CHAPTER 6 Flashcards
The sarcolemma consists of a
true cell membrane, called the
plasma membrane
In turn collect into bundles to form the muscle tendons that then insert into the bones
tendon fibers
Each myofibril (Figure 6-1D and E) is
composed of about
about 1500 adjacent myosin filaments and 3000 actin filaments
which are large polymerized protein molecules that are responsible for the actual muscle contraction.
actin filaments
The thick filaments in the diagrams are
myosin
and the thin
filaments are a
actin
The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z discs is called
sarcomere
The side-by-side relationship between the myosin and actin filaments is difficult to maintain. This is achieved by a large number of filamentous molecules of a protein called
titin
The light bands contain only actin filaments and are called
I bands
called I bands because they are
Isotropic’ to polarized light.
The dark bands contain myosin filaments, as well as the ends of the actin filaments where they overlap the myosin, and are called
A bands
Note also the small projections from the sides of the myosin filaments these are
cross -ridge
called A bands because they are
“anisotropic” to polarized light.
also shows that the ends of the actin filaments are attached to a so-called
Z disc
act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomere will work
springy titin molecules
The spaces between the myofibrils are filled with intracellular fluid called
sarcoplasm
Also in the sarcoplasm surrounding the myofibrils of each muscle fiber is an extensive reticulum called the
Sarcoplasmic reticulum
sarcoplasm, containing large quantities of
potassium
magnesium
phosphate, plus multiple protein
enzymes
acts on a local area of the muscle
fiber membrane to open multiple “acetylcholine-gated” cation channels through protein molecules floating in the membrane
Acetylcholine
At each ending, the nerve secretes a small amount of the neurotransmitter substance
acetylcholine
Also, 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
But what causes the actin filaments to slide inward among the myosin filaments?
This is caused by forces generated by interaction of the cross-bridges from the myosin filaments with the actin filaments.
The two heavy chains wrap spirally around each other to form a double helix called
“tail” of the myosin molecule
The myosin molecule (see Figure 6-6A) is composed of six polypeptide chains
Two heavy chains
Four light chains
One end of each of these chains is folded bilaterally into a globular polypeptide structure called a
myosin head
The protruding arms and
heads together are called
cross-bridges
Each cross-bridge is flexible at two points called
hinges
Another feature of the myosin head that is essential for muscle contraction is that it functions as an A
ATPase enzyme
The backbone of the actin
filament is a double-stranded
F-actin protein molecule
Each strand of the double F-actin helix is composed of polymerized
of the double F-actin helix is composed
of polymerized G-actin
Although the precise manner by 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
“walk-along” theory (or “ratchet”
theory) of contraction.
The actin filament also
contains another protein
tropomyosin
Attached intermittently along the sides of the tropomyosin molecules are still other protein molecules called
troponin
This tilt of the head is called the
power stroke
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
which carries a high-energy phosphate bond similar to the bonds of ATP
phosphocreatine
of glycogen previously stored in the muscle cells.
“glycolysis”
The third and final source of energy is o
This means combining oxygen with the
end products of glycolysis and with various other cellular foodstuffs to liberate ATP
oxidative metabolism
The efficiency of an engine or a motor is calculated as the percentage of energy
input that is converted into work instead of heat.
Maximum efficiency can be realized only when the muscle contracts at a moderate velocity. If the muscle contracts
slowly or without any movement, small amounts of
maintenance heat
Many features of muscle contraction can be demonstrated by
eliciting single
muscle twitches
Muscle contraction is said to be —-
when the muscle does not shorten
during contraction
isometric
when it does shorten but the
tension on the muscle remains constant throughout the contraction.
Isotonic
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.
Myoglobin combines with oxygen and stores it until needed; this also
greatly speeds oxygen transport to the mitochondria. The myoglobin gives the slow muscle a reddish appearance and
the name
red muscle
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
A deficit of red myoglobin in fast
muscle gives it the name
white muscle
All the muscle fibers innervated by
a single nerve fiber are called a
motor unit
means the adding together of individual twitch contractions to increase the intensity of overall muscle contraction.
Summation
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.
by increasing the number of motor units contracting simultaneously, which is called
multiple fiber summation
By increasing the frequency of contraction, which is called
Frequency summation
with the largest motor units often having as much as 50 times the contractile force of the smallest units. This is called
the
size principle.
Even when muscles are at rest, a
certain amount of tautness usually remains. This is called
muscle tone
When the frequency reaches a
critical level, the successive contractions eventually become so rapid that they fuse together and the whole muscle contraction appears to be completely smooth and continuous, This is called
tetanization
That is, the strength of contraction
increases to a plateau, a phenomenon called the
staircase effect, or treppe
an analysis of the lever systems of the body depends on knowledge of
(1) the point of muscle insertion
(2) its distance from the fulcrum of the lever
(3) the length of the lever arm, and
(4) the position of the lever.
The study of different types of muscles, lever systems, and their movements is called
kinesiology
When the total mass of a muscle increases, this is called
muscle hypertrophy
When it decreases, the process is called
muscle atrophy.
Virtually all muscle hypertrophy results from an increase in the number of actin and myosin filaments in
each muscle fiber, causing enlargement of the individual muscle fibers; this is called simply
Fiber hypertrophy
The pathway that appears to account for much of the protein degradation in a muscle undergoing atrophy is the
ATP-dependent ubiquitin-proteasome pathway
is a regulatory protein that basically labels
which cells will be targeted for proteasomal degradation.
Ubiquitin
Proteasomes are
large protein complexes that degrade damaged or unneeded
proteins by
Proteolysis
This increase in fiber number is called f
fiber hyperplasia
The fibrous tissue that replaces the muscle fibers during denervation atrophy also has a tendency to continue
shortening for many months, which is called
contracture
When some but not all nerve fibers to a muscle are destroyed, as commonly
occurs in
poliomyelitis
the remaining nerve fibers branch off
to form new axons that then innervate many of the paralyzed muscle fibers. This causes large motor units called
macromotor units
Several hours after death, all the muscles of the body go into
a state of contracture called
“rigor mortis”
