Neuro anatomy and physiology of masticatory system/certified fixed orthodontic courses by Indian dental academy

NEURO ANATOMY ANDNEURO ANATOMY AND
PHYSIOLOGY OFPHYSIOLOGY OF
MASTICATORY SYSTEMMASTICATORY SYSTEM
The function of masticatory system is
complex
Discriminatory contraction of the various
head and neck muscles is necessary to move
the mandible precisely and allow effective
functioning.
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A highly refined neurologic control system
regulates and coordinates the activities of the
entire masticatory system .
It consist of primarily nerves and muscles hence
the term neuromuscular system.
Muscles:
Motor unit: It is the basic component of
neuromuscular system –consist of number of
muscles fibers that are innervated by6 one
motor neuron .
Each neuron joins with the muscle fiber at the
motor end plate

Activation of neuron – Motor end plate is
stimulated to release small amounts of
acetylcholine-
initiates depolarisation of muscle fibers –
muscle fibers to shorten or contract.
Number of muscle fibers innervated by one
motor neuron varies greatly according to the
function of motor unit.

 Fewer muscle fibers /motor neuron-more precise
movement
 Eg.ciliary muscles(2or3fibers/neuron-controls of
eye-1neuron/1oomuscle fibers-rectus femorus in
leg)
 Similarly muscles of mastication
 The inferior lateral pterygoid muscle has relatively
low muscle fiber-motor neuron ratio therefore
capable of doing more precise adjustments in
length needed to adopt to horizontal changes in
mandibular position

In contrast-masseter-large no of muscle
fibers/motor neuron-more gross functions of
providing the force necessary during
mastication
Muscle:
Hundred to thousand of motor units along
blood vessels and nerves are bundled
together by connective tissue and fascia to
make up muscle.

To understand the effect of these muscles
one must observe the basic skeletal
relationships of head and neck.
The skull is supported in position by
cervical spine in fact cervical spine alone
cannot support the head so far this muscles
are needed to balance it.
If the head is to be maintained in an upright
position so one can see forward’,the
muscles that attach the posterior aspect of
skull to the cervical spine and shoulder
region must contract.

Some of the muscles that serve this function
are the
trapezius,sternocliedomastoid,splenus
capitus and long capitus muscles.
It is possible, however for these muscles to
over contract and direct the line of vision
too far upward.
To counteract this action,an antagonistic
group of muscles exists in anterior region of
the head –masseter (joins mandible to
skull),suprahyoid muscles and infrahyoid
muscles.

– Muscle function: motor unit can carry out only
one action I.contraction or shortening
– Muscle –3 potential functions
– Isotonic contraction-when large no of motor
units in the muscle rare stimulated –contraction
or an overall shortening of muscle occurs-this
type of shortening under a constant load
– Eg.isotonic contraction occurs in the masseter
muscle when the mandible is elevated .forcing
the teeth through bolus of food.

Isometric contraction-when proper no of
motor units contract opposing a given
force,the resultant function of muscle to
hold or stabilize the jaw. This contraction
without shortening is called isometric
contraction eg.it occurs in masseter muscle
when an object is held between the teeth
(pipe or pencil)
Controlled relaxation:when stimulation of
motor unit is discontinued,the fibers of
motor unit relax and return to their normal
length.

By control of this decrease in motor unit
stimulation,a precise muscle lengthening
can occur that allows smooth and deliberate
movement.
Eg.masseter muscle when the mouth opens
to accept a new bolus of food during
mastication.
When these three functions are used, the
muscles of head and neck maintain a
constant desirable head position

Therefore muscles may act as primary
movers of part,they may act in a stabilizing
manner so that a movement will be
coordinated or smooth or they may act as a
fixer of a part so that the part can be
maintained firmly in a given position.
These three types of muscle activities are
present during routine function of head and
neck.

Eccentric contraction-which occurs during
certain conditions-which is injurious to the
muscle tissue.
It refers to the lengthening of muscle at the
same time that it is contracting.
Eg.it occurs with the tissue damage
associated during an extension-flexion
injury(whip-lash injury)
At the precise movement of motor vehicle
accident the cervical muscles contract to
support the head and resist movement.

However the impact is great,the sudden
change in the inertia of head causes it to
move while the muscles contract trying to
support it.
Results in sudden lengthening of muscles
while they are contracting which results in
injury

Neurologic structures:the basic structural
unit of the nervous system is the neuron.
Neuron-composed of a mass of protoplasm
termed cell body and protoplasmic process
from the nerve cell body called axons and
dendrites.
Axon-it is the central core that forms the
essential conducting part of a neuron and it
is an extension of cytoplasm from a nerve
cell.

 Many neurons are grouped together to form a
nerve fiber.
 These neurons are capable of transmitting
electrical and chemical impulses along their axis
enabling information to pass both in and out of the
c.n.s
 Neurons are classified as-(depending on their
location and function)
 Afferent neurons-conducts impulse towards C.N.S
also called AS sensory or receptors –which
receive and convey impulses from receptor
organs.

Efferent neurons-conducts it peripherally
also called as motor neurons-which convey
impulses from c.n.s to effector organ to
produce muscular or secretary effects.
Internuncial neurons or interneurons-they
provide for alternate or distant connections
with many of the cells of the nervous
system.
Receptors-they are structures that detect
changes in environment within the body
and transmit information to c.n.s.

Synapse-where the process of two neurons
are in close proximity-consists of section of
terminal end of the axon of one neuron with
the dendrite zone or cell body of another
neuron.
It can be monosynaptic or polysynaptic.
Sensory receptors-these are neurologic
structures or organs located in all body
tissues that provide information regarding
the status of these
tissues to c.n.s by way of the afferent
neuron.

 Two types of receptors:
 Exteroreceptors-which mediate sensation of
touch.light,pressure,pain,cold,heat etc.
 Nocireceptors-specific for discomfort and pain.
 Proprioceptors-provide information regarding the
position and movement of mandible and
associated oral structures.
 Mechanoreceptors-respond at a higher frequency
to innocous stimuli than to thermal or noxious
stimulation they are tactile receptors

Interoreceptors- they carry information
regarding the status of the internal organs.
Constant input received from all these
receptors allows the cortex and the brain
stem to coordinate action of individual
muscles or muscle groups to create
appropriate response in the individual.

Masticatory system-utilizes four major
types of sensory receptors to monitor the
status of its structures.
Muscle spindles-specialized receptors
organs found in the muscle tissues
Golgi tendon organs-located in tendons
Pacinian corpuscles-located in the
tendons.joints.periosteum.fascia and
subcutaneous tissues.
Nociceptors-found generally through all the
tissues of the masticatory system

Muscle spindles-primarily monitor muscle
length
Skeletal muscle consists of two types of
muscle fibers that is extrafusal and
intrafusal fibers.
Extrafusal fibers-they are contractile and
makeup the bulk of the muscle
Intrafusal fibers-minutely contractile.
A bundle of intrafusal fibers bound by
connective sheath is called muscle spindle.

Primarily monitor tension within the
skeletal muscles.
They are interspersed through the muscles
and aligned parallel with the extrafusal
fibers.
Within each spindle,the nuclei of intrafusal
fibers are arranged in two distinct fashions
Chain like(nuclear type)
Clumped(nuclear bag type)

 There are two types of afferent fibers that supply
the intrafuasal fibers which are classified
according to their diameters .
 The larger fibers conduct impulses at a higher
speed and have lower thresholds.
 Primary endings(annulospiral endings)- they end
central region of the intrafusal fibers which are
larger (1a,a-alpha)
 Secondary ending (flower spray endings)- those
that end in the poles of spindle away from central
region,which are smaller.(2,a-beta)

Since the intrafusal fibers of the muscle
spindles are aligned parallel to the
extrafusal fibers of the muscles,the
intrafusal fibers are stretched as the muscle
is stretched.
This stretch is monitored at the nuclear
chain and nuclear bag region.

Muscle stretch
Activation of annulospiraland flower spray
endings
Afferent neurons carry these neural
impulses to the C.N.S.
(the afferent neurons originating in the
muscle spindles of muscles of mastication
have their cell bodies in the trigeminal
mesencephalic nucleus)

The intrafusal fibers receive efferent
innervations by way of fusimotor nerve
fibers.
The extrafusal fibers receive innervations
by way of efferent motor neurons(most of
these have their cell bodies in the trigeminal
motor nucleus)
Stimulation of these neurons therefore the
group of extrafusal fibers (motor unit )to
contraction
Muscle contraction

Golgi tendon organs-they are located in the
muscle tendon between the muscle fibers
and their attachment to the bone.
They primarily monitor tension.
They occur in series with the muscle
spindles.
Afferent fibers enter near the middle of the
organ and spread out over the the extent of
fibers.

Tension on the tendon stimulates the
receptors in the golgi tendon organ.
Therefore contraction of muscle also
stimulates the organ.
Likewise,an overall stretching of the muscle
creates tension in the tendon and stimulates
the organ.

 Pacinian corpuscles these are large oval organs
made up of concentric lamella of concentric
connective tissue .
 These are widely distributed because of their
frequent location in the joint structures.
 They are considered to serve principally for the
perception of movement and firm pressure.
 They are found in
tendons,joints,periosteum,tendinous
insertions,fascia and subcutaneous tissue.

Nociceptors: these are sensory receptors
that are stimulated by injury and transmit
injury information to the c.n.s by way of the
afferent nerve fibers.
They are located throughout most of the
tissues in the masticatory system.
Several types exist.
Some respond exclusively to noxious,
mechanical and thermal stimuli,others
respond to wide range of stimuli,from
tactile sensations to noxious injury etc.

 the nociceptors primarily function to
monitor the condition,position and
movement of the tissues in the masticatory
system
When conditions exist that are either
potentially harmful or actually cause injury
to the tissues, the nociceptors relay this
information to c.n.s as sensations of
discomfort or pain.

Reflex action-it is the response resulting
from a stimulus that passes as an impulse
along an afferent neuron to a posterior root
or its cranial equivalent;where it is then
transmitted to an efferent neuron leading
back to the skeletal muscle.
A reflex action may be monosynaptic or
polysynaptic
A monosynaptic reflex occurs when the
afferent fiber directly stimulates the efferent
fiber in the CNS.

A polysynaptic reflex is present when the
afferent neuron stimulates one or more
interneurons in the CNS which in turn
stimulate the efferent nerve fibers.
Two general reflex actions are important in
masticatory system.
Myotactic reflex
Nociceptic reflex

 Myotactic reflex-it is monosynaptic jaw reflex
 When a skeletal muscle is quickly stretched,this
protective reflex is elicited and brings about a
contraction of the stretched muscle.
 Sudden stretching of muscle
 Afferent activity from the spindles
 Afferent impulses pass into the brain stem to the
trigeminal motor nucleus by way of the trigeminal
mesencephalic nucleus where the primary afferent
cell bodies are located.

Afferent fibers synapse in trigeminal motor
nucleus with efferent motor neurons.
Efferent fibers carry information to
extrafusal fibers
Muscle contraction
Nociceptive reflex- it is a polysynaptic
reflex to noxious stimuli and hence
considered to be protective.


In masticatory system, this reflex becomes
active when a hard object is suddenly
encountered during mastication.
Unexpected biting on hard object
Sudden noxious stimulus is generated by
over loading the periodontal structures (the
primary afferent nerve fibers carry
information to the trigeminal spinal tract
nucleus,where they synapse with
interneurons)

These interneurons travel to the trigeminal
motor nucleus
Afferent information from the sensory
receptors reaches the interneurons,two
distinct actions occur.
They synapse with efferent neurons in the
trigeminal spinal tract nucleus

 Excitatory interneurons leading to efferent
neurons in trigeminal motor nucleus of the jaw
opening muscles are stimulated
 Causes muscle to contract
 At the same time-the afferent fibers stimulate
inhibitory interneurons,which cause jaw- elevating
muscles to relax.
 The overall result is that the jaw quickly drops and
teeth are pulled away from the object causing
noxious stimuli-this process is called antagonistic
inhibition

 The myotactic reflex protects the masticatory
system from sudden stretching of a muscle.
 The nociceptive reflex protects the teeth and
supportive structures from damage created by
sudden and unusually heavy functional forces .
 These are very complex and controlled in higher
centers.
 Reflex actions play major role in functioning.
eg.mastication,swallowing,gagging
.coughing,speaking.

Influence from higher centersInfluence from higher centers
in brainin brain
 Main sensory nucleus trigeminal nerve-
 Sensory nerve impulses arise from oral receptors
travel to the sensory nucleus of trigeminal nerve.
 From nerve, the impulses are transmitted to the
motor nucleus of trigeminal nerve and to the
cerebral cortex.
 Those transmitted directly to the motor nucleus
are responsible for involuntary movement.

Mesencephalic nucleus of trigeminal nerve-
Changes in tension stimulate proprioceptive
receptors in periodontal membrane,tmj,and
muscles of mastication.
Sensory impulses travel to the
mesencephalic nucleus in the pons.
From here impulses are transmitted to the
motor nucleus and are responsible for
involuntary jaw movements.

Motor cortex of cerebrum-impulses arising
in the cerebral cortex are carried by nerve
fibers to the motor nucleus and are
responsible for voluntary jaw movements.
Globus palladius of corpus striatum-this
center is responsible for postural
adjustments and automatic movements
Cerebellum-impulses from cerebellum go to
the motor nucleus and result in synergism
of various muscles taking part in functional
movements.

Major functions of theMajor functions of the
masticatory systemmasticatory system
 The neuroanatomy and physiology that has been
discussed provide a mechanism by which important
functional movements of the mandible can be
executed
 The functional movements are-
 Mastication
 Swallowing
 Speech
 Respiration
 Aimless contact movements

Mastication- it is defined as the act of
chewing foods.
This act is made up of rhythmic,well
controlled separation and closure of the
maxillary and mandibular teeth.
Each opening and closing movement of
mandible represents a chewing stroke.
The complete chewing stroke has a
movement pattern described as tear shaped

It can be divided into an opening phase and
closing phase
Closing phase –crushing phase and grinding
phase.
Opening phase-the mandible drops
downward from the inter cuspal position to
a point where incisal edges of the teeth are
about 16 to 18 mm apart.
The mandible then moves laterally 5- 6mm
from midline as the closing movement
begins.

 Closing phase- the first phase of closure traps the
food between the teeth and is called as crushing
phase.
 As teeth approaches each other ,the lateral
displacement is lessened so that when the teeth or
only 3mm apart the jaw occupies a position only
3-4mm lateral to the starting position of the
chewing stroke.
 At this point teeth are so positioned that the buccal
cusps of mandibular teeth are almost directly
under buccal cusps maxillary teeth on the side to
which the mandible has been shifted.

As mandible continues to close,the bolus of
food is trapped between the teeth which
begins the grinding phase of closure stroke.
Grinding phase-the mandible is guided by
the occlusal surfaces of teeth back to the
inter cuspal position,which causes the
cuspal inclines of teeth to pass across each
other permitting shearing and grinding of
bolus of food.

Tooth contact during mastication-
When food is initially introduced into the
mouth, few contacts occur.
As the bolus is broken down,the frequency
of tooth contact increases.
In the final stages of mastication,just prior
to swallowing contacts occur during every
stroke.
Two types of contact have been identified.

Gliding contact-which occurs as the cuspal
inclines pass by each other during the
Amount of lateral movement is greater
when food is introduced and then becomes
less as the food is broken down opening and
grinding phases of mastication.
Single contact-which occurs during
maximum intercuspal position.

Factors affecting mandibular movements during
chewing stroke-
Amount of lateral movement is greater when
food is introduced and then becomes less as the
food is broken down.
Consistency of food-hard foods –greater amount
of lateral movement.
Soft foods-lesser lateral movements eg.chewing
on a carrot creates a broader stroke than while
chewing on a cheese.

Tall cusps and deep fossa-promote
predominantly vertical chewing strokes
where as flattened cut worn out teeth
encourage a broader chewing stroke.,
Malocclusion produces an irregular and less
repeatable chewing stroke
Normal persons with good occlusion
masticate with chewing stroke that are well
rounded that have definite borders and less
repeated.

Persons with t.m.j disorders-strokes are
shorter and slower and have an irregular
pathway but repeatable pathways related to
altered functional movement of the condyle
around which the pain is centered.

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