Mastication

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Mastication

• Sitthichai Wanachantararak

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Digestive System

• Functions
• Prehension, ingestion
• Mastication
• Deglutition
• Digestion
• Absorption of nutrients
• Elimination of undigestible/undigested food
  products
• Other

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From Mouth to Stomach

• Mastication (chewing)
• Mixes food with saliva -
• Amylase enzyme that can catalyze the partial
  digestion of starch.
• Deglutition (swallowing)
• Involves 3 phases
• Oral phase is voluntary.
• Pharyngeal and Esophageal phases are involuntary.
• Cannot be stopped.
• Larynx is raised.
• Epiglottis covers the entrance to respiratory
  tract

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Muscles of Mastication
Figure 10.7a
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Muscles of Mastication
Figure 10.7b
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Action of muscles during masticatory movements

• Opening / Depressor jaw muscles
• mylohyoid / digastric / inferior lateral
  pterygoid
• Closing / elevator jaw muscles
• medial pterygoid / superficial masseter /
  tempolaris

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Extrinsic Tongue Muscles
Figure 10.7c
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Tongue

• Consist of 2 groups intrinsic and extrinsic
  muscles.
• Intrinsic muscle change in tongue shape
• Extrinsic muscle (eg. Genioglossus) response for
  protrusion and retrusion of the tongue Three
  major muscles that anchor and move the tongue
• innervated by cranial nerve XII (hypoglossal
  nerve)
• Complete tongue activity occurs in jaw movements
  and respiration, speech, taste, mastication,
  swallowing, and sucking.

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Chewing
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Chewing

• Activity of masticatory muscles during chewing
  reflected
• jaw-tracking devices and EMG
• amplitude
• onset timing
• duration of the chewing cycle
• Variation is related to occlusal contact relation
  and musculoskeletal morphology

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Mastication The crushing grinding

• 1 chewing cycle
• opening closing power stroke
• chewing sequence
• numerous chewing cycle
• chewing sequence could be divided into
• preparatory series
• reduction series
• pre-swallow series

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Chewing
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Mastication The crushing grinding

• opening stroke
• closing stroke / fast stroke
• power stroke
• puncture-crushing
• tooth-tooth contact
• -buccal phase / phase I
• -lingual phase / phase II

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Mascles activity
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Opening

• Start from static intercuspal position, where jaw
  movement pauses for 194 ms in chewing cycle,
• muscle activity begins in the ipsilateral
  inferior head of the lateral pterygoid muscle
  approximately half way through the period of
  tooth contact.
• Follow closely by the action of the contralateral
  inferior lateral pterygoid muscles.
• Both superior and inferior head of the
  lateral pterygoid muscle are active during
  the opening phase.

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Opening

• Early in the opening phase,
• digastric muscles become active and remain until
  maximum opening position
• During the opening phase,
• masseter, temporalis, medial pterygoid,
  and superior head of lateral pterygoid
  muscles are inactive.

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Chewing
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Closing

• At initiation of jaw closing
• the inferior heads of the lateral pterygoid
  muscle ceases their functioning and activity
• initiated in the contralateral medial pterygoid
  muscle

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Closing

• During early closing, contralateral medial
  pterygoid muscle
• more active in wider strokes,
• ceases activity during the intercuspal phase.
• contralateral medial pterygoid controls the
  upward and lateral positions of the mandible

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Closing

• During early closing, contralateral medial
  pterygoid muscle
• more active in wider strokes,
• ceases activity during the intercuspal phase.
• contralateral medial pterygoid controls the
  upward and lateral positions of the mandible

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Closing

• The ipsilateral and contralateral medial
  pterygoid muscles are active
• in the onset of intercuspation when the chewing
  stroke is narrow, i.e., has a minimal lateral
  component
• Activity increases in the anterior and posterior
  temporalis muscle, in the deep and superficial
  masseter muscles, and in the ipsilateral medial
  pterygoid muscle up to the peak 20 to 30 ms
  before the onset of the intercuspal position

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Closing

• anterior and posterior temporalis muscle, in the
  deep and superficial masseter muscles, and in the
  ipsilateral medial pterygoid muscle activity
  declines in activity at the onset of
  intercuspation.
• There appears to be reciprocal action between the
  inferior head of the lateral pterygoid muscle and
  the medial pterygoid muscle in same subject.

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Border Movement
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Clenching

• In vertical affort (clenching in centric
  occlusion), most of the elevator muscles are
  activated maximally.
• In some subjects the medial pterygoid muscle
  activity is low.
• The variation between subjects related to
  occlusal contacts and musculoskeletal morphology.
  
• The inferior head of the lateral pterygoid
  produces little activity or only 25 percent of
  maximum activity compared to the superior head.

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Clenching

• Muscle activity decreases when
• less posterior teeth
• only the incisors in contact
• The digastric muscle
• slightly active during vertical effort with
  intercuspal clenching
• more active during vertical incisive clenching.

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Trigeminal Sensory Pathway
Primary Neurons

• Nociceptor
• Trigeminal nu.
• Tactile
• Motor nu. of V
• Proprioceptive
• Mesencephalic nu.

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Rhythmic jaw movements in mastication

• Chewing is more obviously complicated than
  alternating jaw-opening and jaw-closing reflexes.
  
• Several models have been proposed to account for
  rhythmic jaw movements and sensory input
  interactions with proposed rhythm generators.
• These reflexes perform useful functions when the
  body is in movement and during chewing but their
  characteristics change during the two situations.

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Rhythmic jaw movements in mastication

• Cyclic jaw movements are largely centrally
  programmed and require little in the way of
  proprioceptive control loop.
• mouth is not merely a motor organ, but also a
  sensory perceptual system.

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Trigeminal Pain Pathway
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Jaw-opening reflex

• A simple jaw-opening reflex (JOR) can be evoked
  experimentally by a brisk tap to a tooth
• as well as
• by noxious stimulation of the tooth pulp, facial
  skin, and widespread area in the oral cavity.
• By stimulation of low-threshold afferents in the
  lips or oral mucosa
• by light tactile stimulation of the peroral
  region in a fetus

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Jaw-opening reflex

• The jaw-opening reflex and the trigemino-neck
  reflexes are considered to protect the orofacial
  region against sudden contact with an unforeseen
  object when the body is in motion.
• to protect the soft tissues and lips against
  being bitten during jaw closure
• To against being damaged due to excessive
  occlusal forces if the teeth encounter a hard
  object.

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Periodontal Sensory Pathway

• Proprioceptive from periodontium has cell body in
  Mesencephalic nucleus of V
• Pain in Trigeminal ganglion

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Jaw-opening reflex

• Neurons have cell bodies for mechanoreceptive
  afferents are located in the trigeminal gagnglion
  and in the mesencephalic nucleus of the
  trigeminal nerve.
• The two cell groups appear to have similar
  thresholds for tooth displacement.
• Central projections of primary afferents with
  cell bodies in trigeminal ganglion bifurcation
  and terminate on interneurons in the main sensory
  nucleus (MSN),
• more rostral parts (nucleus oralis or
  interpolaris) of the V spinal nucleus (SpV) and
  on second order neurons in the spinal nucleus
  (SpV).

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Jaw-opening reflex

• These secondary neurons make synaptic connections
  directly or through interneurons with the motor
  neurons of jaw-closing muscles.
• Axon terminals of the mesencephalic nucleus make
  synaptic connections with excitatory and
  inhibitory interneurons in the supratrigeminal
  area and in the trigeminal motor nucleus as well
  as making connections with the reticular
  formation (RF) and the upper cervical segment.
• Intraoral mechanoreceptor pathways involve the
  trigeminal brain stem nuclei and the thalamus to
  the cortex.

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Stretch or Myotatic reflex

• So called Jaw-jerk reflex usually initiated
  experimentally by tapping on the chin.
• Postural or antigravity reflex of jaw-closing
  muscles.
• During locomotion the stretch reflex probably
  helps to maintain
• position of the mandible relative to the maxilla
• postural stability of the mandible

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Stretch or Myotatic reflex

• The reflex is activated when
• muscles that elevate the mandible are stretched
• activate muscle spindle afferents
• conveyed through monosynaptic connections with
  the motoneurons of the trigeminal motor nucleus,
• results in the jaw-closing reflex

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Stretch or Myotatic reflex

• Sensory feed back from the periphery may modulate
  the reflex and other afferent pathways
• reticular formation in brain stem
• V sensory nucleus in brain stem

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Reflexes and chewing interactions

• Simple jaw-opening and jaw-closing reflexes are
  adapted to perform useful functions in two
  different situations, they cannot continue to act
  the same way during mastication.
• during movement of the whole body
• during movements of the jaw
• normal rhythmic jaw movements can take place
  without being interrupted by low threshold
  reflexes evoked by innocuous stimulation of the
  lips, teeth, and mucosa during chewing.

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Reflexes and chewing interactions

• The low-threshold input that can be evoked the
  JOR must be suppressed to allow normal jaw
  movements to occur during chewing.
• The synaptic transmission at the terminals of
  low-threshold primary afferents appears to be
  tonically reduced by presynaptic depolarization
  during chewing.

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Reflexes and chewing interactions

• During jaw closure the amplitude of the JOR
  increases so that a strong stimulus in the
  periphery can interrupt jaw closure to avoid
  damage to the tissues if they are trapped between
  the teeth.
• The protective potential of the JOR occurs in
  those phases pf chewing when injury is likely to
  occur.

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Rhythmic jaw movements

• Neuronal networks located in the brain are
  capable of generating rhythmic activity in
  trigeminal motor systems without peripheral feed
  back.
• The site for the masticatory rhythm generator or
  central pattern generator (CPG) appears to be in
  the brain stem reticular formations (RF).

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Rhythmic jaw movements

• The CPG may modulate directly and indirectly the
  trigeminal motoneuron pool.
• Rhythmic jaw movement (RJM) influence and are
  influenced by orofacial afferents has a
  differential effect on
• the excitability of effector neurons
• influences how information is transmitted.

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Rhythmic jaw movements

• Descending influence on RJM from cortical sites
  occurs. Input may activate the trigeminal motor
  pool during the initial phases of preparing and
  positioning of the food.
• Such inputs also activate the CPG which modulated
  descending inputs from the motor cortex, and acts
  directly on the motor pool to drive RJM.

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Rhythmic jaw movements

• Peripheral input contributions to RJM are
  influences via the central motor program either
• by modulation of motoneuronal excitability
  (stretch reflex)
• by modulation of reflex circuits at the level of
  primary afferents or interneurons.

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Neurological control during mastication

• Coordination between
• sensory feed back from peripheral organ
• CPG Central Pattern Generator neuron in brain
  stem
• higher center
• jaw reflexes

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Motoneuronal Excitation

• During the jaw-opening phase of mastication,
• rhythmic inhibition occurs to inhibit the stretch
  reflex.
• This postsynaptic hyperpolarization appears to be
  responsible for the phasic inhibition of the
  stretch reflex during jaw-opening
• motoneuron pool is inhibited during chewing.
• The muscle spindle feedback is mainly controlled
  by cyclical changes in the membrane potential of
  jaw-closing motoneurons.

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Reflex modulation

• neuron circuits are modulated at the level of
  primary afferent or interneurons.
• modulation of sensory transmission occur through
  neurons in the trigeminal main sensory nucleus in
  the subnucleus oralis, and in the intertrigeminal
  area which lies between the sensory and motor
  nuclei.

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Reflex modulation

• During the masticaory cycle the excitability of
  the jaw-opening reflex interneurons is inhibited
• which receive inputs from low-threshold
  mechanosensitive fields in the face or oral
  cavity,.
• most of the neuron with high threshold fields
  are very excitable during fast and slow jaw
  closing and relatively unexcitable during jaw
  opening.
• Modulation of sensory transmission through the
  subnucleus caudalis is not phase modulated.

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Control of mastication - Sensory
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Control of mastication - Motor
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