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.
EPITHELIUMS
Epithelial Tissue Epithelial tissue covers surfaces, usually has a basement membrane, has little extracellular material, and has no blood vessels. A basement membrane attaches the epithelial cells to underlying tissues. Most epithelia have a free surface, which is not in contact with other cells. Epithelia are classified according to the number of cell layers and the shape of the cells.
- Epitheliums contain no blood vessels. There is normally an underlying layer of connective tissue
- Almost all epitheliums lie on a basement membrane.The basement membrane consists of a basal lamina and reticular lamina. The reticular lamina is connected to the basal lamina by anchoring fibrils. The reticular lamina may be absent in which case the basement membrane consist only of a basal lamina. The basal lamina consists of a - lamina densa in the middle (physical barrier) with a lamina lucida on both sides (+charge barrier),The basement membrane is absent in ependymal cells.The basement membrane is not continuous in sinusoidal capillaries.
- Epitheliums always line or cover something
- Epithelial cells lie close together with little intercellular space
- Epithelial cells are strongly connected to one another especially those epitheliums that are subjected to mechanical forces.
Functions of Epithelium:
→ Simple epithelium involved with diffusion, filtration, secretion, or absorption
→ Stratified epithelium protects from abrasion
→ Squamous cells function in diffusion or filtration
- The tongue is divided into halves by a medial fibrous lingual septum that lies deep to the medial groove.
- In each half of the tongue there are four extrinsic and four intrinsic muscles.
- The lingual muscles are all supplied by the hypoglossal nerve (CN XII).
- The only exception is palatoglossus, which is supplied by the pharyngeal branch of the vagus nerve, via the pharyngeal plexus.
Eye
At week 4, two depressions are evident on each of the forebrain hemispheres. As the anterior neural fold closes, the optic pits elongate to form the optic vesicles. The optic vesicles remain connected to the forebrain by optic stalks.
The invagination of the optic vesicles forms a bilayered optic cup. The bilayered cup becomes the dual layered retina (neural and pigmented layer)
Surface ectoderm forms the lens placode, which invaginates with the optic cup.
The optic stalk is deficient ventrally to contain choroids fissure to allow blood vessels into the eye (hyaloid artery). The artery feeds the growing lens, but will its distal portion will eventually degenerate such that the adult lens receives no hyaloid vasculature.
At the 7th week, the choroids fissure closes and walls fuse as the retinal nerve get bigger.
The anterior rim of the optic vesicles forms the retina and iris. The iris is an outgrowth of the distal edge of the retina.
Optic vesicles induces/maintains the development of the lens vesicle, which forms the definitive lens. Following separation of the lens vesicle from the surface ectoderm, the cornea develops in the anterior 1/5th of the eye.
The lens and retina are surrounded by mesenchyme which forms a tough connective tissue, the sclera, that is continuous with the dura mater around the optic nerve.
Iridopupillary membrane forms to separate the anterior and posterior chambers of the eye. The membrane breaks down to allow for the pupil
Mesenchyme surrounding the forming eye forms musculature (ciliary muscles and pupillary muscles – from somitomeres 1 and 2; innervated by CN III), supportive connective tissue elements and vasculature.
Eyelids
Formed by an outgrowth of ectoderm that is fused at its midline in the 2nd trimester, but later reopen.
Muscles acting on the Temporomandibular Joint
- Movements of the temporomandibular joint are chiefly from the action of the muscles of mastication.
- The temporalis, masseter, and medial pterygoid muscles produce biting movements.
- The lateral pterygoid muscles protrude the mandible with the help from the medial pterygoid muscles and retruded largely by the posterior fibres of the temporalis muscle.
- Gravity is sufficient to depress the mandible, but if there is resistance, the lateral pterygoid, suprahyoid and infrahyoid, mylohyoid and anterior digastric muscles are activated.
| Actions | Muscles | |||
| Depression (Open mouth) |
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| Elevation (Close mouth) |
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| Protrusion (Protrude chin) |
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| Retrusion (Retrude chin) |
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| Side-to-side movements (grinding and chewing) |
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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
Movements of the Temporomandibular Joint
- The two movements that occur at this joint are anterior gliding and a hinge-like rotation.
- When the mandible is depressed during opening of the mouth, the head of the mandible and articular disc move anteriorly on the articular surface until the head lies inferior to the articular tubercle.
- As this anterior gliding occurs, the head of the mandible rotates on the inferior surface of the articular disc.
- This permits simple chewing or grinding movements over a small range.
- Movements that are seen in this joint are: depression, elevation, protrusion, retraction and grinding