. Smooth muscle contraction is very slow. The most important characteristics of muscle
is their ability to contract and relax. There are certain prefixes commonly used in
association to muscular tissue. Those prefixes are myo and mys. Which refers to muscle. While Sacro, such as sarcolema, sarcoplasm
refers to flesh. Smooth muscles are spindle-shaped, uninucleate,
and lack striations. Skeletal muscles are branched with intercalated
discs. They are uninucleate, striated, and they are
involuntary. Skeletal muscles are cylindrical, multinucleate. They are attached to the bones by the means
of special connective tissue called tendons. They are voluntary, which means they’re
under your conscious control. The main function of skeletal muscles is to
produce movement. Skeletal muscles also help in maintaining
posture. They help and stabilizing the bones, joints,
and articulations. And if the body is getting cold, then the
muscles help in generating heat. That is why you shiver when you’re cold. Skeletal muscles are able to relax and contract
because of these following four properties. The first one is excitability that means skeletal
muscles are able to receive a stimulant. That means a trigger or a signal and they
can respond to that signal. Second is contractility. That is the ability to shorten after the stimulus
has been received. Third is, extensibility. That means the ability of the muscles to stretch
after the shortening. And the last one is electricity. That means the ability to the muscles to recoil
and resume. So that means they can shorten and then again
extend. These are the four properties that can help
the muscles to contract and relax. Connective tissue coverings of the skeletal
muscles. One skeletal muscle is known as a muscle fiber. So a single muscle fiber is enclosed by a
membrane that is known as endomysium. Usually muscle cells exist in bundles. So one bundle is known as fascicle. The covering around a fascicle is known as
Perimysium. Several bundles together are enclosed by an
outer covering which is known as the epimysium. So epimysium covers the entire muscle. And then, superficial to the epimysium there’s
another layer known as the fascia. The figure shows a skeletal muscle connected
to the bone by a white tendon. And you can see that the skeletal muscle,
the entire muscle, has several bundles are fascicles. So one of the fascicles has been pulled out,
and then from that one fascicle, one muscle fiber has also been pulled out. So that one muscle fiber is wrapped by endomysium. One fascicle is wrapped by the perimysium. And then all the fascicles together in the
muscle are covered by the epimysium. Around the epimysium is the facia. Skeletal muscles are usually attached to the
bones. Either directly by the tendon or indirectly
by that aponeuroses. They may also be connected to cartilages and
connective tissue coverings. Skeletal muscles have different levels of
organization. One skeletal muscle is formed of many bundles. Those bundles are known as fascicles. Each fascicle is made up of several muscle
cells or muscle fibers. So one muscle cell is known as one muscle
fiber. One muscle fiber has specialized organelles
inside the cells known as myofibrils. Myofibrils are like thin threads. The myofibrils in turn are made up of special
units which can contract. So those are contractile units called sarcomeres. And the sarcomeres in turn are made up of
very thin thread like structures which are known as myofilaments. Primarily the acting and myosin filaments. This slide describes the microscopic anatomy
of a skeletal muscle cell. So a skeletal muscle cell or fiber is in closed
by a cell membrane which is known as the sarcolemma. The cytoplasm inside the cell is called sarcoplasm. The sarcoplasm is filled with different types
of organelles such as multiple nuclei, mitochondria, and there are also long rod like organelles
called myofibrils. Which is shown in the figure here. And there is a special time of smooth endoplasmic
reticulum called Sarcoplasmic reticulum. The function of this Sarcoplasmic reticulum
is to store and release calcium ions. Myofibrils are made up of contractile units
called sarcomeres, which are arranged horizontally along the length of the myofibrils. There are also special transverse tubules,
or T tubules which are formed by side to side extensions of the sarcolemma. These T tubules help in carrying action potential
from the sarcolemma, deep inside the cell. What is the Sarcomere? A sarcomere is defined as the smallest contractile
unit of a muscle fiber which is located between two neighboring Z lines or z discs. Sarcomeres are made up of very thin like structures
called myofilaments. This figure shows the sarcomere. Which is between two zigzag lines those lines
are known as z lines, or Z discs. What is the sarcomere made up of? The sarcomere is made up of thick and thin
myofilaments. Thick myofilaments is mainly made up of a
protein called myosin protein. The thick myofilament has ATPase enzymes. The thin myofilaments are made up three proteins. Actin, troponin, and tropomyosin. The striated appearance of the skeletal muscle
are due to the arrangement of thick and thin myofilaments. Thick myofilament is made of myosin protein. The myosin protein looks like a golf club. It has a tail and a head. The tail is made up of two heavy chains. The head is made up for light chains. The joining between the head and the tail,
the neck region, is like a hinge joint that allows the head to move up and down. The thin myofilament is made up of acting
troponin, and tropomyosin. Three types of proteins. In order to understand the arrangement of
these three proteins imagine a pearl necklace. The string of pearls is the active protein. Now if you take another gold chain and wind
it around the pearl necklace, the gold chain would be the tropomyosin protein. In order for the tropomyosin to not slip,
if you glue it at certain points those drops of glue would be the troponin protein. If you look up a histological section of a
skeletal muscle under the microscope you would see alternate dark and white bands. It would give the appearance of stripes or
striations. This is due to the arrangement of Sarcomeres
in the myofibrils. So the light band is also known as the I band
or isotropic band. This contains only the thin filaments, or
the actin filaments. While the A band or the dark band, know as
anisotropic band. This band is made up of myosin filaments and
also overlapping portions of thin actin filaments. This slide describes the structure of a sarcomere. The sarcomere you see the light band, which
is the light band or the isotropic band. Containing only thin filaments. Then the dark band, or the A band, or anisotropic
band contains both thick filaments, as well as portions of thin filaments. If you look at the figure, right in the center
of the A band, there is a region that actually lacks the overlapping thin filaments. So, this section is lighter than the dark
bands. This central part of know as the H-band. In the center of the H band there is a line
which is known as the M line. Now if you look at the I bands, in the center
of the I bands there is a zigzag line of a protein band, that is known as the Z line. This is where the acting filaments are attached. This figure also illustrates the structure
of a sarcomere.

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