M+O 7 - masticatory control Flashcards
Why do we need to breakdown foodstuffs via mastication?
- preparation for swallowing
- increase surface area for chemical digestion
- release of chemicals for sense of taste
- stimulation of salivary flow
- growth and maintenance of oro-facial tissues
During mastication, what are jaw movements for?
chewing cycles
During mastication, what are cheek movements for?
directing food
During mastication, what are tongue movements for?
- directing food
- crushing food
- mixing food and saliva
During mastication, what are lip movements for?
- (accepting food)
- retaining food: anterior oral seal
- directing food
How long does the chewing cycle last?
0.5-1.2 secs
What are the phases of the chewing cycle?
- opening
- fast closing
- slow closing
- intercuspal phases
(note: many alternative terminologies used)
During what phases of the chewing cycle is food broken down?
slow closing and intercuspal
During the chewing cycle, what is the sequence of muscle activation?
mylohyoid —> digastric —> lateral pterygoid —> temporalis —> masseter —> medial pterygoid
What muscles are activated during opening?
mylohyoid —> digastric —> lateral pterygoid
What muscles are activated during closing?
temporalis —> masseter —> medial pterygoid
(lateral pterygoid also active)
What force can be produced during mastication?
up to 150N
What is the maximum bite force?
500-700N between molars
What are the 3 levels of control of mastication?
- reflex
- pattern generator
- voluntary
What produces pattern of activity for mastication?
chewing centre
What are the 4 parts of the trigeminal nucleus?
- trigeminal main sensory nucleus (top)
- trigeminal nucleus oralis
- trigeminal nucleus interpolaris
- trigeminal nucleus caudalis (bottom)
What is a reflex?
predicable response to a given stimulus
What is a stretch reflex?
- simple
- usually monsynaptic
examples:
- knee jerk reflex
- jaw jerk reflex
What gives the conduction time of a reflex?
neurones speed and distance
What is the minimum synaptic delay?
0.2ms per synapse
What is the stimulus and the receptor of the knee jerk reflex?
stimulus: stretch (via patellar tendon tap)
receptor: muscle spindle
What is the stimulus and the receptor of the jaw jerk reflex?
stimulus: stretch (via chin tap)
receptor: muscle spindle
How many synapses are there in the knee jerk reflex, and what is the latency of the reflex?
synapses: 1
latency: 19-24ms
How many synapses are there in the jaw jerk reflex, and what is the latency of the reflex?
synapses: 1 in V motor nucleus
latency: ~7-8ms
What is the effect of the knee jerk reflex?
contraction of quadriceps femoris
What is the effect of the jaw jerk reflex?
contraction of masseter muscle
What are the tonic roles of stretch reflexes?
- resist gravity
- help maintain jaw posture?
How is the rest/postural position maintained?
- minimal muscle activity? or…
- is it governed by muscle elasticity?
it remains reproducible throughout life in both dentate and edentulous subjects
What are the phasic roles of stretch reflexes?
- load compensation during chewing
- stabilises jaw during vigorous head movements e.g. during running
How many synapses are involved in protective reflexes?
polysynaptic reflexes
>1 synapse
What are protective reflexes often a response to?
a noxious stimulus
examples:
- limb: flexion withdrawal reflex
- jaw: jaw “opening” reflex
What are the stimuli for the jaw “opening” reflexes?
- intra-oral mechanical or noxious
- extra-oral noxious
What is the jaw “opening’’ reflex response in sub-primates (e.g. cats)?
activation of jaw depressors
What is the jaw “opening’’ reflex response in humans?
- inactivation of jaw closers
- i.e. cessation of closing
- inhibitory jaw reflexes
In humans jaw openers not as powerful and jaw closers - can’t compete so jaw closers inhibited
What are the phases of inhibitory jaw reflexes?
early (10-30ms) and late (40-90ms)
these phases will merge together and get bigger is the stimulus is bigger
What is the role of inhibitor jaw reflexes?
- prevent overloading of the masticatory system
- facilitate opening:
- expel noxious material
- minimise damage to intra-/peri-oral structures
- faster than a voluntary reaction
What proves that inhibitory jaw reflexes are a sensitive system?
- they are produced every time our teeth contact
- a tooth movement of only ~9um is enough to produce inhibitory reflex effects in the masseter muscle
What is the stimulus of the jaw unloading reflex?
sudden closure following hard biting
- e.g. a hard/brittle food breaks
What is the response of the jaw unloading reflex?
- inactivation of jaw closing muscles
- activation of jaw opening muscles
What is the result of the jaw unloading reflex?
teeth do not crash together
What is the ‘series of reflexes’ theory of mastication (historical)?
Sir Charles Sherrington (1917)
- e.g. reflex chain theory
- sequence of jaw opening and closing reflexes
- too simplistic - can’t explain all movements
- e.g. lateral movements, changing sides etc…
What are the 4 areas for control of mastication in the brain/body?
higher centres (e.g. cerebral cortex)
—> central neural pattern generator (“chewing centre”)
—> masticatory muscle motorneurones (only these just jaw reflexes)
—> masticatory muscles
