The Muscular System

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 The muscular system is a complex
collection of tissues, each with a different
purpose.
 Understanding the components of the
muscular system, including the various
types of connective tissues, is a good way
to understand how bodies and physical
movement work.
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 Body movement (Locomotion)
 Maintenance of posture
 Respiration
– Diaphragm and intercostal contractions
 Communication (Verbal and Facial)
 Constriction of organs and vessels
– Peristalsis of intestinal tract
– Vasoconstriction of b.v. and other structures (pupils)
 Heart beat
 Production of body heat (Thermogenesis)

 Excitability: capacity of muscle to respond to a
stimulus.
 Contractility: ability of a muscle to shorten and
generate pulling force.
 Extensibility: muscle can be stretched back to its
original length.
 Elasticity: ability of muscle to recoil to original resting
length after stretched.

 Skeletal
– Attached to bones
– Makes up 40% of body weight
– Responsible for locomotion, facial expressions, posture, respiratory
movements, other types of body movement
– Voluntary in action; controlled by somatic motor neurons
 Smooth
– In the walls of hollow organs, blood vessels, eye, glands, uterus, skin
– Some functions: propel urine, mix food in digestive tract,
dilating/constricting pupils, regulating blood flow,
– In some locations, auto rhythmic
– Controlled involuntarily by endocrine and autonomic nervous systems
 Cardiac
– Heart: major source of movement of blood
– Auto rhythmic
– Controlled involuntarily by endocrine and autonomic nervous systems

Muscle attachments
– Most skeletal muscles run from
one bone to another.
– One bone will move – other
bone remains fixed.
Origin – less movable
attachment.
Insertion – more movable
attachment.

Muscle attachments (continued)
– Muscles attach to origins and insertions by
connective tissue
Fleshy attachments – connective tissue fibers are
short.
Indirect attachments – connective tissue forms a
tendon or aponeurosis.
– Bone markings present where tendons meet
bones
Tubercles, trochanters, and crests.

 Composed of muscle cells (fibers), connective tissue, blood
vessels, nerves.
 Fibers are long, cylindrical, and multinucleated.
 Tend to be smaller diameter in small muscles and larger in
large muscles. 1 mm- 4 cm in length.
 Develop from myoblasts; numbers remain constant.
 Striated appearance.
 Nuclei are peripherally located.

 SR is an elaborate, smooth endoplasmic reticulum
– runs longitudinally and surrounds each myofibril
– Form chambers called terminal cisternae on either side of the T-
tubules
 A single T-tubule and the 2 terminal cisternae form a triad
 SR stores Ca++ when muscle not contracting
– When stimulated, calcium released into sarcoplasm
– SR membrane has Ca++ pumps that function to pump Ca++ out of
the sarcoplasm back into the SR after contraction

 Many elongated myosin molecules shaped like golf clubs.
 Single filament contains roughly 300 myosin molecules.
 Molecule consists of two heavy myosin molecules wound together
to form a rod portion lying parallel to the myosin myofilament and
two heads that extend laterally.
 Myosin heads
1. Can bind to active sites on the actins molecules to form cross-bridges.
(Actin binding site)
2. Attached to the rod portion by a hinge region that can bend and
straighten during contraction.
3. Have ATPase activity: activity that breaks down adenosine
triphosphate (ATP), releasing energy. Part of the energy is used to
bend the hinge region of the myosin molecule during contraction

Tropomyosin Tropoin Actin
Myosin

 Thin Filament: composed of 3 major proteins
1. F (fibrous) actin
2. Tropomyosin
3. Troponin
 Two strands of fibrous (F) actin form a double helix extending the
length of the myofilament; attached at either end at sarcomere.
– Composed of G actin monomers each of which has a myosin-
binding site (see yellow dot)
– Actin site can bind myosin during muscle contraction.
 Tropomyosin: an elongated protein winds along the groove of the F
actin double helix.
 Troponin is composed of three subunits:
– Tn-A : binds to actin
– Tn-T :binds to tropomyosin,
– Tn-C :binds to calcium ions.

 Cells are not striated
 Fibers smaller than those in skeletal muscle
 Spindle-shaped; single, central nucleus
 More actin than myosin
 No sarcomeres
– Not arranged as symmetrically as in skeletal muscle,
thus NO striations.
 Caveolae: indentations in sarcolemma;
– May act like T tubules
 Dense bodies instead of Z disks
– Have noncontractile intermediate filaments

Grouped into sheets in walls of hollow organs.
 Longitudinal layer – muscle fibers run parallel to organ’s
long axis.
 Circular layer – muscle fibers run around circumference of
the organ.
 Both layers participate in peristalsis.

 Found only in heart where it forms a thick layer called
the myocardium.
 Striated fibers that branch
 Each cell usually has one centrally-located nucleus
 Fibers joined by intercalated disks
– IDs are composites of desmosomes and gap junctions
– Allow excitation in one fiber to spread quickly to adjoining
fibers
 Under control of the ANS (involuntary) and endocrine
system (hormones)
 Some cells are autorhythmic
– Fibers spontaneously contract (aka Pacemaker cells)

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The Muscular System

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