NEET MDS Lessons
Anatomy
Tongue
Appears at 4th week.
Musculature derived from mesoderm of occipital somites. Precursor muscles cells migrate to region of tongue and are innervated by general sensory efferent fibers of CN XII.
Mucosa derived from anterior endoderm lining arches 1-4; accordingly, innervation depends on arch derivation:
Mucosa of anterior 2/3 of tongue comes from the first arch -> CN V
Mucosa of posterior 1/3 of tongue comes from third and forth arch -> CN IX, X
Special taste of anterior 2/3 of tongue comes from CN VII.
Special taste of posterior 1/3 of tongue comes from CN X.
Tongue freed from floor of mouth by extensive degeneration of underlying tissue. Midline frenulum continues to anchor tongue to floor of mouth.
Thyroid Gland
Develops as in growth of mucosal epithelium located in the midline of the tongue (at foramen cecum). It descends along front of pharyngeal gut, but remains connected to tongue by thyrooglossal duct, which is obliterated later in development. Thyroid gland descends to a point just caudal to laryngeal cartilages.
Facial structures (general)
a) medial nasal prominence forms midline of nose, philtrum and primary palate
b) lateral nasal prominence forms alae of nose
c) maxillary prominence forms cheek region and lateral lip
d) clefts can form at inter-prominence fusion lines
Nose
At the time of anterior neural tube closure, mesenchyme around forebrain, frontonasal prominence (FNP), has smooth rounded extended contour. Nasal placodes (thickening of surface ectoderm to become peripheral neural tissue) develop on frontolateral aspects of FNP. Mesenchyme swells around nasal placode producing a medial and lateral nasal prominence (nasomedial and nasolateral processes). These nasal prominences form the nose.
Mouth
Stomadeum (primitive oral cavity) forms between frontonasal prominence and first pharyngeal arch. The first pharyngeal arch forms the dorsal maxillary prominence and ventral mandibular prominence. The maxillary prominence will merge with medial nasal prominences, pushing them closer to cause fusion. Fused medial nasal prominences will form midline of nose and midline of upper lip (philtrum) and primary palate (first 4 teeth).
Nasolacrimal structures
Maxillary and lateral nasal prominences are separated by deep furrow, the nasolacrimal groove. Ectoderm in floor of groove forms epithelial cord, which detaches from overlying ectoderm. The epithelial cord canalizes to form the nasolacrimal duct. The upper end of the duct widens to form the lacrimal sac. After detachment of the cord, the maxillary and lateral nasal prominences merge with each other, resulting in the formation of a nasolacrimal duct that runs from the medial corner of the eye to the inferior meatus of the nasal cavity.
The maxillary prominences enlarge to form the cheeks and maxillae.
The lateral nasal prominences form the alae of the nose.
Secondary (hard) palate
Main part of definitive palate formed by two palatine shelves derived from intraoral bilateral extensions of the maxillary prominences. These appear at the 6th week. They are directed obliquely downward on each side of the tongue; they move down when mandible gets bigger.
At the seventh week, they ascend to attain a horizontal position, then fuse to form the secondary palate. At the time the palatine shelves fuse, the nasal septum (an outgrowth of median tissue of the frontonasal prominence) grows down and joins the cephalic aspect of the newly formed palate
Anteriorly, shelves fuse with triangular primary palate. The incisive foramen marks the midline between the primary and secondary palate.
External Ear
The auricle is derived from 6 auricular hillocks (mesenchymal proliferations) along the dorsal aspect of arches 1 (top of ear) and 2 (bottom of ear). These fuse to form the definitive auricle. At the mandible grows, the ear is pushed upward and backward from its initial horizontal position on the neck.
The EAM is derived from the 1st pharyngeal arch.
The eardrum (tympanic membrane) is composed of 3 layers of cells: 1) ectodermal epithelial lining of bottom of EAM; 2) endodermal epithelium lining of tympanic cavity; 3) intermediate layer of connective tissue.
The eardrum is composed of multiple cell layers because it represents the first pharyngeal membrane, and thus lies at the junction of the first pharyngeal pouch and cleft.
Middle Ear
The middle ear consists of an auditory tube (from the 1st pharyngeal pouch, along with tympanic cavity) and the ossicles (from pharyngeal arches 1 and 2 cartilage).
The first arch cartilage forms the malleus and incus. The tensor tympani (muscle of the malleus) is derived from the fourth somitomere (associated with the first arch) and is therefore innervated by CN V.
The second arch cartilage forms the stapes. The stapedius (muscles of the stapes) is derived from the sixth somitomere (associated with the second arch) and is therefore innervated by CN VII.
The ossicles are initially embedded in mesenchyme, but in the 8th month, the mesenchyme degenerates and an endodermal epithelial lining of the tympanic cavity envelops the ossicles and connects them to the wall of the cavity in a mesentery-like fashion.
Inner Ear
The inner ear is derived thickening of surface ectoderm on both sides of the hindbrain (otic placodes). The placodes invaginate to form otic vesicles (otocytes). The vesicles then divide into ventral and dorsal components.
The ventral component forms the saccule and cochlear duct.
The dorsal component forms the utricle and semicircular canals and endolymphatic duct.
Cochlear Duct
Derived from an outgrowth of the saccule during the 6th week. The outgrowth penetrates the surrounding mesenchyme in a spiral fashion. The surrounding mesenchyme forms the cartilage and undergoes vacuolization.
The scala vestibule and scale tympani form and surround the cochlear duct. They are filled with periplymp to receive mechanical vibrations of ossicles. The mechanical stimuli activates sensory (ciliary) cells in the cochlear duct.
Semicircular canals
The utricle is initially three flattened outpocketings, which lose the central core. From this three semicircular canals are forms, each at 90 degree angles from one another. Sensory cells arise in the ampulla at one end of each canal, in the utricle and saccule.
The Nose
- The nose is the superior part of the respiratory tract and contains the peripheral organ of smell.
- It is divided into right and left nasal cavities by the nasal septum.
- The nasal cavity is divided into the olfactory area and the respiratory area.
The Eye and Orbit
- The orbit (eye socket) appears as a bony recess in the skull when it is viewed from anteriorly.
- It almost surrounds the eye and their associated muscles, nerves and vessels, together with the lacrimal apparatus.
- The orbit is shaped somewhat like a four-side pyramid lying on its side, with its apex pointing posteriorly and its base anteriorly.
A. Anatomic position-erect body position with the arms at the sides and the palms upward
B. Plane or section
1. Definition-imaginary flat surface formed by an extension through an axis
2. Median plane-a vertical plane. that divides a body into right and left halves
3. Sagittal plane
- Any plane parallel to the median plane
- Divides the body into right and left portions
4. Frontal plane
- Vertical plane that forms at right angles to the sagittal plane
- Divides the body into anterior and posterior sections
- Synonymous with the term coronal plane
5. Transverse plane
- Horizontal plane that forms at right angles to the sagittal and frontal planes
- Divides the body into upper and lower portions
- Synonymous with the term horizontal plane
C. Relative positions
1. Anterior
- Nearest the abdominal surface and the front of the body
- Synonymous with the term ventral
- In referring to hands and forearms, the terms palmar and Volar are used
2. Posterior
- Back of the body
- Synonymous with the term dorsal
3. Superior
- Upper or higher
- Synonymous with the term cranial (head)
4. Inferior
- Below or lower
- Synonymous with the term caudal (tail)
- In referring to the top of the foot and the sole of the foot. the terms dorsal and plantar are used respectively
5. Medial-near to the median plane
6. Lateral-farther away from the median plane
7. Proximal-near the source or attachment
8. Distal-away from the source or. attachment
9. Superficial-near the surface
10. Deep-away from the surface
11. Afferent-conducting toward a structure
12. Efferent-conducting away from a structure
The Cheeks
- The cheeks (L. buccae) form the lateral wall of the vestibule of the oral cavity.
- They have essentially the same structure as the lips with which they are continuous.
- The principal muscular component of the cheeks is the buccinator muscle.
- Superficial to the fascia covering this muscle is the buccal fatpad that gives cheeks their rounded contour, especially in infants.
- The lips and cheeks act as a functional unit (e.g. during sucking, blowing, eating, etc.).
- They act as an oral sphincter in pushing food from the vestibule to the oral cavity proper.
- The tongue and buccinator muscle keep the food between the molar teeth during chewing.
Sensory Nerves of the Cheeks
- These are branches of the maxillary and mandibular nerves.
- They supply the skin of the cheeks and the mucous membrane lining the cheeks.
Connective Tissue
Functions of Connective tissue:
→ joins together other tissues
→ supporting framework for the body (bone)
→ fat stores energy
→ blood transports substances
Connective tissue is usually characterized by large amounts of extracellular materials that separate cells from each other, whereas epithelial tissue is mostly cells with very little extracellular material. The extracellular substance of connective tissue consists of protein fibers which are embedded in ground substance containing tissue fluid.
Fibers in connective tissue can be divided into three types:
→ Collagen fibers are the most abundant protein fibers in the body.
→ Elastic fibers are made of elastin and have the ability to recoil to original shape.
→ Reticular fibers are very fine collagen fibers that join connective tissues to other tissues.
Connective tissue cells are named according to their functions:
→ Blast cells produce the matrix of connective tissues
→ Cyte cells maintains the matrix of connective tissues
→ Clast cells breaks down the matrix for remodeling (found in bone)
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.