Advanced Techniques and Local Anesthesia
Neurophysiology Overview
Trigeminal Nerve
The trigeminal nerve is a predominately sensory nerve with three main branches: the ophthalmic nerve, the maxillary nerve and the mandibular nerve which includes motor function. The cell bodies of the trigeminal nerve form the semilunar or Gasserion Ganglion. This ganglion can be found in Meckle's Cave in the bottom of the middle cranial fascia.
We will limit our discussion to the maxillary and mandibular nerves. The maxillary nerve is exclusively sensory. It exits foramen rotundum at the skull base into the pterygo-palatine fossa where it branches into several branches. It gives off the greater palatine nerve, the naso-palatine nerve, smaller posterior nasal nerves, the zygomatic nerve, and descending posterior/superior alveolar branches.
The anterior/superior alveolar nerve twigs arise just before the exit of the infraorbital nerve through the infraorbital foramen. The superior dental plexus of nerves is formed by the posterior and anterior/superior alveolar branches, and sometimes there are middle superior alveolar branches present as well. The naso-palatine nerve, a terminal nerve, leaves the sphenopalatine ganglion, it passes along the nasal septum to the incisive canal to innervate the anterior part of the hard palate.
The infraorbital nerve provides inferior palpebral branches to the lower eyelid, external nasal branches to the skin of the nose, internal nasal branches to innervate the mucosa of the nose and superior labial branches to innervate the upper lip. The greater palatine leaves the sphenopalatine ganglion, descends through the greater palatine canal, emerging through the greater palatine foramen and innervating the hard palate.
The mandibular nerve is a mixed nerve that has both motor and sensory function, and it exits the foramen ovale into the infratemporal fossa. In the infratemporal fossa, it gives off many branches. It gives off the auriculo-temporal nerve, which supplies the external auditory canal, it passes from the medial around the mandibular condyle providing some supply to the temporomandibular joint.
The motor branches given off include the deep temporal nerve, the buccal and the masseter, and the masseter nerve. Sensory branches include the buccal nerve, the inferior alveolar nerve, which is a mixed nerve. There is a mylohyoid branch, which provides both sensory and motor to the mylohyoid muscle for the mouth and the anterior belly of the digastric muscle, and, of course, there's a mental nerve, which supplies the skin of the chin and the lower lip, and there's a lingual nerve supplying the anterior 2-thirds of the tongue.
The inferior alveoli nerve passes along the medial side of the ramus of the mandible entering the mandibular foramen, and it gives off branches within the bone that innervate teeth and gingiva, and the terminal branches of this nerve are the mental and incisive nerves. The lingual nerve passes together with the inferior alveolar nerve, communicates with chordatympany, picking up parasympathetic secreto-motor fibers to the submandibular, and sublingual glands via the submandibular ganglion, and it also has specialized sensory fibers for taste along the anterior 2-thirds of the tongue. This nerve crosses underneath the submandibular duct, innervating the tongue and the lingual gingiva.
There are many different peripheral nerve fibers. including the A-alpha, A-beta, A-gamma, A-delta. A-delta is involved in pain temperature and pressure. There are B-fibers, which are preganglionic sympathetics, and there's C-fibers, which are also important and provide for pain, temperature, itch, pressure and postganglionic sympathetics. The A-Alpha are heavily myelinated which will increase the conduction velocity to around 70 to 120 meters per second. The C-fibers are not myelinated, and its conduction velocity is 1.2 meters per second.
Nerve Function - Neurochemistry
In a resting state the nerve membrane is slightly permeable to Na+, but freely permeable to K+ and Cl-; therefore, potassium and chloride are in equilibrium across the membrane, but sodium cannot pass through the membrane.
When the nerve is stimulated and depolarization occurs, the ion channels open up, leading to a rapid influx of Na+ into the nerve and an increase in membrane potential from a resting potential of -70 millivolts to -55 millivolts. At this point the nerve fires (depolarizes), permeability increases even more, resulting in an action potential peaking at 40 millivolts. The entire depolarization process occurs within 0.3 milliseconds.
In the next phase, repolarizaton occurs by pumping out sodium, and this occurs through the sodium/potassium. ATPA is a pump that requires the use of energy, I.E., ATP, and it takes about .7 milliseconds for repolarization to occur. During this time there is an absolute refractory period where no matter what the stimulation, the nerve cannot fire again. Later on, during repolarization, there is a relative refractory period, where a much larger stimulation will allow for firing of the nerve.
Impulse spread occurs through two different methods, depending upon the anatomy of the nerve. In unmyelinated nerves, impulse spreads very slowly, traveling at about 1.2 meters per second. In myelinated nerves, it occurs through saltatory conduction from one node of Ranvier to the other, and conduction is very fast.e extracellular fluid, and the intracellular fluid in