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Skeletal Muscle TissueSkeletal Muscle Tissue
Human AnatomyHuman AnatomySonya Schuh-Huerta, Ph.D.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:– 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!
Skeletal Muscle Tissue
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
Muscle fiber(1 cell)
Fascicle (wrapped byperimysium)
Epimysium
Tendon Muscle fiberin middle of a fascicle
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)
Originby directattachment
Muscle contracting
Insertion byindirect attachment
Brachialis
Tendon
Muscle Attachments
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
Photomicrograph: Skeletal muscle at 300x
Nuclei
Striations
Muscle Fiber (cell)
Huge, long cylindrical cells 10–100 m in diameter many centimeters long
Multinucleated cells formed by fusion of many embryonic cells
Striations visible
Microscopic Structure of the Muscle FiberReview
Nucleus
Sarcolemma
Myofilament
I band I bandA band
Thin (actin)filament
Thick (myosin)filament
M lineSarcomere
Myofibril
Microscopic Structure of the Muscle Fiber
=> “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
– 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
MovementThin (actin)
filament
Thick (myosin)filament
Myosinhead
The Sliding Filament Mechanism of Contraction
Ca2+
ATP
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
Nucleus
Nerveimpulse
Myelinated axonof motor neuron
Axon terminal of neuromuscular junction
Sarcolemma ofthe muscle fiber
1 Nerve impulse stimulates the release of the neurotransmitter acetylcholine (ACh) into the synaptic cleft.
Axon terminalof motor neuron
Synaptic vesiclecontaining ACh
Muscle fiber
Synapticcleft
SarcoplasmicReticulum
Ca2+
t tubule
Neuron
The Neuromuscular Junction
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!
2
Motor UnitsSpinal cord
Motor neuroncell body
Muscle
Branching axonto motor unit
Nerve
Motorunit 1
Motorunit 2
Musclefibers
Motor neuronaxon
Axon terminals atneuromuscular junctions
(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 – Obtain energy from aerobic metabolic reactions– Contain a large number of mitochondria– Richly supplied with capillaries– Contract slowly & resistant to fatigue– Fibers are small in diameter
Dark meat?
=> O2 store
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 undergo cell division (to increase number) and enlarge.
Embryonicmesoderm cells
MyoblastsMyotube(immaturemultinucleatemuscle fiber)
Satellite cell
Mature skeletalmuscle fiber
Several myoblasts fuse together to form a myotube.
Myotube matures into skeletal muscle fiber.
1 2 3
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
–The soreness that results:
Delayed Onset Muscle
Soreness (DOMS) The Good Pain!
–The effect Stronger, bigger muscles (that keep working!)
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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 musclesWed Lecture: Muscles of the Body Wed Lab: Finish Muscles & Review