Human Anatomy
Sonya Schuh-Huerta, Ph.D.

• 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!
Coach Sean

• Each muscle is an organ!
– Consists mostly of muscle tissue
– Skeletal muscle also contains:
• Connective tissue
• Blood vessels
• Nerves

• 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

• 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
Tendon
Epimysium
Muscle fiber in middle of a fascicle
Fascicle (wrapped by perimysium)
Muscle fiber
(1 cell)
Muscle Endomysium
(between individual muscle fibers)

• Nerves & blood vessels:
– Each skeletal muscle supplied by branches of
• 1 nerve
• 1 artery
• 1 or more veins

Capillary Network Surrounding Skeletal
Muscle Fibers

• 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 )

• 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 contracting
Origin by direct attachment
Brachialis
Tendon
Insertion by indirect attachment

– 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…

Microscopic Structure of the Muscle Fiber
Review
 Huge, long cylindrical cells

10 –100 m m in diameter
 many centimeters long
 Multinucleated  cells formed by fusion of many embryonic cells
Striations
Nuclei
Muscle
Fiber (cell)
 Striations visible
Photomicrograph:
Skeletal muscle at 300x

Microscopic Structure of the Muscle Fiber
Thin (actin) filament
Thick (myosin) filament
Sarcolemma
I band A band
Sarcomere
I band M line
Nucleus
Myofibril
Myofilament
=> “Bag of pretzels”

• 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 Ca 2+ from the SR!
– Powered by ATP!

The Sliding Filament Mechanism of
Contraction
Thin (actin) filament Movement Ca 2+
ATP
Myosin head
Thick (myosin) filament

The Sliding Filament Mechanism of
Contraction

• 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
 Ca 2+ & ATP
 myosin binds
 ATP  ADP
& P i
 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!

• 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

• 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

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.

Synaptic cleft
Neuron
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 Ca 2+ to be released from the SR!
t tubule
Sarcoplasmic
Reticulum
Muscle fiber
Ca 2+

Spinal cord
Motor unit 1
Motor unit 2
Axon terminals at neuromuscular junctions
Branching axon to motor unit
Muscle
Nerve
Motor neuron cell body
Motor neuron axon
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  ).

• 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

• 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

• Slow oxidative fibers
(slow twitch)
– Red color due to abundant myoglobin => O
2
– Obtain energy from aerobic metabolic reactions store
– Contain a large number of mitochondria
– Richly supplied with capillaries
– Contract slowly & resistant to fatigue
– Fibers are small in diameter
Dark meat?

• 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?

• 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

• Predominately powered by oxidation of fats & carbs
• Fast twitch muscles  operate with little O
2
(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)

• 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)

• 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….

• 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 develops from myoblasts
– Myoblasts fuse to form skeletal muscle fibers
– Skeletal muscles contract by the 7 th week of development
Embryonic mesoderm cells
Myoblasts
Myotube
(immature multinucleate muscle fiber)
Satellite cell
1
Embryonic mesoderm cells undergo cell division
(to increase number) and enlarge.
2
Several myoblasts fuse together to form a myotube.
3
Myotube matures into skeletal muscle fiber.
Mature skeletal muscle fiber

• 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

• 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”)

• 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

• Exercise is Key!!!
-For maintaining healthy muscles, bones, & joints well into old age!
USE IT OR LOSE IT…

• 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

• 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:
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Delayed Onset Muscle
Soreness (DOMS)

The Good Pain!
–The effect 
Stronger, bigger muscles (that keep working!)

• 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