Muscle physiology in orthodontics/certified fixed orthodontic courses by Indian dental academy

MUSCLE PHYSIOLOGY
INDIAN DENTALACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.com

INTRODUCTION
• Muscles as a whole helps to orient the basic
structure of human body. For every movement to
take place an association has to be there between
the bones and muscles.A thorough knowledge of
muscles is a must for proper delivery of
orthodontic and orthopedic treatment .
• It consists of 50% of total body weight.apart from
regulating the body position,it helps to maintain
proper organ volume ,generates heat and propels
fluid.
• Study of muscles is called as myology

TYPES OF MUSCLES
• Skeletal muscle:so named because it moves the
bones of the skeleton.Actions are mainly
voluntary but can act involuntarily to an extent
and are striated.e.g.: stretch reflexes.
• Cardiac muscle:It forms the heart walls and has
auto rythmicity.
• Smooth muscles:Found in the hollow internal
structures like blood vessels and non striated and
mostly always involuntary.

Functions of muscle tissues.
• Produces total and localized body movements.
• Skeletal muscle contractions stabilizes joints and
helps maintain body positions.
• Helps regulate organ volume through smooth
muscles called as sphincters.
• Helps moving substances within the body like
blood.
• Helps produce heat.

Properties of muscle tissues.
• Four functions enable homeostasis:
• Electrical excitability or irritability:is the ability to
respond to certain stimuli by producing
signals,examples are autorythmic electrical signals
or chemical stimuli such as neurotransmitters.
• Contractility is the ability of muscle tissue to
contract forcefully when stimulated by an action
potential.isometric and isotonic contractions.
• Extensibility is the ability to be stretched without
being damaged.
• Elasticity is the ability to return to original length
and shape after contraction and extension.www.indiandentalacademy.com

Microscopic anatomy of a
skeletal muscle fiber
• In embryonic development each skeletal muscle
fiber arises from the fusion of a million
mesodermal cells called as myoblasts.of which
few cells called satellite cells which persists later
in life fuse and cause regeneration.
• Connective tissue components are of three layers
extending from the deep fascia to further protect
and strengthen the skeletal muscle fiber.
• The outermost layer encircling the whole muscle
is the epimysium . Perimysium surrounds groups
of 10-100 individual muscle fibers separating
them into bundles called as fascicles which are
large enough to be seen with the naked eye.www.indiandentalacademy.com

• Each individual muscle
fiber is separated from one
another by the
endomysium.
• Sarcolemma is the fiber’s
plasma membrane and
tiny invaginations tunnel
from the surface to the
center of each muscle
fiber called as ‘T tubules
or transverse’ tubules.
• They are filled with ECF
thus when an action
potential propagates ,it
ensures that all parts of the
muscle is stimulated
simultaneously.

Cell structure
• The cytoplasm called as sarcoplasm contains
glycogen which can be split into glucose used for
ATP synthesis.It also contains myoglobin, an
oxygen binding protein found exclusively in
muscle fibers.
• Myofibrils-Little thread like small contractile
structures about 2micrometer diameter run along
the whole length making the muscle fiber striated.
• Sarcoplasmic reticulum are fluid filled sacs
encircling each myofibril and store calcium and its
release triggers contraction.

Filaments:
• Inside each myofibril
there are even more
smaller structures called
as filaments which are of
two types thick and thin
filaments.
• They are arranged in
compartments called as
sarcomeres which are the
basic functional unit of a
myofibril.
• Filaments overlap causing
zones and bands .The
darker middle portion
extending along the entire
length of the thick
filaments is A band.
I bands are less dense and are the
Thin filaments.Z discs pass through
Center of each I band.

Muscle proteins:
• Contractile proteins consists of actin and myosin.
• Myosin functions as a motor protein,push and pull
their cargo to achieve movement by converting
chemical energy in ATP to mechanical.each
molecule has projections called cross bridges .
• Actin forms the thin filaments,individual
molecules form a helix and each has a myosin
binding site where the head of the myosin can
attach to.

What keeps the myosin and actin
filaments in place?
• The regulatory proteins are tropomysin and
troponin which are part of thin filaments.In
relaxed muscle,myosin blocked from binding to
actin because tropomysin covers the myosin
binding site on actin . Tropomysin instead holds
troponin in place.It helps switch contraction on
and off.
• Titin - filamentous structural proteins accounts for
the elasticity and extensibility of myofibrils and
helps sarcomere return to resting length after
contraction.

General mechanism of muscle
contraction.
• An action potential travels along a motor nerve to
its endings on muscle fibers.at each ending ,the
nerve secretes a small amount of neurotransmitters
called acetylcholine.
• It acts on local areas of the muscle fiber membrane
to open multiple,Acetylcholine gated channels
through protein molecules floating in the
membrane.
• Opening of Acetylcholine gated channels allows
large quantities of sodium ions to flow to interior
of muscle fiber membrane.This initiates an action
potential in the muscle fiber.www.indiandentalacademy.com

Contd…
• This action potential travels along the muscle fiber
membrane the same way it travels in
nerves.Action potential depolarizes the muscle
membrane and travels deeply within the muscle
fiber causing the SR to release large amount of
Ca++ stored.
• Ca++ initiates attractive forces between actin and
myosin filaments causing them to slide alongside
with each other,which is the contractile process.
• After a fraction of a second ,the Ca++ are pumped
back into SR by a Ca++ membrane pump and
remain stored until new muscle potential comes
along;This removal of Ca++ causes muscle
contraction to cease.www.indiandentalacademy.com

EFFECTS OF EXCITATION
ON A MUSCLE FIBER.

The contraction cycle.
• At the onset of contraction the SR releases Ca+
+which binds to troponin and causes the troponin
–tropomyosin complexes to move away from the
myosin binding sites on actin.Once the sites are
free, the contraction cycle – the repeating
sequence of events that causes the filaments to
slide begins.
The contraction cycle - fours steps:
• 1.ATP hydrolysis:the myosin head includes an
ATP binding pocket and an ATPase,an enzyme
that hydrolyzes ATP into ADP and a phosphate
group.this hydrolysis reaction energizes the
myosin head. www.indiandentalacademy.com

Contd…
• 2.attachment of myosin to actin to form cross
bridges.the energized myosin head attaches to the
myosin binding site on actin and then releases the
previously hydrolyzed phosphate group.
• 3.Power stroke.The release of the phosphate group
triggers the power stroke of contraction.the
pocket on the myosin head where ADP is still
bound opens,which rotates the myosin head and
releases the ADP. The myosin head generates
force as it rotates towards the center of the
sarcomere,sliding the thin filament past the thick
towards the middle.www.indiandentalacademy.com

Contraction cycle contd….
• 4.detachment of the
myosin from the actin.
At the end of the power
stroke,the myosin head
remains firmly attached to
actin until it binds another
molecule of ATP.As ATP
binds to the ATP
BINDING POCKET on
the myosin head ,the
myosin head detaches
from actin.
• The contraction cycle
repeats over and over,as
long as ATP is available
and Ca++level near the
thin filament is sufficient.www.indiandentalacademy.com

Sliding mechanism of muscle
contraction• In a relaxed state ends of
actin filaments barely
begin to overlap one
another and lie close to the
myosin filaments.
• Contracted state these
actin filaments are pulled
inward among the myosin
so that their ends
overlap.Also Z discs are
pulled by actin up to the
end of myosin
filaments.This muscle
contraction is called
sliding filament mech.www.indiandentalacademy.com

What causes the actin filaments
to slide inward among the
myosin filaments?
• The mechanical forces generated by the
interaction of cross bridges of myosin with
actin filaments is the reason.when action
potential travels it triggers release of Ca++
in turn it activates the forces between
myosin and actin filaments and contraction
begins.Energy required is derived from
ATP being degraded to ADP.

Inhibition of the actin filaments
by troponin tropomysin complex,
activation by Ca++
• When this complex is present in actin,
binding does not take place.
• Role of Ca++ in inhibition is that when it
combines with the troponin complex,each
molecule can bind with up to 4 Ca++ and
pushes the complex to deeper grooves within
the actin strand.Thus uncovers active actin
site allowing myosin head to attach and
cause contraction.www.indiandentalacademy.com

.
• When a muscle contracts ,work is performed and
energy is required.large amounts of ATP are
cleaved to form ADP during contraction
process,the greater the amount of work performed
by the muscle ,the greater the amount of ATP is
cleaved which is called as the Fenn effect.
• Muscle fibers have three main sources of
ATP:1)creatinine phosphate 2)Anaerobic cellular
respiration 3)aerobic cellular respiration.
Energy sources for muscle
contraction

Creatinine phosphate:
• While at rest muscle fibers produce more ATP
than they need for resting metabolism.some of the
excess ATP is used to synthesize Creatinine
Phosphate,an energy rich molecule unique to the
muscle fibers.The enzyme creatine kinase
catalyses the transfer of one of the high energy
PO4 groups of ATP to creatine,forming creatine
phosphate and ADP.Together CP and ATP
provide enough energy for muscles to contract
maximally for about 15 seconds.

Anaerobic cellular respiration:
• It is a series of ATP producing reactions
that do not require O2.when muscle activity
continues and the supply of CP is
depleted,glucose is catabolised to generate
ATP.
• Glucose easily passes from the blood into
contracting muscle fibers via facilitated
diffusion.

Aerobic cellular respiration
• Muscle activity that lasts longer than half a min
depends increasingly on aerobic cellular
respiration,a series of oxygen requiring
mitochondrial reactions that produce ATP.
• Muscle tissue has two sources of O2: 1)oxygen
that diffuses into muscle fibers from the blood.
2)oxygen released by myoglobin an oxygen
binding protein present in the muscle fibers.
• Aerobic cellular respiration provides enough ATP
for prolonged activity so long as sufficient oxygen
and nutrients are available
• Glycogen/glucose
+ADP+P1+O2H2O+CO2+ATPwww.indiandentalacademy.com

TWITCH CONTRACTION• In myography i.e. muscle
graphing a single brief
threshold stimulus
produces a quick jerk of a
muscle called as twitch
contraction.
• A muscle doesn’t begin
contraction at the instant
of stimulation but rather a
fraction of a sec.there are
3 phases respectively the
latent period,contraction
phase and relaxation
phase.entire twitch lasts
for 1/10th
of a sec.
Twitch rarely happens in the body ,can occur when there's
over activity of the nervous system to stimulate all motor neurons
in a muscle fiber.

TREPPE:THE STAIR CASE
PHENOMENON
• Treppe is the gradual step like increase in the
strength of contraction that can be observed in a
series of twitch contractions that occur about one
second apart.
• A muscle contracts more forcefully after it has
contracted a few times than when it first contracts
– a principle used by athletes to warm up.Possibly
it relates partly to the rise in temp of the active
muscle and partly to their accumulation of
metabolic products.
• After first few stimuli muscle responds to a
considerable number of successive stimuli withwww.indiandentalacademy.com

Treppe contd…
maximal contractions.
Eventually it will respond
with less and less strong
contractions i.e. relaxation
phase shortens and
disappears and muscle
stays in a state of
abnormal contraction-
contracture.
• Repeated stimuli lessens
excitability and
contractility resulting in
muscle fatigue such that
the muscle wont respond
even to the strongest
stimuli. www.indiandentalacademy.com

MUSCLE TONE
• A tonic contraction (tonus)is a continual partial
contraction.At any one moment a small number of
the total fibers in a muscle contract producing
tautness and firmness of the muscle rather than a
recognizable contraction.It’s a characteristic of a
muscle fiber when they are awake.muscles with
less tone are flaccid and more spastic.
• Its maintained by a negative feed back mechanism
centered in the spinal cord.Stretch sensors detect
the info for degree of stretch and send them to the
integrator mechanism in spinal cord.When actual
stretch deviates from set point ,signals are sent via
somatic motor neurons and adjust the strength of
contraction.this subconscious reflex is called
spinal reflex.

ISOTONIC AND ISOMETRIC
CONTRACTIONS• Isotonic contraction is one in which the tone or
tension within a muscle remains the same but the
length of the muscle changes,i.e.,same
tension.Since there’s no /little tension placed on a
muscle in this contraction,all of the energy is used
to pull on the thin filaments and decrease the
length of a fiber’s sarcomere.
• Two types:concentric and eccentric isotonic
contraction.
• In concentric ,a muscle shortens and pulls on
another structure like a tendon .in eccentric is
when the overall length of a muscle increases
during contraction.Repeated eccentricwww.indiandentalacademy.com

Contd…
contractions are more
damaging than concentric
and causes more delayed
onset muscle soreness.
• Isometric contractions:
muscle length remains
same but in which muscle
tension increases,i.e.,same
length.the tension
produced by the power
stroke of myosin cross
bridges cant overcome the
load placed on the muscle.

Graded Strength Principle• Skeletal muscles contract
with varying degrees of
strength at different times-
Graded strength
principle.
• Improved metabolic
conditions that produce
the treppe effect allow a
muscle to increase its
contraction strength.
• Another factor influencing
GSP is number of fibers
contracting
simultaneously.The more
the fibers contracting at
the same time the more
stronger the contraction.www.indiandentalacademy.com

Length Tension Relation
• The maximal strength that a muscle can develop is
directly related to the initial length of its fiber- this
is the length tension relationship.A muscle that
starts contraction from a short initial tension
cannot develop much tension since its already
compressed.
• Conversely overstretched muscle cant develop
much tension because the thick myofilament are
too far away from the thin to effectively pull them
and compress the sarcomere.The strongest
maximal contraction is possible only when the
muscle organ has been stretched to an optimal
length. www.indiandentalacademy.com

TYPES OF SKELETAL
MUSCLE FIBERS.
• Depending on the color of the fibers they are
called red and white muscle fibers.Red muscle
fibers are called so because they contain
myoglobin and more blood capillaries and ATP.
• Also based on enzyme histochemistry,
ATPase,speed of contraction and susceptibilty to
fatigue they are divided into three main
types:1)slow oxidative glycolytic fibers 2)fast
oxidative glycolytic fibers 3)fast glycolytic fibers.

Slow Oxidative Fibers:
• They are the smallest in diameter and thus the
least powerful type.they are dark red because they
contain large amounts of myoglobin and many
blood capillaries.They also have a lot of
mitochondria ,SO fibers generate ATP mainly by
aerobic cellular respiration,which is why they are
called oxidative fibers.
• Has low contraction velocity ,very resistant to
fatigue and capable of prolonged,sustained
contraction for many hours.These slow twitch
,fatigue resistant fibers help maintain posture of
the mandible and mastication of soft
diet.Temporalis,Masseter,ant Medial pterygoid
and Lateral pterygoid muscle all have SO fibers.www.indiandentalacademy.com

Fast Oxidative Glycolytic Fibers
• Intermediate in diameter they contain large
amounts of myoglobin and blood capillaries but
lesser than that of SO fibers.FOG fibers can
generate considerable ATP by aerobic cellular
respiration, which give them a moderately high
resistance to fatigue and also generate ATP by
anaerobic glycolysis.Their contraction velocity is
higher than SO fibers.FOG fibers reach peak
tension quickly than those of SO fibers but are
briefer in duration.Used in walking and sprinting.

Fast Glycolytic Fibers:
• Has the largest diameter and contains the highest
number of myofibrils and this generates the most
powerful contractions.They have low myoglobin
and relatively few blood capillaries and few
mitochondria and appear white in color.
• ATP produced by glycolysis.Contract rapidly and
strongly.These fast twitch fibers are adapted for
intense anaerobic movements of short duration,
but they fatigue quickly.
• More in subjects with high biting forces.Found in
jaw muscles 45% in superior temporalis
muscle,post medial pterygoid and ant digastric
muscle. www.indiandentalacademy.com

STRETCH REFLEX
• Another factor that influences strength of a
skeletal muscle contraction is amount of load
imposed on the muscle.With in certain limits the
heavier the load the stronger the contraction.
• An example:Lift your left hand with your palm up
in front of you and then put a book in your
palm.you can feel the muscle contract more
strongly.This occurs because of a stretch reflex,a
response in which the body tries to maintain a
constancy of muscle length.
• An increase in load threatens to stretch the muscle
beyond the set point length that you are trying to
maintain.Your body exhibits a –ve feed back whenwww.indiandentalacademy.com

Contd..
it detects the increased
stretch caused by an
increased load,feeds the
information back to the
nervous system and in
turns increases stimulation
of the muscle to
counteract the stretch.
• This reflex maintains
constant muscle
length.When load
increases body abandons
this reflex as it can
damage the muscle and
forces the muscle to relax
and drop the load.

MYOTACTIC RELFEX
• The stretch reflex when elicited causes contraction
of the stretched muscle.Muscle stretch receptors
are proprioceptive nerve endings called muscle
spindles.It consists of a bundle of thin intrafusal
muscle fibers within the spindle.
• The long slender ends of the intrafusal fibers are
striated and contractile whereas the nuclear bag
region is noncontractile.The impulses arising from
here are conducted by the group 1A sensory
fibers.These sensory fibers synapse with the motor
neuron called alpha efferent that supply the
extrafusal fibers responsible for the contraction of
the stretched muscle.Therefore this is a
monosynaptic reflex.

Contd….
• The functional
significance is that, it
serves as a mechanism for
maintaining posture.The
stretch of the muscle acts
as a stimulus to elicit the
stretch reflex causing a
sustained contraction of
the stretched muscle
maintaining it upright
against the action of
gravity.
• The same reflex acts in the
mandibular musculature to
maintain the postural rest
position of the mandible
in relation to the maxilla.
In an activator the clinician induces a
A myotactic reflex in the muscles of
mastication by using a bite registration,
thus maintaing the main force provided
through active tension in the stretched
muscles of mastication.www.indiandentalacademy.com

CLASP KNIFE REFLEX
• Also called as autogenic inhibition.if one tries to
flex the the spastic limb of a patient
forcibly,resistance is encountered as soon as the
muscle is stretched.This is a resistance due to the
hyperactive reflex contraction of the muscle in
response to stretch.If flexion is carried further
forcibly a point is reached where all resistance
melts and the rigid limb collapses readily.Because
the resistance of the limb resembles that of a
spring loaded folding knife blade its called clasp
knife reaction i.e. the muscle first resists ,then
relaxes.
• The stimulus necessary to elicit such a reflex is
excessive stretch and when elicited it inhibits
muscular contraction,causing the muscle to relax.

Contd……
• The receptors for this are
the golgi tendon
organs,impulses carried by
the group 1B sensory
fibers.impulses act on the
motor neuron supplying
the stretched muscle.It’s a
disynaptic reflex arc
because an interneuron is
interposed in between the
sensory and motor
neuron.The output of the
motor neuron pool
depends upon the balance
between the two
antagonistic inputs .
Functional significance is to protect
the overload by preventing damaging
contraction against strong stretching
forces.www.indiandentalacademy.com

ACTIVE AND PASSIVE
TENSION.• All the mandibular levator muscles posses
spindles,thus stretch reflexes can be elicited .The
tension resulting from contraction of muscles is
called as ACTIVE TENSION.
• Facial muscles don’t posses spindles they don’t
elicit a stretch reflex.The tension in these muscles
will increase with elongation because of the elastic
properties of the muscle and its investing
tissues.Tension which results from the viscoelastic
properties of the tissue is called as PASSIVE
TENSION.In many muscles,elongation will result
in an increase in both active and passive
tensions.the sum of both tension is called TOTAL
TENSION.

Contd….
• Initially there’s no passive tension,so the total
tension is equal to active tension when the muscle
contracts.As stretching increases the muscle
begins to behave elastically.So passive tension
now begins to add to total tension.As it elongates
further active tension is inhibited while passive
tension continues to increase.On further stretching
active tension suppressed and passive tension
increases exponentially.At this length total tension
equals passive tension.

Contd…
• The total tension in a
muscle is the result of
active tension from the
myotactic reflex and
passive tension from
viscoelastic properties of
the tissues.Moderate bite
registrations in activator
treatment attempt to use
active tension to achieve
correction of
malocclusion.More
extreme vertical openings
in which the mandible is
opened 8mm beyond the
rest position uses passive
tension. www.indiandentalacademy.com

Contd…..
• Many treatment procedures in orthodontics results
in the elongation of jaw and facial
muscles.Expansion of the dental arches stretches
the cheek/lips and increases tension in the
buccinator and orbicularis oris muscle.Increasing
the occlusal vertical dimension in closed bite
malocclusion will stretch the levator muscles.Bite
planes and activators which increase vertical
dimension and advance the mandible increase
tension in both levator and retractor muscles.
• Following immobilization in a shortened muscle
extensibility is decreased i.e its less elastic.During
sleep in jaw muscles stretch reflex is abolished
passive tension generated by appliances.www.indiandentalacademy.com

ELECTROMYOGRAPHY.
• Its used to evaluate the activity of an orofacial
muscle on an electromyogram.The electrical
activity is the easiest to record.Electrodes are
inserted into the skin surface or into the body of a
muscle.They record the membrane potential from
several fibers in a single motor unit which arrives
at different times.At high force levels there’s
interaction of negatively and positively charged
action potentials leading to a leveling of in the
electromyogram causing an interference pattern.
• This gives useful information about when a
muscle is active and defines when activity begins
and ends but its impossible to know how much
activity is being missed.www.indiandentalacademy.com

conclusion
• The functions of oral and facial muscles is
complex in nature.
• Studies have been conducted to know the nature of
these muscles and relationship to various facial
types.
• The knowledge about the relationship between
muscles and facial types is important in
understanding different facial forms and
formulating ideal treatment for the patient.

• Removable orthodontic appliances-Graber and
Neumann.
• Handbook of orthodontics-Robert.E.Moyers.
• Principles of anatomy and physiology-Graboski
and Tortora.
• Review of medical physiology-Ganong.
• Anthony’s textbook of anatomy and physiology-
Gary.A.Thibodeau and Kevin .T.Patton.
REFERENCES

Muscle physiology in orthodontics/certified fixed orthodontic courses by Indian dental academy

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