6- CONTRACTION OF SKELETAL MUSCLE Flashcards
(116 cards)
1
Q
The sarcolemma consists of a
true cell membrane, called the
A
plasma membrane and
an outer coat made up of a thin layer of polysaccharide
material that contains numerous thin collagen fibrils
2
Q
each end of the muscle fiber, this surface layer of the sarcolemma fuses with a
A
tendon fiber
3
Q
in turn collect into bundles to form the muscle tendons that
then insert into the bones
A
tendon fibers
3
Q
Each myofibril composed
-which are large polymerized protein molecules that are responsible for the actual muscle
contraction. T
A
myosin filaments & actin filaments
3
Q
The
thick filaments in the diagrams are
A
myosin
3
Q
thin filaments are
A
actin
3
Q
The light bands contain only actin filaments
and are called –
The light bands contain only actin filaments
and are called
A
I bands
4
Q
The dark bands contain myosin filaments, as
well as the ends of the actin filaments where they overlap the myosin, and are called
-anisotropic to polarized light.
A
A bands
4
Q
small projections from the sides of the myosin filaments
A
These are cross-bridges. It is the interaction
between these cross-bridges and the actin filaments that
causes contraction
4
Q
the ends of the actin filaments are attached to a so-called
A
Z disc
5
Q
which itself
is composed of filamentous proteins different from the
actin and myosin filaments, passes crosswise across the
myofibril and also crosswise from myofibril to myofibril,
attaching the myofibrils to one another all the way across
the muscle fiber
A
5
Q
These bands give skeletal and cardiac muscle their striated appearance.
A
, the entire muscle fiber has light and dark bands
5
Q
The portion of the myofibril (or of the whole muscle
fiber) that lies between two successive Z discs is called
A
sarcomere
6
Q
This is achieved by a large number of filamentous (very springy) molecules of a protein called
A
titin
7
Q
act as a framework that
holds the myosin and actin filaments in place so that the
contractile machinery of the sarcomere will work.
A
springy titin molecules
7
Q
end of the titin molecule is elastic and is attached to the
Z disk, acting as a spring and changing length as the sarcomere contracts and relaxes.
A
7
Q
The spaces
between the myofibrils are filled with intracellular fluid called
A
sarcoplasm
7
Q
sarcoplasm containing large quantities of
A
potassium, magnesium, and phosphate, plus multiple protein
enzymes
8
Q
Also in
the sarcoplasm surrounding the myofibrils of each muscle
fiber is an extensive reticulum the called
-is extremely important in controlling muscle
contraction
A
sarcoplasmic reticulum
9
Q
The acetylcholine acts on a local area of the muscle
fiber membrane to open multiple “acetylcholine-gated”
cation channels through protein molecules
A
10
Q
Opening of the acetylcholine-gated channels allows
A
large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane
11
Q
This causes a
local depolarization that in turn leads to opening of voltage-gated sodium channels.
A
12
Q
The action potential depolarizes the muscle membrane, and much of the action potential electricity
flows through the center of the muscle fiber.
A
it
causes the sarcoplasmic reticulum to release large
quantities of calcium ions that have been stored within
this reticulum
13
Q
The calcium ions initiate attractive forces between
the actin and myosin filaments
A
which is the contractile process
14
After a fraction of a second, the calcium ions are
pumped back into the sarcoplasmic reticulum by a
Ca++ membrane pump
15
the Z discs have been pulled by
the actin filaments up to the ends of the myosin filaments. Thus, muscle contraction occurs by a
sliding filament mechanism.
16
But what causes the actin filaments to slide inward
among the myosin filaments? T
when an action
potential travels along the muscle fiber, this causes the
sarcoplasmic reticulum to release large quantities of calcium ions that rapidly surround the myofibrils.
The calcium ions in turn activate the forces between the myosin
and actin filaments, and contraction begins
17
The myosin molecule is composed of six polypeptide chains
two heavy chains, and four light chains
17
The two heavy
chains wrap spirally around each other to form a double helix, which is called the ----
of the myosin molecule
tail
18
One
end of each of these chains is folded bilaterally into a globular polypeptide structure called a
myosin head
19
providing an arm that extends the head outward from the
body. The protruding arms and
heads together are called
cross-bridges
20
Each strand of the double F-actin helix is composed
of polymerized
--molecules is one molecule of ADP
G-actin molecule
20
Another feature of the myosin head that is essential for muscle contraction is that it functions as an
ATPase enzyme
20
Each cross-bridge
is flexible at two points called
-one where the arm
leaves the body of the myosin filament, and the other where
the head attaches to the arm.
hinges
21
The backbone of the actin filament is a double-stranded
F-actin protein molecule
21
The actin filament also
contains another protein
These molecules are wrapped spirally
around the sides of the F-actin helix.
tropomyosin
22
In the resting state,
the tropomyosin molecules
tropomyosin molecules lie on top of the active sites of
the actin strands so that attraction cannot occur between the
actin and myosin filaments to cause contraction
23
These are actually complexes of three loosely
bound protein subunits, each of which plays a specific
role in controlling muscle contraction.
(troponin I) has a strong affinity for actin,
(troponin T) for tropomyosin, and
third
(troponin C)
for calcium ions.
24
A pure actin filament without the presence of the
troponin-tropomyosin complex (but in the presence of
magnesium ions and ATP)
binds instantly and strongly with the heads of the myosin molecules.
25
which this interaction between
the cross-bridges and the actin causes contraction is still
partly theoretical, one hypothesis for which considerable
evidence exists is the
e “walk-along” theory (or “ratchet”
theory) of contraction.
26
Large amounts of ATP are cleaved to
form ADP during the contraction process; the greater the amount of work performed by the muscle, the greater the amount of ATP that is cleaved, which is called the
Fenn effect
26
The
new alignment of forces causes the head to tilt toward the
arm and to drag the actin filament along with it. This tilt
of the head is called the
power stroke
27
Therefore, the greater the number of cross-bridges in contact with the actin filament
at any given time, the greater the force of contraction
28
what happen, When the troponin-tropomyosin complex binds with
calcium ions
active sites on the actin filament are
uncovered and the myosin heads then bind with these
29
The bond between the head of the cross-bridge and the active site of the actin filament causes a
conformational change in the head, prompting the head to tilt
toward the arm of the cross-bridge.
30
This provides the
power stroke for pulling the actin filament
The bond between the head of the cross-bridge and
the active site of the actin filament causes a conformational change in the head, prompting the head to tilt
toward the arm of the cross-bridge.
31
The energy that activates the power stroke is the energy already
stored, like a “cocked” spring, by the
by the conformational
change that occurred in the head when the ATP molecule was cleaved earlier.
31
Once the head of the cross-bridge tilts, this allows
release of the ADP and phosphate ion that were previously attached to the head.
32
At the site of release of
the ADP, a new molecule of ATP binds. This binding
of new ATP causes
detachment of the head from the
actin
33
After the head has detached from the actin, the new
molecule of ATP is
cleaved to begin the next cycle,
leading to a new power stroke.
That is, the energy again “cocks” the head back to its perpendicular condition, ready to begin the new power stroke cycle.
33
When the cocked head (with its stored energy derived
from the cleaved ATP)
binds with a new active site
on the actin filament, it becomes uncocked and once
again provides a new power stroke.
33
At point D on the diagram, the actin filament has pulled
all the way out to the end of the myosin filament, with
no actin-myosin overlap. At this point, the tension developed by
by the activated muscle is zero
33
- The whole muscle has a large amount of connective tissue in; also, the sarcomeres in different parts of the muscle do not always contract the same amount.
-Therefore, the curve has somewhat different dimensions from those shown for the individual muscle fiber, but it exhibits the same general form for
the slope
slope in the normal range of contraction,
34
that when the muscle is at its normal resting length, which is at a sarcomere length of about
2 micrometers, it contracts upon activation with the approximate maximum force of contraction.
34
However, the
increase in tension that occurs during contraction, called
active tension,
34
decreases as the muscle is stretched beyond its normal length—that is, to a sarcomere length greater
than about
2.2 micrometers.
35
A skeletal muscle contracts rapidly when it contracts
against
no load—to a state of full contraction in about 0.1 second for the average muscle.
36
When loads are applied, the
the
velocity of contraction becomes progressively less as the
load increases,
36
That is, when the load has been increased to equal the maximum force that
the muscle can exert, the velocity of contraction
becomes
zero and no contraction results, despite activation of the
muscle fiber
37
This decreasing velocity of contraction with load is
caused by the fact that
a load on a contracting muscle is a
reverse force that opposes the contractile force caused by
muscle contraction.
37
Therefore, the net force that is available to cause velocity of shortening is
correspondingly reduced
38
When a muscle contracts against a load, it performs
work
38
This means that energy is transferred from the muscle to
the external load to lift an object to a greater height or to
overcome resistance to movement
39
The ATP is split to
form ADP, which transfers energy from the ATP molecule to the contracting machinery of the
muscle fiber.
40
which allows the muscle to continue its contraction.
, the ADP is rephosphorylated to form new
41
There are several sources of the energy for this rephosphorylation
1. phosphocreatine
2. “glycolysis”
3. oxidative metabolism.
42
The first source of energy that is used to reconstitute the ATP is the substance
which carries a
high-energy phosphate bond similar to the bonds of ATP.
phosphocreatine
42
. Therefore, phosphocreatine is instantly cleaved, and its released energy
causes bonding of a new phosphate ion to ADP to reconstitute the ATP.
43
The high-energy phosphate bond of phosphocreatine has
a slightly higher amount of
free energy than that of each
ATP bond
44
However, the total amount of phosphocreatine in the muscle fiber is also very little—
only about five times as great as the ATP.
45
Therefore, the combined energy of both the stored ATP and the phosphocreatine in the muscle is capable of causing
causing maximal muscle contraction for only 5 to 8 seconds.
46
The second important source of energy, which is used
to reconstitute both ATP and phosphocreatine, is
“glycolysis” of glycogen
47
glycolysis” of glycogen previously stored in the
muscle cells
48
liberates energy that is used to convert
ADP to ATP
Rapid enzymatic breakdown of the glycogen to pyruvic
acid and lactic acid
49
the ATP can then be used directly to
to energize additional muscle contraction and also to re-form the
stores of phosphocreatine
50
The importance of this glycolysis mechanism is two fold:
First, the glycolytic reactions can occur even in the absence of oxygen, so muscle contraction can be sustained for many seconds and sometimes up to more than
a minute, even when oxygen delivery from the blood is
not available
Second, the rate of formation of ATP by the glycolytic process is about 2.5 times as rapid as ATP formation in response to cellular foodstuffs reacting with
oxygen.
51
The third and final source of energy is
oxidative metabolism.
51
However, so many end products of glycolysis accumulate in the muscle cells that glycolysis
also loses its capability to sustain maximum muscle contraction after
about 1 minute.
51
This means combining oxygen with the
end products of glycolysis and with various other cellular foodstuffs to liberate ATP.
oxidative metabolism
51
More than 95 percent of
all energy used by the muscles for sustained, long-term
contraction is derived from this source
oxidative metabolism
52
The foodstuffs
that are consumed of oxidative metabolism are
carbohydrates, fats, and protein.
53
For extremely long-term maximal muscle activity—over a period of many hours—by far the greatest proportion
of energy comes from
fats but for periods of 2 to 4 hours,
as much as one half of the energy can come from stored
carbohydrates.
54
The efficiency of an
engine or a motor is calculated as the percentage of energy
input that is converted into
work instead of heat
54
The percentage of the input energy to muscle (the chemical energy in nutrients) that can be converted into work, even
under the best conditions, is less than 25 percent
with the
remainder becoming heat.
55
Maximum efficiency can be realized only when the muscle contracts at a
moderate velocity
55
The reason for this low efficiency is that about one half of the energy in foodstuffs is
lost during the formation of ATP
even then, only 40 to
45 percent of the energy in the ATP itself can later be converted into work.
56
If the muscle contracts
slowly or without any movement, small amounts of
'maintenance heat' are released during contraction, even though
little or no work is performed,
57
Conversely, if contraction is too rapid, large proportions of the energy are used to overcome viscous friction within the muscle itself, and this,
too,
reduces the efficiency of contraction
58
Ordinarily, maximum efficiency is developed when the velocity of contraction is about
about 30 percent of maximum.
59
Many features of muscle contraction can be demonstrated by
eliciting single
muscle twitches
60
This can be accomplished by
instantaneous electrical excitation of the nerve to a muscle
or by passing a short electrical stimulus through the muscle
itself, giving rise to a single, sudden contraction lasting for a
fraction of a second
muscle twitches
60
when it does shorten but the
tension on the muscle remains constant throughout the contraction.
isotonic
60
Muscle contration is said to be -----when the muscle does not shorten
during contraction
isometric
61
the muscle contracts against
a force transducer without decreasing the muscle length
isometric system,
62
the muscle shortens against a fixed load; this is illustrated on the left in the figure, showing a muscle lifting a
pan of weights.
isotonic system
63
The characteristics of isotonic contraction
depend on the load against which the muscle contracts, as
well as the inertia of the load.
63
However, the isometric system records strictly changes in force of muscle contraction itself. Therefore, the isometric system is most often used
when comparing the functional characteristics of different
muscle types.
63
of isometric contractions of
three types of skeletal muscle:
1. an ocular muscle, which has
a duration of isometric contraction of less than 1/50 second;
2. gastrocnemius muscle, which has a duration of contraction of about 1/15 second
3. the soleus muscle, which has
a duration of contraction of about 1/5 second.
64
Ocular movements must be
extremely rapid to maintain fixation of the eyes on specific
objects to provide accuracy of vision.
65
Ocular movements must be
extremely rapid to maintain fixation of the eyes on specific objects to provide accuracy of vision.
66
soleus muscle is concerned principally with slow contraction for continual, long-term support of the body
against gravity.
67
every muscle of the body
is composed of a mixture of so-called
fast and slow muscle
fibers,
68
Muscles that react rapidly, including anterior tibialis, are composed mainly of
“fast” fibers with only small numbers of the slow variety
69
Conversely, muscles such as soleus
that respond slowly but with prolonged contraction are composed mainly of
“slow” fibers
70
Slow Fibers (Type 1, Red Muscle)
(1) Smaller fibers.
(2) Also innervated by smaller nerve fibers.
(3) More extensive
blood vessel system and capillaries to supply extra amounts of oxygen.
(4) Greatly increased numbers of mitochondria, also to support high levels of oxidative metabolism.
(5) Fibers contain large amounts of myoglobin, an iron-containing protein similar to hemoglobin in red blood cells.
71
gives the slow muscle a reddish appearance and
the name red muscle.
myoglobin
72
Fast Fibers (Type II, White Muscle).
(1) Large fibers for
great strength of contraction.
(2) Extensive sarcoplasmic
reticulum for rapid release of calcium ions to initiate contraction.
(3) Large amounts of glycolytic enzymes for rapid
release of energy by the glycolytic process.
(4) Less extensive
blood supply because oxidative metabolism is of secondary
importance.
(5) Fewer mitochondria, also because oxidative
metabolism is secondary
73
A deficit of red myoglobin in fast
muscle gives it the name
white muscle.
74
All the muscle fibers innervated by
a single nerve fiber are called a
a motor unit
75
The muscle fibers in each motor unit are not all bunched together in the muscle but
overlap other motor units in
microbundles of 3 to 15 fibers.
75
overlap other motor units in
microbundles of 3 to 15 fibers. This interdigitation allows the
separate motor units to contract in support of one another
rather than entirely as individual segments
76
means the adding together of individual twitch contractions to increase the intensity of overall muscle contraction
Summation
76
Summation occurs in two
ways:
: (1) by increasing the number of motor units contracting simultaneously, which is called multiple fiber
summation, and
(2) by increasing the frequency of contraction, which is called frequency summation and can
lead to tetanization.
77
by increasing the number of motor units contracting simultaneously, which is called
multiple fiber summation
78
by increasing the frequency of contraction, which is called
frequency summation and can
lead to tetanization.
79
Then, as the strength of the signal
increases, larger and larger motor units begin to be excited as
well, with the largest motor units often having as much as 50
times the contractile force of the smallest units. This is called
size principle
