NEET MDS Lessons
Anatomy
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.
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
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.
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.
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.
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.
The Laryngopharynx
- The laryngeal part of the pharynx lies posterior to the larynx.
- It extends from the superior border of the epiglottis to the inferior border of the cricoid cartilage, where it narrows to become continuous with the oesophagus.
- Posteriorly, the laryngopharynx is related to the bodies of C4 to C6 vertebrae.
- Its posterior and lateral walls are formed by the middle and inferior constrictor muscles, with the palatopharyngeus and stylopharyngeus internally.
- The laryngopharynx communicates with the larynx through the aditus or inlet of the larynx.
- The piriform recess is a small, pear-shaped depression of the laryngopharyngeal cavity on each side of the inlet of the larynx.