NEET MDS Lessons
Anatomy
Walls of the Tympanic Cavity or Middle Ear
- This cavity is shaped like a narrow six-sided box that has convex medial and lateral walls.
- It has the shape of the biconcave lens in cross-section (like a red blood cell).
The Roof or Tegmental Wall
- This is formed by a thin plate of bone, called the tegmen tympani (L. tegmen, roof).
- It separates the tympanic cavity from the dura on the floor of middle cranial fossa.
- The tegmen tympani also covers the aditus ad antrum.
The Floor or Jugular Wall
- This wall is thicker than the roof.
- It separates the tympanic cavity from the superior bulb of the internal jugular vein. The internal jugular vein and the internal carotid artery diverge at the floor of the tympanic cavity.
- The tympanic nerve, a branch of the glossopharyngeal nerve (CN IX), passes through an aperture in the floor of the tympanic cavity and its branches form the tympanic plexus.
The Lateral or Membranous Wall
- This is formed almost entirely by the tympanic membrane.
- Superiorly it is formed by the lateral bony wall of the epitympanic recess.
- The handle of the malleus is incorporated in the tympanic membrane, and its head extends into the epitympanic recess.
The Medial or Labyrinthine Wall
- This separates the middle ear from the membranous labyrinth (semicircular ducts and cochlear duct) encased in the bony labyrinth.
- The medial wall of the tympanic cavity exhibits several important features.
- Centrally, opposite the tympanic membrane, there is a rounded promontory (L. eminence) formed by the first turn of the cochlea.
- The tympanic plexus of nerves, lying on the promontory, is formed by fibres of the facial and glossopharyngeal nerves.
- The medial wall of the tympanic cavity also has two small apertures or windows.
- The fenestra vestibuli (oval window) is closed by the base of the stapes, which is bound to its margins by an annular ligament.
- Through this window, vibrations of the stapes are transmitted to the perilymph window within the bony labyrinth of the inner ear.
- The fenestra cochleae (round window) is inferior to the fenestra vestibuli.
- This is closed by a second tympanic membrane.
The Posterior or Mastoid Wall
- This wall has several openings in it.
- In its superior part is the aditus ad antrum (mastoid antrum), which leads posteriorly from the epitympanic recess to the mastoid cells.
- Inferiorly is a pinpoint aperture on the apex of a tiny, hollow projection of bone, called the pyramidal eminence (pyramid).
- This eminence contains the stapedius muscle.
- Its aperture transmits the tendon of the stapedius, which enters the tympanic cavity and inserts into the stapes.
- Lateral to the pyramid, there is an aperture through which the chorda tympani nerve, a branch of the facial nerve (CN VII), enters the tympanic cavity.
The Anterior Wall or Carotid Wall
- This wall is a narrow as the medial and lateral walls converge anteriorly.
- There are two openings in the anterior wall.
- The superior opening communicates with a canal occupied by the tensor tympani muscle.
- Its tendon inserts into the handle of the malleus and keeps the tympanic membrane tense.
- Inferiorly, the tympanic cavity communicates with the nasopharynx through the auditory tube.
Geniohyoid Muscle
- Origin: Inferior genial tubercles of the mandible.
- Insertion: Anterior surface of the body of the hyoid bone.
- Nerve Supply: Branch of C1 through the hypoglossal nerve (CN XII).
- Arterial Supply: Sublingual branch of the lingual artery.
- Action: Elevates the hyoid bone and depresses the mandible.
Intrinsic muscles
all innervated by recurrent laryngeal nerve except cricothyroid: external laryngeal nerve
adductors of vocal folds: bring folds together at midline
Transverse and oblique arytenoids: pull arytenoids together
Lateral cricoartenoids: spin and slide arytenoids up
only one abductor of vocal folds
Posterior cricoarytenoids—down and up
adjustors of shape and tension of vocal folds
Cricothyroid muscle
o superficial to lateral cricoarytenoid
o tenses vocal folds by tilting thyroid cartilage forward and sliding forward
Thyroartenoid and vocalis muscles
o vocalis: sometimes treated as medial most fibers of thyroartenoid muscle
o different fiber directions
lateral: adduct
medial: change shape of folds
control voice by bring bringing together different parts of folds
o as move from epithelium to vocalis muscle, fold becomes stiffer
o near connections, vocal folds are stiffer
o vocal fold: complex, multilayered vibrator
Appendicular Skeleton
Upper extremity
• Shoulder-clavicle and scapula
Clavicle
Articulates with the manubrium at the sternal end
Articulates with the scapula at the lateral end
Slender S-shaped bone that extends horizontally across the upper part of the thorax
Scapula
Triangular bone with the base upward and the apex downward
Lateral aspect contains the glenoid cavity that articulates with the head of the humerus
Spine extends across the upper part of the posterior surface; expands laterally and
forms the acromion (forms point of shoulder)
Coracoid process projects anteriorly from the upper part of the neck of the scapula
Arm (humerus)
Consists of a shaft (diaphysis) and two ends (epiphyses)
Proximal end has a head that articulates with the glenoid fossa of the scapula
Greater and lesser tubercles lie below the head
Intertubercular groove is located between them; long tendon of the biceps attaches here
Surgical neck is located below the tubercles
o Radial groove runs obliquely on the posterior surface; radial nerve is located here
o Deltoid muscles attaches in a V-shaped area in the middle of the shaft. called the deltoid tuberosity
Distal end has two projections. the medial and lateral epicondyles
Capitulum-articulates with the radius
Trochlea-articulates with the ulqa
Forearm
Radius
Lateral bone of the forearm
Radial tuberosity is located below the head on the medial side
Distal end is broad for articulation with the wrist: has a styloid process on its lateral side
Ulna
Medial side of the forearm
Conspicuous part of the elbow joint (olecranon)
Curved surface that articulates with the trochlea of the humerus is the trochlearnotch
Lateral ide is concave (radial notch); articulates with the head of the radius Distal end contains the styloid process
Distal end contains the styloid process
Hand
Carpal bones (8)
Aranged in two rows of four
Scaphoid. lunate. triquetral. and pisiform proximal row); trapezium. trapezoid.
capitate. and hamate (distal row)
Metacarpal bones (5)
Framework of the hand
Numbered 1 to 5 beginning on the lateral side
Phalanges (14)
Fingers
Three phalanges in each finger; two phalanges in the thumb
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 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.
Endochondral ossification
- A cartilage model exists
- Through intramembraneous ossification in the perichondrium a collar of bone forms around the middle part of the cartilage model
- The perichondrium change to a periostium
- The bone collar cuts off the nutrient and oxygen supply to the chondrocytes in the cartilage model
- The chondrocytes then increase in size and resorb the surrounding cartilage matrix until only thin vertical septae of matrix are left over
- These thin plates then calcify after which the chondrocytes die
- The osteoclasts make holes in the bone collar through which blood vessels can now enter the cavities left behind by the chondrocytes
- With the blood vessels osteoprogenitor cells enter the tissue
- They position themselves on the calcified cartilage septae, change into osteoblasts and start to deposit bone to form trabeculae
- In the mean time the periosteum is depositing bone on the outside of the bone collar making it thicker and thicker
- The trabeculae,consisting of a core of calcified cartilage with bone deposited on top of it, are eventually resorbed by osteoclasts to form the marrow cavity
- The area where this happens is the primary ossification centre and lies in what is called the diaphysis (shaft)
- This process spreads in two directions towards the two ends of the bone the epiphysis
- In the two ends (heads) of the bone a similar process takes place
- A secondary ossification centre develops from where ossification spreads radially
- Here no bone collar forms
- The outer layer of the original cartilage remains behind to form the articulating cartilage
- Between the primary and the secondary ossification centers two epiphyseal cartilage plates remain
- This is where the bone grows in length
- From the epiphyseal cartilage plate towards the diaphysis a number of zones can be identified:
Resting zone of cartilage
Hyaline cartilage
Proliferation zone
Chondrocytes divide to form columns of cells that mature.
Hypertrophic cartilage zone
Chondrocytes become larger, accumulate glycogen, resorb the surrounding matrix so that only thin septae of cartilage remain
Calcification and degeneration zone
The thin septae of cartilage become calcified.
The calsified septae cut off the nutrient supply to the chondrocytes so subsequently they die.
Ossification zone.
Osteoclasts make openings in the bone collar through which blood vessels then invade the spaces left vacant by the chondrocytes that died.
Osteoprogenitor cells come in with the blood and position themselves on the calcified cartilage
septae, change into osteoblasts and start to deposit bone.
When osteoblasts become trapped in bone they change to osteocytes.
Growth and remodeling of bone
Long bones become longer because of growth at the epiphyseal plates
They become wider because of bone formed by the periosteum
The marrow cavity becomes bigger because of resorbtion by the osteoclasts
Fracture repair
When bone is fractured a blood clot forms
Macrophages then remove the clot, remaining osteocytes and damaged bone matrix
The periosteum and endosteum produce osteoprogenitor cells that form a cellular tissue in the fracture area
Intramembranous and endochondral ossification then take place in this area forming trabeculae.
Trabeculae connect the two ends of the broken bone to form a callus
Remodelling then takes place to restore the bone as it was
Joints
The capsule of a joint seals off the articular cavity,
The capsule has two layers
fibrous (outer)
synovial (inner)
The synovial layer is lined by squamous or cuboidal epithelial cells, Under this layer is a layer of loose or dense CT, The lining cells consists of two types:
- A cells
- B cells
They secrete the synovial fluid
They are different stages of the same cell, They are also phagocytic., The articular cartilage has fibres that run perpendicular to the bone and then turn to run parallel to the surface