Muscle Tissue
Anatomy 32
Chapter 10
I.
Muscle tissuemuscle tissue
alone is not an
organ, but a
muscular
structure such as
your bicep is an
organ. A
muscular
structure
incorporates
tendons, fascia,
blood vessels
(connective
tissue), and
nerves (nervous
tissue). There are
three types of
muscle tissue:
skeletal, cardiac,
and smooth.
a. Functions- there are four general functions:
1. movement-skeletal and smooth muscle aid in movement or
bones and fluids.
2. posture maintance- skeletal muscles contract to maintain the
body in a sitting or standing position.
3. joint stabilization- tendons that cross over joints stabilize joint
as the muscle tone (constant low level contraction) places
tension on the tendon.
4. heat generation- muscular contractions generate heat
influencing body temperature.
b. Specialization- just like epithelial and connective tissue have
distinguishing characteristics so does muscle tissue:
1. Able to contract-when long cells shorten simultaneously a
pulling force is created that contracts the muscle- reduce the
overall size. Contractions may cause movement or
stabilization.
2. Able to extend- at the end of a contraction the muscle may
return to its original length by relaxing or extended with the aid
of an opposing muscle.
3. Able to become excitable- muscle cells respond to nerve
impulses by contracting.
4. Elasticity- muscle may be stretched beyond its normal size
and recoil back to it’s normal size.
C. Classification- muscles are classified based on structure and function.
1. Skeletal muscle- this is known as multinucleated, striated, voluntary
muscle that attaches to bones and causes the skeleton to move.
2. Cardiac muscle- this is known as binucleated, branched, striated
involuntary muscle that makes up the wall of the heart.
3. Smooth muscle-this is known as non-striated involuntary muscle
that assists in the movement of internal viscera. It has the capacity to
stretch widely and contract powerfully.
II. Skeletal
Musclearranged in
bundles and
attached to
bones by the
skeleton it
allows the
body to
move. Below
is a
description of
its basic
structure.
A. Basic features of skeletal muscle
– 1. Connective tissue and fascicles- Three sheaths composed of
connective tissue help to organize a muscle. These sheaths extend out
of the muscle area to join the ligaments, they help to transfer the
contracting force of the fiber over to the bone so the joint is moved.
a. The outer-most-layer is epimysium= a sheath of dense connective
tissue that externally surrounds the entire muscle.
b. Bundles within the muscle called fascicles are surrounded by a
sheath of fibrous connective tissue called perimysium.
c. A sheath of reticular fibers called endomysium
surrounds the bundles within the fascicles.
2. Nerves and blood vessels- both structures enter the
muscle as single units and then branch at areas of
connective tissue. Nerves supply the excitability signals
and blood vessels bring in nutrients and remove waste.
3. Muscle attachments- the
origin is the side of a
muscle that attaches to a
bone and when the muscle
contracts it does move that
bone. As the tendon on the
other side of that muscle
crosses over a joint it
connects into an insertion.
The joint it crosses will
move when the muscle
contracts. Some muscles
cross over two or more
joints, when they contract
both joints move.
Tubercles, trochanters, and
crests are raised areas
where muscles attach to
bones as a response to the
pulling force the bone
thickens.
B. Microscopic and functional anatomy of skeletal muscle tissue-This
is a description of the arrangement of filaments that make up muscle
fiber.
1. The skeletal muscle fiber-long cylindrical, large
multinucleated cells created by the fusion of embryonic muscle
cells. The nuclei lie on the sides of the cell right below the
sarcolema (cells membrane)
2. Myofibrils and sarcomeres- the visible stripes result from the
arrangement of myofibrils in the sarcoplasm. Myofibrils are
contractile cellular organelles made up of rows of sarcomeres and
are smaller than muscle fibers (muscle cell) but larger than
myofilaments (protein strands the make up myofibrils). Myofibrils
may be surrounded by mitochondria and sarcoplasm.
a. Sarcomeres: basic contraction unit formed by the
arrangement of thin myofibrils called actin and a thick myofibril
called myosin. The sarcomere boundary is at the Z line (Z discs).
Actin extends from the z line to the middle of the sarcomere. In the
middle of the sarcomere and in between actin strands lies myosin
strands. The myosin length, including the area where actin and
myosin overlap, is the A-band. The H-zone is where only myosin is
present and the I band is the region where only actin is present.
Down the middle of the sarcomere lies the M-line.
3. Mechanisms of contraction-the sliding
filament theory explain how a muscle
contraction happens:
•
a. The contraction is
initiated by the release of calcium from the
terminal cisternae once a nervous impulse
is received.
•
b. Calcium ions bind to troponin on actin.
The troponin-tropomyosin complex moves
exposing the myosin binding sites.
•
c. The ATP (energy molecule) provides
power for myosin to move by “energizing”
the molecule and changing its conformation
(cocked postion).
•
d. The myosin head attaches to the actin
filament (cross bridge) and in a swivel
action pulls the actin filament (power
stroke) .
•
e. A second ATP binds the myosin head
causing it to release actin and return to a
cocked position.
•
f, The myosin binds multiple times pulling
towards the center further and further down
the actin strand. As the filaments slide past
each other the overall size shortens.
•
g. The released calcium isre-absorbed by
the terminal cisternae.
•
h. With the contraction the Z-lines come
closer to each other, the I and H bands
become smaller, the A band and M line
remain the same.
4. Muscle extension- after a contraction the muscle can be
stretched to its original position by the action of an opposing muscle.
5. Muscle fiber length and the force of contraction-each sarcomere
or muscle have an optimal length. The more it is stretched the
greater the ability to produce contraction forces as long as it is not
past the stretch limit which would not allow it to contract. There is a
limit to how much a muscle contracts or stretches and it is monitored
by joint structure.
6. The role of Titin- molecules in sarcomeres that resist
overstretching and attach to myosin.
7. Sarcoplasmic Reticulum and T tubules- the smooth ER of muscle
cells (muscle fibers) is called sarcoplasmic reticulum and run
longitudinal around myofibrils. It stores calcium which are released
for contraction and reabsorbed. T-tubules a continuation of a
sarcolemma is a triad.
8. Types of skeletal muscle fiber-all
muscles in the body contain the three
types of fibers but each varies in its
proportions (this is genetically
determined). These fiber proportions
influence the muscles performance
capabilities.
a. red slow-twitch fibers- high in
myoglobin content and in
mitochondria, and in vascularization.
Not easily fatigued as long as there
is a good oxygen supply, can contract
continuously, do not generate a lot of
power.
b. white fast-twitch fibers- low
myoglobin content and in
mitochondria larger in diameter,
greater capability to produce power.
c. Intermediate fast-twitch fibers- this
fiber type is an intermediate in all
aspects between red and white.
III. Cardiac muscle- myocardium is the muscle of the heart wall, it contracts to
pump blood.
A. Muscle cells are not fused they connect by cell junctions called
intercalated discs and have a branching pattern. Each cell has one to
two nuclei in the center. The cells have sarcomeres which makes the
tissue look striated.
B. This tissue has an abundance of mitochondria to prevent fatigue.
Contractions are also triggered by calcium ions. Not all cells are innervated,
cells can independently have rhythmic contractions. The length of the cells
is proportionally related to the force it produces when it contracts
• IV. Smooth MuscleA tissue formed by
uninucleated spindle
shaped cells found in
six areas of the
body: blood vessel
walls, respiratory
tract, digestive
tubes, urinary
organs, reproductive
organs, and the eye.
A. It exist as two layers with fibers running perpendicular to each
other. One layer, the longitudinal layer is parallel to the axis, the
circular layer is perpendicular. As they alternate contractions they
shorten and constrict the organ.
B. There are no striations and no sarcomeres. Calcium ions signal
contraction and the forces are not high. It can contract for a long
time before fatiguing. Typically cells are not individually innervated
and contraction may be signaled by stretching or hormones.
V. Disorders of muscle tissue
A. Muscular Distrophy- disorder in which muscle is destroyed and
replaced with connective tissue, it is first diagnosed during
childhood. Duchenne muscular dystrophy- muscle degeneration
from pelvis to cranium, predominantly in men, person usually lives to
be 20 years old. It is related to the lack of production of a protein
that influence cell structure. Genetherapy and stem cell research
suggest possible cures.
B. Myofascial pain syndrome-overused or strained postural muscles
cause tightening of muscle bands that twitch when the skin is
touched. This affect about half the population and is treated with
anti-inflammatory drugs and stretching.
C. Fibromyalgia-chronic pain syndrome of lower back or neck
muscles, affects 2% of people, mostly women, treated with
antidepressants, exercise, and pain relievers. It is not truly a muscle
disorder.
VI. Muscle tissue throughout life
1. Mesoderm cells called myoblast fuse to form skeletal muscle
tissues or join at cellular ends to form cardiac and smooth muscle.
2. Cardiac muscle contracts by the week 3 and skeletal muscle by
week 7 of development!
3. Mitosis= skeletal muscle stops dividing once formed but has
limited regenerated capacity in case of injury, cardiac muscle stops
dividing by age 9, and smooth muscle divides as needed and has
great regenerative abilitiy.
4. Muscle tissue is replaced with connective tissue as one ages.
This is called sarcopenia and is reversible with exercise