Neuroanatomy Res Lecture

8/8/2019 Neuroanatomy Res Lecture

1/13

1/9/2011

1

PurposePurpose

To provide a basic knowledge of the

functioning components of the

masticatory system

To understand the general means of

sensation

OutlineOutline

Musculature

Neurological Structures

Functions of the Neuromuscular System

Function of theMasticatory System

Mastication

Swallowing

Speech

MusclesMuscles

Motor Unit- basic component

A number of muscle fibers that are

innervated by one motor neuron. Motor end plate- junction of each

neuron with the muscle fiber

Muscles

Muscles contcont

Activated neurons stimulate the

motor end plate due to the release of

acetylcholine which in initiates

depolarization. This causes

shortening (contraction) of the

muscle fibers.

Muscle Fiber/

Motor Neuron Ratio:

Muscle Fiber/

Motor Neuron Ratio:

Lower: more precise the movement Example: ciliary muscles, lateral

pterygoid

Higher: less precise, more bulkmovement Example: large muscles such as

rectus femoris or the masseter


2/13

1/9/2011

2

AntagonisticAntagonistic

Arrangement ofArrangement of

MusclesMuscles

Always another

muscle thatcounteracts the

action of another

muscle.

Establishes a

balance in any

particular muscle

system.

Muscle FunctionMuscle Function-- can only contractcan only contract

3 Types of Function

1. Isotonic contraction

2. Isometric contraction

3. Controlled relaxation

Isotonic ContractionIsotonic Contraction

Shortening of a muscle under a constant

load

Example- contraction of the masseter

as the mandible moves through a

bolus of food.

Isometric ContractionIsometric Contraction

Contraction of a muscle without

shortening

The muscle is used to hold or stabilize

Example-contraction of the masseterwhen statically holding something

between the teeth, such as a pencil

Controlled RelaxationControlled Relaxation

Occurs when stimulation of the muscle is

ceased.

The fibers of the motor unit relax and

return to their nor mal length.

Example: relaxation of the masseter just

prior to the mouth opening to accept a

bolus of food during mastication.

EccenticEccenticContractionContraction Can be damaging

Happens when a muscle is lengthened, or

stretched, at the same time it is contracting.

Example- a sudden force is applied to

muscle group as it is contracted.

Often seen in whiplash types of incidents

when an outside force causes the sudden

stretch of a contra cted muscle group.


3/13

1/9/2011

3

Neurological StructuresNeurological Structures

Neuron- basic structural unit of the

nervous system

Nerve Fiber- many neurons grouped

together

Parts of theParts of the

NeuronNeuron

a) Nerve cell body-mass

of protoplasm which

contains the nucleus.

b) Dendrites-branched

extensions (usually

several) of the cell

body, conduct impulses

TOWARD the cellbody.

c) Axon-thin, long

process that conductsimpulse away from the

cell body to another

neuron or tissue.

2 locations of nerve cell bodies:2 locations of nerve cell bodies:

1. Within the Central Nervous System

(CNS), they are found in the gray

substance of the spinal cord.

2. Outside the CNS, they are located in

ganglia.

Neurons are named different terms

depending on their location and

function.

Types of neurons:

Afferent Efferent

Interneurons

Sensory (receptor)

Motor

Neuron typesNeuron types

Afferent neurons-

conduct the impulse

toward the CNS-sensory.

Efferent neurons-

conduct the impulse

away from the CNS

towards the periphery-motor.

Interneurons

(internuncial)- conductimpulses completely

within the CNS-

sensory and motor

pathways; function as

connectors

Sensory Neurons- receive and transmit

from receptor organs

second and third order sensory neurons are

the interneuronsand lie within the CNS.

Motor Neurons- convey impulses that

produce muscular or secretory effects.


4/13

1/9/2011

4

SynapseSynapse

Synapse is the area at which one neuron

transmits the impulse to another neuron.

Note- all afferent synapses are within

the gray substance of the CNS. There

are no peripheral connections between

sensory fibers.

Interpretation of sensoryInterpretation of sensory

informationinformation

Needs to be transferred to higher

centers in the CNS for analysis,

either the brainstem or the cortex.

Exception to this rule-

Reflex pathways that have a connectionbetween a sensory afferent and anefferent neuron leading back to thesame organ.

Can be monosynaptic or polysynaptic.

Response is independent of will- nocortex or brain stem influence.

Protective function.

Higher centers of the CNS:Higher centers of the CNS:

Function- to process sensory data then

form an appropriate response.

2Main CNS centers:

1. Brainstem2. Cortex

General brainstem componentsGeneral brainstem components

relating torelating to masticatorymasticatory processes:processes:

Spinal Tract Nucleus

Reticular Formation

Thalamus

Hypothalamus

Limbic Structures

Spinal Tract NucleusSpinal Tract Nucleus

Receives input from sensory afferents

within the oral structures.

Carried by the 5th cranial nerve

(trigeminal)

In contrast, sensory input from other

areas of the body enters the CNS

(spinal cord) by way of spinal nerves.


5/13

1/9/2011

5

Spinal Tract NucleusSpinal Tract Nucleus contcont

The trigeminal spinal tract nucleus is the

masticatory equivalent of the dorsal horn

of the spinal cord.

It is through the dorsal horn that sensory

afferents enter the CNS from areas outside

the oral structures.

Spinal Tract NucleusSpinal Tract Nucleus contcont

Impulses from the trigeminal nerve

synapse with 2nd order neurons in the

spinal tract nucleus within the pons.

It has more sensory functions but has

some motor outputs, too.

Reticular FormationReticular Formation

Transmit impulses through interneurons

to higher centers.

Monitors impulses

(enhancing/inhibiting) before sending upto the brain.

Influences sensory input.

ThalamusThalamus

Central location within the brain.

Receives nearly all sensory information

from the body before moving to higher

centers. Directs these impulses to the proper CNS

areas for appropriate interpretation and

response.

HypothalamusHypothalamus

Small structure located in the base of the

brain.

Functions:

Primarily autonomic regulation of

internal body functions such as body

temperature, hunger and thirst.

Stress can stimulate the hypothalamus

causing increased nociceptive impulses

influencing the brain.

Limbic SystemLimbic System

Ill defined structures that form the inner

border of the cortex.

Functions to control emotional and

behavioral activities.

Anger, rage, docility, depression, anxiety

fear and paranoia.

Thought to be a pleasure-pain center.


6/13

1/9/2011

6

Limbic system stimulation of thehypothalamus can modify the

internal body functions that are

controlled by the hypothalamus.

It plays a major role in pain

problems.

CortexCortex Outer region of the cerebrum

Mostly made up of gray matter neural

tissue.

Responsible for thinking and learning,

requires help from deeper structures of

the brain.

It is where memories are stored and

motor skills are acquired.

Average of 6mm thickness.

Different regions of the cortex have specific functions,many of which have been mapped out.

Sensory ReceptorsSensory Receptors

Neurological structures/organs that

provide information to the CNS through

afferent neurons.

Relays various information concerningthe status of tissue depending on the type

of sensory stimulation.

Types of receptors in theTypes of receptors in the masticatorymasticatory

systemsystem

1. Nociceptors- detect discomfort or pain

2. Proprioceptors-provide information

regarding the position and movement of

the mandible and oral structures.

4 Types of4 Types ofMasticatoryMasticatory SensorySensory

ReceptorsReceptors

1. Muscle Spindles

2. Golgi tendon organs

3. Pacinian corpuscles

4. Nociceptors


7/13

1/9/2011

7

Muscle SpindlesMuscle Spindles

Skeletal muscles consist of two types of

muscle fibers

1. Extrafusal Fibers- make up the bulk of

the muscle and are contractile.

2. Intrafusal Fibers- make up much less

of the muscle mass and are minutely

contractile.

Muscle SpindlesMuscle SpindlesMuscle Spindle-each is composed of intrafusal

fibers bound by a connective tissue sheath.

*Primarily monitor tension within the skeletalmuscles.

*Interspersed and aligned parallel to theextrafusalfibers within the muscle.

Muscle SpindlesMuscle Spindles When the muscle iselongated thespindle is stretched,therefore stimulatingthe afferent activityto the CNS.

In contractedmuscle, the spindlebecomes slack,

therefore sensoryinervationis notstimulated.

Muscle SpindleMuscle Spindle contcont

Receive motor efferent stimulus via

gamma efferents from the CNS.

Cause contraction of the spindles.

2 Ways ofMuscle Spindle2 Ways ofMuscle Spindle

StimulationStimulation

1. Generalized stretching of the entire

muscle (extrafusal fibers).

2. Contraction of the intrafusal fibers via

gamma efferents.

A total shutdown of spindle activity

would occur without gamma

efferent output. This efferent

activity assists in maintaining

muscle contraction.


8/13

1/9/2011

8

GolgiTendon OrgansGolgiTendon Organs

Located in the muscle tendon between the muscle fiber

and bone attachment.

Function to monitor tension.

Receptors are stimulated by tension to the tendon to both

stretching and contraction of the muscle.Inhibitory effect on efferent motor neurons that protects

the muscle from overcontraction/overstretching.

PacinianPacinian

CorpusclesCorpuscles

Oval organs madeup of layers ofconnective tissue,primarily locatedin the joints.

Perception ofmovement andfirm pressure

Stimulated whenapplied pressuredeforms theorgans.

NociceptorsNociceptors

Most are stimulated by injury and conveythis information to the CNS via afferents.

Several types respond to different stimuli:noxious mechanical, thermal, tactile, andinjury.

Mechanoreceptors- low thresholds; detectlight touch, pressure and facial hairmovements.

Primary Function ofPrimary Function of NociceptorsNociceptors

Monitor the condition, position and

movement of the tissues in the

masticatory system.

Alert the CNS to conditions that arepotentially harmful or have already

caused damage to the tissues.

2 types of masticatory reflex pathways

1. Myotatic

2. Nociceptive

M

yotaticRelfexM

yotaticRelfex

Only monosynaptic jaw reflex

Stimulated when the muscle fibers of the

masseter are suddenly stretched causing

an afferent response from the spindles.


9/13

1/9/2011

9

MyotaticMyotatic reflexreflex contcont

This response is transmitted to the

afferents cell body located within the

brainstem (trigeminal mesencephalic

nucleus).

A synapse occurs between the afferent

and a motor efferent which sends the

stimulus back towards the masseter.

MyotaticrelfexcontMyotaticrelfexcont

This reflex can be

demonstrated bytapping

downward on thechin when the jaw

is relaxed.

The efferent then

stimulates the

massetersextrafus

al fibers causing it

to contract.

Muscle TonusMuscle Tonus

Partial contraction of a muscle.

Myotatic reflex prevents the jaw from

dropping by counteracting the forces of

gravity.

Muscle tonusMuscle tonus contcont

Stretching of the muscle spindles

stimulates afferents and therefore causes

the elevator muscles to partially contract.

This relieves the stretch of the musclespindle and establishes a rest position of

the mandible.

CNS influenceCNS influence

Higher center can influence the response

of the myotatic reflex.

The CNS can activate intrafusal muscle

fibers which can lower the response

threshold of the muscle.

CNS influenceCNS influence contcont

Gamma efferent signals from the CNScause the intrafusalfibers to contract.

This lessens the overall muscle stretchrequired for the muscle spindles to produceafferent activity.

Decrease in gamma efferent activity fromthe CNS decreases the myotatic reflex.


10/13

1/9/2011

10

NociceptiveNociceptive ReflexReflex

Polysynaptic reflex to noxious stimuli.

Considered protective.

Masticatory example:

Sudden suppression of the elevator

muscles and activation of the opening

muscles when a hard object is

contacted suddenly during a chewing

stroke.

NocicepticNociceptic reflexreflex

contcont

Occurs through antagonistic

inhibition

In the previous example,

sensory afferents within the

PDL respond to the hardobject.

Impulses are sent to the

trigeminal motor nucleus,

two things occur:

1. Efferents cause the jaw

opening muscles to

contract;

2. Inhibitory interneurons

cause the jaw elevating

muscles to relax.

Influence from higher centersInfluence from higher centers

Cortex

Brainstem

Central Pattern Generator (CPG)

CortexCortex

Determines most actions to CNSstimulation

Appropriate response is influenced by thethalamus, CPG, limbic structures,reticular formation and thehypothalamus before acting

Main area of thought and consciousdecision making

Brain StemBrain Stem

Maintain subconscious body functions

and homeostasis

Respiration

Heart rate

Blood pressure

Digestion

CentralCentral PatterPatterGeneratorGenerator

(CPG)(CPG)

Controls rhythmic muscle activities

Breathing, walking and chewing

The CPG allows the process of masticationto be fine tuned through constant inputfrom the lips, tongue, teeth and PDLs todetermine an appropriate chewing stroke.

This learned pattern is called a muscleengram.


11/13

1/9/2011

11

Higher Center InfluencesHigher Center Influences

Increased emotional stress- fear, anxiety,

frustration, anger

Excites gamma efferent system stimulating themuscle spindles

This lowers the threshold of muscles making themmore sensitive to reflex actions and externalstimuli

Often increases irrelevant muscle activity- bitingfingernails, clenching or bruxing.

PrimaryPrimary FunctionsFunctions of theof theMasticatoryMasticatory SystemSystem

Mastication

Swallowing

Speech

Secondary- respiration and facial

expression

MasticationMastication

Rhythmic and controlled separation and

closure of the maxillary and mandibularteeth

Can be voluntary or involuntary

Tear-shaped chewing stroke

Opening Phase

Closing Phase

*Crushing

*Grinding

Less lateral movement

as food is broken

down.

MasticationMasticationcontcont

Tracing themandibular incisors,they move anterior

during opening andposterior duringclosing.

More lateralmovement init ially,then becomes less asfood is broken down.

Lateral movementincreases with harderfood, decreases withsofter food.

The lips, tongue andcheek work inpositioning the bolusfor breakdown by theteeth.

SwallowingSwallowing

Coordinated movement of the bolus from the oral

cavity into the esophagus, then to the stomach.

Decision to swallow depends on:

Fineness of the food

Intensity of the bolus

Lubrication of the bolus

During swallowing:

Lips are closed

Teeth brought intoMIP stabilizing the mandible


12/13

1/9/2011

12

Types of SwallowTypes of Swallow

Somatic- when teeth are present to stabilize the

mandible.

Visceral- infantile, the mandible is braced by

the tongue placed between the dental arches

Occurs until posterior teeth erupt

SwallowingSwallowing First Stage Voluntary, bolus

is formed. Bolus position on

to the dorsum ofthe tongue andpressed againstthe hard palate.

Lips sealed, teethbrought together

Bolus sent to thepharynx by thecontraction of thetongue.

SwallowingSwallowing Second Stage Peristaltic wave

from thepharyngealconstrictors carriesthe bolus down theesophagus

Nasal cavity issealed by the rise ofthe soft palatetouching theposterior

pharyngeal wall The epiglottiscovers the tracheaand keeps the bolusin the esophagus

SwallowingSwallowing Third Stage

Bolus travels thelength of theesophagus andenters thestomach

Takesapproximately 6 7 seconds

First and second

stage each takeapproximately 1second

ArticulationArticulation

Occurs when a volume of air is forced outof the lungs

Contractions and relaxation of the vocalcords make the pitch

Precise form assumed by the mouthdetermines the resonance and exactarticulation of the sound

Relationship of the lips and tongue

to the palate and teeth produce

sounds formed by various letters.


13/13

1/9/2011

13

SoundsSounds M, B and P- the lips come together and touch

S- incisal edges of the maxillary and mandibularteeth closely approximate but do not touch. Airpassed between the teeth creates the s sound.

D- tip of the tongue touches the palate directlybehind the incisors

Th- the tongue touches the maxillary incisors

F, V- lower lip touches the incisal edges of the

maxillary teeth

K, G- posterior portion of the tongue rises totouch the soft palate.

ConclusionConclusion

The masticatory system is a complex

array of structures which are coordinated

at many levels; the components of which

function together in several processes.

ReferencesReferences OkesonJP,Management ofTemporomandibular

Disorders and Occlusion, 5thed,Mosby 2003

McNeill C, Science and Practice of Occlusion,

Quintessence 1997

Neuroanatomyand Physiology of Occlusion,MAJ Richard

Ritter, 2004

Kawamura Y, Neurophysiological Background of

Occlusion,Periodontics, Jul/Aug 1967

Tortora, Anagnostakos, Principles of Anatomy and

Physiology, 4thed, Biological Sciences Textbooks, 1983

MohlND, A Textbook of Occlusion, Quintessence,

1988

Me and Christi with our nephew,Me and Christi with our nephew,

Micah, saying goodbye the day weMicah, saying goodbye the day we

moved to Georgiamoved to Georgia

Documents

Neuroanatomy Res Lecture