LOCAL ANESTHESIA “Surgical Anatomy”

Home , Auriculotemporal nerve, Buccal nerve, Deep temporal nerves, Frontal nerve, Lacrimal nerve, Masseteric nerve

Local anesthesia

“Surgical anatomy”

 The Anatomical Basis of Dental Pain

 Impulses originating in the nerve-endings of the dental pulp and the supporting structures of the teeth are conveyed to the central nervous system by the second (Maxillary) and the third (Mandibular) divisions of the fifth cranial or trigeminal nerve.

 From cell bodies in the Gasserian ganglion these neural pathways pass to the sensory nucleus of the trigeminal nerve which is situated in the medulla oblongata and extends to the level of second cervical segment of the spinal cord.

 Then they pass via the trigeminal lemniscus to the thalamus and then to the cortex of the brain. Interruption of theses neural pathways at any level abolish the sensation of pain.

 The only nerve with which the dental surgeon is concerned when administrating a local analgesic is the trigeminal nerve (5th cranial nerve), which is the largest of the cranial nerves.

 This nerve has a big sensory and smaller motor roots. It supplies the mandible, maxilla, and associated structures, and also most of the skin of face.

 The motor branch supplies all muscles of mastication (the temporalis, masseter, medial pterygoid, and lateral pterygoid muscles) but not the buccinator, which is considered to be a muscle of facial expression, being innervated by the facial nerve (7th cranial nerve).

 THE OPHALMIC , OR FIRST DIVISION OF THE TRIGRMINAL NERVE

 Ophthalmic nerve, which is entirely sensory, is of little interest to the dental surgeon. It divides into three main branches which pass into the orbit through the superior orbital fissure. These are:

1. The frontal nerve

2. The nasociliary nerve

3. The lacrimal nerve.

1. The frontal nerve. This supplies the forehead and the scalp as far back as the vertex. It also supplies the skin at the root of the nose, the skin and conjunctiva of the upper eyelid, and the mucosal lining of the frontal sinus.

2. The nasociliary nerve. This supplies the cornea and sclera of the eye, the upper and anterior part of the nasal septum, and the lateral wall of the nose.

3. The lacrimal nerve. This is the smallest of the branches. It supplies the gland of the same name (the lacrimal gland), It also supplies part of the conjunctiva.

 THE MAXILLARY, OR SECOND DIVISION OF THE TRIGEMINAL NERVE

 Maxillary division is entirely sensory. It supplies the whole of the maxilla including the teeth and their gingiva, the maxillary sinus, and the mucous membranes of the hard and soft palates, the nasal cavity and the nasopharynx, in addition it innervates the skin of the upper lip, the upper part of the cheek, the lower eyelid, the skin over the adjacent part of the nose, the skin over the anterior part of the temporal region, and that overlying the zygomatic bone.

 Through the sphenopalatine ganglion pass the following main branches:

 Branch to the orbit.

 The short sphenopalatine nerves reach the nose via the sphenopalatine foramen. They supply the upper and posterior part of the lateral wall of the nose.

3. The long sphenopalatine (nasopalatine) nerve passes along the septum, which it supplies, to the floor of the nasal cavity, and then through the incisive canal to emerge at the incisive fossa on to the palate. It supplies the mucoperiosteum, gingiva and alveolar process and the anterior part of the palate from canine to canine.

4. The greater palatine nerve gains the palate by passing through the greater palatine foramen which is situated medially to the second or third molar. It supplies the mucoperiosteum and gingivae of the hard palate from the canine posteriorly and rarely may supply the palatal root of the maxillary first and second molars. This explains why there are occasions when maxillary molars remain sensitive after buccal infiltration have been given and analgesia is not obtained until additional palatal infiltrations have been administrated.

5. The lesser palatine nerves traverse the lesser palatine foramina of the hard palate to innervate the mucosa of the soft palate and the uvula. The palatine nerves (greater and lesser) together supply the whole of the sensory innervation to the mucosa covering the hard and soft palates and also propagate secretory and taste fibres to this region.

6. A small pharyngeal branch passes from the ganglion to reach the nasopharynx.

 THE MANDIBULAR, OR THIRD DIVISION, OF THE TRIGEMINAL NERVE

 This division of the trigeminal nerve is both sensory and motor, supplying all the muscles of mastication. The sensory part innervates the whole of the lower jaw including the teeth and their associated gingivae, the skin overlying the chin, the skin and mucous membrane of the lower lip and part of the cheek, and the anterior two-thirds of the tongue, except for the cicumvallate papillae.

 BRANCHES

• Meningeal branch (nervus spinosus)

• Nerve to medial pterygoid

• Anterior trunk:

 Masseteric nerve

 Deep temporal nerves

 Nerve to lateral pterygoid

 Buccal nerve

• Posterior trunk:

 Auriculotemporal nerve

 Lingual nerve

 Inferior alveolar nerve

 Neurophysiology

 Nerves are groups of cells that act together to be a communication system for the Central Nervous System (CNS).

 The neuron is the cell responsible for transmitting messages from the periphery of the body to the CNS and from the CNS to the periphery of the body.

 Afferent or sensory nerves conduct impulses to the CNS while efferent or motor neurons conduct messages from the CNS to the periphery.

 The sensation of pain is first felt at the skin, tissue, or mucosa and the dendritic zone (made up of free nerve endings) starts the impulse. This impulse follows along an axon to the brain where it is interpreted.

 After the sensory nerve stimulates the CNS, the CNS decides the best action to take and sends its decision via impulse back through motor neurons to activate muscles.

 The axon is a long nerve fiber that runs the whole length of the nerve. It's encased in a nerve membrane called an axolemma. Inside the membrane is a gelatinous substance called axoplasm. Most axolemmas are surrounded by a myelin sheath.

 The most widely held theory on nerve transmission attributes conduction of the nerve impulse to changes in the axolemma, not necessarily the axoplasm or the cell body.

 In a resting nerve, the electrical resistance keeps sodium, potassium and chloride ions from flowing into the axoplasm. Nerves have a resting potential where the inside or axoplasm has a negative electrical potential. When the nerve is stimulated, the electrical potential is slightly decreased (slow polarization). Then at a certain level of electrical potential called the threshold potential or firing threshold, a rapid depolarization of the membrane occurs. The electrical potential is then reversed resulting in the inside of the nerve being electrically positive and the outside negative.

 After depolarization, the membrane gradually polarizes again to the normal resting potential.

 When the nerve is stimulated and an impulse is generated, the conductivity changes and the membrane allows sodium and potassium ions to pass into the axoplasm. This provides the energy for the impulse to continue along the nerve to the CNS.