Download Muscular System

Human Physiology
Assist lecturer: Karrar Abbas
2nd Stage
Lec: 3
Muscular System
The muscular system is an organ system consisting of skeletal, smooth and cardiac
muscles. It permits movement of the body, maintains posture, producing body heat and
circulates blood throughout the body. The muscular system in vertebrates is controlled
through the nervous system, although some muscles (such as the cardiac muscle) can be
completely autonomous. Together with the skeletal system it forms the musculoskeletal
system, which is responsible for movement of the human body.
Muscles
There are three distinct types of muscles: skeletal muscles, cardiac muscles and smooth
(non-striated) muscles. Muscles provide strength, balance, posture, movement, and heat
of the body.
Skeletal muscle
There are approximately 639 skeletal muscles in the human body. Skeletal muscle is a
form of striated muscle tissue which is under the control of the somatic nervous system.
Most skeletal muscles are attached to bones by bundles of collagen fibers known as
tendons.
Skeletal muscle is made up of individual muscle cells or myocytes, known as muscle
fibers. Muscle fibers are cylindrical, and multinucleated.
Muscle fibers are in turn composed of myofibrils. The myofibrils are composed of two
kinds of protein filaments: actin and myosin myofilaments, repeated in units called
sarcomeres, the basic structural and functional units of the muscle fiber, because it is the
smallest portion of skeletal muscle capable of contracting. Each sarcomere extends from
one Z line to another Z line. The arrangement of the actin and myosin filaments is
responsible for the striated appearance of skeletal muscle, and forms the basic machinery
necessary for muscle contraction.
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The light bands contain only actin filaments and are called I bands. The dark bands
contain myosin filaments, as well as the ends of the actin filaments where they overlap
the myosin, and are called A bands.
When the muscle fiber is contracted, the actin filaments completely overlap the myosin
filaments, and the tips of the actin filaments are just beginning to overlap one another.
Smooth muscles
Smooth muscle is an involuntary non-striated muscle, controlled directly by the
autonomic nervous system. Smooth muscle is found within the walls of blood vessels
such as in the aorta and small arteries, arterioles and veins. Smooth muscle is also found
in lymphatic vessels, the urinary bladder, uterus, male and female reproductive tracts,
gastrointestinal tract, respiratory tract and iris of the eye. The structure and function is
basically the same in smooth muscle cells in different organs, but the inducing stimuli
differ substantially, in order to perform individual effects in the body at individual times.
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Smooth muscle is different from skeletal muscle and cardiac muscle in terms of structure,
function and regulation of contraction.
Smooth muscle cells known as myocytes, have a fusiform shape with single nuclei is seen
and, like striated muscle, can tense and relax.
However, smooth muscle tissue tends to demonstrate greater elasticity and function
within a larger length tension curve than striated muscle. This ability to stretch and still
maintain contractility is important in organs like the intestines and urinary bladder.
Cardiac muscle
Cardiac muscle (heart muscle) is involuntary striated muscle that is found in the walls
and histological foundation of the heart, specifically the myocardium, is the muscle tissue
of the heart. Cardiac muscles are distinct from skeletal muscles because the muscle fibers
are laterally connected to each other. Furthermore, as with smooth muscles, they are not
controlling themselves. Heart muscles are controlled by the sinus node influenced by the
autonomic nervous system.
Like skeletal muscle, the primary structural proteins of cardiac muscle are myosin and
actin. The actin filaments are thin, causing the lighter appearance of I bands in striated
muscle, whereas the myosin filament is thicker, lending a darker appearance to A bands.
The cells that constitute cardiac muscle, called cardiomyocytes, contain only three nuclei.
In contrast to skeletal muscle, cardiac muscle cells are typically branch-like instead of
cylindrical.
Coordinated contractions of cardiac muscle cells in the heart propel blood through
circulatory systems.
Neuromuscular junction
Motor neurons are nerve cells that carry action potentials to skeletal muscle fibers. Its
branch forms a junction with a muscle fiber, called a neuromuscular junction or synapse,
is a junction between nerve and muscle by which that a motor neuron is able to transmit a
signal to the muscle fiber, causing muscle contraction.
Motor neurons release acetylcholine (ACh), a small molecule neurotransmitter, when an
action potential reaches the presynaptic terminal of a motor neuron. Acetylcholine
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diffuses across the synaptic and binds to its receptors on the cell membrane of the muscle
fiber, also known as the sarcolemma.
Muscle contraction
Muscle contraction is the activation of tension generating sites within muscle fibers. In
physiology, muscle contraction does not mean muscle shortening because muscle tension
can be produced without changes in muscle length such as holding a heavy book. The
termination of muscle contraction is followed by muscle relaxation, which is a return of
the muscle fibers to their low tension-generating state.
General Mechanism of Muscle Contraction
1. An action potential travels along a motor nerve to its endings on muscle fibers.
2. At each ending, the nerve secretes a small amount of the neurotransmitter substance
acetylcholine.
3. The acetylcholine acts on a local area of the muscle fiber membrane to open multiple
“acetylcholine-gated” channels through protein molecules floating in the membrane.
4. Opening of the acetylcholine-gated channels allows large quantities of sodium ions to
diffuse to the interior of the muscle fiber membrane. This initiates an action potential at
the membrane.
5. The action potential travels along the muscle fiber membrane in the same way that
action potentials travel along nerve fiber membranes.
6. The action potential depolarizes the muscle membrane, and much of the action
potential electricity flows through the center of the muscle fiber. Here it causes the
sarcoplasmic reticulum to release large quantities of calcium ions that have been stored
within this reticulum.
7. The calcium ions initiate attractive forces between the actin and myosin filaments,
causing them to slide alongside each other, which is the contractile process.
8. After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic
reticulum by a Ca++ membrane pump, and they remain stored in the reticulum until a
new muscle action potential comes along; this removal of calcium ions from the
myofibrils causes the muscle contraction to cease.
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1. Action potential travels
along a motor nerve
11. Removal of muscle
contraction
2. Secretes acetylcholine
3. Open multiple acetylcholinegated channels
10. Removal of calcium
ions from the myofibrils
4. Sodium ions diffuse to
the interior of the muscle
fiber membrane
9. Calcium ions are pumped
back
into
sarcoplasmic
reticulum
8. Initiate attractive forces between
the actin and myosin filaments,
causing them to slide alongside
each other
5. Initiates action potential
travels along the muscle
fiber membrane
6. Depolarizes the muscle
membrane, and much of the
action potential electricity
flows through the center of the
muscle fiber
7. Sarcoplasmic reticulum
release large quantities of
calcium ions
Muscle Fatigue
Muscle fatigue, or physical fatigue, is the decline in ability of a muscle to generate force.
It can be a result of vigorous exercise but abnormal fatigue may be caused by barriers to
or interference with the different stages of muscle contraction. There are two main causes
of muscle fatigue:
1. The limitations of a nerve’s ability to generate a suitable signal (neural fatigue).
After a period of maximum contraction, the nerve’s signal reduces in frequency
and the force generated by the contraction diminishes.
2. The reduced ability of the muscle fiber to contract (metabolic fatigue).
Due to the direct or indirect effects of two main factors:
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i. Shortage of fuel (substrates) within the muscle fiber.
ii. Accumulation of substances (metabolites) within the muscle fiber, which interfere
either with the release of calcium (Ca2+) or with the ability of calcium to stimulate
muscle contraction.
Myalgia
Myalgia, or muscle pain, is a symptom of many diseases and disorders. The most
common causes are the overuse injury or strain.
Longer-term myalgias may be indicative of a metabolic myopathy, some nutritional
deficiencies or chronic fatigue syndrome.
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