Download Ch10.Skeletal.Muscle.Tissue.Lecture_1

Skeletal Muscle Tissue
Human Anatomy
Sonya Schuh-Huerta, Ph.D.
Muscle
• Muscle = Latin word for “little mouse”
• Muscle is the primary tissue in the:
– Heart (cardiac muscle)
– Walls of hollow organs (smooth muscle)
• Skeletal muscle
– All of your voluntary muscles
Skeletal Muscle: Basic Function
 Voluntary movement
 Locomotion
 Manipulation of environment
 Facial expression
 Maintenance of posture
 Joint stabilization
 Heat generation
• Muscle contractions produce heat 
helps maintain normal body temperature
Coach Laura
Muscle Tissue: Unique Features
 Contractility
 Excitability
 Extensibility
 Elasticity
Skeletal Muscle Tissue
• Skeletal muscle tissue:
– Packaged into skeletal muscles
– 640 muscles in human body
– Makes up ~40% of body weight
– 15% more dense than fat
– Cells are termed muscle
fibers & are striated!
Coach Sean
Skeletal Muscle
• Each muscle is an organ!
– Consists mostly of muscle tissue
– Skeletal muscle also contains:
• Connective tissue
• Blood vessels
• Nerves
Basic Features of a Skeletal
Muscle
• Connective tissue & fascicles
– Sheaths of connective tissue bind a skeletal
muscle & its fibers together
• Epimysium = dense regular connective tissue
surrounding entire muscle
• Perimysium = surrounds each fascicle
(group of muscle fibers)
• Endomysium = a fine sheath of connective tissue
wrapping each muscle cell
Basic Features of a Skeletal
Muscle
• Connective tissue sheaths are continuous
with tendons
– When muscle fibers contract, pull is exerted
on all layers of connective tissue & tendon
– Sheaths provide elasticity & carry blood
vessels & nerves
Skeletal Muscle: Levels of Organization
Bone
Epimysium
Tendon
Muscle fiber
in middle of
a fascicle
Fascicle (wrapped by
perimysium)
Muscle fiber
(1 cell)
Muscle
Endomysium
(between individual
muscle fibers)
Basic Features of a Skeletal
Muscle
• Nerves & blood vessels:
– Each skeletal muscle supplied by branches of
• 1 nerve
• 1 artery
• 1 or more veins
Capillary Network Surrounding Skeletal
Muscle Fibers
Basic Features of a Skeletal
Muscle
• Nerves & blood vessels:
– Nerves & vessels branch repeatedly
– Smallest nerve branches serve:
• Individual muscle fibers!!!
• Neuromuscular junction  signals the muscle to
contract (also called motor endplate)
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 attachment (directly
attached to the bone)
• Insertion = more movable attachment (attached to
bone by a tendon)
Muscle Attachments
Muscle contracting
Origin
by direct
attachment
Brachialis
Tendon
Insertion by
indirect attachment
Muscle Attachments
– Muscles attach to origins & insertions by CT
• Fleshy attachments (direct)  CT fibers are short
• Indirect attachments  CT forms a tendon or
aponeurosis
– Bone markings present where tendons meet
bones
• Tubercles, trochanters, & crests…
Muscle
At the Microscopic Level
Microscopic Structure of the Muscle Fiber
Review
 Huge, long cylindrical cells
 10–100 mm in diameter
Striations
 many centimeters long
Nuclei
 Multinucleated  cells
formed by fusion of many
embryonic cells
Muscle
Fiber (cell)
 Striations visible
Photomicrograph: Skeletal muscle at 300x
Microscopic Structure of the Muscle Fiber
Thin (actin)
filament
I band
Thick (myosin)
filament
A band
Sarcomere
Sarcolemma
I band
M line
Myofibril
Nucleus
Myofilament
=> “Bag of pretzels”
Mechanism of Contraction
• 2 major types of contraction
– Concentric contraction  muscle shortens
to do work
• Most common type of contraction
– Eccentric contraction  muscle generates
force as it lengthens
• Muscle acts as a “brake” to resist gravity
• “Down” portion of a pushup is an example
The Sliding Filament Mechanism
of Contraction
– Explains concentric contraction
• Myosin heads attach to thin filaments at both ends
of a sarcomere
– Then pull thin filaments toward the center of sarcomere
• Thin & thick filaments do NOT shorten, the
sarcomere shortens!
– Initiated by release of Ca2+ from the SR!
– Powered by ATP!
The Sliding Filament Mechanism of
Contraction
Thin (actin)
filament
Movement
Ca2+
ATP
Myosin
head
Thick (myosin)
filament
The Sliding Filament Mechanism of
Contraction
Sliding Filament Mechanism
• Contraction changes the striation pattern
– Fully relaxed  thin filaments partially overlap
thick filaments
– Contraction  thin filaments completely
overlap thick filaments, & Z discs move closer
together
• Sarcomere shortens
• I bands shorten, H zone disappears
• A band remains same length
The Molecular Components
 Ca2+ & ATP
 myosin binds
 ATP  ADP
& Pi
 powerstroke
 new ATP binds
 release &
recovery stroke
 myosin binds
again
Sliding Filament Mechanism in Action!
How your muscles contract…
animation by www.encognitive.com
Functional Anatomy
of Skeletal Muscle
• Muscle extension
– Muscle is stretched by a movement opposite
of that which contracts it
• Muscle fiber length & force of contraction
– Greatest force produced when a fiber starts
out slightly stretched
– Myosin heads can pull along the entire length
of the thin filaments  do more work &
generate greater contraction!
The Role of Titin
• Titin  a spring-like molecule (protein) in
sarcomeres (= strong like spider’s silk)
– Resists overstretching
– Holds thick filaments in place
– Unfolds when muscle is stretched
Innervation of Skeletal Muscle
• Motor neurons innervate skeletal muscle
tissue
– Neuromuscular junction  the point where
nerve ending & muscle fiber meet
– Axon terminals  ends of axons
• Store neurotransmitters (Acetylcholine)
– Synaptic cleft  space between axon
terminal & sarcolemma
The Neuromuscular Junction
Nerve
impulse
Nucleus
Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
1 Nerve impulse
stimulates the release
of the neurotransmitter
acetylcholine (ACh)
into the synaptic cleft.
The Neuromuscular Junction
Neuron
Synaptic
cleft
Axon terminal
of motor neuron
Synaptic vesicle
containing ACh
2 Ach binds to its
receptor & causes Na+ to
flow into the cell & the
membrane to depolarize.
This triggers massive Ca2+
to be released from the SR!
t tubule
Sarcoplasmic
Reticulum
Muscle fiber
Ca2+
Motor Units
Spinal cord
Motor Motor
unit 1 unit 2
Axon terminals at
neuromuscular
junctions
Branching axon
to motor unit
Nerve
Motor neuron
cell body
Motor
neuron
axon
Muscle
Muscle
fibers
(a) Axons of motor neurons extend from the spinal cord to
the muscle. There each axon divides into a number of
axon terminals that form neuromuscular junctions with
muscle fibers scattered throughout the muscle.
(b) Branching axon
terminals form
neuromuscular
junctions, one
per muscle fiber
(photomicrograph
110).
Quiz – What is this?
Types of Skeletal Muscle Fibers
• Skeletal muscle fibers are categorized
according to 2 characteristics
– How they manufacture energy (ATP)
– How quickly they contract!
• Oxidative fibers  produce ATP
aerobically
• Glycolytic fibers  produce ATP
anaerobically by glycolysis
Types of Skeletal Muscle Fibers
• Skeletal muscle fibers are divided into 3
classes:
• Slow oxidative fibers
– Red slow oxidative fibers
• Fast glycolytic fibers
– White fast glycolytic fibers
• Fast oxidative fibers
– Intermediate fibers
Types of Skeletal Muscle Fibers
• Slow oxidative fibers (slow twitch)
–
–
–
–
–
–
Red color due to abundant myoglobin => O2 store
Obtain energy from aerobic metabolic reactions
Contain a large number of mitochondria
Richly supplied with capillaries
Contract slowly & resistant to fatigue
Dark meat?
Fibers are small in diameter
Types of Skeletal Muscle Fibers
• Fast glycolytic fibers (fast twitch)
–
–
–
–
–
Contain little myoglobin & few mitochondria
About twice the diameter of slow-oxidative fibers
Contain more myofilaments & generate more power!!!
Depend on anaerobic pathways (glycolysis)
Contract rapidly & tire quickly
White meat?
Types of Skeletal Muscle Fibers
• Fast oxidative fibers
– Have an intermediate diameter
– Contract quickly like fast glycolytic fibers
– Are oxygen-dependent (aerobic pathways)
– Have high myoglobin content & rich supply of
capillaries
– Somewhat fatigue-resistant
– More powerful than slow oxidative fibers
Physiology of Skeletal Muscles
• Predominately powered by
oxidation of fats & carbs
• Fast twitch muscles  operate with little O2 (anaerobic) to
break down glucose & produce ATP (energy) for quick
powerful bursts of contraction, but tire quickly…
–By-product is lactic acid (“Feel the burn!”)
–Lactic acid build-up (lactic acid threshold or blood lactate accumulation)
Disorders of Muscle Tissue
• Muscle tissues experience few disorders
– Heart muscle is exception (many problems)
– Skeletal muscle
• Remarkably resistant to infection!
– Smooth muscle
• Problems stem from external irritants (things we
breathe in & eat)
Disorders of Muscle Tissue
• Muscular dystrophy
– A group of inherited muscle destroying
diseases
• Affected muscles enlarge with fat & connective
tissue
• Muscle fibers & muscles degenerate – can lead to
paralysis & death
• Types of muscular dystrophy
– Duchenne muscular dystrophy
– Myotonic dystrophy
– Others….
Disorders of Muscle Tissue
• 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, & headache
Muscle Tissue Throughout Life
• Muscle tissue develops from myoblasts
– Myoblasts fuse to form skeletal muscle fibers
– Skeletal muscles contract by the 7th week of
development
Embryonic
mesoderm cells
Myoblasts
Myotube
(immature
multinucleate
muscle fiber)
Satellite
cell
1 Embryonic
2 Several
3 Myotube
mesoderm
cells undergo
cell division
(to increase
number) and
enlarge.
myoblasts
fuse together
to form a
myotube.
matures into
skeletal
muscle fiber.
Mature
skeletal
muscle
fiber
Muscle Tissue Throughout Life
• Satellite cells
– Surround skeletal muscle fibers
– Resemble undifferentiated myoblasts
– Fuse into existing muscle fibers to help them
grow
• Cardiac muscle
– Pumps blood in embryo end of week 3
What happens with age?
• With increased age:
– Amount of connective tissue
increases in muscles
– Number of muscle cells decreases
• Loss of muscle mass with age:
– Decrease in muscular strength is
50% by age 80 (sarcopenia = “muscle wasting”)
What happens with age?
• Can your body produce new muscle cells?
• NO. -Or very few.
– Not many functional stem cells in muscle tissue
The Body’s Capacity for Regeneration:
Adult Stem Cells
– Good to excellent:
• Epithelial tissue, bone, dense irregular connective
tissue, blood, fat
– Moderate:
• Smooth muscle, dense regular connective tissue
– Weak:
• Skeletal muscle, cartilage
– None or almost none:
• Cardiac muscle, nervous tissue
Human stem cells, S. Huerta
What can you do about this?
• Exercise is Key!!!
-For maintaining healthy muscles,
bones, & joints well into old age!
USE IT OR LOSE IT…
Effects of Exercise on the Muscles
• So how do you gain more (or maintain)
muscle? Especially, when you’re fighting
an uphill battle with age?
• Strength training, lifting weights,
& core exercises (3X week+)
• Healthy balanced diet (with
protein!)
• Proper recovery time & rest
• This prevents loss of muscle
cells, & increases the strength & size of the individual cells
How does this happen?
• With strength/weight training exercise:
–Tiny microscopic tears occur
–An inflammation & healing
response triggers the growth
of more myofilaments &
myofibrils that pack within
the muscle cell
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–The soreness that results:
Delayed Onset Muscle
Soreness (DOMS)  The Good Pain!
–The effect  Stronger, bigger muscles (that keep working!)
What kind of exercise?
• ALL KINDS – those who do multi-sports, cross-training, switching
between activities, develop greater muscle strength, overall fitness,
& keep the muscles constantly adjusting & being challenged
• Also, have less injuries due to wear-and-tear of doing one
repetitive sport
Questions…?
What’s Next?
Lab: Skeletal muscles
Wed Lecture: Muscles of the Body
Wed Lab: Finish Muscles