Download Chapter 9 Muscle

Chapter 9 Muscle
Functions of Skeletal Muscle
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movement
maintain posture
stabilize joints
heat production
Gross anatomy
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muscle cell
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= muscle fiber
myofibrils
fascicle
many myofibrils
cylindrical units within each cell
contain sarcomeres
= bundle
many cells
muscle
many fascicles
microanatomy
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sarcolemma
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T tubules
cell membrane
run deep into SR
stores Ca++
sarcoplasmic reticulum
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surrounds myofibrils
terminal cisternae
sarcoplasm
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= cytoplasm
glycogen
myoglobin
sarcomere
unit of contraction
sarcomere
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unit of contraction
many along length of myofibril
Z disc
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“striations”
ends of sarcomere
A band (dark)
myosin and overlapping actin
I band
non-overlapping actin
(light)
contractile proteins
actin ; myosin
contractile proteins
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thick filament
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myosin
tail
shaft
heads
2 binding sites for actin
thin filament
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actin
helical chain of actin molecules
myosin binding sites
tropomyosin
covers actin – myosin binding sites
troponin
binds tropomyosin to actin
has Ca++ binding sites
Contraction of Sarcomere
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sarcomere gets shorter ; not the filaments
Sliding Filament Model
actin and myosin slide past each other
Z discs get closer = sarcomere shortens
Ca stimulates filament sliding
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Ca binds to troponin
troponin changes shape
pulls tropomyosin off the actin
exposes actin
actin and myosin bind
Who does what to who ?
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myosin vs ADP
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w/ ADP
myosin extended
w/o ADP
myosin bends
actin vs ATP
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ATP
knocks actin off myosin
Actin
knocks ADP off myosin
Ca
actin and myosin bind
put it together
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resting
myosin extended w/ ADP
stimulus
Ca
cross bridge formation
myosin and actin bind
actin knocks off ADP
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power stroke
w/o ADP, myosin bends
myosin bend pulls actin
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detachment
ATP binds to myosin head
breaks bond with actin
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“cocking” of myosin heads
ATP  ADP + P
ADP extends myosin
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repeats
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some myosin always in contact with actin
cycle repeats several times –
pulling actin further
1 power stroke shortens muscle about 1%
muscles shorten ~ 30 – 35 % of their resting length
continues as long as Ca++ and ATP present
Neuromuscular junction
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= motor neuron + muscle fiber
control cell = neuron
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neurotransmitter
transport
axon terminal
acetylcholine (Ach)
synapse
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target cell
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(post-synaptic cell) muscle
motor end plate
sarcolemma at synapse
• receptors
• action potential
for Ach
sarcolemma
Neuromuscular excitation
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GOAL:
sarcomere contraction
Muscle vs Nerve :
NT depolarizes target cell
effect of action potential:
target cell = nerve
NT release
target cell = muscle
Ca from sarcoplasmic reticulum
Sarcolemma
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polarized
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Na channels
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+ outside / - inside
chemically-gated
at motor end plate
voltage-gated
receptors
sarcolemma, T-tubules
for Ach
motor end plate only
Sarcolemma acts like an axon
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polarized
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Ach opens Na ligand-gated channel
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at rest
+ outside / - inside
Na / K
nature wants ?
depolarization
+ in / - ouside
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Na inside opens voltage-gated Na channels
action potential
entire sarcolemma and T-tubules
same as axon
action potential causes calcium release
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T-tubules next to terminal cisternae
T tubules depolarize
Na+ inside opens voltage gated Ca channels in SR
sarcoplasmic reticulum releases Ca++
excitation-contraction coupling – short version
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neuron stim sarcolemma
action potential
Ca released
sarcomere shortens
excitation-contraction coupling
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neuron depolarizes
depolarization reaches axon terminal
rush of Ca++ into axon terminal
Ca++ causes release of Acetylcholine into synapse
Ach binds with receptors on motor endplate
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Ach opens ligand-gated Na channels
Na+ inside opens voltage-gated Na+
action potential along sarcolemma and T-tubules
Na+ inside causes sarcoplasmic reticulum to release Ca ++
Ca++ binds to troponin
pulls tropomyosin off actin
actin and myosin bind
cross bridge
myosin bends
power stroke
sarcomere shortens
How do you stop this darn thing?
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Acetylcholinesterase destroys Ach at motor end plate
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Na channels close
sarcolemma repolarizes
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Na - K+ pump
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Ca++ pump
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Ca++ into SR via active transport
troponin and tropomyosin cover actin
ATP - detachment
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myosin extends
sarcomere lengthens
ATP uses
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Na pump
Ca pump
break myosin and actin bond
Motor unit
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motor neuron + muscle cells it stimulates
1 neuron has several branches of its axon
strength of contraction
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more motor units =
stronger contraction of muscle
recruitment
control of movements
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small motor units (4-10)
fine control
fingers
large motor units (100)
poor control
thigh
alternation
one stimulus
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twitch = single contraction due to a single stimulus
3 parts:
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latent period
time from excitation to contraction
no change in myogram
~ 3 ms
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contraction
begin contraction to max. force
myogram increases
~ 10-100 ms
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relaxation
myogram decreases
sarcomere relaxes
~ 10-100 ms
myogram
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recording of muscle activity
tension , not voltage
muscles vary in speed and length of twitch
size of motor unit
repeated stimuli
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graded response =
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wave summation
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varied strength of contraction
number of motor units
repeat stim w/o full relaxation
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2 twitch is a stronger contraction (summed)
increased Ca++ available
tetanus:
smooth, sustained contraction
normal muscle contraction
stronger stimuli
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motor unit summation
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in vivo:
more neurons
lab:
more electricity (mV)
recruitment
threshold stimulus
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minimum stim to cause contraction
maximal stimulus
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strongest stim that increases force of contraction
all motor units recruited
muscle tone
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muscle tone
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slight, constant contraction of all skeletal muscles
posture
stabilize joints
heat production
treppe
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“warming up”
treppe
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gradual increase strength of 1st few contractions
increase Ca++
and enzyme activity
length - strength
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resting length vs strength
ideal resting length
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80 – 120 % of resting length
too short
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sarcomeres already short
too long
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actin and myosin too far away
force of muscle contraction
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affected by:
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recruitment
size of muscle fibers
wave summation (frequency)
muscle length
types of muscle contraction
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muscle tension
force
load
weight of object (bone)
isometric contraction
tension w/o movement
isotonic contraction
tension w/ movement
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concentric
tension while shortening
eccentric
tension while lengthening
Energy production
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stored ATP
3 – 4 seconds
creatine phosphate (CP)
10 – 15 sec
– creatine-P
+ ADP  creatine + ATP
CPK
new ATP:
– glycolysis
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anaerobic respiration
• fastest, but only 2 ATP made
• strenuous activity
30 – 40 sec
• when decreased O2 and blood flow
• lactic acid
– aerobic respiration =
cell respiration
• glucose + O2  ATP + CO2 + H2O + heat
• 36 ATP made
• mild, or prolonged activity
fuel for ATP
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fatty acids
main source at rest
glycogen
stored in muscle
moderate to heavy exercise
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glucose
blood
minimal source in muscle
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pyruvic acid
liver converted lactic acid
replaces ATP after exercise
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oxygen
myoglobin
hemoglobin
effects of ATP use
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fatigue
low ATP
lactic acid - low pH inhibits enzymes
inability to contract
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contractures
lack ATP
can’t detach cross-bridges
decrease blood flow
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Oxygen debt
replace O2 stored in myoglobin
O2 for lactic acid  pyruvic acid
increased respiratory rate
decreased pH
CO2 , lactic acid
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increase body temp
types of skeletal muscle fibers
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slow oxidative fibers (type I)
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aerobic (cell respiration)
myoglobin ; mitochondria
slow , prolonged contraction
little fatigue
(red)
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fast glycolytic fibers (type II x)
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anaerobic
little myoglobin or mitochondria
(pale)
fast contraction ; quick fatigue
fast oxidative fibers (type II a)
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(pink)
intermediate speed, strength, and fatigue
Exercise
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endurance exercise
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aerobic
increase mitochondria , myoglobin , capillaries
slow, oxidative fibers
less fatigue
no increase mass
resistance exercise
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increase myofibrils , not # muscle cells
= hypertrophy
stores glycogen
split ends theory
atrophy
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disuse
nerve damage
homeostatic responses to exercise
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vasodilation
increase O2 , glucose
remove CO2
increase heart rate
same
increase respiration
remove CO2 , raise pH
remove heat
repay oxygen debt
acid-base mechanisms
remove H+ , raise pH
sweat
decrease body heat
thirst
replace water loss
smooth muscle
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no sarcomeres
network of sliding thick and thin filaments
Ca++ stim sliding
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SR and caveoli (pouch of extracellular Ca++)
gap junctions
no fatigue
slow contraction
low ATP requirement
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neural stim
acetylcholine
norepinephrine
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other controls
α ß
hormones
O2 , CO2 , pH
histamine , paracrines
cardiac muscle
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see heart chapter
diseases
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Muscular Dystrophy
myasthenia gravis
atrophy