NEET MDS Lessons
Anatomy
Innervation of the Skin
- Innervation of the skin is mainly through the three branches of the trigeminal nerve (CN V).
- Some skin over the angle of the mandible and anterior and posterior of the auricle is supplied by the great auricular nerve from the cervical plexus.
- Some cutaneous branches of the auricular branch of the facial nerve also supplies skin on both sides of the auricle.
- The trigeminal nerve is the general sensory nerve to the head, particularly the face, and is the motor nerve to the muscles of mastication.
The Ophthalmic Nerve
- This is the superior division of the trigeminal nerve, the smallest of the three branches and is wholly sensory.
- The ophthalmic nerve divides into three branches: the nasociliary, frontal and lacrimal just before entering the orbit through the superior orbital fissure.
- The nasociliary nerve supplies the tip of the nose through the external nasal branch of the anterior ethmoidal nerve.
- The frontal nerve is the direct continuation of CN V1 and divides into two branches, the supraorbital and supratrochlear.
- The supratrochlear nerve supplies the middle part of the forehead.
- The supraorbital nerve supplies the lateral part and the front of the scalp.
- The lacrimal nerve, the smallest of the main ophthalmic branches, emerges over the superolateral orbital margin to supply the lacrimal gland and the lateral part of the upper eyelid.
The Maxillary Nerve
- This is the intermediate division of the trigeminal nerve.
- It has three cutaneous branches.
- The infraorbital nerve is the largest terminal branch of the maxillary nerve.
- It passes through the infraorbital foramen and breaks up into branches that supplies the skin on the lateral aspect of the nose, upper lip and lower eyelid.
- The zygomaticofacial nerve, a small branch of the maxillary, emerges from the zygomatic bone from a foramen with the same name.
- It supplies the skin over the zygomatic bone.
- The zygomaticotemporal nerve emerges from the zygomatic bone from foramen of the same name.
- It supplies the skin over the temporal region.
The Mandibular Nerve
- This is the inferior division of the trigeminal nerve.
- Of the three division of the trigeminal nerve, CN V3 is the only one that carries motor fibres (to the muscles of mastication).
- The main sensory branches of the mandibular nerve are the buccal, auriculotemporal, inferior alveolar and lingual nerves.
- The buccal nerve is a small branch of the mandibular that emerges from deep to the ramus of the mandible.
- It supplies the skin of the cheek over the buccinator muscle, the mucous membrane lining the cheek, and the buccal surface of the gingiva.
- The auriculotemporal nerve passes medially to the neck of the mandible and then turns superiorly, posterior to its head and anterior to the auricle. It then crosses over the root of the zygomatic process of the temporal bone, deep to the superficial temporal artery.
- It supplies the auricle, external acoustic meatus, tympanic membrane, and the skin in the temporal region.
- The inferior alveolar nerve is the large terminal branch of the posterior division of the mandibular nerve (the lingual nerve is the other terminal branch).
- It enters the mandible through the mandibular foramen to the mandibular canal. In the canal, it gives branches to the mandibular teeth.
- Opposite the mental foramen, this nerve divides into the mental nerve and the incisive nerve.
- The incisive nerve supplies the incisor teeth, the adjacent gingiva and the mucosa of the lower lip.
- The mental nerve emerges from the mental foramen and supplies the skin of the chin and the skin and mucous membrane of the lower lip and gingiva.
- The lingual nerve is the smaller terminal branch of the mandibular nerve.
- It supplies the general sensory fibres to the anterior two-thirds of the tongue, the floor of the mouth and the gingivae of the mandibular teeth.
The Temporomandibular Joint
- This articulation is a modified hinge type of synovial joint.
- The articular surfaces are: (1) the head or condyle of the mandible inferiorly and (2) the articular tubercle and the mandibular fossa of the squamous part of the temporal bone.
- An oval fibrocartilaginous articular disc divides the joint cavity into superior and inferior compartments. The disc is fused to the articular capsule surrounding the joint.
- The articular disc is more firmly bound to the mandible than to the temporal bone.
- Thus, when the head of the mandible slides anterior on the articular tubercle as the mouth is opened, the articular disc slides anteriorly against the posterior surface of the articular tubercle
Intramembranous ossification
- Flat bones develop in this way (bones of the skull)
- This type of bone development takes place in mesenchymal tissue
- Mesenchymal cells condense to form a primary ossification centre (blastema)
- Some of the condensed mesenchymal cells change to osteoprogenitor cells
- Osteoprogenitor cells change into osteoblasts which start to deposit bone
- As the osteoblasts deposit bone some of them become trapped in lacunae in the bone and then change into osteocytes
- Osteoblasts lie on the surface of the newly formed bone
- As more and more bone is deposited more and more osteocytes are formed from mesenchymal cells
- The bone that is formed is called a spicule
- This process takes place in many places simultaneously
- The spicules fuse to form trabeculae
- Blood vessels grow into the spaces between the trabeculae
- Mesenchymal cells in the spaces give rise to hemopoetic tissue
- This type of bone development forms the first phase in endochondral development
- It is also responsible for the growth of short bones and the thickening of long bones
The Arteries of the Face
- The superficial arteries are derived from the external carotid arteries.
The Facial Artery
- This is the chief artery of the face.
- It arises from the external carotid artery and winds its way to the inferior border of the mandible, just anterior to the masseter muscle.
- It hooks around the inferior border of the mandible and grooves the bone. Here the artery is superficial, just beneath the platysma and its pulsation can be felt.
- In its course over the face to the medial angle of the eye, the facial artery crosses the mandible, buccinator muscle, and maxilla.
- It lies deep to the zygomaticus major.
- The facial artery ends by sending branches to the lip and side of the nose.
- The part of the artery that runs along the side of the nose to supply the eyelids is called the angular artery.
The Superficial Temporal Artery
- This artery is the smaller of the two terminal branches of the external carotid artery (the other is the maxillary artery).
- It begins deep to the parotid gland, posterior to the neck of the mandible, and ascends superficial to the posterior end of the zygomatic process of the temporal bone. It then enters the temporal fossa.
- The superficial temporal artery ends in the scalp by dividing into the frontal and parietal branches.
- Pulsation of this artery can be felt by compressing the root of the zygomatic process of the temporal bone.
The Transverse Facial Artery
- This small artery arises from the superficial temporal artery before it emerges from the parotid gland.
- It crosses the face superficial to the masseter muscle, about a fingerbreadth inferior to the zygomatic arch.
- It divides into numerous branches that supply the parotid gland and duct, the masseter muscle, and the skin of the face.
- It anastomoses with branches of the facial artery.
The Medial Wall of the Orbit
- This wall is paper-thin and is formed by the orbital lamina or lamina papyracea of the ethmoid bone, along with contributions from the frontal, lacrimal, and sphenoid bones (L. papyraceus, "made of papyrus" or parchment paper).
- There is a vertical lacrimal groove in the medial wall, which is formed anteriorly by the maxilla and posteriorly by the lacrimal bone.
- It forms a fossa for the lacrimal sac and the adjacent part of the nasolacrimal duct.
- Along the suture between the ethmoid and frontal bones are two small foramina; the anterior and posterior ethmoidal foramina.
- These transmit nerves and vessels of the same name.
Structure of the Nasal Septum
- This part bony, part cartilaginous septum divides the chamber of the nose into two narrow nasal cavities.
- The bony part of the septum is usually located in the median plane until age 7; thereafter, it often deviates to one side, usually the right.
- The nasal septum has three main components: (1) the perpendicular plate of the ethmoid bone; (2) the vomer, and (3) the septal cartilage.
- The perpendicular plate, which forms the superior part of the septum, is very thin and descends from the cribiform plate of the ethmoid bone.
- The vomer, which forms the posteroinferior part of the septum, is a thin, flat bone. It articulates with the sphenoid, maxilla and palatine bones.
- Bones begin to form during the eighth week of embryomic life in the fibrous membranes (intramembranous ossification) and hyaline cartilage (endochondral ossification)