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.
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
-> This bone forms much of the base and posterior aspect of the skull.
-> It has a large opening called the foramen magnum, through which the cranial cavity communicates with the vertebral canal.
-> It is also where the spinal cord becomes continuous with the medulla (oblongata) of the brain stem.
-> The occipital bone is saucer-shaped and can be divided into four parts: a squamous part (squama), a basilar part (basioccipital part), and two lateral parts (condylar parts).
-> These four parts develop separately around the foramen magnum and unite at about the age of 6 years to form one bone.
-> On the inferior surfaces of the lateral parts of the occipital bone are occipital condyles, where the skull articulates with C1 vertebra (the atlas) at the atlanto-occipital joints.
-> The internal aspect of the squamous part of the occipital bone is divided into four fossae: the superior two for the occipital poles of the cerebral hemispheres, and the inferior two, called cerebellar fossae, for the cerebellar hemispheres.
-> This is a wedge-shaped bone (G. sphen, wedge) is located anteriorly to the temporal bones.
-> It is a key bone in the cranium because it articulates with eight bones (frontal, parietal, temporal, occipital, vomer, zygomatic, palatine, and ethmoid).
-> It main parts are the body and the greater and lesser wings, which spread laterally from the body.
-> The superior surface of its body is shaped like a Turkish saddle (L. sella, a saddle); hence its name sella turcica.
-> It forms the hypophyseal fossa which contains the hypophysis cerebri or pituitary gland.
-> The sella turcica is bounded posteriorly by the dorsum sellae, a square plate of bone that projects superiorly and has a posterior clinoid process on each side.
-> Inside the body of the sphenoid bone, there are right and left sphenoid sinuses. The floor of the sella turcica forms the roof of these paranasal sinuses.
-> Studies of the sella turcica and hypophyseal fossa in radiographs or by other imaging techniques are important because they may reflect pathological changes such as a pituitary tumour or an aneurysm of the internal carotid artery. Decalcification of the dorsum sellae is one of the signs of a generalised increase in intracranial pressure.
The External Ear
- The auricle (L. auris, ear) is the visible, shell-like part of the external ear.
- It consists of a single elastic cartilage that is covered on both surfaces with thin, hairy skin.
- The external ear contains hairs, sweat glands, and sebaceous glands.
- The cartilage is irregularly ridged and hollowed, which gives the auricle its shell-like form.
- It also shapes the orifice of the external acoustic meatus.
The Ear Lobule
- The ear lobule (earlobe) consists of fibrous tissue, fat and blood vessels that are covered with skin.
- The arteries are derived mainly from the posterior auricular artery and the superficial temporal artery.
- The skin of the auricle is supplied by the great auricular and auriculotemporal nerves.
- The great auricular nerve supplies the superior surface and the lateral surface inferior to the external acoustic meatus with nerve fibres from C2.
- The auriculotemporal nerve supplies the skin of the auricle superior to the external acoustic meatus.
The External Acoustic Meatus
- This passage extends from the concha (L. shell) of the auricle to the tympanic membrane (L. tympanum, tambourine). It is about 2.5 cm long in adults.
- The lateral 1/3 of the S-shaped canal is cartilaginous, whereas its medial 2/3 is bony.
- The lateral third of the meatus is lined with the skin of the auricle and contains hair follicles, sebaceous glands, and ceruminous glands.
- The latter glands produce cerumen (L. cera, wax).
- The medial two-thirds of the meatus is lined with very thin skin that is continuous with the external layer of the tympanic membrane.
- The lateral end of the meatus is the widest part. It has the diameter about that of a pencil.
- The meatus becomes narrow at its medial end, about 4 mm from the tympanic membrane.
- The constricted bony part is called the isthmus.
- Innervation of the external acoustic meatus is derived from three cranial nerves:
- The auricular branch of the auriculotemporal nerve (derived from the mandibular, CN V3).
- The facial nerve (CN VII) by the branches from the tympanic plexus.
- The auricular branch of the vagus nerve (CN X).
The Tympanic Membrane
- This is a thin, semi-transparent, oval membrane at the medial end of the external acoustic meatus.
- It forms a partition between the external and middle ears.
- The tympanic membrane is a thin fibrous membrane, that is covered with very thin skin externally and mucous membrane internally.
- The tympanic membrane shows a concavity toward the meatus with a central depression, the umbo, which is formed by the end of the handle of the malleus.
- From the umbo, a bright area referred to as the cone of light, radiates anteroinferiorly.
- The external surface of the tympanic membrane is supplied by the auriculotemporal nerve.
- Some innervation is supplied by a small auricular branch of the vagus nerve (CN X); this nerve may also contain some glossopharyngeal and facial nerve fibres.
Mylohyoid Muscle
- Origin: Mylohyoid line of the mandible.
- Insertion: Median raphe and body of the hyoid bone.
- Nerve Supply: Nerve to mylohyoid (branch of the trigeminal nerve, CN V3).
- Arterial Supply: Sublingual branch of the lingual artery and submental branch of the facial artery.
- Action: Elevates the hyoid bone, base of the tongue, and floor of the mouth; depresses the mandible.
Levator Palpebrae Superioris Muscles
- This is a thin, triangular muscle that elevates the upper eyelid.
- It is continuously active except during sleeping and when the eye is closing.
- Origin: roof of orbit, anterior to the optic canal.
- Insertion: this muscle fans out into a wide aponeurosis that inserts into the skin of the upper eyelid. The inferior part of the aponeurosis contains some smooth muscle fibres that insert into the tarsal plate.
- Innervation: the superior fibres are innervated by the oculomotor nerve (CN III), and the smooth muscle component is innervated by fibres of the cervical sympathetic trunk and the internal carotid plexus.
Illnesses involving the Levator Palpebrae Superioris
- In third nerve palsy, the upper eyelid droops (ptosis) and cannot be raised voluntarily.
- This results from damage to the oculomotor nerve (CN III), which supplies this muscle.
- If the cervical sympathetic trunk is interrupted, the smooth muscle component of the levator palpebrae superioris is paralysed and also causes ptosis.
- This is part of Horner's syndrome.
The Rectus Muscles
- There are four rectus muscles (L. rectus, straight), superior, inferior, medial and lateral.
- These arise from a tough tendinous cuff, called the common tendinous ring, which surrounds the optic canal and the junction of the superior and inferior orbital fissures.
- From their common origin, these muscles run anteriorly, close to the walls of the orbit, and attach to the eyeball just posterior to the sclerocorneal junction.
- The medial and lateral rectus muscles attach to the medial and lateral sides of the eyeball respectively, on the horizontal axis.
- However, the superior rectus attaches to the anterosuperior aspect of the medial side of the eyeball while the inferior rectus attaches to the anteroinferior aspect of the medial side of the eye.
The Oblique Muscles
The Superior Oblique Muscle
- This muscle arises from the body of the sphenoid bone, superomedial to the common tendinous ring.
- It passes anteriorly, superior and medial to the superior and medial rectus muscles.
- It ends as a round tendon that runs through a pulley-like loop called the trochlea (L. pulley).
- After passing though the trochlea, the tendon of the superior oblique turns posterolaterally and inserts into the sclera at the posterosuperior aspect of the lateral side of the eyeball.
The Inferior Oblique Muscle
- This muscle arises from the maxilla in the floor of the orbit.
- It passes laterally and posteriorly, inferior to the inferior rectus muscle.
- It inserts into the sclera at the posteroinferior aspect of the lateral side of the eyeball.