Chapter 10:
Muscular Tissue
Copyright 2009, John Wiley & Sons, Inc.
Muscular Tissue
Chapter 10












Overview of Muscular Tissue
Skeletal Muscle Tissue
Contraction and Relaxation of Skeletal Muscle Fibers
Muscle Metabolism
Control of Muscle Tension
Types of Skeletal Muscle Fibers
Exercise and Skeletal Muscle Tissue
Cardiac Muscle Tissue
Smooth Muscle Tissue
Regeneration of Muscle Tissue
Development of Muscle
Aging and Muscular Tissue
Copyright 2009, John Wiley & Sons, Inc.
Overview of Muscular Tissue

Types of Muscular Tissue

The three types of muscular tissue





Skeletal
Cardiac
Smooth
(see video)
Skeletal Muscle Tissue


So named because most skeletal muscles move bones
Skeletal muscle tissue is striated:


Skeletal muscle tissue works mainly in a voluntary manner


Alternating light and dark bands (striations) as seen when examined
with a microscope
Its activity can be consciously controlled
Most skeletal muscles also are controlled subconsciously to
some extent

Ex: the diaphragm alternately contracts and relaxes without
conscious control
Copyright 2009, John Wiley & Sons, Inc.
Overview of Muscular Tissue

Cardiac Muscle Tissue



Found only in the walls of the heart
Striated like skeletal muscle
Action is involuntary



Contraction and relaxation of the heart is not consciously
controlled
Contraction of the heart is initiated by a node of tissue called
the “pacemaker”
Smooth Muscle Tissue

Located in the walls of hollow internal structures



Blood vessels, airways, and many organs
Lacks the striations of skeletal and cardiac muscle
tissue
Usually involuntary
Copyright 2009, John Wiley & Sons, Inc.
Overview
of Muscular
Tissue
Copyright 2009, John Wiley & Sons, Inc.
nucleus
TYPE: Skeletal muscle
DESCRIPTION: Bundles of
cylindrical, long, striated contractile
cells; many
mitochondria; often reflex-activated
but can be consciously controlled
LOCATIONS: Partner of skeletal
bones, against which it exerts great
force
FUNCTION: Locomotion, posture;
head, limb movements
Fig. 28-8a, p.474
junction between abutting cells
TYPE: Cardiac muscle
DESCRIPTION: Unevenly striated,
fused-together cylindrical cells that
contract as a unit owing to signals
at gap junctions between them
LOCATIONS: Heart wall
FUNCTION: Pump blood forcefully
through circulatory system
Fig. 28-8b, p.474
Cardiac Muscle Tissue

Principal tissue in the heart wall

Intercalated discs connect the ends of cardiac muscle fibers to
one another


Cardiac muscle tissue contracts when stimulated by its own
autorhythmic muscle fibers





Gap junctions allow muscle action potentials to spread from one
cardiac muscle fiber to another
Continuous, rhythmic activity is a major physiological difference
between cardiac and skeletal muscle tissue
Contractions lasts longer than a skeletal muscle twitch
Have the same arrangement of actin and myosin as skeletal
muscle fibers
Mitochondria are large and numerous
Depends on aerobic respiration to generate ATP


Requires a constant supply of oxygen
Able to use lactic acid produced by skeletal muscle fibers to make
ATP
Copyright 2009, John Wiley & Sons, Inc.
Glucose
Copyright 2009, John Wiley & Sons, Inc.
Gycogen
Glycogenin
Overview of Muscular Tissue

Functions of Muscular Tissue
 Producing Body Movements
 Walking and running
 Stabilizing Body Positions
 Posture
 Moving Substances Within the Body
 Heart muscle pumping blood
 Moving substances in the digestive tract
 Generating heat
 Contracting muscle produces heat
 Shivering increases heat production
Copyright 2009, John Wiley & Sons, Inc.
Overview of Muscular Tissue

Properties of Muscular Tissue

Properties that enable muscle to function
and contribute to homeostasis

Excitability


Contractility


Ability to contract forcefully when stimulated
Extensibility


Ability to respond to stimuli
Ability to stretch without being damaged
Elasticity

Ability to return to an original length
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue

Connective Tissue Components
 Fascia


Epimysium



Separates individual muscle fibers from one another
Tendon


Surrounds numerous bundles of fascicles
Separates 10-100 muscle fibers into bundles called fascicles
Endomysium


The outermost layer
Perimysium


Dense sheet or broad band of irregular connective tissue that
surrounds muscles
Cord that attach a muscle to a bone
Aponeurosis

Broad, flattened tendon
Copyright 2009, John Wiley & Sons, Inc.
Skeletal
Muscle
Tissue
Copyright 2009, John Wiley & Sons, Inc.
Sarcomere
Skeletal Muscle Tissue

Nerve and Blood Supply
Neurons that stimulate skeletal muscle
to contract are somatic motor neurons
 The axon of a somatic motor neuron
typically branches many times



Each branch extending to a different
skeletal muscle fiber
Each muscle fiber is in close contact
with one or more capillaries
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue

Microscopic Anatomy

The number of skeletal muscle fibers is
set before you are born


Most of these cells last a lifetime
Muscle growth occurs by hypertrophy

An enlargement of existing muscle fibers
Testosterone and human growth
hormone stimulate hypertrophy
 Satellite cells retain the capacity to
regenerate damaged muscle fibers

Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue

Sarcolemma


Transverse (T tubules)



The plasma membrane of a muscle cell
Tunnel in from the plasma membrane
Muscle action-potentials travel through the T tubules
Sarcoplasm, the cytoplasm of a muscle fiber


Sarcoplasm includes glycogen used for synthesis of
ATP and a red-colored protein called myoglobin
which binds oxygen molecules
Myoglobin releases oxygen when it is needed for
ATP production
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue

Myofibrils


Sarcoplasmic reticulum (SR)




Membranous sacs which encircles each myofibril
Stores calcium ions (Ca++)
Release of Ca++ triggers muscle contraction
Filaments




Thread like structures which have a contractile
function (I.e., Contractile Organelle)
Function in the contractile process
Two types of filaments (Thick and Thin)
There are two thin filaments for every thick filament
Sarcomeres


Compartments of arranged filaments
Basic functional unit of a myofibril
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue
Copyright 2009, John Wiley & Sons, Inc.
Copyright 2009, John Wiley & Sons, Inc.
Copyright 2009, John Wiley & Sons, Inc.
Copyright 2009, John Wiley & Sons, Inc.
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue

Z discs



A band




Lighter, contains thin filaments but no thick filaments
Z discs passes through the center of each I band
H zone


Darker middle part of the sarcomere
Thick and thin filaments overlap
I band


Separate one sarcomere from the next
Thick and thin filaments overlap one another
Center of each A band which contains thick but no thin filaments
M line

Supporting proteins that hold the thick filaments together in the H
zone
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle
Copyright 2009, John Wiley & Sons, Inc.
Contraction
and
Relaxation
of Skeletal
Muscle
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue

Muscle Proteins

Myofibrils are built from three kinds of
proteins

1) Contractile proteins


2) Regulatory proteins


Generate force during contraction
Switch the contraction process on and off
3) Structural proteins



Align the thick and thin filaments properly
Provide elasticity and extensibility
Link the myofibrils to the sarcolemma
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue

Contractile Proteins

Myosin





Thick filaments
Functions as a motor protein which can achieve motion
Convert ATP to energy of motion
Projections of each myosin molecule protrude outward (i.e.,
myosin head)
Actin




Thin filaments
Actin molecules provide a site where a myosin head can attach
Tropomyosin and troponin are also part of the thin filament
In relaxed muscle




Myosin is blocked from binding to actin
Strands of tropomyosin cover the myosin-binding sites
Calcium ion binding to troponin moves tropomyosin away from
myosin-binding sites
Allows muscle contraction to begin as myosin binds to actin
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue

Structural Proteins

Titin



Stabilize the position of myosin
accounts for much of the elasticity and extensibility of
myofibrils
Dystrophin

Links thin myofibrils to the sarcolemma
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle

The Sliding Filament Mechanism
Myosin heads attach to and “walk” along
the thin filaments at both ends of a
sarcomere
 Progressively pulling the thin filaments
toward the center of the sarcomere
 Z discs come closer together and the
sarcomere shortens
 Leading to shortening of the entire
muscle

Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle
Contraction and Relaxation of Skeletal Muscle
Contraction and
Relaxation of
Skeletal Muscle
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle

The Contraction Cycle

The onset of contraction begins with the SR
releasing calcium ions into the muscle cell

Where they bind to actin opening the myosin
binding sites
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle

The contraction cycle consists of 4 steps

1) ATP hydrolysis (=ADP + P)


2) Formation of cross-bridges


Myosin head attaches to the myosin-binding site on actin
3) Power stroke


Hydrolysis of ATP reorients and energizes the myosin head
During the power stroke the crossbridge rotates, sliding the
filaments
4) Detachment of myosin from actin



As the next ATP binds to the myosin head, the myosin
head detaches from actin
The contraction cycle repeats as long as ATP is available
and the Ca++ level is sufficiently high
Continuing cycles applies the force that shortens the
sarcomere
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle
Key:
= Ca2+
1 Myosin heads
hydrolyze ATP and
become reoriented
and energized
ADP
P
P
ATP
ATP
4 As myosin heads
bind ATP, the
crossbridges detach
from actin
Contraction cycle continues if
ATP is available and Ca2+ level in
the sarcoplasm is high
2 Myosin heads
bind to actin,
forming
crossbridges
ADP
ADP
3 Myosin crossbridges
rotate toward center of the
sarcomere (power stroke)
Contraction and Relaxation of Skeletal Muscle

Excitation–Contraction Coupling





An increase in Ca++ concentration in the muscle starts
contraction
A decrease in Ca++ stops it
Action potentials causes Ca++ to be released from the
SR into the muscle cell
Ca++ bind to troponin and moves tropomyosin away
from the myosin-binding sites on actin allowing crossbridges to form
The muscle SR membrane contains Ca++ pumps to
return Ca++ back to the SR quickly


Decreasing calcium ion levels
As the Ca++ level in the cell drops, myosin-binding
sites are covered and the muscle relaxes
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle
Contraction and Relaxation of Skeletal Muscle

Length–Tension Relationship

The forcefulness of muscle contraction
depends on the length of the sarcomeres

When a muscle fiber is stretched there is less
overlap between the thick and thin filaments
and tension (forcefulness) is diminished

When a muscle fiber is shortened the
filaments are compressed and fewer myosin
heads make contact with thin filaments and
tension is diminished
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle
Contraction
and
Relaxation
of Skeletal
Muscle
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle

The Neuromuscular Junction


Neuromuscular junction (NMJ)


Chemical released by the initial cell communicating with the second cell
Synaptic vesicles


Gap that separates the two cells
Neurotransmitter


Where communication occurs between a somatic motor neuron and a
muscle fiber
Synaptic cleft


Action potentials arise at the interface of the motor neuron and muscle
fiber
Synapse


Motor neurons have a threadlike axon that extends from the brain or
spinal cord to a group of muscle fibers
Sacs suspended within the synaptic end bulb containing molecules of
the neurotransmitter acetylcholine (Ach)
Motor end plate


The region of the muscle cell membrane opposite the synaptic end bulbs
Contain acetylcholine receptors
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle

Nerve impulses elicit a muscle action potential in
the following way

1) Release of acetylcholine


2) Activation of ACh receptors



Binding of ACh to the receptor on the motor end plate opens an ion
channel
Allows flow of Na+ to the inside of the muscle cell
3) Production of muscle action potential



Nerve impulse arriving at the synaptic end bulbs causes many
synaptic vesicles to release ACh into the synaptic cleft
The inflow of Na+ makes the inside of the muscle fiber more
positively charged triggering a muscle action potential
The muscle action potential then propagates to the SR to release its
stored Ca++
4) Termination of ACh activity

ACh effects last only briefly because it is rapidly broken down by
acetylcholinesterase (AChE)
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle
Copyright 2009, John Wiley & Sons, Inc.
Axon collateral of
somatic motor neuron
Axon terminal
Nerve impulse
Synaptic vesicle
containing
acetylcholine
(ACh)
Synaptic
end bulb
Sarcolemma
Axon terminal
Synaptic
end bulb
Motor
end
plate
Neuromuscular
junction (NMJ)
Synaptic cleft
(space)
Sarcolemma
Myofibril
(b) Enlarged view of the
neuromuscular junction
(a) Neuromuscular junction
1 1ACh is released
from synaptic vesicle
Synaptic end bulb
Synaptic cleft
(space)
4 ACh is broken down
2
Ach
2 ACh binds to ACh
receptor
Junctional fold
Motor end plate
Na+
3 Muscle action
potential is produced
(c) Binding of acetylcholine to ACh receptors in the motor end plate
Contraction and Relaxation of Skeletal Muscle
Copyright 2009, John Wiley & Sons, Inc.
Nerve
impulse
1 Nerve impulse arrives at
axon terminal of motor
neuron and triggers release
of acetylcholine (ACh).
2 ACh diffuses across
synaptic cleft, binds
to its receptors in the
motor end plate, and
triggers a muscle
action potential (AP).
ACh receptor
3 Acetylcholinesterase in
Synaptic vesicle
synaptic cleft destroys
filled with ACh
ACh so another muscle
action potential does not
arise unless more ACh is
released from motor neuron.
Muscle action
potential
Transverse tubule
4 Muscle AP travelling along
transverse tubule opens Ca2+
release channels in the
sarcoplasmic reticulum (SR)
membrane, which allows
calcium ions to flood into the
sarcoplasm.
SR
Ca2+
9 Muscle relaxes.
8 Troponin–tropomyosin
complex slides back
into position where it
blocks the myosin
binding sites on actin.
5 Ca2+ binds to troponin on
the thin filament, exposing
the binding sites for myosin.
Elevated Ca2+
Ca2+ active
transport pumps
7 Ca2+ release channels in
SR close and Ca2+ active
transport pumps use ATP
to restore low level of
Ca2+ in sarcoplasm.
6 Contraction: power strokes
use ATP; myosin heads bind
to actin, swivel, and release;
thin filaments are pulled toward
center of sarcomere.
Contraction and Relaxation of Skeletal Muscle

Botulinum toxin


Blocks release of ACh from synaptic vesicles
May be found in improperly canned foods


A tiny amount can cause death by paralyzing respiratory
muscles
Used as a medicine (Botox®)





Strabismus (crossed eyes)
Blepharospasm (uncontrollable blinking)
Spasms of the vocal cords that interfere with speech
Cosmetic treatment to relax muscles that cause facial wrinkles
Alleviate chronic back pain due to muscle spasms in the
lumbar region
Copyright 2009, John Wiley & Sons, Inc.
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle

Curare




A plant poison used by South American Indians on
arrows and blowgun darts
Causes muscle paralysis by blocking ACh receptors
inhibiting Na+ ion channels
Derivatives of curare are used during surgery to relax
skeletal muscles
Anticholinesterase


Slow actions of acetylcholinesterase and removal of
ACh
Can strengthen weak muscle contractions


Antidote for curare poisoning
Terminate the effects of curare after surgery
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism
Production of ATP in Muscle Fibers
A huge amount of ATP is needed to:
 Power the contraction cycle
 Pump Ca++ into the SR
The ATP inside muscle fibers will power
contraction for only a few seconds
ATP must be produced by the muscle fiber
after reserves are used up
Muscle fibers have three ways to produce ATP
 1) From creatine phosphate
 2) By anaerobic cellular respiration
 3) By aerobic cellular respiration
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism
Creatine Phosphate (15 secs)
Excess ATP is used to synthesize creatine
phosphate
 Energy-rich molecule
Creatine phosphate transfers its high energy
phosphate group to ADP regenerating new
ATP
Creatine phosphate and ATP provide enough
energy for contraction for about 15 seconds
Creatine = small amino acid like molecule
Muscle Metabolism
 Anaerobic Respiration (30-40 secs)
 Series of ATP producing reactions that do not require
oxygen
 Glucose is used to generate ATP when the supply of
creatine phosphate is depleted
 Glucose is derived from the blood and from glycogen
stored in muscle fibers
 Glycolysis breaks down glucose into molecules of pyruvic
acid and produces two molecules of ATP
 If sufficient oxygen is present, pyruvic acid formed by
glycolysis enters aerobic respiration pathways producing a
large amount of ATP
 If oxygen levels are low, anaerobic reactions convert
pyruvic acid to lactic acid which is carried away by the
blood and can be used by the heart…
 Anaerobic respiration can provide enough energy for about
30 to 40 seconds of muscle activity
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism
 Aerobic Respiration (>10 min, 90% of ATP)
 Activity that lasts longer than half a minute depends on aerobic
respiration
 Pyruvic acid entering the mitochondria is completely oxidized
generating




ATP
carbon dioxide
Water
Heat
 Each molecule of glucose yields up to 36 molecules of ATP
 Muscle tissue has two sources of oxygen
 1) Oxygen from hemoglobin in the blood
 2) Oxygen released by myoglobin in the muscle cell
 Myoglobin and hemoglobin are oxygen-binding proteins
 Aerobic respiration supplies ATP for prolonged activity
 Aerobic respiration provides more than 90% of the needed ATP
in activities lasting more than 10 minutes
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism
Muscle Fatigue
Inability of muscle to maintain force of
contraction after prolonged activity
Factors that contribute to muscle fatigue
Inadequate release of calcium ions from the SR
Depletion of creatine phosphate (ATP~)
Insufficient oxygen
Depletion of glycogen and other nutrients
Buildup of lactic acid and ADP
Failure of the motor neuron to release enough
acetylcholine
Central fatigue
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism
Oxygen Consumption After Exercise
After exercise, heavy breathing continues and
oxygen consumption remains above the
resting level
Oxygen debt (”pay back” AKA - Recovery
oxygen uptake)
 The added oxygen that is taken into the body after
exercise
This added oxygen is used to restore muscle
cells to the resting level in three ways
 1) to convert lactic acid into glycogen (liver)
 2) to synthesize creatine phosphate and ATP
 3) to replace the oxygen removed from myoglobin
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension

The tension or force of muscle cell
contraction varies

Maximum Tension (force) is dependent on




The rate at which nerve impulses arrive
The amount of stretch before contraction
The nutrient and oxygen availability
The size of the motor unit
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension

Motor Units





Consists of a motor neuron and the muscle fibers it stimulates
The axon of a motor neuron branches out forming neuromuscular
junctions with different muscle fibers
A motor neuron makes contact with about 150 muscle fibers
Contract in unison
Control of precise movements consist of many small motor units




Muscles that control voice production have 2 - 3 muscle fibers per
motor unit
Muscles controlling eye movements have 10 - 20 muscle fibers per
motor unit
Muscles in the arm and the leg have 2000 - 3000 muscle fibers per
motor unit
The total strength of a contraction depends on the size of the
motor units and the number that are activated
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension

Twitch Contraction



The brief contraction of the muscle fibers in a
motor unit in response to an action potential
Twitches last from 20 to 200 msec L
Latent period (2 msec)



A brief delay between the stimulus and muscular
contraction
The action potential sweeps over the sarcolemma
and Ca++ is released from the SR
Contraction period (10–100 msec)



Ca++ binds to troponin
Myosin-binding sites on actin are exposed
Cross-bridges form
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension

Relaxation period (10–100 msec)



Ca++ is transported into the SR w/ ATP
Myosin-binding sites are covered by tropomyosin
Myosin heads detach from actin w/ATP present



Muscle fibers that move the eyes have contraction periods
lasting 10 msec
Muscle fibers that move the legs have contraction periods
lasting 100 msec
Refractory period

When a muscle fiber contracts, it temporarily cannot
respond to another action potential


Skeletal muscle has a refractory period of 5 milliseconds
Cardiac muscle has a refractory period of 300 milliseconds
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension
Stim 20-30 times/sec
Stim 80-100 times/sec
Control of Muscle Tension
Copyright 2009, John Wiley & Sons, Inc.
Motor Unit Recruitment



Alternation of motor unit use by the muscle
Allows for delayed fatigue (some MUs rest
others contract)
Prevention of “jerky” movements
Control of Muscle Tension

Muscle Tone




A small amount of tension in the muscle due
to weak/involuntary contractions of motor units
Small groups of motor units are alternatively
active and inactive in a constantly shifting
pattern to sustain muscle tone
Muscle tone keeps skeletal muscles firm
Keep the head from slumping forward on the
chest
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension

Types of Contractions

Isotonic contraction




The tension developed remains constant while the
muscle changes its length
Used for body movements and for moving objects
Picking a book up off a table
Isometric contraction


The tension generated is not enough for the object
to be moved and the muscle does not change its
length
Holding a book steady using an outstretched arm
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers

Muscle fibers vary in their content of
myoglobin

Red muscle fibers





Have a high myoglobin content
Appear darker (dark meat in turkey legs and thighs)
Contain more mitochondria
Supplied by more blood capillaries
White muscle fibers


Have a low content of myoglobin
Appear lighter (white meat in turkey breasts)
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers


Muscle fibers contract at different speeds, and
vary in how quickly they fatigue
Muscle fibers are classified into three main types



1) Slow oxidative fibers
2) Fast oxidative-glycolytic fibers
3) Fast glycolytic fibers
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers

Slow Oxidative Fibers (SO fibers)





Smallest in diameter
Least powerful type of muscle fibers
Appear dark red (more myoglobin)
Generate ATP mainly by aerobic cellular respiration
Have a slow speed of contraction




Twitch contractions last from 100 to 200 msec
Very resistant to fatigue
Capable of prolonged, sustained contractions for
many hours
Adapted for maintaining posture and for aerobic,
endurance-type activities such as running a marathon
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers

Fast Glycolytic Fibers (FG fibers)










Largest in diameter
Generate the most powerful contractions
Have low myoglobin content
Relatively few blood capillaries
Few mitochondria
Appear white in color
Generate ATP mainly by glycolysis
Fibers contract strongly and quickly
Fatigue quickly
Adapted for intense anaerobic movements of short
duration

Weight lifting or throwing a ball
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers

Fast Oxidative–Glycolytic Fibers (FOG fibers)







Intermediate in diameter between the other two types
of fibers
Contain large amounts of myoglobin and many blood
capillaries
Have a dark red appearance
Generate considerable ATP by aerobic cellular
respiration
Moderately high resistance to fatigue
Generate some ATP by anaerobic glycolysis
Speed of contraction faster than SO


Twitch contractions last less than 100 msec
Contribute to activities such as walking and sprinting
Copyright 2009, John Wiley & Sons, Inc.
Types of
Skeletal
Muscle
Fibers
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers

Distribution and Recruitment of
Different Types of Fibers


Most muscles are a mixture of all three types
of muscle fibers
Proportions vary, depending on the action of
the muscle, the person ’s training regimen,
and genetic factors



Postural muscles of the neck, back, and legs have
a high proportion of SO fibers
Muscles of the shoulders and arms have a high
proportion of FG fibers
Leg muscles have large numbers of both SO and
FOG fibers
Copyright 2009, John Wiley & Sons, Inc.
Exercise and Skeletal Muscle Tissue

Ratios of fast glycolytic and slow oxidative
fibers are genetically determined

Individuals with a higher proportion of FG
fibers


Excel in intense activity (weight lifting, sprinting)
Individuals with higher percentages of SO
fibers

Excel in endurance activities (long-distance
running)
Copyright 2009, John Wiley & Sons, Inc.
Exercise and Skeletal Muscle Tissue

Various types of exercises can induce
changes in muscle fibers

Aerobic exercise transforms some FG fibers
into FOG fibers


Endurance exercises do not increase muscle mass
Exercises that require short bursts of strength
produce an increase in the size of FG fibers

Muscle enlargement (hypertrophy) due to increased
synthesis of thick and thin filaments and more
myofibrils
Copyright 2009, John Wiley & Sons, Inc.
TYPE: Smooth muscle
DESCRIPTION: Contractile cells
tapered at both ends; not striated
LOCATIONS: Wall of arteries,
sphincters, stomach, intestines,
urinary bladder, many other soft
internal organs
FUNCTION: Controlled constriction;
motility (as in gut); arterial blood
flow
Smooth Muscle Tissue




Usually activated involuntarily
Action potentials are spread through the fibers
by gap junctions
Fibers are stimulated by certain
neurotransmitter, hormone, or autorhythmic
signals
Found in the






Walls of arteries and veins
Walls of hollow organs
Walls of airways to the lungs
Muscles that attach to hair follicles
Muscles that adjust pupil diameter
Muscles that adjust focus of the lens in the eye
Copyright 2009, John Wiley & Sons, Inc.
Smooth Muscle Tissue

Microscopic Anatomy of Smooth
Muscle

Contains both thick filaments and thin
filaments





Not arranged in orderly sarcomeres
No regular pattern of overlap thus not striated
Contain only a small amount of stored Ca++
Filaments attach to dense bodies and stretch
from one dense body to another
Dense bodies


Function in the same way as Z discs
During contraction the filaments pull on the dense bodies
causing a shortening of the muscle fiber
Copyright 2009, John Wiley & Sons, Inc.
Smooth
Muscle
Tissue
Copyright 2009, John Wiley & Sons, Inc.
Smooth
Muscle
Tissue
Copyright 2009, John Wiley & Sons, Inc.
Smooth Muscle Tissue

Visceral (single-unit) SMT


Tubular arrangements in small arteries, veins,
hollow organs
Multiunit SMT

Large arteries, airways, arrector pili, iris, ciliary
process of lens in eye.
Copyright 2009, John Wiley & Sons, Inc.
Smooth Muscle Tissue

Physiology of Smooth Muscle




Contraction lasts longer than skeletal muscle
contraction
Contractions are initiated by Ca++ flow primarily from
the interstitial fluid
Ca++ move slowly out of the muscle fiber delaying
relaxation
Able to sustain long-term muscle tone


Prolonged presence of Ca++ in the cell provides for a state of
continued partial contraction
Important in the:


Gastrointestinal tract where a steady pressure is maintained on
the contents of the tract
In the walls of blood vessels which maintain a steady pressure on
blood
Copyright 2009, John Wiley & Sons, Inc.
Smooth Muscle Tissue

Physiology of Smooth Muscle

Most smooth muscle fibers contract or relax in
response to:

Action potentials from the autonomic nervous
system


In response to stretching


Food in digestive tract stretches intestinal walls initiating
peristalsis
Hormones


Pupil constriction due to increased light energy
Epinephrine causes relaxation of smooth muscle in the airways and in some blood vessel walls
Changes in pH, oxygen and carbon dioxide levels
Copyright 2009, John Wiley & Sons, Inc.
Regeneration of Muscular Tissue

Hyperplasia


An increase in the number of fibers
Skeletal muscle has limited regenerative abilities


Growth of skeletal muscle after birth is due mainly to
hypertrophy
Satellite cells divide slowly and fuse with existing
fibers



Cardiac muscle can undergo hypertrophy in
response to increased workload


Assist in muscle growth
Repair of damaged fibers
Many athletes have enlarged hearts
Smooth muscle in the uterus retain their capacity
for division
Copyright 2009, John Wiley & Sons, Inc.
Aging and Muscular Tissue

Aging






Brings a progressive loss of skeletal muscle mass
A decrease in maximal strength
A slowing of muscle reflexes
A loss of flexibility
With aging, the relative number of slow
oxidative fibers appears to increase
Aerobic activities and strength training can
slow the decline in muscular performance
Copyright 2009, John Wiley & Sons, Inc.