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Lecture on muscular system
1. 1
9. Muscular System:
Histology and Physiology
Prepared by: Mirza Anwar Baig
M.Pharm (Pharmacology)
Anjuman I Islam's Kalsekar Technical Campus,
School of Pharmacy.
New Panvel,Navi Mumbai
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3. 3
Functions of Muscular System
1.Body movement (Locomotion)
2.Maintenance of posture
3.Respiration
Diaphragm and intercostal contractions
4.Communication (Verbal and Facial)
5.Constriction of organs and vessels
1.Peristalsis of intestinal tract
2.Vasoconstriction of b.v. and other structures (pupils)
6.Heart beat
7.Production of body heat (Thermogenesis)
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4. 4
Properties of Muscle
1.Excitability: capacity of muscle to respond
to a stimulus
2.Contractility: ability of a muscle to shorten
and generate pulling force
3.Extensibility: muscle can be stretched back
to its original length
4.Elasticity: ability of muscle to recoil to
original resting length after stretched
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5. 5
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,
– 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
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6. 6
Connective Tissue Sheaths
• Connective Tissue of a Muscle
– Epimysium. Dense regular, 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. 6
8. 8
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
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9. 9
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
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10. 10
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 to 4 cm
in length
• Develop from myoblasts;
numbers remain constant
• Striated appearance
• Nuclei are peripherally located10
11. 11
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) 11
12. 12
Sarcoplasmic Reticulum (SR)
• SR is fluid-filled system of membranous sacs
– 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
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15. 15
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; 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
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17. 17
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:
• Can bind to active sites on the actin molecules to form
cross-bridges. (Actin binding site)
• Attached to the rod portion by a hinge region that can
bend and straighten during contraction.
• 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.
19. 19
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.
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26. 26
Sliding Filament Mechanism
Cross bridge interaction between
actin and myosin brings about
muscle contraction by means of
the sliding filament mechanism.
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27. 27
Sliding Filament Mechanism
• Increase in Ca2+ starts filament sliding
• Decrease in Ca2+ turns off sliding
process
• Thin filaments on each side of
sarcomere slide inward over stationary
thick filaments toward center of A band
during contraction
• As thin filaments slide inward, they pull
Z lines closer together
• Sarcomere shortens 27
28. 28
Sliding Filament Mechanism
• All sarcomeres throughout muscle
fiber’s length shorten
simultaneously
• Contraction is accomplished by
thin filaments from opposite sides
of each sarcomere sliding closer
together between thick filaments.
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29. 29
Power Stroke
• Activated cross bridge bends toward
center of thick filament, “rowing” in
thin filament to which it is attached
– Sarcoplasmic reticulum releases Ca2+
– Myosin heads bind to actin
– Hydrolysis of ATP transfers energy to
myosin head and reorients it
– Myosin heads swivel toward center of
sarcomere (power stroke)
– ATP binds to myosin head and detaches
it from actin
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30. 30
Contraction- Relaxation Steps
Requiring ATP
• Splitting of ATP by myosin ATPase
provides energy for power stroke of cross
bridge
• Binding of fresh molecule of ATP to myosin
lets bridge detach from actin filament at
end of power stroke so cycle can be
repeated
• Active transport of Ca2+ back into
sarcoplasmic reticulum during relaxation
depends on energy derived from
breakdown of ATP
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31. 31
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
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33. 33
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
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34. 34
Motor Unit: The Nerve-Muscle
Functional Unit contd...
• 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)
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40. 40
Major Events in Neuromuscular
Transmission
• Motor neuron depolarization causes action potential to
travel down the nerve fiber to the neuromuscular
junction (1).
• Depolarization of the axon terminal causes an influx of
Ca2+ (2) which triggers fusion of the synaptic vesicles (3)
and release of neurotransmitter (Acetylcholine; ACh) (4).
• ACh diffuses across the synaptic cleft and binds to
post/ synaptic ACh receptor (AChR) located on the
muscle fiber at the motor end/ plate (5).
• Binding of ACh to AChRs opens the channels causing an
influx of Na (5), depolarization of the sarcolemma that
travels down the t/ tubules (6) and ultimately causes
the release of Ca2+ from the sarcoplasmic reticulum /
CONTRACTION.
• Unbound ACh in synaptic cleft defuses away or is
hydrolyzed (inactivated) by acetylcholinesterase (AChE)
(7).
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41. 41
Muscle Response to Strong Stimuli
• Muscle force depends
upon the number of
fibers stimulated
– More fibers contracting
results in greater muscle
tension
• Muscles can continue to
contract unless they run
out of energy
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42. 42
How Do Muscles Get Energy?
• Initially, muscles use
stored ATP for energy
– ATP bonds are broken to
release energy
– Only 4–6 seconds
worth of ATP is stored
by muscles
• After this initial time, other
pathways must be utilized
to produce ATP 42
44. 44
1. Creatine Phosphate (high-energy molecule)
• Muscle cells store CP
• CP transfers energy to ADP, to
regenerate ATP by direct
phosphorylation of ADP
• Creatine synthesize in
liver,pancrease,kidneys
• The enzyme creatine kinase
forms CP from creatine and
ADP
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45. 45
2.Anaerobic Respiration
• Anaerobic glycolysis and
lactic acid formation
– Reaction that breaks
down glucose without
oxygen
– Glucose is broken down
to pyruvic acid to
produce some ATP
– Pyruvic acid is converted
to lactic acid
• This reaction is not as
efficient, but is fast
– Huge amounts of glucose
are needed
– Lactic acid produces
muscle fatigue
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46. 46
3.Aerobic Respiration
• Glucose is broken
down to carbon
dioxide and water,
releasing energy (ATP)
• This is a slower
reaction that requires
continuous oxygen
• A series of metabolic
pathways occur in the
mitochondria
46
47. 47
Muscle Fatigue & Oxygen Debt
• When a muscle is fatigued, it is
unable to contract even with a
stimulus
• Common cause for muscle
fatigue is oxygen debt
– Oxygen must be “repaid” to
tissue to remove oxygen
deficit
– Oxygen is required to get rid
of accumulated lactic acid
• Increasing acidity (from lactic
acid) and lack of ATP causes
the muscle to contract less
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48. 4848
Smooth Muscle
• Located in the blood vessels, the
respiratory tract, the iris of the eye,
the gastro-intestinal tract
• The contractions are slow and
uniform
• Functions to alter the activity of
various body parts to meet the needs
of the body at that time
• Is fatigue resistant
• Activation is involuntary
49. 49
Smooth
Muscle
• 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.
• Dense bodies instead of Z
disks
– Have noncontractile
intermediate filaments
49
50. 50
Smooth Muscle
Figure 9.24
• 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
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51. 5151
Cardiac Muscle
• Has characteristics of both skeletal
and smooth muscle
• Functions to provide the contractile
activity of the heart
• Contractile activity can be gradated
(like skeletal muscle)
• Is very fatigue resistant
• Activation of cardiac muscle is
involuntary (like smooth muscle)
52. 52
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 ( Pacemaker cells) 52
54. 54
Cardiac Muscle and Heart Function
• Cardiac muscle fibers are striated
– sarcomere is the functional unit
• Fibers are branched; connect to
one another at intercalated discs.
The discs contain several gap
junctions
• Nuclei are centrally located
• Abundant mitochondria
• SR is less abundant than in
skeletal muscle, but greater in
density than smooth muscle
• Sarcolemma has specialized ion
channels that skeletal muscle
does not – voltage/ gated Ca2+
channels
• Fibers are not anchored at ends;
allows for greater sarcomere
shortening and lengthening
54
56. 56
How are cardiac contractions started? Cardiac
conduction system
• Specialized muscle cells “pace”
the rest of the heart; cells
contain less actin and myosin,
are thin and pale microscopically
• Sinoatrial (SA) node; pace of
about 65 bpm
• Internodal pathways connect SA
node to atrioventricular (AV)
node
• AV node could act as a
secondary pacemaker;
autorhythmic at about 55 bpm
• Bundle of His
• Left and right bundle branches
• Purkinje fibers; also
autorhythmic at about 45 bpm
ALL CONDUCTION FIBERS
CONNECTED TO MUSCLE FIBERS
THROUGH GAP JUNCTIONS IN THE
INTERCALATED DISCS
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57. 57
Muscle tone:
• Muscle tone (tonos=tension), a small amount of tautness
or tension in the muscle due to weak, involuntary
contractions of its motor units.
• Muscle tone keeps skeletal muscles firm, but it does not
result in a force strong enough to produce movement.
For example,
1. Upright position of head : when the muscles in the back of
the neck are in normal tonic contraction, they keep the head
upright and prevent it from slumping forward on the chest.
2. Gastrointestinal tract: where the walls of the digestive organs
maintain a steady pressure on their contents.
3. Walls of blood vessels: plays a crucial role in maintaining
blood pressure.
58. 58
Disorders of Muscle tone:
• Hypotonia refers to decreased or lost muscle tone.
• Such muscles are said to be flaccid.
• Flaccid muscles are loose and appear flattened rather
than rounded.
• Certain disorders of the nervous system and disruptions
in the balance of electrolytes (especially sodium,
calcium, and, to a lesser extent, magnesium) may result
in flaccid paralysis, which is characterized by loss of
muscle tone, loss or reduction of tendon reflexes, and
atrophy (wasting away) and degeneration of muscles.
59. 59
Hypertonia refers to increased muscle tone and is
expressed in two ways: spasticity or rigidity.
1.Spasticity is characterized by increased muscle tone
(stiffness)
• Certain disorders of the nervous system and
electrolyte disturbances such as those previously
noted may result in spastic paralysis, partial
paralysis in which the muscles exhibit spasticity.
2. Rigidity refers to increased muscle tone in which
reflexes are not affected.
60. 60
Types of Muscle Contractions
• Isometric contractions
– Tension in the muscles
increases
– The muscle is unable to
shorten or produce
movement
• Isotonic contractions
– Myofilaments are able to
slide on each other
during contractions
– The muscle shortens
and movement occurs
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