Download How Muscles Contract

Chapter 9
The Muscular System
Skeletal Muscle Structure
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Tendon –
Fascia – outermost
covering; covers entire
muscle & continuous
w/tendon; separates
muscle from adjacent
muscles
Skeletal Muscle Structure
Coverings:
 Epimysium – covers entire muscle
(under fascia)
 Perimysium –
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Endomysium –
covers each fiber (cell)
Sarcolemma-
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 –
2. myosin –
Skeletal Muscle Structure
Muscle Fiber (muscle cell)
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Cisternae of SR –
enlarged portions
Transverse tubules
(T-tubules) –
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 –
• I band – on either
side of Z line; actin
filaments only
• H zone –
•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
Synaptic vesicles –
store neurotransmitter;
most common – acetylcholine (ACh)
Electron Micrograph
Neuromuscular Junction
4 Proteins in Muscle Cells:
Troponin & Tropomyosin
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4 proteins are found in muscle cells: actin, myosin,
troponin & tropomyosin
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troponin –
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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
•
• Transverse tubules (T-tubules) carry impulse
deep into muscle fibers
•
• Ca²+ bind to troponin, tropomyosin moves,
exposing binding sites on actin filaments
Sliding Filament Theory
(How Muscles Contract )
• Linkages form b/t actin & myosin
•
• 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 –
Sliding Filament Theory
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Ca²+ binds to troponin
Tropomyosin
slides out of the way
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Sarcomeres
shorten & muscle
contracts
Sliding Filament Theory
Energy for Muscle Contraction
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ATP (adenosine triphosphate)
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
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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 –
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
•
•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 –
•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 –
•Contraction
•Relaxation –
Patterns of Contraction
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a) Muscle Twitch –
single contraction
b) Staircase Effect
Patterns of Contraction
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c) Summation – when
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the 2nd stimulus occurs
during the relaxation
pd. of 1st contr.; the
2nd contr. generates
more force
d) Tetany-
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 –
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 –
Origin & Insertion
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Origin –
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 –
Antagonist – resists the
prime mover & causes
movement in the opposite
direction (ex. - triceps)
Structural Differences of
3 Types of Muscle
Skeletal Muscle
Smooth Muscle
Cells elongated
w/multiple
nuclei/cell
Cells spindleshaped w/1
nucleus/cell
No T-tubules
Striated/voluntary
Cardiac Muscle
T-tubules lg.;
releases lg. amts.
of Ca++; can
contract longer
(Ca channel
blockers)
Striated/invol.
Functional Differences of
3 Types of Muscle
Skeletal Muscle
Needs nerve impulse for
contraction
Smooth Muscle
Cardiac Muscle
Displays rhythmicity &
cells stimulates each
other (as in peristalsis)
Ca+² binds to
calmodulin
Ca+² binds to troponin
Not affected by
hormones
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 slowly, tires
slowly (long distance)
Fewer mitochondria
Less myoglobin
More myoglobin
Red muscle
Composes smaller
muscles (eyes, hands,
etc.)
Levers
• Parts of a lever:
wt., force, pivot
3 types of levers:
• 1st class –
• 2nd class – P-W-F
(wheelbarrow)
• 3rd class –
•
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)