Junior Dentist » Human Anatomy

What are the muscles responsible for Facial Expressions

Varun — Sat, 28 Jan 2012 07:06:37 +0000

Some Features of the Muscles of Facial Expression are:

Subcutaneous
Develop from the Mesoderm of the2nd Branchial arch
They are supplied by the facial nerve
All of them are inserted into the skin

The Muscles which lead to various expressions:

Smiling and laughing – Zygomaticus Major
Sadness – Levator labii superioris, levator angulioris
Grief – Depressor Anguli Oris
Anger – Dilator naris and depressor septi
Frowning – Corrugator Supercilii, Procerus
Horror, terror, fright – Platysma
Surprise – Frontalis
Doubt – Mentalis
Grinning – Risorius
Contempt – Zygomaticus Minor
Closing the mouth – Orbicularis Oris
Whistling – Buccinator, Orbicularis Oris

Muscles of Facial Expression are the Facial muscles which with the help of minute and major contractions and relaxation lead to appearance of facial expressions.

Masticatory Reflexes or Jaw Reflexes

Varun — Mon, 26 Dec 2011 20:19:20 +0000

Reflex is a highly stereotyped automated response to a specific stimulus

Masticatory reflexes are type of stretch reflexes brought about by activation of muscle spindles in the muscles

Types of Jaw Reflexes:

Jaw Closing Reflex / Jaw Jerk Reflex
Jaw Opening Reflex
Jaw Unloading Reflex
Tooth Contact Reflexes
Horizontal Jaw Reflexes

Jaw Closing Reflex / Jaw Jerk Reflex:

A stretch reflex similar to knee jerk reflex
It is a monosynaptic reflex generated by stretching muscle spindles in the masseter muscle
Latency period 7-12ms
Demonstration: A sharp downward tap on the chin when the mandible is held loosely in the rest position results in contraction of the masseter muscle to bring the teeth into occlusion

Jaw Opening Reflex:

Polysynaptic reflex
Occurs as a result of mechanical or electrical stimulation of the lips, oral mucosa or teeth
A slight opening movement occurs due to inhibition of activity in the mandibular elevators without simultaneous contraction of the depressors

Jaw Unloading Reflex:

It is a protective reflex that occurs when the jaw is suddenly unloaded
Eg: When a stone comes inside mouth with food while mastication.
On sudden encounter with the hard object, mastication is stopped.
This is due to reflex inhibition of elevators & reflex excitement of jaw depressors
Due to receptors in periodontal ligament
Protects teeth from damage

Tooth Contact Reflex:

Reflex changes that occur in the elevator muscles when the upper & lower teeth are snapped together
There is transient activation followed by a silent period & then a phase of increased & decreased activity in the elevators
No effects on the depressors

Horizontal Jaw Reflexes:

Lateral, protrusive & retrusive reflex mandibular reflexes

Masticatory Cycle Phases

Varun — Mon, 26 Dec 2011 19:58:16 +0000

The pathway of mandible during chewing is referred to as chewing cycle.

Characterized by opening, closing, retrusive, protrusive & lateral jaw movements

The chewing cycle can take many forms; classically tear drop shape when viewed in frontal or saggital plane

There are about 15 chews in a series from the time of food entry until swallowing

Avg jaw opening during chewing is between 16-20mm

Duration of each cycle – 0.8 – 1.0 sec

The 3 phases Of Masticatory Cycle:

Opening phase (mandible is depressed)
Closing Phase (mandible is elevated)
Occlusal / Intercuspal phase

Opening phase:

Intake of bolus in the mouth – mouth opens by reflex inhibition of elevators

Isotonic contraction of depressors

Condyles move forward & downward

Divided into two types:

Slow opening
Rapid opening

Closing Phase:

In the beginning of this phase – elevators show isotonic contraction

In the end of this phase – elevators (masseter) shows isometric contraction

Gradual transition from isotonic contraction to isometric contraction

Condyles -

On working side – moves back almost horizontally
On balancing side – moves upwards & backwards in reverse direction

Intercuspal phase:

Tooth to tooth contact occurs
Path of mandibular closure determined by slide of mandibular teeth along the cuspal inclines of maxillary teeth
First cuspal contact B-B, then L-L, then cusps glide sideways

The 3 Possibilities:

Failure to penetrate the intervening food particle even after power stroke
Slow penetration of food particle & tooth contact
Sudden breakage of food particle leading to unloading reflex & separation of occluding surfaces

What are Masticatory Forces

Varun — Mon, 26 Dec 2011 19:29:04 +0000

Masticatory Forces are the Forces exerted during mastication with variations in region, persons, age, sex, food habits & race. The muscles of mastication are the main cause for these forces they contract and relax and lead to the production of force for Mastication. It is measured and sometimes termed as Bite force as it is the force generated on biting.

The Various Masticatory forces during mastication:

The average maximum sustainable biting force is 756N{170 pounds}.
Average masticatory force:
Males – 520N; Females – 340 N
Normal dentition – 80N;
Dentures – 64N
Molar region: 400-890N
Premolar region: 222-445N
Cuspid region: 133-334N
Incisor region: 89-111N
Masticatory force measured by Gnathodynamometer

What are the Factors influencing Bite Force:Particular tooth

Dietary consistency
Degree of chronic periodontal disease
Jaw separation
Tooth – cusp configuration
Natural / artificial teeth
Biting practice

Sutures Of Skull

Varun — Thu, 24 Nov 2011 02:56:47 +0000

Human skull has around 20 Sutures which means that the number of bones fusing together to form sutures are 40 and including the number of single bones in the Skull the number of bones in the human Skull during birth are 45 and the number of bones in adulthood are 22 bones.

Sutures:

Def: Sutures are Junctions or lines of articulation between adjacent bones of skull

What is a Suture:

A suture is a type of fibrous joint which only occurs in the skull (or “cranium”). They are bound together by Sharpey’s fibres. A tiny amount of movement is permitted at sutures, which contributes to the compliance and elasticity of the skull.

What are the various Sutures Present in the Skull:

Coronal suture – Junction between frontal and parietal bones
Lambdoid suture – Junction between parietal and occipital bones
Occipitomastoid suture – Junction between sphenoid and temporal bones
Parietomastoid suture – Junction between parietal and temporal bones
Sphenofrontal suture – Junction between sphenoid and frontal bones
Sphenoparietal suture – Junction between sphenoid and parietal bones
Sphenosquamosal suture – Junction between sphenoid and temporal bones
Sphenozygomatic suture - Junction between sphenoid and zygomatic bones
Squamosal suture – Junction between the parietal and the temporal bone
Zygomaticotemporal suture – Junction between Zygomatic and temporal bones
Zygomaticofrontal suture – Junction between Zygomatic and Frontal bone