Transcript Slide 1

Mastication
Sitthichai Wanachantararak
Digestive System
• Functions
– Prehension, ingestion
– Mastication
– Deglutition
– Digestion
– Absorption of nutrients
– Elimination of undigestible/undigested food products
– Other
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
Muscles of Mastication
Figure 10.7a
Muscles of Mastication
Figure 10.7b
Action of muscles during masticatory
movements
• Opening / Depressor jaw muscles
– mylohyoid / digastric / inferior lateral pterygoid
• Closing / elevator jaw muscles
– medial pterygoid / superficial masseter / tempolaris
Extrinsic Tongue Muscles
Figure 10.7c
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.
Chewing
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
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
Chewing
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
Mascles activity
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.
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.
Chewing
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
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
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
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
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.
Border Movement
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.
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.
Trigeminal Sensory Pathway
Primary Neurons
• Nociceptor
– Trigeminal nu.
• Tactile
– Motor nu. of V
• Proprioceptive
– Mesencephalic nu.
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.
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.
Trigeminal Pain Pathway
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
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.
Periodontal Sensory Pathway
• Proprioceptive from
periodontium has
cell body in
Mesencephalic
nucleus of V
• Pain in Trigeminal
ganglion
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).
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.
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
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
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
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.
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 lowthreshold primary afferents appears to be tonically
reduced by presynaptic depolarization during chewing.
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.
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).
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.
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.
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.
Neurological control during mastication
• Coordination between
– sensory feed back from peripheral organ
– CPG :Central Pattern Generator neuron in
brain stem
– higher center
– jaw reflexes
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.
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.
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.
Control of mastication - Sensory
Control of mastication - Motor
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