development of face Understanding facial development is not only fascinating from a biological perspective but also crucial in various fields, including medicine (craniofacial surgery, orthodontics), forensics, and anthropology.

The development of the human face is a marvel of coordinated cellular activity, transforming a simple group of cells into the intricate and expressive structure we know. This process occurs entirely within the first eight weeks of embryonic development, with most of the sculpting happening between weeks four and six.

1. Topic - Jaw Reflex Contents - Overview of mastication Muscles of Mastication Reflex Jaw Reflex Types of Jaw Reflex Sahil Khairnar Roll no..... 49

2. Mastication:- ☆ Mastication of food is the initial stage in the process of digestion. ☆ Large pieces of food are reduced to smaller for swallowing. ☆ The food is broken apart and the surface area increased for the efficient action of digestive enzymes and to facilitate solubilisation of food substances in saliva to stimulate taste receptors.

3. Action of muscles during masticatory movements :- ☆ Opening / Depressor jaw muscles - mylohyoid - diagastric - Inferior Lateral pterygoid ☆ Closing/ Elevator jaw muscles - Medial pterygoid - Superficial Masseter - Temporalis

4. Reflex :- ☆ Reflex is a highly stereotype automated response to a specific stimulus. ☆ Masticatory reflexes are a type of stretch reflex brought about by activation of muscle spindles in muscles associated with the mandibular joint. ☆ There are multiple types of jaw reflexes which are seen during normal speech, any trauma, chewing, etc.

5. Jaw Reflex ☆ Jaw reflexes refer to the involuntary responses of the muscles in the jaw to certain stimuli. ☆ These reflexes are mediated by the trigeminal nerve, which is the fifth cranial nerve responsible for sensory input and motor control of the muscles involved in chewing and biting.

6. Types of jaw reflexes:- Jaw Closing reflex Jaw opening reflex Jaw unloading reflex Tooth contact reflex Horizontal Jaw reflex

7. 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

8. ☆Muscle spindle in the Masseter suddenly stretched ☆ Afferent nerve activity is generated in the spindles ☆ afferent impulses passed to the mesencephalic nucleus in the brain stem ☆ Afferent fibres synapse with the efferent motor neuron leading back to the fibres of the Masseter ☆ Muscle will contract

9. This reflex also counteracts the force of the gravity which would act to lower the jaw and separate the articular surface of the temporomandibular joint. As all the elevator muscles are maintained in a mild state of the of contraction called muscle tone.

10. 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

11. Jaw unloading reflex:- •It is a protective reflex that occurs when the jaw is suddenly unloaded
•E.g.: When a hard substance or food comes between occlusion while mastication.
•This reflex is seen when the food or substance suddenly breaks or slips or collapses, this reflex prevents the jaws from closing with a force and thus prevents any injury or damage to the teeth etc.
• This is due to reflex inhibition of elevators & reflex excitement of jaw depressors
• Due to receptors in periodontal ligament
• Protects teeth from damage

12. 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

13. Horizontal jaw reflex:- It is also called as Horizontal Mandibular reflex which is seen when a labiolingual force is applied to the maxillary central incisor resulting in a lateral movement of the mandible to the contralateral side.
• Lateral, protrusive and retrusive reflex mandibular reflexes
•The reflex is known to be initiated by a rapidly adapting periodontal receptor

14. Sahil Khairnar Roll no.......49