The Musculature

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The Musculature

• RDN 803 Principles of Occlusion
• Reading Mohl, et al., Chapter 7
• W.D. McCall, Jr., Ph.D.
• These slides are available athttp//www.acsu.buf
  falo.edu/wdmccall/

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Topics to be Covered

• I. Properties of muscle
• II. Muscles of mandibular function
• III. Electromyography
• IV. Some trigeminal reflexes
• V. Selected dental controversies

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I. Properties of Muscle

• Common with limb muscle
• Length-tension curve
• Force-velocity curve
• Determinants of muscle force
• Peculiar to jaw muscles
• Anatomy of origins and insertions
• Location of some sensory cell bodies
• Organization of some reflexes

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The Length-Tension Experiment
2. Stimulate nerve to muscle

1. Set muscle length

3. Record force
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• Features
• Tetanic force gt twitch force
• Fast slow fibers differ
• Part of force is passive

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The Force-Velocity Experiment
1. Set weight to lift
2. Stimulate muscle
3. Measure velocity
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Feature Faster is weaker
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• Features
• Delay from action potential to twitch
• More force from a shower of action potentials

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Determinants of Muscle Force

• Length
• Velocity
• Delay from electrical to mechanical events
• Number of motor units recruited
• Frequency of stimulation
• Anatomy of origin and insertion

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II. Muscles of mandibular function

• Jaw closing muscles
• Masseter
• Temporalis
• Medial pterygoid
• Jaw opening muscles
• Anterior digastric
• Lateral pterygoid

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Origin zygomatic process of the maxilla and
inferiorborder of zygomatic arch Insertion ang
le of the mandibleinferior, lateral side of
ramus Innervation masseteric
nerve Function elevate mandible
Masseter
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Temporalis
Origin temporal fossa andtemporal
fascia Insertion coronoid process ofthe
mandible Innervation temporal
nerve Function elevate mandible
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Medial Pterygoid
Origin pterygoid fossa and medial surface of
lateral pterygoid plate Insertion ramus and
angle of the mandible Innervation medial
pterygoid nerve Function elevate mandible

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Sphenoid
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Origin of Medial Pterygoid
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Lateral Pterygoid -- Inferior Head
Origin lateral surface of lateral pterygoid
plate Insertion anterior neck of
themandible Innervation branch of masseteric
or buccal nerve Function pull condyle and
disk along the eminence
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Origin of Lateral Pterygoid, Inferior head
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Lateral Pterygoid -- Superior Head
Origin infratemporal fossa of greater sphenoid
wing Insertion anterior neck of
themandible Innervation branch of masseteric
or buccal nerve Function pull condyle and
disk along the eminence
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Superior Head of Lateral Pterygoid
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Origin posterior digastricmastoid notch of
temporalbone Insertion anterior
digastriclingual, inferior border ofthe
mandible Innervation Posterior
digastric--facial nerve Anterior
digastric--mylohyoid, mandibular
nerve Function Lower mandible, raise hyoid
Digastric
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ANTERIOR DIGASTRIC
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THE BIG PICTURE
FUNCTION EXAMPLE NEURAL CIRCUIT LOCATION
Protection Jaw jerk, Reflex Brain stem Jaw
opening, Gagging
Rhythmic Chewing, Pattern generator Brain
stem activity Breathing, Walking
Complex Speech Cortex function
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III. Electromyography

1. Mechanism
2. Electrodes
3. Electronics
4. Caveats
5. Strengths

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A. MECHANISM Muscle Action Potentials
Extracellular about 90 ?V
Across membrane 90 mV
EMG signal is attenuated at the skin by about 1000
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B. ELECTRODES
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Caveats

1. Electrical activity must be calibrated to be
   directly related to muscle force.
2. Jaw muscles are redundant six jaw closers, and
   they all are active in closing. (You cannot
   control the muscles independently. The forces on
   the jaw must satisfy Newtons equations.)

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Determinants of Muscle ForceItem Seen in EMG?

• A. Recruitment Yes
• B. Frequency Yes
• C. Length No
• D. Velocity No

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Strengths of Electromyography

1. Know precisely when muscle is active.
2. Know roughly how active.
3. Insight into activity of motor neuron.

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IV. Some trigeminal reflexes

1. Jaw closing reflex
2. Jaw opening reflex
3. Blink reflex
4. Tongue reflex
5. Gagging
6. Modulating influences

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Types of Reflexes

1. Postural
2. Protective
3. Cardiovascular
4. Respiratory
5. Digestive
6. Humoral

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The Jaw Jerk Reflex (Postural)
R.M. Bradley, Figure 10-12
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The Jaw Opening Reflex (Protective)
R.M. Bradley, Figure 10-13
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V. Selected Controversies

1. Rest position passive elasticity vs. active
   contraction
2. Mastication alternating reflexes vs. central
   pattern generator
3. Electronic devices for TMD diagnosis

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A. Rest position hypotheses
1. Passive elasticity 2. Active contraction a.
Stretch reflex b. TMJ receptors c. Airway
patency
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Rest Position Evidence

• Passive Elasticity
• Length-tension curve
• Negative EMG search

• Stretch reflex
• Sleep in chair
• MN inhibition in sleep
• Positive needle EMG
• Positive surface EMG

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Surface EMG,mV
Vertical Jaw Opening, mm
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B. Mastication hypotheses

• Hypotheses
• Alternating reflexes (Sherrington, 1917)
• Central pattern generator (Lund, 1971)
• Significance part of the theme Be skeptical
  and be critical

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C. Electronic devices for TMD diagnosis

• Hypotheses
• Resting EMG greater in TMD patients
• (No convincing evidence)
• 2. Jaw position differs in TMD patients
• (No convincing evidence)

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Summary

• Properties of muscle
• II. Muscles of mandibular function
• III. Electromyography
• IV. Two trigeminal reflexes
• V. Selected dental controversies

These slides are available athttp//www.acsu.buf
falo.edu/wdmccall/