Download How Muscles Contract

Chapter 9
The Muscular System
Skeletal Muscle Structure
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Tendon – connect
muscle to bone
Fascia – outermost
covering; covers entire
muscle & continuous
w/tendon; separates
muscle from adjacent
muscles
Aponeuroses- connect muscle to
muscle
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Skeletal Muscle Structure
Coverings:
 Epimysium – covers entire muscle
(under fascia)
 Perimysium – covers
muscle bundle (fascicle)
 Endomysium –
covers each fiber (cell)
 Sarcolemma – cell
membrane
Skeletal Muscle Structure
Skeletal Muscle Structure – Cont.
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Sarcoplasmic reticulum (SR) channels for transport
Myofibrils – threads that compose muscle fibers; contain
protein filaments:
1. actin – thin
2. myosin – thick
Skeletal Muscle Structure
Muscle Fiber (muscle cell)
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Cisternae of SR –
enlarged portions
Transverse tubules
(T-tubules) –
important
in muscle contraction
Sarcoplasm – cytoplasm
Breakdown of Skeletal Muscle
Parts of a Sarcomere (functional
unit of a muscle)
Parts of a Sarcomere
•Z lines – end points
• M line – middle
• I band – on either
side of Z line; actin
filaments only
• H zone – on either
side of M line;
myosin filaments only
• A band – overlapping
actin & myosin filaments
Parts of a Sarcomere
Neuromuscular Junction – junction
b/t motor neuron & muscle
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Motor neuron – carries
impulse from brain or
spinal cord
to muscle
Motor end plate – end
of muscle fiber; many
nuclei & mitochondria located here
Neuromuscular Junction
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Neurotransmitters
(ntm) chemicals that
help carry impulses
Motor unit – 1 motor
neuron & fibers that
it stimulates
Synaptic vesicles –
store neurotransmitter;
most common – acetylcholine (ACh)
Electron Micrograph
Neuromuscular Junction
Neuromuscular Junction
Animation
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Neuromuscular Junction Animation
4 Proteins in Muscle Cells:
Troponin & Tropomyosin
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4 proteins are found in muscle cells: actin, myosin,
troponin & tropomyosin
troponin – appear
as globules; provide
a binding site for Ca+²
tropomyosin –
appear as ribbons;
cover the myosin
cross-bridge
binding sites in a
relaxed muscle
Sliding Filament Theory
(How Muscles Contract)
• Muscle fiber stimulated by release of ACh
from synaptic vesicles of neuron
• ACh causes impulse to travel to muscle cell
membrane
• Transverse tubules (T-tubules) carry impulse
deep into muscle fibers
• Sarcoplasmic reticulum releases Ca ions
(Ca²+)
• Ca²+ bind to troponin, tropomyosin moves,
exposing binding sites on actin filaments
Cross Bridge Animation
• cross bridge animation
Sliding Filament Theory
(How Muscles Contract )
• Linkages form b/t actin & myosin
• Actin filaments move inward,
shortening the sarcomere
• Muscle fiber relaxes when Ca²+ are
transported back to S.R.
• The enzyme cholinesterase (or
AChesterase) decomposes ACh
Sliding Filament Theory
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Relaxed muscle – binding sites on actin
are covered by tropomyosin
Sliding Filament Theory
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Ca²+ binds to troponin
Tropomyosin
slides out of the way
Myosin binds to actin &
pulls inward
Sarcomeres
shorten & muscle
contracts
Sliding Filament Animation
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sliding filament animation
Sliding Filament Theory
Energy for Muscle Contraction
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ATP (adenosine triphosphate)
provides the energy for muscle contraction
When ATP is converted to ADP (adenosine
diphosphate) by losing the last phosphate,
energy is released.
Energy for Muscle Contraction
• Cells depend on cellular respiration
of glucose to synthesize ATP
•An additional source is creatine phosphate
Energy for Muscle Contraction
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Creatine phosphate stores excess
energy
Can be used to convert ADP back into
ATP
Anaerobic respiration (in the
absence of O2) provides few ATP’s,
while aerobic resp. (in the presence of
O2) provides many ATP’s
Creatine Phosphate
High amts. of ATP - ATP is used to
synthesize CP, which stores energy
for later use.
Low amts. of ATP – CP is used
to resynthesize ATP.
Importance of Myoglobin
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l.a. carried by blood to
liver; liver can convert
l.a. to glucose, but
requires ATP (ATP being
used for muscle contraction)
myoglobin – stores O2
in muscle cells; gives
muscle its red color
Aerobic vs. Anaerobic
Respiration
Aerobic vs. Anaerobic Respiration
Carried by blood to liver;
liver can convert l.a. to
glucose, but requires ATP
(ATP being used for
muscle contraction)
Imp. b/c blood supply
during muscle contr.
may decrease
As l.a. accumulates, O2 debt occurs
Oxygen Debt
•Strenuous exercise leads to O2 deficiency &
lactic acid buildup
•ATP provides energy for muscle contraction
•Amt. of O2 needed to convert accumulated
l.a. to glucose & restore ATP levels = O2 debt
•L.A. accumulation leads to muscle fatigue
b/c pH of muscle cell is lowered & muscle
cannot contract
Muscle Cramp
•Muscle cramp – fatigued muscle has lack of ATP
needed to move Ca+² back into S.R.; cross
bridges not broken
•Rigor mortis – takes up to 72 hrs. to occur;
sarcolemma becomes more permeable to Ca+² &
ATP levels insufficient
Myogram
•Pattern or graph of
a muscle contraction
•A single contraction is
called a muscle twitch
•3 parts:
•Latent (lag) phase –
brief pd. of delay
b/t when the stimulus is applied & actual
contraction occurs
•Contraction
•Relaxation – return to
original state
Patterns of Contraction
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a) Muscle Twitch –
single contraction
b) Staircase Effect
many stimuli closely
spaced w/complete
relaxation in b/t; each
contraction generate
incr. force
Patterns of Contraction
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c) Summation – when
the 2nd stimulus occurs
during the relaxation
pd. of 1st contr.; the
2nd contr. generates
more force
d) Tetany – when
twitches fuse into
1 sustained contr.
Muscle Facts
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If a muscle is stimulated twice in quick
succession, it may not respond the 2nd time –
called refractory period
Threshold – the minimum stimulus needed
to cause a contraction
All-or-none – increasing the strength of the
stimulation does NOT incr. the degree of
contraction (a muscle contracts completely or
not at all)
More Facts
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Incr. stimulation from motor neurons causes
a greater # of motor units to contract & vice
versa
Called recruitment of motor units
Incr. the rate of stimulation also incr. the
degree of contraction
Muscle tone – a sustained contraction
caused by nerve impulses from s.c. to a small
# of muscle fibers in the back, neck, etc.;
maintains posture
Origin & Insertion
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Origin – end of muscle
that attaches to stationary
bone
Insertion – end of muscle
that attaches to moving
bone
During contr., insertion
is pulled toward origin
Muscle Functions in
Groups
Prime mover – responsible
for most of the movement
(ex.- biceps)
Synergist – aids the prime
mover
Antagonist – resists the
prime mover & causes
movement in the opposite
direction (ex. - triceps)
Structural Differences of
3 Types of Muscle
Skeletal Muscle
Smooth Muscle
Cardiac Muscle
Cells elongated
w/multiple
nuclei/cell
Cells spindleshaped w/1
nucleus/cell
Cells branching
w/1 nucleus/cell
T-tubules present
No T-tubules
T-tubules lg.;
releases lg. amts.
of Ca++; can
contract longer
(Ca channel
blockers)
Striated/voluntary Non-striated/invol.
Striated/invol.
Functional Differences of
3 Types of Muscle
Skeletal Muscle
Smooth Muscle
Cardiac Muscle
Needs nerve impulse for
contraction
Displays rhythmicity &
cells stimulates each
other (as in peristalsis)
Displays rhythmicity &
self-excitation
Ca+² binds to troponin
Ca+² binds to
calmodulin
Ca+² binds to troponin
Not affected by
hormones
Hormones may affect
contraction
Hormones may affect
rate of contr.
Contracts & relaxes
rapidly
Slower to contract but Contracts & relaxes at a
can maintain contraction
certain rate
longer
Functional Differences Continued
Skeletal Muscle
Not affected by
stretching
Smooth Muscle
Cardiac Muscle
Stretching of fibers
Remains in a
may stimulate
refractory pd. until
contr.
contraction ends
(tetany won’t
(ex.-stomach)
occur)
Fast Twitch vs. Slow Twitch
Muscle
Fast Twitch
Slow Twitch
Contracts quickly, tires
easily (sprinter)
Fewer mitochondria
Contracts slowly, tires
slowly (long distance)
More mitochondria
Less myoglobin
More myoglobin
White muscle
Red muscle
Composes smaller
muscles (eyes, hands,
etc.)
Composes lg. muscles
(legs, back, etc.)
Levers
• Parts of a lever:
wt., force, pivot
3 types of levers:
• 1st class – W-P-F
(seesaw/scissors)
• 2nd class – P-W-F
(wheelbarrow)
• 3rd class – W-F-P
(forceps)
Bones & Muscles as Levers
• Forearm bends – 3rd
class lever (biceps
attaches at a pt. on
the radius below the
elbow joint)
• Forearm straightens
- 1st class lever
((triceps attaches at a
pt. on the ulna
above the elbow joint)
Bones & Muscles as Levers
Standing on tip-toe –
2nd class lever
(P-W-F)