1. 1
Dr. Mohammed Alruby
Muscles
Part 1
Prepare by
Dr. Mohammed Alruby

2. 2
Dr. Mohammed Alruby
Histology of muscles
Physiology of muscles
Muscles development
Orofacial muscles
- Facial muscles
- Jaw muscles
- Portal muscles
Methods of studying muscles
Muscle changes during growth
Muscle function and facial development

3. 3
Dr. Mohammed Alruby
Histology of muscles
The structural and functional unit of the muscles is the muscle fiber
Muscle fiber: elongated cylinder measure about 10 to 80 microns in thickness and from 1 to 15cm
in length
= Each muscle fiber contains an acidophilic granular cytoplasm (sarcoplasm) that rich in:
Glycogen, mitochondria, Golgi apparatus, protein (actin, myosin, tropomyosin),
Large number of myofibrils (sarcostyles) which responsible for muscle contraction
= the muscle fiber is covered by thick membrane called (sarcolemma) and surrounded by CT called
(endomysium)
= the muscle fibers are coalescing together to form bundles; each bundles are covered by C T
septa called perimysium
= the muscle bundles are coalescing together to the whole muscle which is covered by CT fascia
called epimysium, these CT contain: blood vessel, lymph vessel, and nerves, that firmly attach the
muscle bundles to each other and attach the whole muscle to its tendon
= the myofibrils (sarcostyles)are the contractile units of the muscle, in skeletal muscle they are
transversely striated due to presence of dark and light bands
The dark bands are formed of thick myosin filaments rich in Ca, the light bands are formed of thin
actine filaments rich in water, there is a pale line in at the center of dark band called (Henesen’s
disk), There is dark line at the center of light bands called (Krauses membrane) or Z line
The distance between the two lines called (sarcomere) which is a contraction unit of the muscle.
During the muscle contraction there the Sarcomere is shortened due to sliding of the light bands
over the dark bans. The energy required for contraction is derived from transformation of ATP --
---- ADP
Physiology of muscles:
Man has 639 muscles, composed of 6 billion muscle fibers, each fiber has 1000 fibrils, which means
that there are 6000 billion fibrils at work at one of time or another.
Elasticity: muscle can be stretched behind its original length and return to the original shape after
relaxation (normal muscle can be elongated about 6/10 of its length
Contractility: it is the ability of muscle to shorten its length under nerve impulse, this contraction
is stimulated by acetyl choline, glycogen is partially oxidized to provides energy and lactic acid
that carried away by blood stream
Excessive accumulation of lactic acid can produce fatigue
Isometric contraction: (stretching): the muscle is simply resisting the external forces without actual
shortening
Isotonic contraction: there is an actual shortening of the muscle, the strength of isometric
contraction is much greater than that of isotonic contraction as the strength of contraction increase
as the muscle being closer to the resting length
Muscle tone: is the state of mild constant tension responsible for maintenance of various postures
Stretch reflex: the reflex contraction of healthy muscle result from pull on its tendon
All or non-law (Sherrington law): the individual muscle fibers have no variable contraction but
they either relaxed or in maximum contraction, so that the strength of contraction depends on:
- The number of muscle fibers involved in the contraction
- Frequency of stimuli
- The discharge from the individual muscle receptors (muscle spindle)

4. 4
Dr. Mohammed Alruby
Neuromuscular reflexes:
Reflexes is the basic unit of all neuromuscular activities, it is composed of receptors, afferent
neuron, synaptic joint in the CNS, efferent neuron, effector organ
Types of neuromuscular reflexes:
1- Conditioned reflexes: include all acquired reflexes that have been learned for example,
thumb sucking
2- Unconditioned reflexes: include all reflexes that normally present at birth and are
responsible for vital functions as: respiration, swallowing, sucking, sneezing, cough,
gagging, tongue and mandibular postures.
3- Reflexes appearing with normal growth and development: as:
Mature swallowing, mastication
Muscle learning:
The inherited muscle behavior is very difficult to be changed, but muscle can be learned anew
conditioned reflexes through muscle training, the learning process involve three important steps:
1- The brain must have a clear image about the task to be performed (repeated training put
the new reflexes into memory), this step involves conscious understanding of new skill
2- A new pathway must achieve for the new reflexes; this step involves reinforcement of new
reflexes by repeated training
3- Central control of the new reflex must pass through the higher center of the brain (cerebral
cortex), this step involves transition of the new skill to the unconscious level
Muscles development:
Prenatal development:
The muscle grows by increase in size and amount of CT septa (endomysium, perimysium,
epimysium) as well as cell division (increase in number of fibers)
= Striated muscles begin to differentiate at 7th
WIU, typical muscle fibers are seen at 22 WIU
Normal muscle activity begins by the end of 7th
MIU and still incomplete in extremities until after
birth
= The muscle increase by 59 folds from 4th
MIU to birth and by 40 folds from birth to the middle
of 3rd
decade in postnatal life. This means that muscles achieve most of their growth in prenatal
life.
Post-natal development:
Growth of muscles is rapid in infancy and early childhood, slower and regular in the middle and
late childhood and again become more rapid before and during puberty.
There is constant increase in the bulk of muscles from birth until maturity, this increase not equal
in all muscles, the muscles of head show the smallest increments of the postnatal growth.
From birth until 20 years old:
- The facial muscles increase by 4 folds
- Upper limbs by 16 folds
- Lower limb by 24 folds
N: B:
In the middle of IUL, skeletal muscles represent 1/6 of total body weight
At birth = 1/5 to ¼ of total body weight
In early adolescent = 1/3 of total body weight
At maturity: it equal 2/5 of total body weight

5. 5
Dr. Mohammed Alruby
Orofacial muscles
Origin:
Some of the orofacial muscles are originated from the mesodermal myoblast of the first branchial
arch
Muscles of mastication developed in relation of the Meckel’s cartilage, attaching the forming
mandible
Genioglossus muscle arise from the membranous of the mandible and extended to tongue
Geniohyoid muscle arise from the membranous of the mandible and extended to hyoid pre-
cartilage
Nerve supply:
Facial muscles innervated by the 7th
cranial nerve
The jaw musculature innervated by the fifth cranial nerve
Oral and pharyngeal muscle are innervated by fifth, ninth, tenth, cranial nerve
Types:
facial muscles, jaw muscles, portal muscles
1- Facial muscles:
Introduction:
The primary function of the facial muscles is expression of emotion; Coleman 1981 contends that
human is capable of 7000 possible facial expression
= It is important in maintenance of postures of facial structures, paralysis of the orbicularis oculi
leads to dropping of the lower eyelids and drying of the conjunctiva
Paralysis of the orbicularis oris muscles lead to angular chelosis
= According to proffit 1987, the lips and the buccinator muscles opposed the tongue, contribute to
postural equilibrium of the teeth
= The facial muscles also contribute to stabilization of the mandible during infantile swallowing
and in chewing and swallowing in the edentulous and occlusally compromised adult.
= patient with short upper lip or excessively proclined upper incisors compensate by elevation of
the lower lip through the action of the mentalis muscle to establish an anterior seal during
swallowing
= facial muscles also play an important role in both visual and spoken communication, the position
of the chin and the inclination of the mandibular plane are controlled by the balance between the
suprahyoid and the orofacial muscles. Since the posterior border of the ramus undergo gradual
remodeling and maintains its normal inclination
== N: B: Lider and Aronson reported that, the child with large adenoids may develop a mandible
with a more open gonial angle, and later he observed that reduction of the angle occurs after
adenoidectomy
Types of orofacial muscles:
1- Orbicularis oris:
Consists of numerous layers of muscle fibers surrounding the orifice of the mouth
Origin: some of these fibers lie in other facial muscles and inserted into the lips. Other fibers
developed from the mesenchymal tissues of the lips
Action: compression of the lips producing distal force upon the incisors

6. 6
Dr. Mohammed Alruby
2- Buccinator muscle: the muscle that coat the cheek
Origin:
Upper fibers: alveolar margin of maxilla above 3 molar teeth
Lower fibers: alveolar margin of mandible below 3 molar teeth
Middle fibers: the pterygo- mandibular ligament (raphe)
N: B: pterygo mandibular ligament give attachment to the buccinator muscle anterior and to
superior constrictor muscle of the pharynx posterior
Insertion:
Upper fiber: into the upper lip
Lower fiber: into the lower lip
Middle fibers: decussate at the angle of the mouth and pass to the upper and lower lip
Actions:
a- Draw the corner of the mouth laterally, pulling the lips against the teeth and flatten the
cheek
b- Exert inward force against the teeth to balance the very powerful force of the tongue
c- Aids in swallowing by providing seal with the buccal teeth and prevent food from being
entrapped in the mucobuccal sulcus during mastication
d- Aids in whistling and blowing the trumpet
e- Buccinator mechanism with superior constrictor and orbicularis oris muscle form a
continuous band encircling the dentition and thus preventing them from carried for forward
by the tongue and other occlusal forces
N: B: --- there is a collection of fat on the bucco-pharyngeal fascia called buccal fate of pad
on the outer surface of buccinator. It is larger on the suckling infant
3- Orbicularis oculi:
formed of 3 parts:
a- Orbital part: surrounding the orbital opening, act to close the eye tightly to avoid danger
b- Palpebral part: lies in the eyelids, closing the eye gently
c- Lacrimal part: surrounds the lacrimal sac, aspirate tears from conjunctival sac to the
lacrimal sac
4- Quadratus labii superius: elevator superior alcanasi
Origin: from the root of the nose
Insertion: alar cartilage of the upper lip
Action: raise the upper lip and the wings of the nose
5- Quadratus labii inferius:
Origin: from the mandible below the canine and premolars
Insertion: lower lip
Action: pull the lower lip downward
6- Caninus: elevator anguli oris
Origin: from canine fossa of the maxilla
Insertion: orbicularis oris muscle
Action: raise the corner of the mouth, draw it mesially
7- Zygomaticus minor:
Origin: from the zygomatic bone just behind the zygomatico maxillary suture

7. 7
Dr. Mohammed Alruby
Insertion: upper lip
Action: raise the upper lip
8- Zygomaticus major:
Origin: from zygomatic bone in front of zygomatico temporal suture
Insertion: the angle of the mouth
Action: pull the angle of the mouth upward and laterally as in smiling
9- Incisive labii superius:
Origin: from maxilla above the canine and lateral incisor
Insertion: into the orbicularis oris of upper canine
10-Incisive labii inferius:
Origin: arise from the mandible below the canine and lateral incisor
Insertion: into the orbicularis oris of lower lip
Action: both superius and inferius are used to draw the corner of mouth mesially
11-Resorius muscle:
Origin: from the subcutaneous tissue over the parotid gland
Insertion: superficially at the corner of the mouth
Action: pull the corner of the mandible laterally to produce the sardonic expression, it is a
continuation of the platysma muscle in the face
12-Triangularis muscle:
Origin: in the mandible below the canine, premolar, and first molar teeth
Insertion: into the orbicularis oris
Action: draw the corner of the mouth downward
13-Mentalis muscle:
Origin: in the mandible below the lateral incisors
Insertion: skin of the chin
Action: draw the skin of the chin upward
Wrinkle the skin of chin
Draw the lower lip upward
2- Jaw muscles:
Jaw muscles are often designated as elevator and depressor or protractor and retractor,
EMG studies of postural position have shown that the infra mandibular group muscles are more
active than the elevator muscle
== the superior head pf lateral pterygoid muscles are illustrative of muscle’s multiple function.
Calculation of this muscle vector about the instantaneous center of rotation suggest that its action
early in opening and late in closure is to stabilize the joint
== the functional protraction of the mandible in posture class II malocclusion that is called
(Sunday bite) does not lead to skeletal correction
== Moyers stated, {{what is the difference in adaptive stimulus between naturally occurring
protrusive posture and that resulting from functional appliance? We do not know, but finding out
would have a great clinical consequence, perhaps it is the frequent return of the mandible to more

8. 8
Dr. Mohammed Alruby
dorsal position during mastication and swallowing which cancel the biologic signals to joint
structures to adapt
Types:
1- Masseter muscle:
Origin: super-facial head: from the anterior 2/3 of the zygomatic arch
Deep head: from the posterior and middle parts of the zygomatic arch
Insertion: lateral surface of ramus of mandible
Action: elevation and protraction of the mandible
Superfacial fibers are used in incising
The whole muscle in addition to the temporal muscles used in forceful contract at molars
2- Temporal muscle:
Origin: from the temporal fossa of the temporal bone
Insertion: into the medial surface of the coronoid process of the ramus
Actions:
a- Elevation and retrusion of the mandible.
b- It is chiefly responsible for keeping the teeth in position and is constantly active when the
person is in an upright position
c- The anterior vertical fibers elevate the mandible upward, middle diagonal and posterior
horizontal fibers elevate and retract the mandible to varying degree depending upon the
direction of fibers involved in the contraction
d- The posterior part of temporal muscle plays an important role in maintaining the
mandibular postures and stabilization of the TMJ during rest as it closures to the joint
e- Assist the lateral pterygoid muscle in the lateral movement by contralateral contraction on
the other side.
3- The lateral pterygoid muscle:
Origin: upper head: pterygoid plates of the greater wing of sphenoid bone and squama of infra
temporal crest
Lower head: lateral surface of lateral pterygoid plate of the sphenoid
Insertion: upper head: the capsule of TMJ and the intra radicular disc
Lower head: anterior aspect of condylar neck
Actions: depress the mandible
Pull the mandible forward and downward
Unilateral contraction moves the mandible laterally
Side to side movement by bilateral contraction
4- Medial pterygoid muscle:
It has superfacial and deep heads embracing the lower head of lateral pterygoid muscle
Origin: superfacial head: from maxillary tuberosity behind the last molar tooth
Deep head: from the medial surface of lateral pterygoid plate
Insertion: the two heads fuse and are inserted into the rough medial surface of the angle of the
mandible
Actions: bilateral contraction elevate the mandible
Unilateral contraction moves the mandible from side to side
Protrude the mandible

9. 9
Dr. Mohammed Alruby
Adjustments of masticatory muscles:
= the origins of masseter, internal pterygoid, and vertical fiber of the temporal muscle are
separated by the mandibular condyle from their insertions
= the origins of lateral pterygoid and horizontal fibers of the temporal muscles are separated by
the growth of spheno-occipital synchondrosis
= continuing growth of cartilage makes room for the developing muscles of mastication between
the vertebral column and posterior portion of the facial skeleton
= lateral growth of zygomatic arches also provides space for the muscles of mastication
Summary of the masticatory actions:
= the masseter, temporalis, and medial pterygoid muscles are elevator muscles used for closing
the jaws, incising and grinding the foods
= the infra mandibular muscles: regulate the position of mandible in space
Acts against gravity
Responsible for mandibular postures by their continuous unconscious tone
= the posterior fibers of temporal muscle: stabilize the TMJ
Regulate the position of the mandible during closure
= retraction of mandible is achieved by contraction of posterior fibers of temporal muscle assisted
by digastric and Geniohyoid muscles
= protrusion of mandible is achieved by bilateral contraction of lateral pterygoid muscle assisted
by anterior fibers of masseter
= side to side movement is achieved by contraction of lateral or medial pterygoid muscle on one
side and the contralateral contraction of temporal muscle on the other side
= the mouth opening is achieved by contraction of lateral pterygoid muscle assisted by suprahyoid
and digastric muscles
N: B:
Reciprocal innervation and inhibition phenomena exist between the elevator and the depressor
muscles as: when the elevator muscles contract, there will be a controlled relaxation of the
depressor muscle to allow smooth closing but not jerky
3- Portal muscles:
The term portal area was coined by Bosma 1975 to denote the upper alimentary and respiratory
tract, this muscles including the muscles of tongue, soft palate, pharyngeal pillar, pharynx proper
and larynx
The muscles of portal area are serve the multiple functions of postures, respiration and feeding
Muscles of tongue:
Origin: the tongue developed from the ventral mucous membrane of the 1st
four branchial arches
Development: anterior 2/3 formed by fusion of three process, two lingual process and one median
process called tuberculum impar
Posterior 1/3: mesenchymal condensation elevates from the second and third branchial arches to
form copula (hypo branchial eminence). The copula and the endodermal mucosa of 2nd
, 3rd
and 4th
branchial arches form the mucosa covering the posterior third of the tongue
A- Extrinsic muscles:
Origin: developed from occipital somite region and migrate to the area of developing tongue by
myotomes migration

10. 10
Dr. Mohammed Alruby
1- Hyoglossus muscle:
Origin: from the hyoid bone
Insertion: into the lateral side of the tongue
Action: depress the tongue downward
2- Chondroglossus muscle:
It is a small muscle which sometimes considered as a part of Hyoglossus muscle
Origin: from the lesser cornu of hyoid bone
Insertion: into the tongue
Action: depress the tongue
3- Genioglossus muscle:
Origin: from the superior genial tubercle of the mandible
Insertion: into the medial part of the tongue along its whole length
Action: protrude the tongue forward
Depress the middle of the dorsal surface makes it as a concavity
4- Styloglossus muscle:
Origin: from the styloid process and adjacent part of stylomandibular ligament
Insertion: into the side of the tongue
Action: raise the tongue upward and draw it back
5- Palatoglossus muscle:
Origin: palatine aponeurosis
Insertion: lateral side of the tongue
Action: elevate the posterior part of the tongue
N: B;
Palatine aponeurosis: fibrous sheath attached to the posterior border of the hard palate, it thick
and strong anterior but thin posterior
B- Intrinsic muscles:
1- Superior longitudinal group: shorten the tongue
2- Inferior longitudinal group: shorten the tongue
3- Transverse longitudinal group: narrowing and elongate the tongue
4- Vertical longitudinal group: flatten and widen the tongue
Muscles of pharynx:
1- Circular muscles:
a- Superior constrictor muscle:
Origin: medial pterygoid plate, pterygoid Hamulus,
pterygo-mandibular raphe, lateral side of tongue
Insertion: pharyngeal tubercle
Action: constrict the pharynx
b- Middle constrictor muscle:
Origin: lesser and greater connu of hyoid bone
Insertion: pharyngeal tubercle
Action: constrict the pharynx
c- Inferior constrictor muscle:

11. 11
Dr. Mohammed Alruby
Origin: oblique line of thyroid cartilage and arch of cricoid cartilage
Insertion: pharyngeal tubercle
Action: constrict the pharynx
2- Longitudinal muscles:
a- Salpingo-pharyngeous:
Origin: cartilage of auditory tube
Insertion: lateral wall of pharynx
Action: elevate the pharynx in swallowing
b- Palato-pharyngeous:
Origin: posterior border of hard palate and palatal aponeurosis
Insertion: lateral and posterior wall of the pharynx
Action: elevate the pharynx in swallowing
c- Stylo-pharyngeous:
Origin: styloid process
Insertion: upper and posterior surface of thyroid cartilage
Action: elevate the pharynx in swallowing
N: B:
The bolus of food passes down through the pharynx by contraction of the superior and middle
constrictor muscles in second stage of deglutition. In third stage, the inferior constrictor muscle
contracts and relaxed to push the bolus down to the esophagus
Muscles of palate:
1- Levator palati:
Origin: petrous part of temporal bone, cartilaginous part of auditory tube
Insertion: palatine aponeurosis
Action: elevate the soft palate
2- Tensor palati:
Origin: scaphoid fossa of the sphenoid bone, cartilaginous part of auditory tube
Insertion: palatine aponeurosis
Action: pull the soft palate to its side
The muscles of two side acting together to stretch the soft palate, and depress it during the
1st
stage of deglutition
3- Palatoglossus:
Origin: palatal aponeurosis
Insertion: lateral and posterior surface of tongue
Action: depress the palate
4- Palato-pharyngeous:
Origin: anterior part: arise from the posterior border of hard palate and under the surface of
aponeurosis
Posterior part: upper surface of aponeurosis
Insertion: the two parts unite together and inserted to the posterior border of thyroid cartilage
Action: elevate the pharynx in swallowing

12. 12
Dr. Mohammed Alruby
5- Musculus uvulae:
Origin: posterior nasal spine of palatine bone
Insertion; mucosa of uvulae
Action: elevate the uvulae
Methods of studying muscles
1- Anatomic method:
a- Gross section:
The old method for studying the muscles, the shape, boundaries, origin and insertion provide
insight into possible force vector of muscle, but estimated force vector for the whole muscle have
limited value in predicting the real force vector, because the muscles are never fully contractile in
normal function
b- Histologic:
Another traditional method of studying muscles, two major classifications are based on the
concentration of oxidative enzymes and /or ATPase present in muscle fiber
== on the basis of oxidative enzyme strains, fiber have been designated as: oxidative or glycolytic
and functionally correlated with speed of contraction and susceptibility to fatigue
Type I:
Fiber are associated with small, low tension, slowly contracting motor unit
These units are resistant to fatigue and richly supplied with capillary – this type depend on good
circulation
= fibers are found in significant proportion 30% only in the digastric muscle
Type II:
Fiber associated with large, high tension, rapidly contracting unit
1- Fatigue resistance and good capillary circulation: for maximum effort and long duration
2- Fatigue sensation and poor capillary circulation: for maximum effort and short duration
= found in all jaws muscles and present in high proportions 45% in:
- Superior posterior temporalis
- Posterior medial pterygoid
- Anterior digastric muscle
2-Functional method:
a- Force and pressure:
Pressures of the lips, cheeks and tongue against the teeth have been measured by number of
investigators
Masticatory, swallowing, and maximal biting forces have been recorded on teeth
b- EMG:
The most instrument used for evaluating the activity of oro-facial muscles, the electrical activity is
easiest to record, electrode may be placed on the skin over the muscle or inserted into the body of
the muscles
3- Behavioral:
Mastication can be visualized as consolidation of simple elements such as jaw opening, jaw
closing, jaw translation, tongue protraction, tongue retraction
Other behavior such as swallowing appears to emerge as fully developed reflex to which other
elements is added

13. 13
Dr. Mohammed Alruby
In all animals:
it appears that some behavior is predetermined and unlearned
nest building of birds is predetermined while singing is learned from birds of the same species
If sustaining behavior such as ventilation and swallowing are usually considered predetermined
The bolus propelling component of swallowing appear to be predetermined while anterior seal and
stabilization component are learned
Muscles changes during growth
Friel 1926, has shown correlation between the growth of muscles of mastication and the
development of dentition, the muscles develop most rapidly after puberty when the deciduous teeth
are replaced by the permanent dentition
Skull deformity can be produced by pressure, removal paralysis of muscles, deformity is the result
of changes in direction of growth of the developing skull
Atrophy of the masticatory muscles due to injury or old age is responsible for change in shape of
the mandible
Subtleny found that soft tissue of the nose continuous to grow in downward and forward direction
from age 1 to 18 years
Continuous adjustment of muscle attachment during growth:
= According to the type of attachment during growth, the muscles are divided into two groups:
1- Periosteal attachment:
The muscle is directly attached to the fibrous layer of the periosteum; the muscles shift its
attachment directly during growth
2- Tendinous attachment:
The muscle is attached to the bone through a tendon
The muscles shift its tendon by alternative bone deposition and resorption, the bone resorption
freed the tendon and the muscle become temporarily periosteal, so the muscle can shift its tendon
during growth maintaining its normal position after growth is ceases the muscle tendon is attached
again to the bone
N: B:
= The mandible is attached to the cranium by the:
- muscles of mastication.
- sphenomandibular
- stylomandibular
- capsular ligament
= the suprahyoid muscles are attaches are attached to inferior and lingual surface of the mandible
and to the hyoid bone, where bone resorption is found in relation to tendinous attachment of
muscle, resorption frees the muscle from the bone
= muscles can become temporarily periosteal in attachment and can shift relative to bone growth
maintaining their normal position, this is particularly true of muscle attached at the growing ends
of the mandible
= when resorption ceases, the muscles may become reattached directly to the bone by tendinous
fibers
N: B:

14. 14
Dr. Mohammed Alruby
Growth at the anterior end of each half of the mandible, until the symphyseal sutures is obliterated
in the later part of the 1st
year. Gradually tends to separate the anterior belly of digastric and
Geniohyoid muscles
The tendinous insertion of the temporal muscle is gradually freed from the bone of anterior border
of the ramus of the mandible which is resorbed to make room for permanent molar eruption and
the development of the alveolar process around these teeth
The attachments of the internal pterygoid shifts during growth of the mandible and expand as the
ramus increase in size by bone deposition along its posterior border
Muscle function and facial development
Prenatal muscle function:
= with the differentiation of the neuromuscular system in the fetus, certain fetal movement are
initiated in the forming muscles
= when bone formation begins the neuromuscular contraction are effective in skeletal development
= muscle function begins in intrauterine life:
- influences the structural arrangement of the bones
- induce the development of osseous process on the external surface of the bones.
These changes may be brought about by bone apposition or resorption and are determined
by endochondral growth center
Post-natal muscle function:
= from the age of 6 to 10 years of age, muscle development proceeds at steady rate
= from age 10 to 15 years, development is less rapid
= muscles developed rapidly in association with replacement of the deciduous dentition by the
permanent one
= failure of orbicularis oris to keep the lip closed in absence of other manifest causes may be due
to a state of imbalance of the facial muscles secondary to mal-relationship of the jaws
Nearly all humans show activity of their muscles of mastication during sleep. Contraction of
muscles of mastication range from few to over 250 contractions within 8 sleep hours
= force of pressure exerted during sleep ranges from 10 to 20 kg
N: B:
Phasic activities such as mastication, swallowing and speech are less likely to cause structural
adaptation unless the force are heavy and /or act over long periods of time
Bruxism: is an example of phasic activity which can cause structural changes in:
- mastication
- teeth
- periodontal ligament
- alveolar bone