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.
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| Motor Innervation | All muscles by hypoglossal nerve (CN XII) except palatoglossus muscle (by the pharyngeal plexus) | ||
| General Sensory Innervation |
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| Special Sensory Innervation |
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The Oropharynx
- The oral part of the pharynx has a digestive function.
- It is continuous with the oral cavity through the oropharyngeal isthmus.
- The oropharynx is bounded by the soft palate superiorly, the base of the tongue inferiorly, and the palatoglossal and palatopharyngeal arches laterally.
- It extends from the soft palate to the superior border of the epiglottis.
The Palatine Tonsils
- These are usually referred to as "the tonsils".
- They are collections of lymphoid tissue the lie on each side of the oropharynx in the triangular interval between the palatine arches.
- The palatine tonsils vary in size from person to person.
- In children, the palatine tonsils tend to be large, whereas in older persons they are usual small and inconspicuous.
- The visible part of the tonsil is no guide to its actual size because much of it may be hidden by the tongue and buried in the soft 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.
The Palatopharyngeus Muscle
- Superior attachment: hard palate 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.
Skeletal Muscle: 1-40 cm long fibres, 10- 60 µm thick, according to myoglobin content there are:
Red fibres: lots of myoglobin, many mitochondriam slow twitching - tire slowly
White fibres: less myoglobin, less mitochondria, fast twitching - tire quickly
Intermediate fibres:
mixture of 2 above
Most muscles have all three - in varying ratios
Structure of skeletal muscle:
Light Microscopy: Many nuclei - 35/mm, Nuclei are oval - situated peripheral, Dark and light bands
Electron Microscopy: Two types of myofilaments
Actin
- 5,6 nm
3 components:
-actin monomers,
-tropomyosin - 7 actin molecules long
- troponin
actin monomers form 2 threats that spiral
- tropomyosin - lie in the groove of the spiral
- troponin - attach every 40 nm
- one end attach to the Z line
- other end goes to the middle of the sarcomere
- Z line consists of á actinin
Myosin:
- 15 nm
- 1,6 µm long
- The molecule has a head and a tail
- tails are parallel
- heads project in a spiral
- in the middle is a thickening
- thin threats bind the myosin at thickening (M line)
Contraction:
A - band stays the same, I - band, H - bands become narrower
Myosin heads ratchet on the actin molecule
Sarcolemma: 9 nm thick, invaginate to form T-tubule,
myofibrils - attach to the sarcolemma
Sarcoplasmic Reticulum:
specialized smooth EPR, Consists of T-tubules, terminal sisternae and sarcotubules
It is speculated that there are gap junctions between the T-tubule and terminal sisterna
An impulse is carried into the fiber by the T-tubule from where it goes to the rest of the sarcoplasmic reticulum
Connective tissue coverings of the muscle
Endomycium around fibres, perimycium around bundles and epimycium around the whole muscle
Blood vessels and nerves in CT
CT goes over into tendon or aponeurosis which attaches to the periosteum
Nerves:
The axon of a motor neuron branches and ends in motor end plates on the fiber
Specialized striated fibres called spindles (stretch receptors) form sensory receptors in muscles telling the brain how far the muscle has stretched
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Ossification
- Intramembranous-found in the flat bones of the face
- Mesenchymal cells cluster and form strands
- Strands are cemented in a uniform network. Which is known as osteoid
- Calcium salts are deposited; osteoid is converted to bone
- Trabeculae are formed and make cancellous bone with open spaces known as marrow cavities
- Periosteum forms on the inner and outer surfaces of the ossification centers
- Surface bone becomes compact bone
- Endochondral-primary type of ossification In the human
The Palate
- The palate forms the arched roof of the mouth and the floor of the nasal cavities.
- The palate consists of two regions: the anterior 2/3 or bony part, called the hard palate, and the mobile posterior 1/3 or fibromuscular part, known as the soft palate.