2. 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
3. Properties of Muscle
• Excitability: capacity of muscle to
respond to a stimulus
• Contractility: ability of a muscle to
shorten with force
• Extensibility: muscle can be stretched
to its normal resting length and beyond
to a limited degree
• Elasticity: ability of muscle to recoil to
original resting length after stretched
4. Types of Muscle
• Skeletal
– Attached to bones
– 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
5. Connective
Tissue
• Layers of Connective Tissue
– Epimysium. Dense collagen fibers that
surround the whole 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.
6. Nerve and Blood Vessel Supply
• Motor neurons: stimulate
muscle fibers to contract. Nerve
cells with cell bodies in brain or
spinal cord; axons extend to
skeletal muscle fibers through
nerves
• Axons branch so that each
muscle fiber is innervated
• 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
7. 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 due to light and
dark banding
8. 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
– Actin filaments (thin filaments)
– 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)
9. Sarcoplasmic Reticulum (SR)
• SR is an elaborate, smooth endoplasmic
reticulum that mostly 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 has Ca++ pumps that function to pump
Ca++ out of the sarcoplasm back into the SR
12. Sarcomeres: Z
Disk to Z Disk
• Sarcomere - repeating functional units
of a myofibril
– About 10,000 sarcomeres per
myofibril, end to end
– 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; length of thick
filaments
• 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
14. Myosin
(Thick)
Myofilament
• 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
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
15. Actin (Thin)
Myofilaments
• 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.
16. 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
17. 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
InterActive Physiology®: Muscular PPLLAAYY System: Sliding Filament Theory
18. Neuromuscular Junction
• The neuromuscular junction is formed from:
– Axonal endings, which have small membranous
sacs (synaptic vesicles) that contain the
neurotransmitter acetylcholine (ACh)
– The motor end plate of a muscle, which is a
specific part of the sarcolemma that contains ACh
receptors and helps form the neuromuscular
junction
• Though exceedingly close, axonal ends and
muscle fibers are always separated by a space
called the synaptic cleft
20. 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
21. Motor Unit: The Nerve-Muscle Functional
Unit
Figure 9.12 (a)
22. 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)
23. Smooth
Muscle
• Not striated, fibers smaller than those
in skeletal muscle
• Spindle-shaped; single, central nucleus
• More actin than myosin
– 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 as in
skeletal muscle; have noncontractile
intermediate filaments
25. Types of Smooth Muscle
• Visceral or unitary: cells in sheets;
function as a unit
– Numerous gap junctions; waves of contraction
– Often autorhythmic
• Multiunit: cells or groups of cells act as
independent units
– Sheets (blood vessels); bundles (arrector pili and
iris); single cells (capsule of spleen)
26. Cardiac Muscle
• Found only in heart
• Striated fibers that branch
• Each cell usually has one 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)
27. Developmental Aspects
• Muscle tissue develops from embryonic mesoderm
called myoblasts
• Multinucleated skeletal muscles form by fusion of
myoblasts
• The growth factor agrin stimulates the clustering of
ACh receptors at newly forming motor end plates
• As muscles are brought under the control of the somatic
nervous system, the numbers of fast and slow fibers are
also determined
• Cardiac and smooth muscle myoblasts do not fuse but
develop gap junctions at an early embryonic stage
28. 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
29. 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
30. 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
