Muscular System

Question 1

Overview of muscle tissue

Answer 1

• Types: Different cells, location,
different control
1. Skeletal muscle
2. Smooth muscle
3. Cardiac muscle
• Terminology: muscle fibers,
“myo”, “mys”, “sarco” pertain to
muscle

Question 2

Properties of skeletal muscle tissue

Answer 2

• Electrically excitable
– Respond to certain stimuli by
producing action potentials
– Stimulated by chemical
neurotransmitters released by
nervous system
• Respond with mechanical contraction
– Develops tension as proteins slide past each other

Question 3

Skeletal muscle tissue

Answer 3

• Location: (general)
– In skeletal muscles attached to
skeleton
– Most abundant
• Cells: very large! (muscle fibers)
– long and slender
– multinucleated
– striated (striped); dark- light bands
– Amitotic
• Voluntary control
– Subject to conscious control
• Somatic motor nervous system
– Some subconscious control:
diaphragm, reflexes, postural
muscles

Question 4

Location

Answer 4

Attached to skeleton

Question 5

Control

Answer 5

Voluntary- somatic
motor nervous system

Question 6

Shape of fibers (cells)

Answer 6

Elongated, cylindrical,
blunt ends: FIBERS

Question 7

Striations

Answer 7

Yes

Question 8

of nuclei/cell

Answer 8

Many

Question 9

Location of nuclei

Answer 9

Periphery

Question 10

Function

Answer 10

Movement, heat,
posture

Question 11

Functions of skeletal muscle

Answer 11

• Composition of muscle as organ:
– Skeletal muscle tissue, connective
tissue, nerves, blood vessels
• Functions:
– Movement of skeleton
– Maintain body posture
– Support and protect soft tissue
– Guard entrances and exits
– Thermoregulation
– Communication
– Energy storage…lots of proteins

Question 12

Skeletal muscle connective tissue

Answer 12

• Major connective tissue: fascia
– Tough sheet of connective tissue
• Below superficial fascia
(subcutaneous fat layer)
– Functions:
• Anchors to surrounding
tissue
• Separates individual muscles
• Binds together muscle
groups of similar function
• Fills spaces between muscles
• Contains nerves and blood,
lymph vessels supplying
muscle

• 3 more connective tissue
layers internal to fascia
– Epimysium: surrounds
whole muscle
– Perimysium: surrounds
fascicle (fiber bundles)
• “grain” of the meat
– Endomysium: surrounds/
between individual
muscle fibers
• Contains myosatellite
cells
• Nerve supply and extensive
blood vessel branches
throughout

Question 13

Skeletal muscle tissue attachments

Answer 13

• Indirect attachment to
bones: epi-, peri-, and
endomysium come together
– Tendons: bundle
attached to bone
periosteum or fascia of
other muscles
– Aponeurosis: sheet
maybe to >1 bone
• Direct attachments: collagen
fibers fused directly to bone
periosteum or cartilage
perichondrium

Question 14

Nerve and blood supply

Answer 14

• Nervous system controls skeletal
muscle activity
– Somatic motor neurons – nerve
cells from brain/spinal cord that
stimulate skeletal muscle fibers
within muscle
• 1 muscle fiber controlled by
only 1 nerve cell axon (nerve
ending)
• 1 axon controls multiple fibers
• Blood supply is extensive!
– Need rich blood supply!
• Oxygen, nutrients in (artery)
– Make, use lots of ATP!!!
• Waste, heat out (vein)

Question 15

Skeletal muscle and fibers

Answer 15

• Skeletal muscle (organ) with epimysium around it
• Muscle made of fascicles with perimysium around them
• Fascicles made of bundles of muscle fibers (cells) with endomysium
around them

Question 16

Skeletal muscle cells- muscle fibers!!

Answer 16

• Large, multinucleate, non-dividing
– Myoblast fusion forms multinucleated muscle fibers
– Purpose of many nuclei?
– Limited repair possible -myosatellite cells (in endomysium)
Some:
100μm wide
12 in. long !!

Question 17

Skeletal muscle fiber structure

Answer 17

• Sarcolemma: plasma membrane
– Characteristic transmembrane potential, negative ICF vs. ECF
– Electrically excitable cells
• Transverse (T) tubules: invaginations (folding in) of sarcolemma
– Quick spread of action potential (electrical signal) allows for
synchronous muscle fiber contraction

Question 18

Muscle fiber structure

Answer 18

• Sarcoplasm: cytoplasm
– Nuclei!
– Mitochondria (ATP)
– Myofibrils: protein filament bundles
– Glycogen!
– Myoglobin: O2 storage,
pigmented
Muscle fiber structure
– Sarcoplasmic reticulum
• Muscle cell ER
• Stores Ca2+, keeps
cytoplasmic Ca2+ low
• Terminal cisternae:
enlarged SR ends
against T tubules
• Triad: 2 terminal
cisternae +
1 T-tubule

Question 19

Skeletal muscle fiber structure

Answer 19

• Myofibril
– Contractile organelle of
skeletal muscle fiber
– Parallel to each other
– SR and T tubules encircle
– Other organelles squeezed
in between myofibrils
• Myofibril contains 2 types of
myofilaments (protein
filaments)
– Thin filaments w/ actin
– Thick filaments w/myosin

• Sarcomeres of myofibril
– Smallest contractile unit of
skeletal muscle fiber myofibril
– Repeating units of actin and
myosin myofilaments
– Boundaries formed by Z lines
– Dark – light areas = striations

Question 20

Anatomy of a sarcomere

Answer 20

• Sarcomere ends are called Z discs /lines
• Striations within sarcomere:
– Darker area: A band
• Thick myosin filaments present
• H- zone – center of A band
containing only myosin
– M-line: Middle of H zone
• Zone of overlap: thin actin and
thick myosin filaments overlap
– Lighter area: I band
• Z line/disc
• Thin actin filaments
• No thick myosin filaments

• Sarcomere:
– Z disc to Z disc
– 2- ½ I bands
– 1- A band

Question 21

Thin actin filaments

Answer 21

• Actin: contractile protein
– Attached to Z discs
– Actin molecules each
have active site for
myosin binding
• Tropomyosin: regulatory
protein
– In resting muscle: Covers
myosin binding sites on
actin
• Troponin: regulatory protein
with 3 parts for binding:
– Tropomyosin
– Actin
– Ca 2

Question 22

Thick myosin filaments

Answer 22

• Myosin: contractile protein
– Motor protein
– Head and tail
• Tail: interacts with
other myosin
molecules
• Head: two globular
subunits (ATPase)
– “Business end”
– Binds ATP, uses
for energy
– Forms cross-
bridges with actin
during
contraction

Question 23

Structural protein

Answer 23

• Titin (connectin):
– Extends from Z line to M line through thick myosin- stabilizes thick
filament
– Provides some elasticity: like elastic SPRING
Titin

Question 24

Contraction: filaments sliding

Answer 24

• Contraction = cross bridge formation
– Myosin heads bind actin, pull actin
filaments
• Thin actin filaments slide past
thick myosin towards M line
• Amount of overlap changes, but
thin and thick filaments do not
change length!
– H, I bands: become smaller→
disappear
– Zones of overlap – larger
– Z lines- closer
– A band width: same
• Shorter sarcomeres= shorter myofibrils=
shorter muscle fiber= contraction!

Question 25

Requirements
for
contraction

Answer 25

1. Neuron stimulation of
   muscle fiber
2. Generation, spread of
   electrical current (action
   potential) along muscle
   fiber sarcolemma
3. Rise in muscle fiber
   intracellular Ca2+ -triggers
   contraction
   Excitation-contraction
   coupling

Question 26

Transmembrane potential

Answer 26

• Transmembrane potential:
– Uneven (+/-) charges across sarcolemma
membrane - negative inside sarcoplasm vs
outside
– Muscle fiber stimulated by neuron activity: Na+
moves in, K+ moves out = change in
transmembrane potential
– Excitation (action potential produced)-
contraction coupling

Question 27

Neuromuscular junctions

Answer 27

• Neural control of skeletal muscle
contraction
– Axon of somatic motor neuron in
brain, spinal cord extends out to
skeletal muscle fiber
• Action potential (electrical
signal) travels along axon of
neuron
• Converted to chemical signal
(ACh) to stimulate muscle fiber
• Produces action potential
(electrical signal) in muscle fiber
which results in contraction
– Neuromuscular junction (NMJ):
• Communication site between
neuron- muscle cell

• Neuromuscular junction (NMJ): (type of synapse- communication site
between neuron and another cell (ie. muscle fiber, neuron))
1. Axon terminal (neuron)
• Synaptic vesicles with chemical neurotransmitter: acetylcholine (ACh)
2. Synaptic cleft: small liquid-filled space between neuron and muscle fiber
3. Motor end plate (muscle cell sarcolemma)
• One muscle fiber controlled by only one motor neuron
• Sarcolemma in folds

Question 28

Muscle excitation

Answer 28

• Excitation: electrical
change (action
potential) in the
neuron axon leads
to action potential in
the muscle fiber
1. Neuronal action
potential (electrical
signal) arrives at
neuron’s axon
terminal:
– Causes Ca2+ entry
into terminal

2. Ca2+ entering neuron axon terminal causes release of acetylcholine
   (chemical signal) from neuron axon terminal into cleft
   – Electrical signal (AP) has been turned into chemical signal (ACh)
   – ACh diffuses across synaptic cleft to motor end plate
   – ACh binds to ACh receptors on protein ion channel in muscle fiber
   sarcolemma (motor end plate)
3. When Ach binds sarcolemma protein ion channel (type of ligand-gated
   ion channel) –channel open
   – A lot of Na+ moves in, some K+ moves out – why?
   – Extracellular Na+ > intracellular Na+ inside muscle fiber SO………… Na+
   rushes into sarcoplasm!! = depolarization (more + charge inside fiber)
4. Na+ entry starts action
   potential (AP) in sarcolemma
   – Na+ entry causes more
   voltage sensitive Na+
   channels to open
   – Forms AP and spreads along
   sarcolemma (propagation)
   • Continues in one
   direction as more
   voltage sensitive ion
   channels open
   – AP spread (excitation) will
   lead to sliding of actin and
   myosin filaments
   (contraction): Excitation-
   contraction coupling!! (stay
   tuned)

Question 29

Ca2+ : the coupler!

Answer 29

• Both Ca2+ and energy (ATP) needed for contraction
• Calcium in general
– Intracellular cytoplasmic [Ca2+] kept low
– Stored in sarcoplasmic reticulum (SR)….terminal cisternae
– Important for muscle contraction: released from terminal cisternae into
sarcoplasm

Question 30

Excitation-
contraction
coupling

Answer 30

• AP moves along sarcolemma and
down T-tubules
– AP causes Ca2+ channels in
sarcoplasmic reticulum to
open
– Ca2+ released from SR
terminal cisternae
– Large amount of Ca2+ enters
sarcoplasm at zones of
overlap
• Excitation coupled to
contraction: effect of Ca2+

Question 31

Muscle fiber
contraction

Answer 31

• Resting sarcomere before
contraction starts
– Myosin head energized
(cocked position)
– Energy stored in head as
a result of prior ATP
breakdown
• Will be used to
power contraction
– Tropomyosin covers
myosin binding sites on
actin

1. Contraction cycle start
   – Ca2+ from SR terminal cisternae
   released within zone of overlap
2. Active sites exposed
   – Ca2+ binds troponin
   • Changes troponin shape
   • Tropomyosin rolled off
   myosin binding sites on
   actin
   • Allows actin and myosin to
   interact
3. Cross bridge formation:
   “attach”
   – Myosin heads (which
   were in cocked position
   from previous cycle) bind
   to exposed active sites on
   actin
   – Form cross-bridges
4. Powerstroke: “pull”
   – ADP and Pi released
   – Cocked myosin head
   pivots
   – Pulls on actin, slides actin
   past myosin filaments
   towards M line
5. Cross bridge detaches:
   “release”
   – Another ATP binds to
   myosin to cause cross-
   bridge release
   – Myosin and actin link
   broken
   – If active sites on actin still
   exposed: bind another
   myosin
6. Myosin reactivation: “reset”
   – ATP split by ATPase in
   myosin head →energy
   used to re-cock myosin

Question 32

End of muscle excitation: relaxation

Answer 32

• Neural stimulus ends
• AChE breaks down ACh
• Membrane returns to normal
resting transmembrane
potential
End of muscle excitation: relaxation

Question 33

Relaxation

Answer 33

• SR Ca2+ channels close, Ca2+ returns to SR
– Ca2+ pumped (requires ATP!) into
terminal cisternae
– Takes longer than release: relaxation
takes longer than contraction
• Troponin-tropomyosin return to normal
– Active sites covered, no cross bridges
• Rigor mortis?
• Note: ATP needed for contraction and
relaxation!

Question 34

Tension –
length
relationship

Answer 34

• Starting sarcomere length affects tension (force of contraction) produced
• Affects number of cross bridges that can form
• Optimal starting sarcomere length? – 100% resting length and moderate
range around (80-120% resting length) -allows optimal overlap of filament
• Starting sarcomere length too short or too long? – can not form optimal #
of cross bridges, affects filaments sliding/tension

Question 35

Twitch

Answer 35

• Understand contraction of muscle fiber
to understand contraction of skeletal
muscle!!
• Muscle twitch: contraction of muscle
fiber (or skeletal muscle)
– In response to motor neuron
stimulation
– Produces tension: pulling force
• Measure of force of muscle
contraction
– Single twitch: not enough tension to
perform work (ie. move object
(load))

Question 36

Twitch contraction in a muscle fiber

Answer 36

• Latent period
– No tension yet but excitation!
– AP spreads, SR releases Ca2+
• Contraction period
– Tension (force of contraction)
peaks
– Ca2+ binding troponin
– Myosin binding sites on actin
exposed- cross bridges form
– If tension> load, muscle
shortens
• Relaxation period- longer
– Ca2+ pumped into SR
– Cross bridges detach,
tropomyosin covers active sites
– Tension returns to resting

Question 37

Factors that affect
fiber tension

Answer 37

• Amount of tension produced by 1
muscle fiber varies, depends on:
– Frequency of stimulation –
affects Ca2+ in cytosol
– How stretched muscle was
before stimulation: length-
tension relationship
• Resting produces optimal #
of x-bridges
– Temperature: “warming up”
benefits

Question 38

Frequency of stimulation

Answer 38

• Repeated stimulation (frequency of stimulation) of muscle fiber by motor
neuron results in twitches with greater tension: Wave summation
– Waves of contraction add together
– No time to pump all Ca2+ back into SR, Ca2+ increases with each stimulus
• Types of wave summation:
– Incomplete (unfused) tetanus: lower rate of stimulation, partial
relaxation occurs
– Complete (fused) tetanus: high rate of stimulation, no relaxation
before next stimulation

Question 39

Skeletal muscle
tension

Answer 39

• Amount of tension in whole muscle,
depends on:
– Amount of tension in individual fibers
(frequency of stimulation)
– # of muscle fibers contracting at same
time (recruitment)

Question 40

Motor
units

Answer 40

• Motor unit: Somatic motor neuron + all muscle fibers it controls (more
than 1 fiber!)
• Fibers not clustered together- spread throughout entire muscle
• Motor units differ in # of muscle fibers
– Size of motor unit reflects control
• Few fibers= precise control, less force (eye)
• Many fibers= less control, more force (leg)
• In 1 muscle = mixture of motor units

Question 41

Motor unit
recruitment
and tension

Answer 41

• Motor unit recruitment:
increase in # of active motor
units
– Lifting a feather – few
motor units recruited
– Lifting a box of books –
more motor units
stimulated

Question 42

Muscle relaxation

Answer 42

• Remember what happens at muscle fiber level!
• Muscle returns to original length:
1. Elastic forces
• Recoiling
• Pull of tendons, ligaments
2. Opposing muscle contractions: antagonistic pairs
3. Gravity

Question 43

Energy for muscle activity at rest

Answer 43

• Resting muscle fibers:
– Low ATP demand – some ATP storage but not significant (5-6 sec)
– Fatty acids used
– Some glucose stored as glycogen
– ATP used to convert creatine to creatine phosphate: quick “energy”
storage

Question 44

Energy for peak activity (max exertion)

Answer 44

• Glucose = energy source
• 5-6 seconds (like 50m dash) – immediate energy needed!
– Immediate energy source: ATP and Creatine phosphate (CP)
• 50-60 seconds (like 400m run) – short term, quick energy needed!
– High demand for ATP, but oxygen diffusion into mitochondria slow
– Most from CP and anaerobic glycolysis, little from aerobic metabolism
– Drawbacks: ↑ lactic acid produced, pH ↓, inefficient ATP production,
rapid fatigue results

Question 45

Energy for moderate activity

Answer 45

• Mins- hrs (moderate pace long run)- long term, continual energy needed
– Increased demand for ATP, no excess ATP made
– Mitochondria meets demand if O2 supply sufficient – aerobic
metabolism of glycogen!
– O2 consumption ↑, blood flow to muscles ↑ to meet demand
– If glycogen low, other nutrients used
– Glycogen, lipids and amino acid depleted? = muscle fatigue

Question 46

Muscle fatigue

Answer 46

• Muscle contraction can not
continue even with nervous
stimulation
– Not well understood!- CNS or
muscle level?
– Lactic acid, H+ build up: pH ↓
– Ca2+ release and uptake
– Decreased O2 available
– Energy sources depleted (CP,
glycogen, other substrates)
– Hyperthermia
– Psychological basis

Question 47

Muscle recovery

Answer 47

• Return body to pre-exercise conditions
– Lactic acid recycled to pyruvate – make ATP, rebuild glycogen
– Excess post-exercise oxygen consumption (EPOC) – “O2 debt”
• Increased amount of O2 inhale to help return
• The more ATP required for recovery, the more O2 needed
• Correct blood pH: lactic acid recycled, CO2 exhaled
• Replace O2 on hemoglobin in blood, myoglobin in muscle
– Heat elimination
• Delayed onset muscle soreness (DOMS) – small muscle tears? Injury to
tendons, body adapts after and becomes stronger

Question 48

Skeletal muscle fiber types

Answer 48

• Typed by:
– Time to reach peak tension (fast vs. slow)
– Major metabolic pathways for forming ATP (glycolytic vs. oxidative)
• Types:
– Fast glycolytic: (fast fibers)
• WHITE muscle fibers
– Slow oxidative (slow fibers)
• RED (dark) muscle fibers

• Red Slow-oxidative
– Slow contraction
– Small diameter fibers- fewer myofibrils
– Less forceful contractions
– More mitochondria-red
– Extensive blood supply- dark
• More O2 - Aerobic metabolism
– More myoglobin: red
• Red-colored O2-binding/ releasing
protein
– Slow to fatigue: prolonged sustained
contractions for hours= endurance
• Maintain posture, marathon running
(endurance)

• White Fast-glycolytic
– Majority in body
– Largest diameter, most
myofibrils: powerful
– Faster contractions
– Less myoglobin, fewer blood
vessels, few mitochondria
– Lots of glycogen for
glycolysis: anaerobic
– Fatigue easily
– Best suited for strong,
powerful, short contractions
• Anaerobic exercise:
weight lifting, sprinting,
swing bat

Question 49

Muscles and fiber types

Answer 49

• White muscles: pale
– Fast fibers more abundant
– Speed
– Turkey breast “white meat”
• Red muscle: reddish
– Slow fibers more abundant
– Endurance
– Turkey legs “dark meat”
• Humans:
– One muscle usually mixture
– % genetically determined
– But fibers in motor unit = SAME!… Refer back to motor unit
recruitment
– Aerobic physical conditioning can affect muscle fiber performance

Question 50

Physical conditioning

Answer 50

• Anaerobic conditioning
– Short duration, high intensity,
frequent: i.e. weight lifting, sprint
training
– Promote strength, speed and power
– Contraction of fast-glycolytic fibers
– Adaptations cause muscles to get
bigger (hypertrophy) – due to ↑ # of
fiber myofibrils NOT fibers!
?
• Aerobic conditioning
– Longer activity, moderate intensity, endurance training (ie. marathon)
– Must be supported by oxidative phosphorylation (O2, mitochondria)
– Adaptations to support metabolism, some fast fibers physiologically be
more like slower fibers
– No hypertrophy