MUSCLE TISSUE
A.
TYPES OF MUSCLE
Although bones and joints provide leverage and the framework for the body, they
cannot move the body.
What causes motion?
Motion results from the alternating contraction (shortening) and relaxation
of muscles.
The prime function of muscle is to convert chemical energy (ATP) into
mechanical energy that can be used to …?
Generate force, perform work, and produce movements
Name and briefly describe the three types of muscle tissue?
Skeletal muscle -- Skeletal muscle is striated muscle and is voluntary (it
can be controlled consciously). It can only be stimulated by the
nervous system.
Cardiac muscle -- Cardiac muscle is striated muscle. It is involuntary (no
conscious control). It can be stimulated or inhibited by either the
nervous or the endocrine systems.
Smooth muscle -- Smooth muscle is nonstriated and also involuntary. It
also can be stimulated or inhibited by either the nervous or
endocrine function.
B.
FUNCTIONS OF MUSCLE
Identify and briefly describe the three functions of muscle tissue.
Motion -- Motion can be obvious body movements or less noticeable
motions such as heartbeat and gut movement.
Stabilize body positions and regulate organ volume -- Sustained
contractions of skeletal muscle maintain body posture without
creating noticeable movement. Sustained contractions of smooth
muscle prevent outflow from hollow organs and maintain them at an
appropriate volume.
Thermogenesis -- A by-product of muscle contraction is heat production
and is therefore important in homeostasis of body temperature.
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C.
CHARACTERISTICS OF MUSCLE
List and define the four basic characteristics of muscle tissue.
Excitability -- Also known as irritability. This is the ability to receive and
respond to certain stimuli by producing electrical messages.
Contractility -- Contractility is the ability to shorten and thicken (contract),
thus generating force to do work.
Extensibility -- Extensibility is the ability to stretch without damaging the
tissue.
Elasticity -- Elasticity is the ability to return to original shape after
contraction or extension.
D.
ANATOMY AND INNERVATION OF SKELETAL MUSCLE TISSUE
1.
CONNECTIVE TISSUE COMPONENTS
What is fascia?
Fascia refers to a sheet or broad band of fibrous connective tissue
beneath the skin or around muscles and organs of the body. There
are two types.
Describe superficial fascia.
Superficial fascia (subcutaneous layer or hypodermis) lies
immediately deep to the skin. It is composed of adipose and
areolar tissues. Superficial fascia has four functions:
1.
2.
3.
4.
Store fat and therefore water
Insulation
Mechanical protection
Pathway for nerves and blood vessels
Describe deep fascia.
Deep fascia is formed of dense irregular connective tissue. It lines
the body wall and extremities and holds muscles together,
separating them into functional groups.
Deep fascia allows free movement of muscles, carries nerves,
blood vessels, and lymph vessels, and fills the spaces between
muscles.
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What is the function of the connective tissue specializations that surround
skeletal muscle cells?
These connective tissues protect and strengthen the muscles, as
well as provide attachment of the muscle to surrounding structures.
All the layers are continuous with each other and surrounding deep
fascia.
Identify the following:
Epimysium -- Epimysium is the outermost layer of deep fascia,
wrapping the entire muscle.
Perimysium -- Perimysium is formed by invaginations of the
epimysium, dividing the muscle into bundles of cells called
fascicles (fasciculi).
Endomysium -- Endomysium is formed by invaginations of the
perimysium that penetrate the fascicle and wrap each
muscle cell, completely insulating it from the others.
Tendon -- All three layers may extend beyond the muscle as a cord
of dense connective tissue, called a tendon, that attaches
the muscle to the periosteum of a bone.
Tendon sheath -- Some tendons, particularly those in high stress
areas like the ankle and wrist, are wrapped in a layer of
synovial membrane, forming a tendon (synovial) sheath.
Aponeurosis -- When the three connective tissue components
extend from the muscle as a flat sheet, rather that a round
cord, they are called an aponeurosis.
2.
THE MOTOR UNIT
What is a motor neuron?
A motor neuron delivers the nervous stimulus that ultimately causes
a muscle tissue to contract.
Define the concept of a motor unit?
One motor neuron plus ALL of the skeletal muscle cells it
stimulates is called a motor unit. On average, a single motor
neuron makes contact and thus stimulates about 150 individual
skeletal muscle cells. All the cells contract and relax together, as a
unit.
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How are motor units different for precise and gross movements?
Muscles that control precise body movements may have as few as
2-3 muscle cells per motor neuron (eye muscles), while muscles
that control gross body movements may have as many as 2,000
muscle cells per motor neuron (gluteal muscles).
How is the total strength of a muscle determined?
Stimulation by its motor neuron results in the simultaneous
contraction of all the skeletal muscle cells in the motor unit.
Accordingly, the total strength of any particular muscle is
determined by the total number of motor units being used at any
given time.
3.
THE NEUROMUSCULAR JUNCTION
Describe the neuromuscular junction.
Excitable cells (muscle and nerve) make contact and communicate
with one another at specialized regions called synapses.
At each synapses a small gap, called the synaptic cleft, separates
the two excitable cells.
The first cell, the motor neuron, communicates with the second cell,
the skeletal muscle cell, across the synaptic cleft via a chemical
messenger called a neurotransmitter.
The type of synapse formed between the motor neuron and the
skeletal muscle cell is called the neuromuscular (myoneural)
junction.
At the synapse, the motor neuron branches into clusters of bulbshaped axon terminals (end bulbs), each cluster forming a synapse
with a group of muscle cells (motor unit).
The region of the muscle cell membrane that participates in the
synapse with the axon terminal is the motor end plate.
Describe the use of acetylcholine.
Within each axon terminal are many membrane-enclosed vesicles
called synaptic vesicles containing thousands of neurotransmitter
molecules.
The neurotransmitter used exclusively by motor neurons for
skeletal muscles is acetylcholine (ACh).
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The nerve impulse (1) reaching the axon terminal, triggers
exocytosis of the synaptic vesicles (2), releasing the ACh into the
synaptic cleft.
ACh diffuses through the extracellular fluid in the synaptic cleft and
approaches the motor end plate.
On the motor end plate are ACh receptors (30-40 million). These
are integral proteins specific for ACh. They recognize the molecule
and bind specifically to it, causing Na+ channels to open (3).
Binding of ACh to its receptors on the motor end plate initiates an
electrical message in the motor end plate, and therefore in the
muscle cell membrane (4).
In most skeletal muscles there is a single neuromuscular junction
per cell, located near the cell’s midpoint.
Stimulation of the membrane in this way spreads from the midpoint
of the muscle cell towards its ends and therefore causes almost
simultaneous contraction of all parts of the cell.
4.
MICROSCOPIC ANATOMY OF MUSCLE
Describe each of the following concerning skeletal muscle cells.
Myofiber -- Within a typical skeletal muscle there are thousands of
individual, very long, cylindrical cells called muscle fibers
(myofibers), bundled together as fascicles. They lie in
parallel to one another, ranging in size from 10-100 microns
in diameter and up to 10 cm in length.
Sarcolemma -- The sarcolemma is the cell membrane of a muscle
cell. It surrounds the sarcoplasm, or muscle fiber cytoplasm.
Mitochondria -- Mitochondria lie in rows throughout the muscle
fiber, located close to the muscle proteins that use ATP for
the contraction-relaxation sequence.
Nuclei -- Skeletal muscle cells are multinucleated, due to fusion of
precursor cells during embryogenesis; the nuclei are located
along the periphery of the cell, out of the way of the
contractile elements within the sarcoplasm.
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a.
MYOFIBRILS
What are myofibrils? Where are they located? What is their
composition? What is responsible for the alternating light and dark
striations (bands) seen in skeletal and cardiac muscles?
At higher magnification the sarcoplasm appears to be stuffed
with small threads called myofibrils.
Myofibrils lie in parallel to each other and extend lengthwise
throughout the extent of the myofiber.
The prominent alternating light and dark bands evident in
myofibrils give skeletal muscle cells their characteristic
striations.
Myofibrils are the contractile elements of skeletal muscle.
Each is 1-2 microns in diameter and consists of three even
smaller structures called myofilaments.
These myofilaments do not extend the length of the
myofiber, but rather are stacked into repeating units
(compartments) called sarcomeres
What is a sarcomere?
The sarcomere is the functional (contracting) unit of skeletal
muscle.
What is the Z disc (line)?
A dense material called the Z disc is found at each end of a
sarcomere, separating it from the next sarcomere in line.
Where are thin and thick myofilaments located in the sarcomere?
Extending from each Z disc towards the middle of the
sarcomere are the thin myofilaments.
Suspended within the sarcoplasm, between the thin
myofilaments, and not attached to the z discs, are the thick
myofilaments.
What forms the striations of skeletal muscle?
The alternating areas of thin myofilaments, followed by areas
of overlapping thin and thick myofilaments, are responsible
for the striations seen in skeletal muscle.
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Describe the thin myofilament by describing the following proteins:
Actin -- The main component of a thin myofilament is actin,
each molecule of which looks like a kidney bean.
Individual molecules of actin are linked together to
form the actin filament that is twisted to form a helical
strand. On each molecule of actin within the helical
strand is a myosin-binding site upon which the thick
myofilaments will attach.
Tropomyosin-troponin complex -- Also present on the thin
myofilament are two regulatory molecules called
tropomyosin and troponin. In relaxed muscle, the
tropomyosin-troponin complex covers the myosinbinding sites on the actin molecules. This blocks the
myosin-binding sites and prevents the attachment of
the thick myofilaments, thus preventing contraction of
the sarcomere.
Describe the thick myofilament by describing the following:
Myosin -- Each thick myofilament is composed of about 200
molecules of a protein called myosin. A molecule of
myosin is shaped like two golf clubs twisted together.
The tail of the molecule extends to the center of each
sarcomere. The projecting “head,” called a cross
bridge, extends out towards the thin myofilaments.
Arrangement of molecules -- Tails of adjacent myosin
molecules lie parallel to each other, forming the
“shaft” of the thick myofilament, while the “heads”
project around the shaft in a spiraling fashion.
Titan -- A third component of the sarcomere is the elastin
filament (also known as titan). The role of titan is to
anchor the thick myofilaments in position and to play
a role in recovery of the resting sarcomere length
when a muscle cell is stretched or contracted.
b.
SARCOPLASMIC RETICULUM AND TRANSVERSE TUBULES
Describe the sarcoplasmic reticulum and calcium flux.
A fluid-filled system of tubules called the sarcoplasmic
reticulum encircles each myofibril. In a relaxed muscle cell,
the sarcoplasmic reticulum stores calcium ions by
sequestering them from the sarcoplasm. Calcium ions
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released through calcium channels back into the sarcoplasm
around the thin and thick myofilaments trigger muscle
contraction.
Describe transverse tubules.
Transverse tubules (T tubules) are tunnel- like enfolding of
the sarcolemma. They penetrate the myofiber at right
angles to the sarcoplasmic reticulum and the myofilaments.
T tubules are open to the outside of the muscle fiber and are
therefore filled with extracellular fluid.
What is a muscle triad?
On both sides of a T-tubule are dilated end sacs of the
sarcoplasmic reticulum called the terminal cisternae. A Ttubule, together with its two terminal cisternae, is called a
muscle triad.
E.
CONTRACTION OF SKELETAL MUSCLE
1.
SLIDING FILAMENT MECHANISM
a.
ROLE OF CALCIUM AND REGULATOR PROTEINS
b.
THE POWER STROKE AND THE ROLE OF ATP
Describe the sliding filament theory of muscular contraction.
In the 1950's it was proposed that skeletal muscle shortens during
contraction because the thin and thick myofilaments slide past one
another. This model is known as the sliding filament mechanism.
During muscle contraction, the thick myofilaments attach to, then
pull on, the thin myofilaments, causing them to slide inward towards
each other.
As the cross bridges apply force to the thin myofilaments, the thin
myofilaments move towards the center of the sarcomere. This may
occur so far that their tips overlap each other.
As the thin myofilaments “slide” inward, they pull the Z discs
towards each other, and the sarcomere shortens, but the lengths of
the myofilaments stay the same.
The shortening of the sarcomeres, all in series, causes shortening
of the whole muscle fiber, and ultimately of the entire muscle itself,
producing force used for work.
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Describe the electrical and chemical events of skeletal muscle contraction.
Acetylcholine released by the motor neuron at the neuromuscular
junction diffuses across the synaptic cleft and binds to its receptors
on the motor end plate. This binding initiates formation of an
electrical message in the sarcolemma that spreads in all directions,
passing down the transverse tubules and out into the sarcoplasmic
reticulum, causing calcium channels there to open, allowing
calcium to diffuse into the sarcoplasm. Calcium does two things:
(1) it binds to troponin, causing the tropomyosin-troponin
complex to move, exposing the myosin binding sites
on the actin;
(2) it activates ATP on the myosin heads, causing them to
bind to the actin.
Binding of myosin heads to actin causes the hinge regions to tilt,
pulling the thin myofilaments across the thick myofilaments, thus
causing the sarcomeres to shorten and the muscle cells to contract.
2.
RELAXATION
Describe the electrical and chemical events of skeletal muscle relaxation.
Two changes are necessary to permit a muscle to relax after it has
contracted. ACh is rapidly broken down in the synaptic cleft by the
enzyme acetylcholinesterase (AChE) present on the motor end
plate; this stops the generation of the muscle membrane electrical
message. Secondly, calcium ions are pumped into the
sarcoplasmic reticulum, where they are bound to the molecule
calsequestrin, thus removing them from the sarcoplasm. Without
calcium, the tropomyosin-troponin complex moves back over the
actin, covering the myosin-binding sites. This prevents binding of
myosin cross bridges to actin. Release of the actin by the cross
bridges allows the thin myofilaments to slip back to their resting
position so that the sarcomere resumes its resting length. The
muscle cell is now relaxed.
3.
MUSCLE TONE
What is muscle tone?
Sustained, small contractions of motor units give skeletal muscles a
firmness known as muscle tone.
How is it accomplished?
At any given moment a few motor units are contracting, while all the
others remain relaxed.
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How is muscle tone essential for maintaining posture?
It gives the muscle firmness without producing movement. This is
essential for maintaining posture.
Why does the muscle not fatigue?
This is accomplished without fatigue due to asynchronous firing of
motor units. Different units are activated at different times.
4.
MUSCLE METABOLISM
Describe each of the following energy systems in a muscle cell:
Phosphagen system -- The phosphagen energy system, which
utilizes stored ATP and creatinine phosphate, provides
enough energy for skeletal muscles to contract maximally for
about 15 seconds. It is used for maximal short bursts of
activity.
Glycogen-lactic acid system -- When stored ATP and creatinine
phosphate are depleted, the cell catabolizes glucose to
create new ATP. A series of reactions known as glycolysis
creates 2 ATPs for each glucose and does so anaerobically
(without oxygen). This provides energy for an additional 3040 seconds.
Aerobic system -- For continued activity and with sufficient oxygen,
mitochondria completely catabolize glucose to carbon
dioxide, water, and a net 36 ATPs. When the oxygen supply
is outstripped by the activity, glucose is incompletely
catabolized to lactic acid. This leads to fatigue and oxygen
debt.
5.
HOMEOSTASIS OF BODY TEMPERATURE
Describe the role of skeletal muscle in thermogenesis and the
homeostasis of body temperature.
Skeletal muscle is responsible for generating a great deal of heat in
a very short time in response to decreased body temperature.
During contraction, 20-30% of energy created is used. The balance
is lost as body heat and is used to help maintain normal body
temperature.
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A decrease in body temperature is detected by thermoreceptors in
the hypothalamus of the brain and the skin. In response, the
hypothalamic neurons initiate two events to occur.
1.
2.
Widespread shunting of blood from the body core to
arterio-venous plexuses of the skin.
Involuntary increase in skeletal muscle tone, causing
shivering.
This involuntary thermogenesis returns body temperature to within
normal homeostatic range, and the process is turned off by
negative feedback.
F.
ADJUSTING MUSCLE TENSION
What is the all-or-none principle of muscle?
The all-or-none principle states that individual muscle fibers contract fully
and completely for the existing conditions, or do not contract at all.
What is a threshold stimulus?
A threshold stimulus is one that initiates the formation of an electrical
impulse in the sarcolemma.
How, then, does the all-or-none principle fit with the concept of the motor unit?
A motor neuron delivers a threshold stimulus to all of the cells of the motor
unit and each cell contracts fully and completely. There is no partial
contraction of the motor unit.
If the all-or-none principle is true, how can a muscle vary the force it generates?
The amount of force (tension) that a muscle can generate is dependent upon four
factors. Identify them.
1.
2.
3.
4.
frequency of stimulation of muscle fiber by motor neurons.
length of muscle fibers before they begin to contract.
the number of muscle fibers contracting at any one given time (number of
activated motor units)
structural components of the muscle itself.
1.
TWITCH
What is a twitch contraction?
A twitch contraction is a brief contraction of all muscle fibers in a
motor unit in response to a single threshold stimulus from a motor
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neuron. As seen by the myogram, there are three distinct phases
in a simple twitch contraction.
Describe the following periods:
Latent -- The latent period is the time from when the stimulus is
applied to the beginning of the actual shortening of the
sarcomere.
Contraction -- The contraction period is the time during which the
myofilaments are sliding, the sarcomere is shortening, and
the muscle fiber is generating force.
Relaxation -- The relaxation period is the time during which
acetylcholinesterase is destroying acetylcholine at the
synapse and the sarcomere is returning to its resting
strength.
Refractory -- When a muscle fiber receives a threshold stimulus, it
temporarily loses its excitability and cannot be stimulated
again. This is called the refractory period.
2.
FREQUENCY OF STIMULATION
a.
TETANUS
Describe wave summation.
If a second stimulus is applied to an excited muscle cell after
the refractory period, but before the cell has finished resting
from the first stimulus, the second contraction will be greater
than the first. This phenomenon, in which stimuli arrive at
different times and cause larger contractions, is called wave
summation.
What is incomplete tetany?
If a muscle receives multiple stimuli very quickly (20-30 per
second), it can only relax partially before its next wave
summation. The result is a sustained contraction called
incomplete tetany, during which the muscle is contracting
maximally but relaxing only partially.
What is complete tetany?
If frequency of stimulation is increased to 80-100 per
second, no relaxation period occurs at all between
contractions. This is complete tetany. Both types of tetany
occur as a result of the continued addition of calcium ions to
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the sarcoplasm without benefit of sequestering calcium
between contractions.
How are most voluntary movements accomplished?
Most voluntary movements are accomplished using shortterm tetanic contractions, rather than simple twitch
contractions. This allows the movements to be smooth and
sustained over time.
b.
STAIRCASE EFFECT (TREPPE)
What are treppe contractions?
When a skeletal muscle has been fully rested and is then
stimulated repeatedly by identical stimuli too far apart to
cause wave summation, each of the first few contractions is
a little stronger than the last. This phenomenon is known as
the staircase effect or treppe.
Explain this phenomenon.
After the first few contractions, the muscle reaches its peak
performance and can undergo its strongest contractions.
One explanation may be similar to that for tetanus -- build-up
of calcium ions in the sarcoplasm during the repeated
stimulations. It may also be in part due to “loosening up” of
the connective tissue elements.
3.
NUMBER OF MUSCLE CELLS CONTRACTING
What is recruitment?
Recruitment is the process of increasing the number of active motor
units within a given skeletal muscle.
How is recruitment accomplished?
Recruitment involves activating more and more motor units from
the inactive population in order that the whole muscle can generate
more force during contraction.
Of what use is recruitment
In addition to increasing strength of a muscle, it is also important
because it works with short-term tetanic contractions to produce
smooth movements rather than a series of jerky ones.
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4.
ISOTONIC AND ISOMETRIC CONTRACTIONS
Compare isotonic with isometric contractions. Give examples.
Isotonic contractions occur when you move a constant load through
the range of motions possible at a joint. In this type of contraction,
the tension within the muscle stays the same during the
contraction, while the length of the fibers shortens. (For example:
most movements of the body.)
Isometric contractions occur when the muscle does not or cannot
shorten, but the tension within it greatly increases. (Example:
holding a book in a steady position, outstretched away from the
body; the book stretches the arm and shoulder; muscle contraction
counteracts the stretch. There is no motion.)
G.
CARDIAC MUSCLE
Describe cardiac muscle.
The principal constituent of the heart is cardiac muscle. It is striated like
skeletal muscle, with the same arrangement of thin and thick
myofilaments. Unlike skeletal muscle, each cardiac muscle cell has a
single nucleus, there is no insulating endomysium, and the cells branch
freely.
What are intercalated discs? What is a functional syncytium?
Adjacent cell membranes contact one another at thickened areas called
intercalated discs; it is through these structures that adjoining cells
communicate. This feature allows the cells to act as one, a network of
muscle cells ( functional syncytium). If one cell is stimulated all cells of the
network become stimulated.
How is cardiac muscle controlled?
Unlike skeletal muscle, cardiac muscle stimulates itself to contract; it can
then be regulated by the nervous and endocrine systems.
H.
SMOOTH MUSCLE
Describe smooth muscle.
Smooth muscle is non-striated and involuntary muscle whose cells are
spindle-shaped with a single nucleus. Each cell contains thin and thick
myofilaments, but not arranged into sarcomeres. This permits the walls of
hollow organs to stretch without increasing the tension within the cells.
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Compare visceral smooth muscle with multiunit smooth muscle.
Visceral (single-unit) smooth muscle is the most common type. It is found
wrapped in sheets that form part of the walls of hollow organs. The cells
are tightly bound to each other to form a continuous network (functional
syncytium), so that stimulation of one cell results in stimulation of the
entire network.
Multiunit smooth muscle consists of individual smooth muscle cells, each
with its own motor neuron, so that each can contract independent of the
others. Multiunit smooth muscle is found in the walls of blood vessels, the
arrector pili of hair follicles, and in the iris of the eye.
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