NEET MDS Synopsis
Induction of Local Anesthesia
Oral and Maxillofacial SurgeryInduction of Local Anesthesia
The induction of local anesthesia involves the administration of a local
anesthetic agent into the soft tissues surrounding a nerve, allowing for the
temporary loss of sensation in a specific area. Understanding the mechanisms of
diffusion, the organization of peripheral nerves, and the barriers to anesthetic
penetration is crucial for effective anesthesia management in clinical practice.
Mechanism of Action
Diffusion:
After the local anesthetic is injected, it begins to diffuse from
the site of deposition into the surrounding tissues. This process is
driven by the concentration gradient, where the anesthetic moves from an
area of higher concentration (the injection site) to areas of lower
concentration (toward the nerve).
Unhindered Migration: The local anesthetic
molecules migrate through the extracellular fluid, seeking to reach the
nerve fibers. This movement is termed diffusion, which is the passive
movement of molecules through a fluid medium.
Anatomic Barriers:
The penetration of local anesthetics can be hindered by anatomical
barriers, particularly the perineurium, which is the
most significant barrier to the diffusion of local anesthetics. The
perineurium surrounds each fascicle of nerve fibers and restricts the
free movement of molecules.
Perilemma: The innermost layer of the perineurium,
known as the perilemma, also contributes to the barrier effect, making
it challenging for local anesthetics to penetrate effectively.
Organization of a Peripheral Nerve
Understanding the structure of peripheral nerves is essential for
comprehending how local anesthetics work. Here’s a breakdown of the components:
Organization of a Peripheral Nerve
Structure
Description
Nerve fiber
Single nerve cell
Endoneurium
Covers each nerve fiber
Fasciculi
Bundles of 500 to 1000 nerve fibres
Perineurium
Covers fascicule
Perilemma
Innermost layer of perinuerium
Epineurium
Alveolar connective tissue supporting fasciculi andCarrying nutrient
vessels
Epineural sheath
Outer layer of epinuerium
Composition of Nerve Fibers and Bundles
In a large peripheral nerve, which contains numerous axons, the local
anesthetic must diffuse inward toward the nerve core from the extraneural site
of injection. Here’s how this process works:
Diffusion Toward the Nerve Core:
The local anesthetic solution must travel through the endoneurium
and perineurium to reach the nerve fibers. As it penetrates, the
anesthetic is subject to dilution due to tissue uptake and mixing with
interstitial fluid.
This dilution can lead to a concentration gradient where the outer
mantle fibers (those closest to the injection site) are blocked
effectively, while the inner core fibers (those deeper within the nerve)
may not be blocked immediately.
Concentration Gradient:
The outer fibers are exposed to a higher concentration of the local
anesthetic, leading to a more rapid onset of anesthesia in these areas.
In contrast, the inner core fibers receive a lower concentration and are
blocked later.
The delay in blocking the core fibers is influenced by factors such
as the mass of tissue that the anesthetic must penetrate and the
diffusivity of the local anesthetic agent.
Clinical Implications
Understanding the induction of local anesthesia and the barriers to diffusion
is crucial for clinicians to optimize anesthesia techniques. Here are some key
points:
Injection Technique: Proper technique and site
selection for local anesthetic injection can enhance the effectiveness of
the anesthetic by maximizing diffusion toward the nerve.
Choice of Anesthetic: The selection of local anesthetic
agents with favorable diffusion properties can improve the onset and
duration of anesthesia.
Monitoring: Clinicians should monitor the effectiveness
of anesthesia, especially in procedures involving larger nerves or areas
with significant anatomical barriers.
Thrombolytic Agents
Pharmacology
Thrombolytic Agents:
Tissue Plasminogen Activator (t-PA, Activase)
t-PA is a serine protease. It is a poor plasminogen activator in the absence of fibrin. t-PA binds to fibrin and activates bound plasminogen several hundred-fold more rapidly than it activates plasminogen in the circulation.
Streptokinase (Streptase)
Streptokinase is a protein produced by β-hemolytic streptococci. It has no intrinsic enzymatic activity, but forms a stable noncovalent 1:1 complex with plasminogen. This produces a conformational change that exposes the active site on plasminogen that cleaves a peptide bond on free plasminogen molecules to form free plasmin.
Urokinase (Abbokinase)
Urokinase is isolated from cultured human cells.Like streptokinase, it lacks fibrin specificity and therefore readily induces a systemic lytic state. Like t-PA, Urokinase is very expensive.
Contraindications to Thrombolytic Therapy:
• Surgery within 10 days, including organ biopsy, puncture of noncompressible vessels, serious trauma, cardiopulmonary resuscitation.
• Serious gastrointestinal bleeding within 3 months.
• History of hypertension (diastolic pressure >110 mm Hg).
• Active bleeding or hemorrhagic disorder.
• Previous cerebrovascular accident or active intracranial bleeding.
Aminocaproic acid:
Aminocaproic acid prevents the binding or plasminogen and plasmin to fibrin. It is a potent inhibitor for fibrinolysis and can reverse states that are associated with excessive fibrinolysis.
Agonist, Antagonist, and Partial Agonists
Pharmacology
Agonist, Antagonist, and Partial Agonists
Agonists: molecules that activate receptors. A drug that mimics the body's own regulatory processes.
Antagonists: produce their effects by preventing receptors activation by endogenous regulatory molecules and drugs. Block activation of receptors by agonists.
Noncompetive Antagonist: Bind irreversibly to receptors, and reduce the maximal response that an agonist can elicit.
Competitive Antagonist: Bind reversibly to receptors, competing with agonists for binding sites.
Partial Agonists: Have moderate intrinsic activity, the maximal effect that a partial agonist can produce is lower than that of a full agonist. Act as antagonists as well as agonists.
The Orbital Margin
AnatomyThe Orbital Margin
The frontal, maxillary and zygomatic bones contribute equally to the formation of the orbital margin.
The supraorbital margin is composed entirely of the frontal bone.
At the junction of its medial and middle thirds is the supraorbital foramen (sometimes a notch), which transmits the supraorbital nerves and vessels.
The lateral orbital margin is formed almost entirely of the frontal process of the zygomatic bone.
The infraorbital margin is formed by the zygomatic bone laterally and the maxilla medially.
The medial orbital margin is formed superiorly by the frontal bone and inferiorly by the lacrimal crest of the frontal process of the maxilla.
This margin is distinct in its inferior half only.
The Optic Nerve
Anatomy
This is the second cranial nerve (CN II) and is the nerve of sight.
MANDIBULAR FIRST BICUSPID
Dental Anatomy
MANDIBULAR FIRST BICUSPID
Facial: The outline is very nearly symmetrical bilaterally, displaying a large, pointed buccal cusp. From it descends a large, well developed buccal ridge.
Lingual: This tooth has the smallest and most ill-defined lingual cusp of any of the premolars. A distinctive feature is the mesiolingual developmental groove
Proximal: The large buccal cusp tip is centered over the root tip, about at the long axis of this tooth. The very large buccal cusp and much reduced lingual cusp are very evident. You should keep in mind that the mesial marginal ridge is more cervical than the distal contact ridge; each anticipate the shape of their respective adjacent teeth.
Occlusal: The occlusal outline is diamond-shaped. The large buccal cusp dominates the occlusal surface. Marginal ridges are well developed and the mesiolingual developmental groove is consistently present. There are mesial and distal fossae with pits,
Contact Points: When viewed from the facial, each contact area/height of curvature is at about the same height.
Root Surface:-The root of the mandibular first bicuspid is usually single, but on occasion can be bifurcated (two roots).
Periodontium
Dental Anatomy
The periodontium consists of tissues supporting and investing the tooth and includes cementum, the periodontal ligament (PDL), and alveolar bone.
Parts of the gingiva adjacent to the tooth also give minor support, although the gingiva is Not considered to be part of the periodontium in many texts. For our purposes here, the groups Of gingival fibers related to tooth investment are discussed in this section.
Endocrine System
Physiology
The endocrine system along with the nervous system functions in the regulation of body activities. The nervous system acts through electrical impulses and neurotransmitters to cause muscle contraction and glandular secretion and interpretation of impulses. The endocrine system acts through chemical messengers called hormones that influence growth, development, and metabolic activities