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Muscular System Functions
• 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)

Properties of Muscle
• 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

Types of Muscle
• 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, autorhythmic
– Controlled involuntarily by endocrine and autonomic nervous systems
• Cardiac
– Heart: major source of movement of blood
– Autorhythmic
– Controlled involuntarily by endocrine and autonomic nervous systems

Connective Tissue Sheaths
• Connective Tissue of a Muscle
– Epimysium. Dense regular c.t. surrounding entire muscle
• Separates muscle from surrounding tissues and organs
• Connected to the deep fascia
– Perimysium. Collagen and elastic fibers surrounding a
group of muscle fibers called a fascicle
• Contains b.v and nerves
– Endomysium. Loose connective tissue that surrounds
individual muscle fibers
• Also contains b.v., nerves, and satellite cells (embryonic stem cells
function in repair of muscle tissue
• Collagen fibers of all 3 layers come together at each
end of muscle to form a tendon or aponeurosis.

Nerve and Blood Vessel Supply

• Motor neurons
– stimulate muscle fibers to contract
– Neuron axons branch so that each muscle fiber (muscle cell)
is innervated
– Form a neuromuscular junction (= myoneural junction)
• Capillary beds surround muscle fibers
– Muscles require large amts of energy
– Extensive vascular network delivers necessary
oxygen and nutrients and carries away metabolic
waste produced by muscle fibers

Basic Features of a Skeletal Muscle
• Muscle attachments
– Most skeletal muscles
run from one bone to
another
– One bone will move –
other bone remains
fixed
• Origin – less
movable attach-
ment
• Insertion – more
movable attach-
ment

Basic Features of a Skeletal
Muscle
• 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

Skeletal Muscle Structure
• 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

Muscle Fiber Anatomy
• Sarcolemma - cell membrane
– Surrounds the sarcoplasm (cytoplasm of fiber)
• Contains many of the same organelles seen in other cells
• An abundance of the oxygen-binding protein myoglobin
– Punctuated by openings called the transverse tubules (T-
tubules)
• Narrow tubes that extend into the sarcoplasm at right angles
to the surface
• Filled with extracellular fluid
• Myofibrils -cylindrical structures within muscle fiber
– Are bundles of protein filaments (=myofilaments)
• Two types of myofilaments
1. Actin filaments (thin filaments)
2. Myosin filaments (thick filaments)
– At each end of the fiber, myofibrils are anchored to the inner
surface of the sarcolemma
– When myofibril shortens, muscle shortens (contracts)

Sarcoplasmic Reticulum (SR)
• 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

Sarcoplasmic Reticulum (SR)

Figure 9.5

• Sarcomere - repeating functional units
of a myofibril
Sarcomeres: Z – About 10,000 sarcomeres per
myofibril, end to end
Disk to Z Disk – Each is about 2 µm long
• Differences in size, density, and
distribution of thick and thin filaments
gives the muscle fiber a banded or
striated appearance.
– A bands: a dark band; full length of thick
(myosin) filament
– M line - protein to which myosins attach
– H zone - thick but NO thin filaments
– I bands: a light band; from Z disks to ends
of thick filaments
• Thin but NO thick filaments
• Extends from A band of one sarcomere to
A band of the next sarcomere
– Z disk: filamentous network of protein.
Serves as attachment for actin
myofilaments
– Titin filaments: elastic chains of amino
acids; keep thick and thin filaments in
proper alignment

• Many elongated myosin molecules
Myosin •
shaped like golf clubs.
Single filament contains roughly 300

(Thick)
myosin molecules
• Molecule consists of two heavy myosin
molecules wound together to form a
Myofilament rod portion lying parallel to the
myosin myofilament and two heads
that extend laterally.
• Myosin heads
1. Can bind to active sites on the
actin 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

• Thin Filament: composed of 3 major
proteins
1. F (fibrous) actin
2. Tropomyosin Actin (Thin)
3. Troponin
• Two strands of fibrous (F) actin
form a double helix extending the
Myofilaments
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.

Sliding Filament Model of
Contraction
• Thin filaments slide past the thick ones
so that the actin and myosin filaments
overlap to a greater degree
• In the relaxed state, thin and thick
filaments overlap only slightly
• Upon stimulation, myosin heads bind to
actin and sliding begins

Sliding Filament Model of
Contraction
• Each myosin head binds and detaches
several times during contraction, acting
like a ratchet to generate tension and
propel the thin filaments to the center
of the sarcomere
• As this event occurs throughout the
sarcomeres, the muscle shortens

PLAY InterActive Physiology®: Muscular System: Sliding Filament Theory

Neuromuscular Junction
• Region where the motor neuron stimulates the
muscle fiber
• The neuromuscular junction is formed by :
1. End of motor neuron axon (axon terminal)
• Terminals have small membranous sacs (synaptic
vesicles) that contain the neurotransmitter acetylcholine
(ACh)
2. The motor end plate of a muscle
• A specific part of the sarcolemma that contains ACh
receptors
• Though exceedingly close, axonal ends and muscle
fibers are always separated by a space called the
synaptic cleft

Neuromuscular Junction

Figure 9.7 (a-c)

Motor Unit: The Nerve-
Muscle Functional Unit
• A motor unit is a motor neuron and all the
muscle fibers it supplies
• The number of muscle fibers per motor unit
can vary from a few (4-6) to hundreds
(1200-1500)
• Muscles that control fine movements
(fingers, eyes) have small motor units
• Large weight-bearing muscles (thighs, hips)
have large motor units

Motor Unit: The Nerve-Muscle Functional
Unit

Figure 9.12 (a)

Motor Unit: The Nerve-
Muscle Functional Unit

• Muscle fibers from a motor unit are spread
throughout the muscle
– Not confined to one fascicle
• Therefore, contraction of a single motor unit
causes weak contraction of the entire muscle
• Stronger and stronger contractions of a muscle
require more and more motor units being
stimulated (recruited)

Smooth • Cells are not striated
• Fibers smaller than those in skeletal
Muscle 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

Smooth Muscle
• 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

Figure 9.24

Smooth Muscle
• Is innervated by autonomic nervous system (ANS)
• Visceral or unitary smooth muscle
– Only a few muscle fibers innervated in each group
– Impulse spreads through gap junctions
– Who sheet contracts as a unit
– Often autorhythmic
• Multiunit:
– Cells or groups of cells act as independent units
– Arrector pili of skin and iris of eye

Cardiac Muscle
• 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)

Cardiac Muscle Tissue

Figure 10.10a

Disorders of Muscle Tissue
• Muscle tissues experience few disorders
– Heart muscle is the exception
– Skeletal muscle – remarkably resistant to
infection
– Smooth muscle – problems stem from
external irritants

• Muscular dystrophy – a group of
inherited muscle destroying disease
– Affected muscles enlarge with fat and
connective tissue
– Muscles degenerate
• Types of muscular dystrophy
– Duchenne muscular dystrophy
– Myotonic dystrophy

• Myofascial pain syndrome – pain is
caused by tightened bands of muscle
fibers
• Fibromyalgia – a mysterious chronic-
pain syndrome
– Affects mostly women
– Symptoms – fatigue, sleep abnormalities,
severe musculoskeletal pain, and headache

Developmental Aspects:
Regeneration
• Cardiac and skeletal muscle become amitotic, but can
lengthen and thicken
• Myoblast-like satellite cells show very limited
regenerative ability
• Cardiac cells lack satellite cells
• Smooth muscle has good regenerative ability
• There is a biological basis for greater strength in men
than in women
• Women’s skeletal muscle makes up 36% of their body
mass
• Men’s skeletal muscle makes up 42% of their body
mass

Developmental Aspects: Male
and Female
• These differences are due primarily to
the male sex hormone testosterone
• With more muscle mass, men are
generally stronger than women
• Body strength per unit muscle mass,
however, is the same in both sexes

Developmental Aspects: Age
Related
• With age, connective tissue increases and
muscle fibers decrease
• Muscles become stringier and more sinewy
• By age 80, 50% of muscle mass is lost
(sarcopenia)
• Decreased density of capillaries in muscle
• Reduced stamina
• Increased recovery time
• Regular exercise reverses sarcopenia

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