NEET MDS Lessons
Anatomy
The Nasopharynx
- The nasal part of the pharynx has a respiratory function.
- It lies superior to the soft palate and is a posterior extension of the nasal cavity.
- The nose opens into the nasopharynx via to large posterior apertures called choanae.
- The roof and posterior wall of the nasopharynx form a continuous surface that lies inferior to the body of the sphenoid bone and the basilar part of the occipital bone.
- In the mucous membrane of the roof of the posterior wall of the nasopharynx is a collection of lymphoid tissue, known as the pharyngeal tonsil (commonly known as the adenoids).
- The pharyngeal orifice of the auditory tube is on the lateral wall of the nasopharynx, 1 to 1.5 cm posterior to the inferior concha, and level with the superior border of the palate.
- The orifice is directed inferiorly and has a hood-like tubal elevation over it called the torus of the auditory tube or the torus tubarius (L. torus, swelling).
- Extending inferiorly from the torus is a vertical fold of mucous membrane, known as the salpingopharyngeal fold.
- The collection of lymphoid tissue in the submucosa of the pharynx, posterior to the orifice of the auditory tube, is known as the tubal tonsil.
- Posterior to the torus and the salpingopharyngeal fold, there is a slit-like lateral projection of the pharynx called the pharyngeal recess.
- It extends laterally and posteriorly.
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.
->The sides and base of the skull are formed partly by these bones.
->Each bone consists of four morphologically distinct parts that fuse during development (squamous, petromastoid, and tympanic parts and the styloid process).
->The flat squamous part is external to the lateral surface of the temporal lobe of the brain.
->The petromastoid part encloses the internal ear and mastoid cells and forms part of the base of the skull.
->The tympanic part contains the bony passage from the auricle (external ear), called the external acoustic meatus. The petromastoid part also forms a portion of the bony wall of the tympanic cavity (middle ear). The meatus and tympanic cavity are concerned with the transmission of sound waves.
->The slender, pointed styloid process of the temporal bone gives attachment to certain ligaments and muscles (e.g., the stylohyoid muscle that elevates the hyoid bone).
->The temporal bone articulates at sutures with the parietal, occipital, sphenoid, and zygomatic bones.
->The zygomatic process of the temporal bone unites with the temporal process of the zygomatic bone to form the zygomatic arch. The zygomatic arches form the widest part of the face.
->The head of the mandible articulates with the mandibular fossa on the inferior surface of the zygomatic process of the temporal bone.
->Anterior to the mandibular fossa is the articular tubercle.
->Because the zygomatic arches are the widest parts of the face and are such prominent facial features, they are commonly fractured and depressed. A fracture of the temporal process of the zygomatic bone would likely involve the lateral wall of the orbit and could injure the eye.
The Articular Capsule
- The capsule of this joint is loose.
- The thin fibrous capsule is attached to the margins of the articular area on the temporal bone and around the neck of the mandible.
The Scalp
- The scalp consists of five layers of soft tissue.
- It extends from the superior nuchal line on the posterior aspect of the skull of the supraorbital margins.
- Laterally, the scalp extends into the temporal fossa to the level of the zygomatic arches.
Layers of the Scalp
- The scalp proper is composed of three fused layers. It is separated from the pericranium by loose connective tissue.
- Because of this potential areolar cleavage plane, the scalp is fairly mobile.
- Each letter of the word "S C A L P" serves as a memory key for one of the layers of the scalp: Skin, Connective Tissue, Aponeurosis Epicranialis, Loose Areolar Tissue and Pericranium.
Layer 1: Skin
- Hair covers the scalp in most people.
- The skin of the scalp is thin, especially in elderly people, except in the occipital region.
- The skin contains many sweat and sebaceous glands and hair follicles.
- The skin of the scalp has an abundant arterial supply and good venous and lymphatic drainage systems.
Layer 2: Connective Tissue
- This is a thick, subcutaneous layer of connective tissue and is richly vascularised and innervated.
- It attaches the skin to the third layer of the scalp.
- Fat is enclosed in lobules between the connective fibres.
Layer 3: Aponeurosis Epicranialis
- This is a strong membranous sheet that covers the superior aspect of the cranium.
- This aponeurosis is the membranous tendon of the fleshy bellies of the epicranius muscle.
- The epicranius muscle consists of four parts: two occipital bellies, occipitalis and two frontal bellies, frontalis that are connected by the epicranial aponeurosis.
Layer 4: Loose Areolar Tissue
- This is a subaponeurotic layer or areolar or loose connective tissue.
- It is somewhat like a sponge because it contains innumerable potential spaces that are capable of being distended by fluid.
- It is this layer that allows free movement of the scalp proper, composed of layers 1-3.
Layer 5: Pericranium
- This is a dense layer of specialised connective tissue.
- The pericranium is firmly attached to the bones by connective tissue fibres called Sharpey’s fibres, however, they can be fairly easily stripped from the cranial bones of living persons, except where they are continuous with the fibrous tissues of the cranial sutures.
Nerves of the Palate
- The sensory nerves of the palate, which are branches of the pterygopalatine ganglion, are the greater and lesser palatine nerves.
- They accompany the arteries through the greater and lesser palatine foramina, respectively.
- The greater palatine nerve supplies the gingivae, mucous membrane, and glands of the hard palate.
- The lesser palatine nerve supplies the soft palate.
- Another branch of the pterygopalatine ganglion, the nasopalatine nerve, emerges from the incisive foramen and supplies the mucous membrane of the anterior part of the hard palate.
Muscles of the Soft Palate
The Levator Veli Palatini (Levator Palati)
- Superior attachment: cartilage of the auditory tube and petrous part of temporal bone.
- Inferior attachment: palatine aponeurosis.
- Innervation: pharyngeal branch of vagus via pharyngeal plexus.
- This cylindrical muscle runs inferoanteriorly, spreading out in the soft palate, where it attaches to the superior surface of the palatine aponeurosis.
- It elevates the soft palate, drawing it superiorly and posteriorly.
- It also opens the auditory tube to equalise air pressure in the middle ear and pharynx.
The Tensor Veli Palatini (Tensor Palati)
- Superior attachment: scaphoid fossa of medial pterygoid plate, spine of sphenoid bone, and cartilage of auditory tube.
- Inferior attachment: palatine aponeurosis.
- Innervation: medial pterygoid nerve (a branch of the mandibular nerve).
- This thin, triangular muscle passes inferiorly, and hooks around the hamulus of the medial pterygoid plate.
- It then inserts into the palatine aponeurosis.
- This muscle tenses the soft palate by using the hamulus as a pulley.
- It also pulls the membranous portion of the auditory tube open to equalise air pressure of the middle ear and pharynx.
The Palatoglossus Muscle
- Superior attachment: palatine aponeurosis.
- Inferior attachment: side of tongue.
- Innervation: cranial part of accessory nerve (CN XI) through the pharyngeal branch of vagus (CN X) via the pharyngeal plexus.
- This muscle, covered by mucous membrane, forms the palatoglossal arch.
- The palatoglossus elevates the posterior part of the tongue and draws the soft palate inferiorly onto the tongue.
- Superior attachment: hard palatThe Palatopharyngeus Musclee and palatine aponeurosis.
- Inferior attachment: lateral wall of pharynx.
- Innervation: cranial part of accessory nerve (CN XI) through the pharyngeal branch of vagus (CN X) via the pharyngeal plexus.
- This thin, flat muscle is covered with mucous membrane to form the palatopharyngeal arch.
- It passes posteroinferiorly in this arch.
- This muscle tenses the soft palate and pulls the walls of the pharynx superiorly, anteriorly and medially during swallowing.
The Musculus Uvulae
- Superior attachment: posterior nasal spine and palatine aponeurosis.
- Inferior attachment: mucosa of uvula.
- Innervation: cranial part of accessory through the pharyngeal branch of vagus, via the pharyngeal plexus.
- It passes posteriorly on each side of the median plane and inserts into the mucosa of the uvula.
- When the muscle contracts, it shortens the uvula and pulls it superiorly.