Chapter 9 - Muscle
Human Physiology, pages 267 - 310
A.
B.
C.
Chapter 9 - Introduction
Movement is a characteristic of living things
1.
Cell division and migration
2.
Cilia used in many ways
Muscle types
1.
Skeletal (striated) - voluntary body movement
2.
Smooth - involuntary, visceral etc.
3.
Cardiac - heart only
SECTION A - SKELETAL MUSCLE
II.
Structure
A.
Muscle fiber  a single muscle cell, Figure 9-1, page 269
Cylindrical tube ranging from 10 to 100 m in diameter and a
length of up to 30 cm
2.
It is derived from several myoblasts and is therefore
multinucleated
Skeletal muscle fiber differentiation is completed at birth -- no new
fibers are produced from myoblasts after that
1.
A damaged muscle fiber can not regenerate
2.
New fibers can develop from satellite cells that are located
adjacent to fibers
3.
Can not restore 100% function, however
A muscle is composed of many fibers
1.
Tendons attach muscle to bone an ligaments attach bone
to bone - generally
2.
Myofibrils - fibers are composed of myofibrils which occupy
80% of the volume
3.
Filaments (myofilaments ) - myofibrils are composed of
filaments arranged as sarcomeres and contain two
filamentous proteins, actin and myosin
1.
B.
C.
a)
4.
5.
Thick filaments  myosin
b)
Thin filaments  actin
Striations are regular and defined, Figure 9-2 & 3, page 270
(longitudinal view)
Ultrastructure , Figure 9-4, page 271; Figure 9-5, page 272
a)
A band - central portion of sarcomere and is mostly
myosin
b)
Z line - anchor of each thin filament and two Z lines
are a sarcomere
c)
I band - between two the ends of the A bands
(spans two sarcomeres)
d)
H zone - an area in the A band and is accounted for
by the space where the thin filaments don't touch,
only thick filaments are found in the H zone
e)
M line - thin line made up of where thick filaments
are anchored to one another
Page 1 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
6.
Titin
a)
b)
c)
d)
It is the largest protein known, a single chain of
nearly 27,000 amino acids, with a molecular weight
of about 3 million.
Few other proteins have molecular weights greater
than 200,000. Fittingly, the muscle protein is called
titin, and its size has made determining its entire
sequence difficult.
The sequence reveals a brand-new type of structural
protein's springiness. What's more, the length of this
motif can vary, and that may explain why some
muscles are more elastic than others. Other parts of
the protein's sequence suggest how it may perform
another key function, acting as a ruler to aid the
precise placement of proteins within muscle fibers.
The structure may also shed light on how titin
contributes to the ability of muscles to spring back
after they are stretched. As muscles expand and
contract, the sarcomere changes length, mostly in
the I band.
Page 2 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
Stretching titan. (Upper panel) In a relaxed, unstretched sarcomere the 100
immunoglobulin domains and PEVK domain to the titan I-band segment can span the I
band without unfolding. This native titin is flexible and exerts a weak force as an entropic
spring. At larger stretch (within physiological range) the PEVK domain unravels and
there are then two entropic springs in series: the still-native immunoglobulin segments
and the much more flexible extended polypeptide chain of the PEVK. (Lower panel) At
extreme stretch and higher force, titin immunoglobulin domains unravel one at a time. A
shows a segment of five folded-immunoglobulin domains colored alternately green and
blue, their boundaries indicated by lines. In C through F, two domains successively
unfold to extended polypeptide, which is indicated in red. In G the molecule is retracted,
exerting a weak force as an entropic spring. Immunoglobulin domains do not refold until
the molecule is extensively retracted H. The inset plots the force during extension (red
line) and retraction (blue line).
The lower panel of the figure illustrates titan's behavior under these high stresses. A
~100-nm length of titin, comprising 25 immunoglobulin domains (five are shown), is
stretched between two tethers. When the tethers are very close to each other the titin will
be randomly coiled. As the molecule is extended it behaves as an entropic spring; a small
force is needed to extend it to A. As the titin reaches full extension, the force of the
entropic spring increases steeply. At B the force is starting to pull out the weakest pair of
 strands (arrow). An important conclusion emerging from the new results (1-3) is that
domain unraveling requires a very high force sustained for only a short distance to initiate
the process. When the  strands have been displaced ~0.5 nm (9), the domain is greatly
destabilized, it rapidly unravels (C, in figure), and the force drops. As the polypeptide
Page 3 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
extends, the process is repeated and another domain unravels (D to F). At this point, the
experimenter retracts the tether back toward zero. The force of the entropic spring drops
rapidly as the strand collapses into a random coil (G). However, even the weak force at G
prevents the renaturation of the domain. Only when the tether is almost completely
retracted (H) do the domains start to renature (arrow).
The conventional picture of titin is that it operates as a pair of springs on either side of
the myosin band, keeping it centered in the sarcomere. [HN10]This picture now needs
rethinking in light of the enormous hysteresis revealed in the new studies, especially
when immunoglobulin domains are unfolded. This property is completely unlike a true
spring, which is largely reversible. In titin, unfolding immunoglobulin domains may
provide a reservoir of extra length in case of extreme stretch. However, there is no
obvious way to recover this stretch and refold the domain unless the force is completely
relaxed and the stretch retracted. Could chaperones be involved in refolding the
immunoglobulin domains after severe stretch?
In summary, the titin I-band segment accommodates physiological stretch by first
straightening, but not unfolding, the immunoglobulin segments, and unfolding the PEVK
domain. The unfolded PEVK may act more as a leash than a spring, exerting forces
greater than 5 pN (the force generated by a single myosin molecule) only near 80% of its
maximal extension. It requires extreme force to unfold immunoglobulin domains. Once
unfolded, these domains cannot exert a significant force, and must be renatured by other
means.
D.
III.
Cross section , Figure 9-6, page 273
1.
Cross bridges from myosin to actin
2.
One myosin surrounded by six actins
3.
Three myosin surround each actins
4.
The cross bridges are the force-generating sites in muscle
cells; Figure 9-7, page 273
Molecular Mechanisms of Contraction
A.
Sliding-filament mechanism
1.
Figure 9-8 page 274 - the two sets of filaments slide past
one another and do not change size
a)
A band stays constant
b)
Width of I band and H zone shorten
2.
What produces the movement?
a)
Cross bridges attach to actin and undergo a
configurational change which causes the shortening;
Figure 9- 9, page 274
b)
Cross bridges are independent and at any one
instant in time only about 50% are attached
3.
Figure 9-10 and Figure 9-11, page 274, 275
a)
Thin filament - two helical chains of globular actin
molecules
b)
Thick filament - myosin molecule with globular
head, i.e. like a lollipop, and the head has an actin
binding site and an ATPase site (it is an enzyme, an
ATPase)
4.
The contraction cycle, Figure 9-12, page 276
Page 4 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
a)
B.
The hydrolysis of ATP dissociates cross bridges
from previous cycle and provides energy for next
cycle by "cocking" the myosin
b)
The "cocking" is the activation of myosin, so called
"high energy" myosin
c)
Upper left - calcium ion activates the cross bridge
attachment to actin (calcium ion is not shown in this
figure but will be included later)
d)
Cross bridge movement occurs, therefore tension is
created and back to bottom right in Figure 9-12
where ATP is once again added to undo attachment
and release myosin cross bridges for another cycle
5.
Rigor mortis - this occurs when ATP is not available for
uncoupling and muscle stays contracted
Role of troponin , tropomyosin , and calcium in contraction
1.
Two proteins are involved in the regulation of muscle fiber
contraction - troponin and tropomyosin
a)
Tropomyosin is a rod shaped molecule that is
approximately seven actin molecules long
b)
The tropomyosin lies on the actin and covers the
actin binding sites thus preventing binding
c)
The tropomyosin is held in this position by another
protein called troponin - that is troponin is inhibiting
tropomyosin from not blocking the actin binding
sites
2.
Figure 9-13, page 277 - the tropomyosin uncovers the actin
binding sites when calcium ion "appears" in the cytosol and
inhibits troponin from inhibiting tropomyosin
(AN INHIBITOR OF AN INHIBITOR IS AN EXCITER)
C.
Calcium inhibits troponin which had been inhibiting
tropomyosin and that uncovers the actin binding sites.
Excitation-contraction coupling
1.
What occurs between the presence of an action potential in
the muscle membrane and the cross bridge movement?
a)
Figure 9-14, page 278 - mechanical events are
occurring after the action potential is over
b)
The action potential causes the release of calcium
ion which binds to troponin - this inhibits troponin
from inhibiting tropomyosin and actin binds with
myosin etc.
2.
Sarcoplasmic reticulum - Figure 9-15 A&B, page 279
a)
Sarcoplasmic reticulum is modified endoplasmic
reticulum (unit membrane) and surrounds each
myofibril
b)
The structure is repeated for sarcomere to
sarcomere
Page 5 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
c)
3.
4.
At the end of each segment there are two enlarged
sac-like regions called lateral sacs which store the
calcium ion
d)
The t-tubule structure lies at the A-I junction and its
lumen is continuous with the extracellular medium
surrounding the fiber
e)
The t-tubule can propagate action potentials and
cause calcium ion release
(1)
The action potential triggers the release of
calcium ion, probably by opening calcium
ion channels in the membrane of the
lateral sac
(2)
The calcium ion then diffuses the short
distance to the troponin
(3)
The calcium ion is pumped back into the
lateral sac by a Ca-ATPase system thus
relaxation of the muscle requires ATP
f)
Summary diagram - Figure 9-16, page 280
Functions of ATP in skeletal muscle, Table 9-1, page 281
a)
Hydrolysis of ATP provides energy for cross-bridge
movement
b)
Binding of ATP to myosin breaks the link formed
between action and myosin during the contraction
cycle - this step does not provide energy
c)
Hydrolysis of ATP by Ca-ATPase in the
sarcoplasmic reticulum provides the energy for the
active transport of calcium ions into the lateral sacs
of the reticulum, lowering cytosolic calcium to prerelease levels, ending the contract, and allowing the
muscle fiber to relax.
Membrane excitation: The neuromuscular junction
a)
Motor unit → a motor neuron plus all the muscle
fibers it innervates, Figure 9-17, page 281
(1)
The fibers are scattered throughout the
muscle are not adjacent to each other
(2)
A muscle is made up and controlled via
many motor units depending on the muscle
function
b)
How are action potentials initiated in the plasma
membranes of muscle fibers? - depends on type of
muscle
(1)
Stimulation by a nerve fiber
(2)
Stimulation by hormones and local chemical
agents
(3)
Spontaneous electrical activity within the
membrane itself
c)
The branches of the motor neuron end at synaptic
junctions know as motor end plates or
Page 6 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
neuromuscular junctions , Figure 9-18, page 281
and Figure 9-19, page 282
(1)
Membrane bound vesicles contain
acetylcholine
(2)
The neuron terminus has voltage-sensitive
calcium channels which allow calcium ions
to diffuse into the axon terminus
(3)
The entrance of the calcium ion triggers
exocytosis releasing acetylcholine into the
synaptic cleft
(4)
The transmitter, acetylcholine , binds to
receptors on post-synaptic membrane and
opens sodium and potassium channels sodium influxes faster than potassium
effluxes and depolarization occurs - called
EPP  end-plate potential
(5)
EPP are much larger in potential than
EPSPs - one can depolarize muscle
membrane and cause propagation of action
potential
SUMMARY ON PAGE 283, TABLE 9-2
IV.
Mechanics of single-fiber contraction
A.
Terminology
1.
Tension - force exerted by a contracting muscle
2.
Load - the force exerted on the muscle (tension and load are
opposing forces)
3.
Isometric contraction - when a muscle develops tension but
does not change length, Figure 9-20a, page 285
4.
Isotonic contraction - load on the muscle is constant but the
muscle is shortening Figure 9-20b, page 285
B.
Twitch contractions
1.
Twitch - the mechanical response to a single action potential
2.
Latent period - in an isometric contraction before tension
begins to develop
3.
Contraction time - time interval from end of latency to the
peak tension
4.
Relaxation time - time interval form peak to tension equal
zero
5.
Compare isotonic and isometric
a)
Both the duration and velocity of shortening in an
isotonic twitch depend on the magnitude of the load,
Figure 9-21, page 286
b)
The greater the load the shorter the duration and
slower the velocity, Figure 9-22, page 286
C.
Frequency-tension relation
1.
Summation , Figure 9-23, page 287
Page 7 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
a)
V.
If two stimuli are close enough together one can add
to the other
b)
Result is a larger than "normal" twitch
2.
Tetanus - repetitive stimulation, Figure 9-24, page 2871
a)
Unfused tetanus - partial relaxation occurs
b)
Fused tetanus - no oscillations
3.
Series elastic component - all the elements in a muscle fiber
other than the active (actin , myosin ) which have elastic
properties, i.e. act like rubber bands
a)
When a muscle twitch begins the active components
begin to react and create tension
b)
As the active components are reacting then the
passive elastic components are "catching up" to the
active
D.
Length-tension relation
1.
Figure 9-25, page 288
2.
Optimal length - the length at which the fiber develops the
greatest tension
a)
Stretch too much reduces tension
b)
Too short reduces tension
3.
Important in cardiac muscle
Skeletal-Muscle Energy Metabolism
A.
Introduction
1.
ATP hydrolysis performs three functions
a)
Energy for cross bridge movement
b)
Unbinding cross bridges
c)
The calcium pump
2.
Figure 9-26, page 289 - ways in which muscle fiber can form
ATP
a)
Phosphorylation of ADP by creatine phosphate
(1)
Rapid, involves creatine kinase
CP + ADP  C + ATP
In the resting muscle there is about five
times more CP than ATP
Oxidative phosphorylation of ADP in the
mitochondria
(1)
At moderate levels of activity ATP can be
formed from oxidative phosphorylation
(2)
Limited by amount of oxygen to cell, the
availability of fuel molecules such as
glucose and fatty acids and the rates at
which enzymatic pathways can metabolize
fuels
Substrate phosphorylation of ADP, primarily by the
glycolytic pathway in the cytosol
(2)
(3)
b)
c)
Page 8 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
(1)
(2)
(3)
When activity reaches about 70% of
maximal anaerobic metabolism takes over
Must have adequate input of fuel, however
Muscle glycogen can be broken down with a
corresponding increase in lactic acid
production
Must pay back “oxygen debt ”
(4)
Muscle fatigue
1.
Defined as the inability of a muscle to continue to maintain
tension - Figure 9-27, page 290
2.
Types of fatigue - depends on type of muscle fiber and how
stimulated
a)
Fatigue created by rapid, high frequency
contractions is quickly reversed such as in weight
lifting
b)
Fatigue developed from long, sustained contractions
such as long distance running is not rapidly reversed
3.
Cause of fatigue?
a)
A failure of some step in excitation-contraction
coupling appears to be the most likely cause
(1)
Conduction failure
(2)
Lactic acid buildup
(3)
Inhibition of cross-bridge cycling
b)
Most significant is psychological fatigue - it is
possible for a muscle to perform far more than the
person demands (that is the danger of drugs such
as amphetamines)
c)
Central Command Fatigue : Another type of fatigue
quite different from muscle fatigue is due to failure of
the appropriate regions of the cerebral cortex to
send excitatory signals to the motor neurons. This is
called central command fatigue , and it may cause
an individual to stop exercising even though the
muscles are not fatigued. An athlete's performance
depends not only on the physical state of the
appropriate muscles but also upon the "will to win,"
that is, the ability to initiate central commands to
muscles during a period of increasingly distressful
sensations.
VI.
Types of Skeletal Muscle Fibers
A.
Myoblasts - mononucleated, undifferentiated muscle cells that fuse
to form muscle fibers - a skeletal muscle fiber is composed of
several embryonic cells, thus the multinucleate characteristic
B.
After formation the fiber is innervated
C.
Skeletal muscle fibers come in three basic types based on maximal
velocity of shortening and the manner in which ATP is produced
1.
A fast fiber is a fast because of the speed at which it can
complete the contraction cycle (myosin -ATPase activity)
2.
A slow fiber does it slower (different kind of ATPase)
B.
Page 9 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
D.
ATP production
1.
Some fibers have many mitochondria and a large amount of
myoglobin (a respiratory pigment)
a)
These fibers have a high capacity for oxidative
phosphorylation and require a larger amount of
oxygen and fuels
b)
The presence of the myoglobin gives the fibers a red
color thus the name red muscle for muscles that
have predominantly fast fibers
2.
Other fibers have fewer mitochondria but a large supply of
glycolytic enzymes and a greater capacity  to store
glycogen
a)
These fibers can produce ATP in the relative
absence of oxygen and are therefore not as
vascularized
b)
These are termed white muscle fibers because they
lack appreciable amounts of myoglobin
E.
Three types of fibers are distinguished
1.
Slow-oxidative fibers - low myosin -ATPase activity with a
high oxidative capacity - most resistant to fatigue, smaller
diameter
2.
Fast-oxidative fibers - high myosin -ATPase activity with a
high oxidative capacity - intermediate resistance to fatigue,
smaller diameter
3.
Fast-glycolytic fibers - high myosin -ATPase activity with a
high glycolytic capacity - fatigue rapidly, large diameter
4.
Figure 9-28, page 292
5.
Rate of fatigue development , Figure 9-29, page 293
F.
Characteristics, Table 9-3, page 294
1.
All of the fibers in a motor unit are of the same type
2.
Combinations of the three fiber types can result in whole
muscles with different abilities
a)
Large postural muscle are primarily oxidative slow
fibers
b)
Arm muscle are primarily glycolytic fast fibers
VII.
Contraction of Whole Muscles
A.
Control of muscle tension depends on two factors, Table 9-4, page
294, Figure 9-30 a & b, page 294
1.
Tension developed by each contracting fiber depend on four
factors
a)
Action potential frequency (frequency-tension
relationship) producing summations and tetanus
b)
Changes in fiber length (length-tension)
c)
State of fatigue
d)
The fiber type (the larger the muscle fiber diameter
the greater the tension created)
Page 10 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
2.
Number of muscle fibers in the muscle that are contracting
at any given time
a)
The number of fibers contracting depends on the
number of motor units and the ratio of fibers to motor
neurons
b)
The rate of action potentials can be increased or
decreased to increase or decrease the amount of
tension
c)
Recruitment refers to the "calling in" of more motor
units - like filling a bucket with water, 2 or 3 hoses
can fill faster than just one
d)
The smaller the ratio of fibers to motor units the
greater the control such as in the extraocular eye
muscle where the ratio is about 13:1
e)
Large postural muscles can have ratios of up to
1,700:1 and have little control
f)
Tension can be altered depending on the muscle
fiber type being recruited
(1)
Weak contractions  oxidative slow
(2)
Stronger demand  oxidative fast
(3)
Strongest demand  glycolytic fast
g)
The fibers fire asynchronously which "smooths" out
the muscle contraction, i.e., not usually jerky
B.
Control of shortening velocity - controlled by the same factors that
control total muscle tension
C.
Muscle adaptation to exercise
1.
If the neuron is damaged the fiber will atrophy
a)
after the neuron is damaged the sensitivity to
acetylcholine spreads from an area directly under
the motor end-plate to the entire fiber
b)
the fiber becomes progressively smaller - this is
called denervation atrophy
2.
Disuse of a muscle will lead to disuse atrophy as when
wearing a cast
3.
Certain types of exercise can lead to muscle hypertrophy
a)
Fibers do not undergo mitosis but can enlarge
b)
The major increase in size of a whole muscle is the
increased vascularization
c)
Increase is also due to a lesser part on increased
protein being laid down in the fibers already present
4.
The types of fibers respond to different types of exercises
5.
Can't change nature of fibers in a muscle but can selectively
exercise and increase capacities of those present
6.
New fibers are not produced
D.
Lever action of muscles, Figures 11-31, 32, 33, 34 page 297, 298
VIII.
Skeletal muscle disease - not responsible
Page 11 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
SECTION B - SMOOTH MUSCLE
IX.
Structure
A.
Characteristics
1.
Utilize actin , myosin , ATP and calcium ion but differently
than skeletal muscle
2.
They lack striations thus the name smooth
3.
All are mononucleated
4.
Controlled by the autonomic nervous system and are mostly
involuntary
5.
Don't necessarily have the one to one neuron-muscle fiber
relationship
B.
Smooth muscle structure
1.
Spindle shaped, a single nucleus and range in size from 2 to
10 m - Figure 9-36, page 303
2.
Bundles of smooth muscle fibers are usually arranged in two
layers - an outer longitudinal and inner circular layer
3.
Smooth muscle cytoplasm
a)
Filled with filament arranged longitudinally, Figure 937, page 303
b)
Three types of filaments are present but they are not
neatly arranged like skeletal muscle
(1)
Thick myosin
(2)
Thin actin anchored by dense bodies
(3)
Intermediate size filaments that seem to act
as an elastic framework or cytoskeleton
c)
Contain about 1/3 the myosin as skeletal muscle
fibers but twice the actin
(1)
Ratio of 16:1 thin filaments for each thick
filament
(2)
Despite difference the optimal tension per
cross sectional unit is the same in smooth
and skeletal
d)
Length-tension - smooth muscle be stretched to a
much greater extent than skeletal muscle and still
develop tension
C.
Contraction and its control (Excitation-contraction coupling)
1.
Troponin is NOT present in smooth muscle fibers
a)
Calcium controls cross bridge activity by regulating
the activity of protein kinase (myosin light-chain
kinase) that phosphorylates myosin
b)
Sequence, Figure 9-38, page 304
(1)
Calcium binds to calmodulin
(2)
The calcium-calmodulin complex binds to a
protein kinase activating it
(3)
Myosin is then phosphorylated
c)
Sources of calcium into cytosol
Page 12 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
(1)
2.
3.
4.
Some is released from sarcoplasmic
reticulum within the fiber
(2)
Rest comes from extracellular fluid by
diffusion through calcium channels in the
plasma membrane
d)
Because of slow myosin -ATPase activity smooth
muscle twitches are generally slower than skeletal
e)
Smooth muscles exhibit tonus
f)
Inputs influencing smooth muscle contractile activity
- Table 9-5, page 306
(1)
Spontaneous activity
(2)
Neurotransmitters released by autonomic
neurons
(3)
Hormones
(4)
Locally induced changes
(5)
Stretching
g)
A smooth muscle fiber contraction can be graded
(not all-or-none) by varying the amount of cytosolic
calcium
h)
The depolarization is also due to an influx of calcium
ion and NOT sodium ion
Spontaneous electrical activity
a)
Some will generate action potentials in the absence
of neural or hormonal input
b)
The membrane oscillates, Figure 9-39, page 306 - it
produces a pacemaker potential
c)
Slow wave potentials - cyclic changes due to
changes in sodium pumping which then result in
bursts of action potentials
d)
Not all smooth muscle fibers have slow wave or
pacemaker potentials
Nerves and hormones
a)
Possibilities are to be innervated by both
sympathetic and parasympathetic or by only one of
the two; some have no innervation at all but are
influenced by hormones
b)
No specialized motor-end plate but rather branches
and varicosities
c)
The transmitters kind of "slop" over the cells, Figure
9-40, page 307
d)
Net effect is a sum of distance, concentration,
excitability and counteracting agents
e)
Some hormones and neurotransmitters can have
depolarize one smooth muscle and have the
opposite effect on another
Local factors - paracrines, acidity, oxygen concentration,
osmolarity and ion composition of the extracellular fluid can
also alter smooth muscle tension by affecting cytosolic
Page 13 of 14
Chapter 9 - Muscle
Human Physiology, pages 267 - 310
calcium concentration - this is adaptive in some instances
and damaging in others
Classification of smooth muscle - two groups  single unit and
multiunit
1.
Single-unit smooth muscle - examples are intestinal tract,
the uterus and small-diameter blood vessels
a)
The whole muscle responds to a stimulation as a
single unit
b)
Each muscle fiber is linked to adjacent fibers by gap
junctions
c)
Some cells in single-unit are pacemaker cells
d)
The contractile activity of single-unit can be altered
by nerves, hormones and local factors
e)
Innervation - Figure 9-41, page 308, the pacemakers
are usually innervated and controlled
f)
Single-unit contraction can often be induced by
stretching
2.
Multiunit smooth muscle - examples are large airways to
the lungs, large arteries and attached hairs in the skin
a)
Have few gap junctions
b)
Each fiber responds independently of its neighbors
thus the name multiunit
c)
Multiunit are richly innervated by the autonomic
nervous system,
d)
Action potentials do not occur in most multiunit
smooth muscles even though depolarization and
contraction occurs
e)
Stretch has no effect on multiunit
Summary - Table 9-6, page 309
D.
X.
Page 14 of 14