Sunday 9 October 2016

Nerve supply of teeth

Nerve supply of teeth

Trigeminal nerve



The trigeminal nerve is the largest cranial nerve (Fig. A).

It leaves the anterior aspect of the pons as a small motorroot and a large sensory root, and it passes forward, out ofthe posterior cranial fossa, to reach the apex of the petrouspart of the temporal bone in the middle cranial fossa. Here,the large sensory root expands to form the trigeminalganglion(Fig. A).

The trigeminal ganglion lies within a pouch of dura mater called the trigeminalcave.

The motor root of the trigeminal nerve is situated below the sensory ganglion and is completely separate from it. The ophthalmic (V1), maxillary (V2), and mandibular (V3) nerves arise from the anterior border of the ganglion (Fig. A).




Figure (A) Branches of trigeminal nerve



Maxillary Nerve 


The maxillary nerve arises from the trigeminal ganglion inthe middle cranial fossa.
It passes forward in the lateral wall of the cavernous sinus and leaves the skull through the foramen rotundum and crosses the pterygopalatine fossa to it enter the orbit through the inferior orbital fissure. It then continues as the infraorbital nerve in the infraorbital groove, and emerges on the face throughthe infraorbital foramen. It gives sensory fibers to the ski of the face and the side of the nose.





Figure (B) Branches of maxillary nerve






Branches

  • Meningeal branches
  • Zygomatic branch, which divides into thezygomaticotemporal and the zygomaticofacial nervesthat supply the skin of the face. The zygomaticotemporalbranch gives parasympathetic secretomotor fibers tothe lacrimal gland via the lacrimal nerve.
  • Ganglionic branches, which are two short nerves thatsuspend the pterygopalatine ganglion in the pterygopalatinefossa. They contain sensory fibersthat have passed through the ganglion from the nose,the palate, and the pharynx. They also contain postganglionicparasympathetic fibers that are going to the lacrimalgland.
  • Posterior superior alveolar nerve (Fig. B), whichsupplies the maxillary sinus as well as the upper molarteeth and adjoining parts of the gum and the cheek
  • Middle superior alveolar nerve (Fig. B), which suppliesthe maxillary sinus as well as the upper premolarteeth, the gums, and the cheek
  • Anterior superior alveolar nerve (Fig. B), whichsupplies the maxillary sinus as well as the upper canineand the incisor teeth
  • Pterygopalatine Ganglion
  • The pterygopalatine ganglion is a parasympathetic ganglion,which is suspended from the maxillary nerve in thepterygopalatine fossa. It is secretomotor to thelacrimal and nasal glands.
  • Branches
  • Orbital branches, which enter the orbit through theinferior orbital fissure
  • Greater and lesser palatine nerves, whichsupply the palate, the tonsil, and the nasal cavity
  • Pharyngeal branch, which supplies the roof of thenasopharynx





Mandibular Nerve 


The mandibular nerve is both motor and sensory (Figs.11.11 and 11.65). The sensory root leaves the trigeminalganglion and passes out of the skull through the foramenovale to enter the infratemporal fossa. The motor root ofthe trigeminal nerve also leaves the skull through the foramenovale and joins the sensory root to form the trunk ofthe mandibular nerve, and then divides into a small anteriorand a large posterior division (Fig. 11.66).


Branches from the Main Trunk of the Mandibular Nerve

  • Meningeal branch
  • Nerve to the medial pterygoid muscle, which suppliesnot only the medial pterygoid, but also the tensor velipalatini muscle.
  • Branches from the Anterior Division of theMandibular Nerve
  • Masseteric nerve to the masseter muscle
  • Deep temporal nerves to the temporalis muscle
  • Nerve to the lateral pterygoid muscle
  • Buccal nerve to the skin and the mucous membrane ofthe cheek (Fig. A). The buccal nerve does not supplythe buccinator muscle (which is supplied by the facialnerve), and it is the only sensory branch of the anteriordivision of the mandibular nerve.
  • Branches from the Posterior Division of theMandibular Nerve
  • Auriculotemporal nerve, which supplies the skin of theauricle (Fig. A), the external auditory meatus, thetemporomandibular joint, and the scalp. This nerve alsoconveys postganglionic parasympathetic secretomotorfibers from the otic ganglion to the parotid salivarygland.
  • Lingual nerve, which descends in front of the inferioralveolar nerve and enters the mouth (Fig. A). It then runs forward on the side of the tongueand crosses the submandibular duct. In its course, it isjoined by the chorda tympani nerve,and it supplies the mucous membrane of theanterior two thirds of the tongue and the floor of themouth.
  • It also gives off preganglionic parasympatheticsecretomotor fibers to the submandibular ganglion.
  • Inferior alveolar nerve (Fig. A), whichenters the mandibular canal to supply the teeth of thelower jaw and emerges through the mental foramen(mental nerve) to supply the skin of the chin. Before entering the canal, it gives off the mylohyoidnerve, which supplies the mylohyoidmuscle and the anterior belly of the digastricmuscle.
  • Communicating branch, which frequently runs fromthe inferior alveolar nerve to the lingual nerveThe branches of the posterior division of the mandibularnerve are sensory (except the nerve to the mylohyoid muscle).










DENTAL PAIN PATHWAY


Otic Ganglion

The otic ganglion is a parasympathetic ganglion that islocated medial to the mandibular nerve just below theskull, and it is adherent to the nerve to the medial pterygoidmuscle. The preganglionic fibers originate in the glossopharyngealnerve, and they reach the ganglion via thelesser petrosal nerve. The postganglionicsecretomotor fibers reach the parotid salivary gland via theauriculotemporal nerve. (Fig. A)

C L I N I C A L N O T E S

Injury to the Lingual Nerve:

The lingual nerve passes forward into the submandibularregion from the infratemporal fossa by running beneath theorigin of the superior constrictor muscle, which is attachedto the posterior border of the mylohyoid line on the mandible.Here, it is closely related to the last molar tooth and is liableto be damaged in cases of clumsy extraction of an impactedthird molar








Taste Bud. Its Histology for under graduate dental students

Taste Bud: The Histology

Tongue


The tongue is a mass of striated muscle covered by mucosa, which manipulates ingested material during mastication and swallowing.

 The muscle fibers are oriented in all directions, allowing a high level of mobility. 

Connective tissue between the small fascicles of muscle is penetrated by the lamina propria, which makes the mucous membrane strongly adherent to the muscular core.

The lower surface of the tongue is smooth, with typical lining mucosa.

The dorsal surface is irregular, having hundreds of small protruding papillae of various types on its anterior two-thirds and the massed lingual tonsils on the posterior third, or root of the tongue (Figure (a)).

The papillary and tonsillar areas of the lingual surface are separated by a V-shaped groove called the sulcus terminalis.

The lingual papillae are elevations of the mucous membrane that assume various forms and functions. There are four types (Figure (A)):
  1. Filiform papillae (Figure (A)) are very numerous, have an elongated conical shape, and are heavily keratinized, which gives their surface a gray or whitish appearance. They provide a rough surface that facilitates movement of food during chewing.
  2. Fungiform papillae (Figure (A)) are much less numerous, lightly keratinized, and interspersed among the filiform papillae. They are mushroom-shaped with well-vascularized and innervated cores of lamina propria.
  3. Foliate papillae consist of several parallel ridges on each side of the tongue, anterior to the sulcus terminalis, but are rudimentary in humans, especially older individuals.
  4. Vallate (or circumvallate) papillae (Figure (A)) are the largest papillae, with diameters of 1 to 3 mm. Eight to twelve vallate papillae are normally aligned just in front of the terminal sulcus. Ducts of several small, serous salivary (von Ebner) glands empty into the deep, moatlike groove surrounding each vallate papilla. This provides a continuous flow of fluid over the taste buds that are abundant on the sides of these papillae, washing away food particles so that the taste buds can receive and process new gustatory stimuli. Secretions from these and other minor salivary glands associated with taste buds contain a lipase that prevents the formation of a hydrophobic film on these structures that would hinder gustation.




(A)Dorsal surface of tongue




Taste buds are ovoid structures within the stratified epithelium on the tongue’s surface, which sample the general chemical composition of ingested material 


(Figures (A) and (B)). 

Approximately 250 taste buds are present on the lateral surface of each vallate papilla, with many others present on fungiform and foliate (but not the keratinized filiform) papillae.

 They are not restricted to papillae and are also widely scattered elsewhere on the dorsal and lateral surfaces of the tongue, where they are also continuously flushed by numerous minor salivary glands.


A taste bud has 50 to 100 cells, about half of which are elongated gustatory (taste) cells, which turn over with a 7- to 10-day life span.

 Other cells present are slender (b) Micrograph shows a single very large vallate papilla with two distinctive features: many taste buds (TB) around the sides and several small salivary glands (Gl) emptying into the cleft or moat formed by the elevated mucosa surrounding the papilla.

 These glands continuously flush the cleft, renewing the fluid in contact with the taste buds.
 The base of each bud rests on the basal lamina and is entered by afferent sensory axons that form synapses with the gustatory cells.

 At the apical ends of the gustatory cells, microvilli project toward a 2-μm-wide opening in the structure called the taste pore.

 Molecules (tastants) dissolved in saliva contact the microvilli through the pore and interact with cell surface taste receptors (Figure (B)).

Taste buds detect at least five broad categories of  tastants: 

  1.  Sodium ions (salty);
  2.  Hydrogen ions from acids (sour);
  3.  Sugars and related compounds (sweet)
  4.  Alkaloids and certain toxins (bitter); and 
  5.  Amino acids such as glutamate and aspartate (umami; Jap. umami, savory).


 Salt and sour tastes are produced by ion channels and the other three taste categories are mediated by G-protein–coupled receptors.

 Receptor binding produces depolarization of the gustatory cells, stimulating the sensory nerve fibers that send information to the brain for processing.

 Conscious perception of tastes in food requires olfactory and other sensations in addition to taste bud activity.




(B) Histology of taste bud





Taste and its pathway

Taste: Taste allows person to select food according to its desire.

Taste Buds: these are mainly present on the dorsal surface of the tongue but few 
present on palate, pharynx, glottis.

Size: 50-70 milli microns

Number: 10000

30-80 cells are present in each taste bud.

Particular type of substances activates particular sense receptor known as taste modalities i.e sweet, salty, sour, bitter.

Taste areas in brain:
1.   Insular Cortex
2.   Somatosensory cortex



Taste reflex:

Taste reflex is concerned with the saliva secretion. Large number of nerve impulses from nucleus tactussolitarius are transmitted directly into the superior and inferior salivary nucleus, from there signals are transmitted to the salivary glands to help the control of secretion of saliva during ingestion of food. This is taste reflex.

Cell types of taste buds:

1.   Type-I cell/light cell – secrete pit substances
2.   Type-II cells/Dark cell – contains numerous vesicles
3.   Type-III cell/ Intermediate cell- synapse with numerous nerve fibers
4.   Type-IV cell/basal/precursor cell – production of new cells



There are two types of cells involved in generation of action potential:

·        Transducing
·        Gustducin

Factors affecting taste:

1.   Temperature of oral cavity
2.   Hormonal
3.   Genetic
4.   Age
5.   Anaesthetic drugs (Local)
6.   Use of tobacco, pan etc







Taste pathway:

Taste pathway is concerned with transmission of taste sensation to the brain from tongue as explained by the figure below:





(C) Diagrammatic representation of taste pathway